# Reactor Netty Reference Guide

Stephane Maldini
@smaldini
Violeta Georgieva
@violeta_g_g
version 1.0.6

## 1. About the Documentation

This section provides a brief overview of `Reactor Netty` reference documentation. You do not need to read this guide in a linear fashion. Each piece stands on its own, though they often refer to other pieces.

### 1.1. Latest Version and Copyright Notice

The `Reactor Netty` reference guide is available as `HTML` documents. The latest copy is available at https://projectreactor.io/docs/netty/release/reference/index.html

Copies of this document may be made for your own use and for distribution to others, provided that you do not charge any fee for such copies and further provided that each copy contains this `Copyright Notice`, whether distributed in print or electronically.

### 1.2. Contributing to the Documentation

The reference guide is written in Asciidoc, and you can find its sources at https://github.com/reactor/reactor-netty/tree/master/docs/asciidoc.

If you have an improvement, we will be happy to get a pull request from you!

We recommend that you check out a local copy of the repository so that you can generate the documentation by using the `asciidoctor` Gradle task and checking the rendering. Some of the sections rely on included files, so `GitHub` rendering is not always complete.

| To facilitate documentation edits, most sections have a link at the end that opens an edit `UI` directly on `GitHub` for the main source file for that section. These links are only present in the `HTML5` version of this reference guide. They look like the following link: Suggest Edit to About the Documentation.

### 1.3. Getting Help

There are several ways to reach out for help with `Reactor Netty`. You can:

- Get in touch with the community on Gitter.

- Ask a question on stackoverflow.com at `reactor-netty`.

- Report bugs in `Github` issues. The repository is the following: reactor-netty.

| All of `Reactor Netty` is open source, including this documentation.

Suggest Edit to "About the Documentation"

## 2. Getting Started

This section contains information that should help you get going with `Reactor Netty`. It includes the following information:

- Introducing Reactor Netty

- Prerequisites

- Understanding the BOM and versioning scheme

- Getting Reactor Netty

### 2.1. Introducing Reactor Netty

Suited for Microservices Architecture, `Reactor Netty` offers backpressure-ready network engines for `HTTP` (including Websockets), `TCP`, and `UDP`.

### 2.2. Prerequisites

`Reactor Netty` runs on `Java 8` and above.

It has transitive dependencies on:

- Reactive Streams v1.0.3

- Reactor Core v3.x

- Netty v4.1.x

### 2.3. Understanding the BOM and versioning scheme

`Reactor Netty` is part of the `Project Reactor BOM` (since the `Aluminium` release train). This curated list groups artifacts that are meant to work well together, providing the relevant versions despite potentially divergent versioning schemes in these artifacts.

| The versioning scheme has changed between 0.9.x and 1.0.x (Dysprosium and Europium).

Artifacts follow a versioning scheme of `MAJOR.MINOR.PATCH-QUALIFIER` while the BOM is versioned using a CalVer inspired scheme of `YYYY.MINOR.PATCH-QUALIFIER`, where:

- `MAJOR` is the current generation of Reactor, where each new generation can bring fundamental changes to the structure of the project (which might imply a more significant migration effort)

- `YYYY` is the year of the first GA release in a given release cycle (like 1.0.0 for 1.0.x)

- `.MINOR` is a 0-based number incrementing with each new release cycle

  - in the case of projects, it generally reflects wider changes and can indicate a moderate migration effort

  - in the case of the BOM it allows discerning between release cycles in case two get first released the same year

- `.PATCH` is a 0-based number incrementing with each service release

- `-QUALIFIER` is a textual qualifier, which is omitted in the case of GA releases (see below)

The first release cycle to follow that convention is thus `2020.0.x`, codename `Europium`. The scheme uses the following qualifiers (note the use of dash separator), in order:

- `-M1`..`-M9`: milestones (we don’t expect more than 9 per service release)

- `-RC1`..`-RC9`: release candidates (we don’t expect more than 9 per service release)

- `-SNAPSHOT`: snapshots

- no qualifier for GA releases

| Snapshots appear higher in the order above because, conceptually, they’re always "the freshest pre-release" of any given PATCH. Even though the first deployed artifact of a PATCH cycle will always be a -SNAPSHOT, a similarly named but more up-to-date snapshot would also get released after eg. a milestone or between release candidates.

Each release cycle is also given a codename, in continuity with the previous codename-based scheme, which can be used to reference it more informally (like in discussions, blog posts, etc…​). The codenames represent what would traditionally be the MAJOR.MINOR number. They (mostly) come from the Periodic Table of Elements, in increasing alphabetical order.

| Up until Dysprosium, the BOM was versioned using a release train scheme with a codename followed by a qualifier, and the qualifiers were slightly different. For example: Aluminium-RELEASE (first GA release, would now be something like YYYY.0.0), Bismuth-M1, Californium-SR1 (service release would now be something like YYYY.0.1), Dysprosium-RC1, Dysprosium-BUILD-SNAPSHOT (after each patch, we’d go back to the same snapshot version. would now be something like YYYY.0.X-SNAPSHOT so we get 1 snapshot per PATCH)

### 2.4. Getting Reactor Netty

As mentioned earlier, the easiest way to use `Reactor Netty` in your core is to use the `BOM` and add the relevant dependencies to your project. Note that, when adding such a dependency, you must omit the version so that the version gets picked up from the `BOM`.

However, if you want to force the use of a specific artifact’s version, you can specify it when adding your dependency as you usually would. You can also forego the `BOM` entirely and specify dependencies by their artifact versions.

#### 2.4.1. Maven Installation

The `BOM` concept is natively supported by `Maven`. First, you need to import the `BOM` by adding the following snippet to your `pom.xml`. If the top section (`dependencyManagement`) already exists in your pom, add only the contents.

    <dependencyManagement> (1)
        <dependencies>
            <dependency>
                <groupId>io.projectreactor</groupId>
                <artifactId>reactor-bom</artifactId>
                <version>Dysprosium-SR10</version> (2)
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

1 | Notice the `dependencyManagement` tag. This is in addition to the regular `dependencies` section.
2 | As of this writing, `Dysprosium-SR10` is the latest version of the `BOM`. Check for updates at https://github.com/reactor/reactor/releases.

Next, add your dependencies to the relevant reactor projects, as usual (except without a `<version>`). The following listing shows how to do so:

    <dependencies>
        <dependency>
            <groupId>io.projectreactor.netty</groupId>
            <artifactId>reactor-netty-core</artifactId> (1)
            (2)
        </dependency>
    </dependencies>
    <dependencies>
        <dependency>
            <groupId>io.projectreactor.netty</groupId>
            <artifactId>reactor-netty-http</artifactId>
        </dependency>
    </dependencies>

1 | Dependency on `Reactor Netty`
2 | No version tag here

#### 2.4.2. Gradle Installation

The `BOM` concept is supported in Gradle since version 5. The following listing shows how to import the `BOM` and add a dependency to `Reactor Netty`:

    dependencies {
        // import a BOM
        implementation platform('io.projectreactor:reactor-bom:Dysprosium-SR10') (1)

        // define dependencies without versions
        implementation 'io.projectreactor.netty:reactor-netty-core' (2)
        implementation 'io.projectreactor.netty:reactor-netty-http'
    }

1 | As of this writing, `Dysprosium-SR10` is the latest version of the `BOM`. Check for updates at https://github.com/reactor/reactor/releases.
2 | There is no third `:` separated section for the version. It is taken from the `BOM`.

#### 2.4.3. Milestones and Snapshots

Milestones and developer previews are distributed through the `Spring Milestones` repository rather than `Maven Central`. To add it to your build configuration file, use the following snippet:

Milestones in Maven

    <repositories>
    	<repository>
    		<id>spring-milestones</id>
    		<name>Spring Milestones Repository</name>
    		<url>https://repo.spring.io/milestone</url>
    	</repository>
    </repositories>

For Gradle, use the following snippet:

Milestones in Gradle

    repositories {
      maven { url 'https://repo.spring.io/milestone' }
      mavenCentral()
    }

Similarly, snapshots are also available in a separate dedicated repository (for both Maven and Gradle):

-SNAPSHOTs in Maven

    <repositories>
    	<repository>
    		<id>spring-snapshots</id>
    		<name>Spring Snapshot Repository</name>
    		<url>https://repo.spring.io/snapshot</url>
    	</repository>
    </repositories>

-SNAPSHOTs in Gradle

    repositories {
      maven { url 'https://repo.spring.io/snapshot' }
      mavenCentral()
    }

### 2.5. Support and policies

The entries below are mirroring https://github.com/reactor/.github/blob/master/SUPPORT.adoc

#### 2.5.1. Do you have a question?

| Search Stack Overflow first; discuss if necessary

If you’re unsure why something isn’t working or wondering if there is a better way of doing it please check on Stack Overflow first and if necessary start a discussion. Use relevant tags among the ones we monitor for that purpose:

- `reactor-netty` for specific reactor-netty questions

- `project-reactor` for generic reactor questions

If you prefer real-time discussion, we also have a few Gitter channels:

- `reactor` is the historic most active one, where most of the community can help

- `reactor-core` is intended for more advanced pinpointed discussions around the inner workings of the library

- `reactor-netty` is intended for netty-specific questions

Refer to each project’s README for potential other sources of information.

We generally discourage opening GitHub issues for questions, in favor of the two channels above.

#### 2.5.2. Our policy on deprecations

When dealing with deprecations, given a version `A.B.C`, we’ll ensure that:

- deprecations introduced in version `A`.`B`.`0` will be removed no sooner than version `A`.`B+1`.`0`

- deprecations introduced in version `A`.`B`.`1+` will be removed no sooner than version `A`.`B+2`.`0`

- we’ll strive to mention the following in the deprecation javadoc:

  - target minimum version for removal

  - pointers to replacements for the deprecated method

  - version in which method was deprecated

| This policy is officially in effect as of January 2021, for all modules in `2020.0` BOMs and newer release trains, as well as `Dysprosium` releases after `Dysprosium-SR15`.

| Deprecation removal targets are not a hard commitment, and the deprecated methods could live on further than these minimum target GA versions (ie. only the most problematic deprecated methods will be removed aggressively).

| That said, deprecated code that has outlived its minimum removal target version may be removed in any subsequent release (including patch releases, aka service releases) without further notice. So users should still strive to update their code as early as possible.

Suggest Edit to "Getting Started"

## 3. TCP Server

`Reactor Netty` provides an easy to use and configure `TcpServer`. It hides most of the `Netty` functionality that is needed to create a `TCP` server and adds `Reactive Streams` backpressure.

### 3.1. Starting and Stopping

To start a `TCP` server, you must create and configure a `TcpServer` instance. By default, the `host` is configured for any local address, and the system picks up an ephemeral port when the `bind` operation is invoked. The following example shows how to create and configure a `TcpServer` instance:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/create/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()   (1)
    				         .bindNow(); (2)

    		server.onDispose()
    		      .block();
    	}
    }

1 | Creates a `TcpServer` instance that is ready for configuring.
2 | Starts the server in a blocking fashion and waits for it to finish initializing.

The returned `DisposableServer` offers a simple server API, including `disposeNow()`, which shuts the server down in a blocking fashion.

#### 3.1.1. Host and Port

To serve on a specific `host` and `port`, you can apply the following configuration to the `TCP` server:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/address/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .host("localhost") (1)
    				         .port(8080)        (2)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Configures the `TCP` server host
2 | Configures the `TCP` server port

### 3.2. Eager Initialization

By default, the initialization of the `TcpServer` resources happens on demand. This means that the `bind operation` absorbs the extra time needed to initialize and load:

- the event loop groups

- the native transport libraries (when native transport is used)

- the native libraries for the security (in case of `OpenSsl`)

When you need to preload these resources, you can configure the `TcpServer` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/warmup/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		TcpServer tcpServer =
    				TcpServer.create()
    				         .handle((inbound, outbound) -> inbound.receive().then());

    		tcpServer.warmup() (1)
    		         .block();

    		DisposableServer server = tcpServer.bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Initialize and load the event loop groups, the native transport libraries and the native libraries for the security

### 3.3. Writing Data

In order to send data to a connected client, you must attach an I/O handler. The I/O handler has access to `NettyOutbound` to be able to write data. The following example shows how to attach an I/O handler:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/send/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .handle((inbound, outbound) -> outbound.sendString(Mono.just("hello"))) (1)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Sends `hello` string to the connected clients

### 3.4. Consuming Data

In order to receive data from a connected client, you must attach an I/O handler. The I/O handler has access to `NettyInbound` to be able to read data. The following example shows how to use it:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/read/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .handle((inbound, outbound) -> inbound.receive().then()) (1)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Receives data from the connected clients

### 3.5. Lifecycle Callbacks

The following lifecycle callbacks are provided to let you extend the `TcpServer`:

Callback | Description
--- | ---
`doOnBind` | Invoked when the server channel is about to bind.
`doOnBound` | Invoked when the server channel is bound.
`doOnChannelInit` | Invoked when initializing the channel.
`doOnConnection` | Invoked when a remote client is connected
`doOnUnbound` | Invoked when the server channel is unbound.

The following example uses the `doOnConnection` and `doOnChannelInit` callbacks:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/lifecycle/Application.java

    import io.netty.handler.logging.LoggingHandler;
    import io.netty.handler.timeout.ReadTimeoutHandler;
    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;
    import java.util.concurrent.TimeUnit;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .doOnConnection(conn ->
    				             conn.addHandler(new ReadTimeoutHandler(10, TimeUnit.SECONDS))) (1)
    				         .doOnChannelInit((observer, channel, remoteAddress) ->
    				             channel.pipeline()
    				                    .addFirst(new LoggingHandler("reactor.netty.examples")))(2)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | `Netty` pipeline is extended with `ReadTimeoutHandler` when a remote client is connected.
2 | `Netty` pipeline is extended with `LoggingHandler` when initializing the channel.

### 3.6. TCP-level Configurations

This section describes three kinds of configuration that you can use at the TCP level:

- Setting Channel Options

- Using a Wire Logger

- Using an Event Loop Group

#### 3.6.1. Setting Channel Options

By default, the `TCP` server is configured with the following options:

./../../reactor-netty-core/src/main/java/reactor/netty/tcp/TcpServerBind.java

    TcpServerBind() {
    	Map<ChannelOption<?>, Boolean> childOptions = new HashMap<>(2);
    	childOptions.put(ChannelOption.AUTO_READ, false);
    	childOptions.put(ChannelOption.TCP_NODELAY, true);
    	this.config = new TcpServerConfig(
    			Collections.singletonMap(ChannelOption.SO_REUSEADDR, true),
    			childOptions,
    			() -> new InetSocketAddress(DEFAULT_PORT));
    }

If additional options are necessary or changes to the current options are needed, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/channeloptions/Application.java

    import io.netty.channel.ChannelOption;
    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

You can find more about `Netty` channel options at the following links:

- Common ChannelOption

- Epoll ChannelOption

- KQueue ChannelOption

- Socket Options

#### 3.6.2. Using a Wire Logger

Reactor Netty provides wire logging for when the traffic between the peers has to be inspected. By default, wire logging is disabled. To enable it, you must set the logger `reactor.netty.tcp.TcpServer` level to `DEBUG` and apply the following configuration;

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/wiretap/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .wiretap(true) (1)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Enables the wire logging

By default, the wire logging uses AdvancedByteBufFormat#HEX_DUMP when printing the content. When you need to change this to AdvancedByteBufFormat#SIMPLE or AdvancedByteBufFormat#TEXTUAL, you can configure the `TcpServer` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/wiretap/custom/Application.java

    import io.netty.handler.logging.LogLevel;
    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;
    import reactor.netty.transport.logging.AdvancedByteBufFormat;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .wiretap("logger-name", LogLevel.DEBUG, AdvancedByteBufFormat.TEXTUAL) (1)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Enables the wire logging, AdvancedByteBufFormat#TEXTUAL is used for printing the content.

#### 3.6.3. Using an Event Loop Group

By default, the `TCP` server uses an “Event Loop Group,” where the number of the worker threads equals the number of processors available to the runtime on initialization (but with a minimum value of 4). When you need a different configuration, you can use one of the LoopResource`#create` methods.

The default configuration for the `Event Loop Group` is the following:

./../../reactor-netty-core/src/main/java/reactor/netty/ReactorNetty.java

    /**
     * Default worker thread count, fallback to available processor
     * (but with a minimum value of 4)
     */
    public static final String IO_WORKER_COUNT = "reactor.netty.ioWorkerCount";
    /**
     * Default selector thread count, fallback to -1 (no selector thread)
     */
    public static final String IO_SELECT_COUNT = "reactor.netty.ioSelectCount";
    /**
     * Default worker thread count for UDP, fallback to available processor
     * (but with a minimum value of 4)
     */
    public static final String UDP_IO_THREAD_COUNT = "reactor.netty.udp.ioThreadCount";
    /**
     * Default quiet period that guarantees that the disposal of the underlying LoopResources
     * will not happen, fallback to 2 seconds.
     */
    public static final String SHUTDOWN_QUIET_PERIOD = "reactor.netty.ioShutdownQuietPeriod";
    /**
     * Default maximum amount of time to wait until the disposal of the underlying LoopResources
     * regardless if a task was submitted during the quiet period, fallback to 15 seconds.
     */
    public static final String SHUTDOWN_TIMEOUT = "reactor.netty.ioShutdownTimeout";

    /**
     * Default value whether the native transport (epoll, kqueue) will be preferred,
     * fallback it will be preferred when available
     */
    public static final String NATIVE = "reactor.netty.native";

If changes to the these settings are needed, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/eventloop/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.resources.LoopResources;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		LoopResources loop = LoopResources.create("event-loop", 1, 4, true);

    		DisposableServer server =
    				TcpServer.create()
    				         .runOn(loop)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

### 3.7. SSL and TLS

When you need SSL or TLS, you can apply the configuration shown in the next listing. By default, if `OpenSSL` is available, `SslProvider.OPENSSL` provider is used as a provider. Otherwise `SslProvider.JDK` is used. Switching the provider can be done through `SslContextBuilder` or by setting `-Dio.netty.handler.ssl.noOpenSsl=true`.

The following example uses `SslContextBuilder`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/security/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;
    import reactor.netty.tcp.TcpSslContextSpec;

    import java.io.File;

    public class Application {

    	public static void main(String[] args) {
    		File cert = new File("certificate.crt");
    		File key = new File("private.key");

    		TcpSslContextSpec tcpSslContextSpec = TcpSslContextSpec.forServer(cert, key);

    		DisposableServer server =
    				TcpServer.create()
    				         .secure(spec -> spec.sslContext(tcpSslContextSpec))
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

#### 3.7.1. Server Name Indication

You can configure the `TCP` server with multiple `SslContext` mapped to a specific domain. An exact domain name or a domain name containing a wildcard can be used when configuring the `SNI` mapping.

The following example uses a domain name containing a wildcard:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/sni/Application.java

    import io.netty.handler.ssl.SslContext;
    import io.netty.handler.ssl.SslContextBuilder;
    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    import java.io.File;

    public class Application {

    	public static void main(String[] args) throws Exception {
    		File defaultCert = new File("default_certificate.crt");
    		File defaultKey = new File("default_private.key");

    		File testDomainCert = new File("default_certificate.crt");
    		File testDomainKey = new File("default_private.key");

    		SslContext defaultSslContext = SslContextBuilder.forServer(defaultCert, defaultKey).build();
    		SslContext testDomainSslContext = SslContextBuilder.forServer(testDomainCert, testDomainKey).build();

    		DisposableServer server =
    				TcpServer.create()
    				         .secure(spec -> spec.sslContext(defaultSslContext)
    				                             .addSniMapping("*.test.com",
    				                                     testDomainSpec -> testDomainSpec.sslContext(testDomainSslContext)))
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

### 3.8. Metrics

The TCP server supports built-in integration with `Micrometer`. It exposes all metrics with a prefix of `reactor.netty.tcp.server`.

The following table provides information for the TCP server metrics:

metric name | type | description
--- | --- | ---
reactor.netty.tcp.server.data.received | DistributionSummary | Amount of the data received, in bytes
reactor.netty.tcp.server.data.sent | DistributionSummary | Amount of the data sent, in bytes
reactor.netty.tcp.server.errors | Counter | Number of errors that occurred
reactor.netty.tcp.server.tls.handshake.time | Timer | Time spent for TLS handshake

These additional metrics are also available:

`ByteBufAllocator` metrics

metric name | type | description
--- | --- | ---
reactor.netty.bytebuf.allocator.used.heap.memory | Gauge | The number of the bytes of the heap memory
reactor.netty.bytebuf.allocator.used.direct.memory | Gauge | The number of the bytes of the direct memory
reactor.netty.bytebuf.allocator.used.heap.arenas | Gauge | The number of heap arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.direct.arenas | Gauge | The number of direct arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.threadlocal.caches | Gauge | The number of thread local caches (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.tiny.cache.size | Gauge | The size of the tiny cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.small.cache.size | Gauge | The size of the small cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.normal.cache.size | Gauge | The size of the normal cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.chunk.size | Gauge | The chunk size for an arena (when `PooledByteBufAllocator`)

The following example enables that integration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/metrics/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .metrics(true) (1)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Enables the built-in integration with Micrometer

When TCP server metrics are needed for an integration with a system other than `Micrometer` or you want to provide your own integration with `Micrometer`, you can provide your own metrics recorder, as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/metrics/custom/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.channel.ChannelMetricsRecorder;
    import reactor.netty.tcp.TcpServer;

    import java.net.SocketAddress;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .metrics(true, CustomChannelMetricsRecorder::new) (1)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}

1 | Enables TCP server metrics and provides `ChannelMetricsRecorder` implementation.

### 3.9. Unix Domain Sockets

The `TCP` server supports Unix Domain Sockets (UDS) when native transport is in use.

The following example shows how to use UDS support:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/server/uds/Application.java

    import io.netty.channel.unix.DomainSocketAddress;
    import reactor.netty.DisposableServer;
    import reactor.netty.tcp.TcpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				TcpServer.create()
    				         .bindAddress(() -> new DomainSocketAddress("/tmp/test.sock")) (1)
    				         .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Specifies `DomainSocketAddress` that will be used

Suggest Edit to "TCP Server"

## 4. TCP Client

Reactor Netty provides the easy-to-use and easy-to-configure `TcpClient`. It hides most of the Netty functionality that is needed in order to create a `TCP` client and adds Reactive Streams backpressure.

### 4.1. Connect and Disconnect

To connect the `TCP` client to a given endpoint, you must create and configure a `TcpClient` instance. By default, the `host` is `localhost` and the `port` is `12012`. The following example shows how to create a `TcpClient`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/create/Application.java

    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()      (1)
    				         .connectNow(); (2)

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Creates a `TcpClient` instance that is ready for configuring.
2 | Connects the client in a blocking fashion and waits for it to finish initializing.

The returned `Connection` offers a simple connection API, including `disposeNow()`, which shuts the client down in a blocking fashion.

#### 4.1.1. Host and Port

To connect to a specific `host` and `port`, you can apply the following configuration to the `TCP` client. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/address/Application.java

    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com") (1)
    				         .port(80)            (2)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Configures the `TCP` host
2 | Configures the `TCP` port

### 4.2. Eager Initialization

By default, the initialization of the `TcpClient` resources happens on demand. This means that the `connect operation` absorbs the extra time needed to initialize and load:

- the event loop group

- the host name resolver

- the native transport libraries (when native transport is used)

- the native libraries for the security (in case of `OpenSsl`)

When you need to preload these resources, you can configure the `TcpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/warmup/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		TcpClient tcpClient =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .handle((inbound, outbound) -> outbound.sendString(Mono.just("hello")));

    		tcpClient.warmup() (1)
    		         .block();

    		Connection connection = tcpClient.connectNow(); (2)

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Initialize and load the event loop group, the host name resolver, the native transport libraries and the native libraries for the security
2 | Host name resolution happens when connecting to the remote peer

### 4.3. Writing Data

To send data to a given endpoint, you must attach an I/O handler. The I/O handler has access to `NettyOutbound` to be able to write data.

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/send/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .handle((inbound, outbound) -> outbound.sendString(Mono.just("hello"))) (1)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Sends `hello` string to the endpoint.

### 4.4. Consuming Data

To receive data from a given endpoint, you must attach an I/O handler. The I/O handler has access to `NettyInbound` to be able to read data. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/read/Application.java

    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .handle((inbound, outbound) -> inbound.receive().then()) (1)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Receives data from a given endpoint

### 4.5. Lifecycle Callbacks

The following lifecycle callbacks are provided to let you extend the `TcpClient`.

Callback | Description
--- | ---
`doAfterResolve` | Invoked after the remote address has been resolved successfully.
`doOnChannelInit` | Invoked when initializing the channel.
`doOnConnect` | Invoked when the channel is about to connect.
`doOnConnected` | Invoked after the channel has been connected.
`doOnDisconnected` | Invoked after the channel has been disconnected.
`doOnResolve` | Invoked when the remote address is about to be resolved.
`doOnResolveError` | Invoked in case the remote address hasn’t been resolved successfully.

The following example uses the `doOnConnected` and `doOnChannelInit` callbacks:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/lifecycle/Application.java

    import io.netty.handler.logging.LoggingHandler;
    import io.netty.handler.timeout.ReadTimeoutHandler;
    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;
    import java.util.concurrent.TimeUnit;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .doOnConnected(conn ->
    				             conn.addHandler(new ReadTimeoutHandler(10, TimeUnit.SECONDS))) (1)
    				         .doOnChannelInit((observer, channel, remoteAddress) ->
    				             channel.pipeline()
    				                    .addFirst(new LoggingHandler("reactor.netty.examples")))(2)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | `Netty` pipeline is extended with `ReadTimeoutHandler` when the channel has been connected.
2 | `Netty` pipeline is extended with `LoggingHandler` when initializing the channel.

### 4.6. TCP-level Configurations

This section describes three kinds of configuration that you can use at the TCP level:

- Channel Options

- Wire Logger

- Event Loop Group

#### 4.6.1. Channel Options

By default, the `TCP` client is configured with the following options:

./../../reactor-netty-core/src/main/java/reactor/netty/tcp/TcpClientConnect.java

    TcpClientConnect(ConnectionProvider provider) {
    	this.config = new TcpClientConfig(
    			provider,
    			Collections.singletonMap(ChannelOption.AUTO_READ, false),
    			() -> AddressUtils.createUnresolved(NetUtil.LOCALHOST.getHostAddress(), DEFAULT_PORT));
    }

If additional options are necessary or changes to the current options are needed, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/channeloptions/Application.java

    import io.netty.channel.ChannelOption;
    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

You can find more about `Netty` channel options at the following links:

- Common ChannelOption

- Epoll ChannelOption

- KQueue ChannelOption

- Socket Options

#### 4.6.2. Wire Logger

Reactor Netty provides wire logging for when the traffic between the peers has to be inspected. By default, wire logging is disabled. To enable it, you must set the logger `reactor.netty.tcp.TcpClient` level to `DEBUG` and apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/wiretap/Application.java

    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .wiretap(true) (1)
    				         .host("example.com")
    				         .port(80)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Enables the wire logging

By default, the wire logging uses AdvancedByteBufFormat#HEX_DUMP when printing the content. When you need to change this to AdvancedByteBufFormat#SIMPLE or AdvancedByteBufFormat#TEXTUAL, you can configure the `TcpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/wiretap/custom/Application.java

    import io.netty.handler.logging.LogLevel;
    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;
    import reactor.netty.transport.logging.AdvancedByteBufFormat;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .wiretap("logger-name", LogLevel.DEBUG, AdvancedByteBufFormat.TEXTUAL) (1)
    				         .host("example.com")
    				         .port(80)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Enables the wire logging, AdvancedByteBufFormat#TEXTUAL is used for printing the content.

#### 4.6.3. Event Loop Group

By default the `TCP` client uses an “Event Loop Group”, where the number of the worker threads equals the number of processors available to the runtime on initialization (but with a minimum value of 4). When you need a different configuration, you can use one of the LoopResource`#create` methods.

The following listing shows the default configuration for the Event Loop Group:

./../../reactor-netty-core/src/main/java/reactor/netty/ReactorNetty.java

    /**
     * Default worker thread count, fallback to available processor
     * (but with a minimum value of 4)
     */
    public static final String IO_WORKER_COUNT = "reactor.netty.ioWorkerCount";
    /**
     * Default selector thread count, fallback to -1 (no selector thread)
     */
    public static final String IO_SELECT_COUNT = "reactor.netty.ioSelectCount";
    /**
     * Default worker thread count for UDP, fallback to available processor
     * (but with a minimum value of 4)
     */
    public static final String UDP_IO_THREAD_COUNT = "reactor.netty.udp.ioThreadCount";
    /**
     * Default quiet period that guarantees that the disposal of the underlying LoopResources
     * will not happen, fallback to 2 seconds.
     */
    public static final String SHUTDOWN_QUIET_PERIOD = "reactor.netty.ioShutdownQuietPeriod";
    /**
     * Default maximum amount of time to wait until the disposal of the underlying LoopResources
     * regardless if a task was submitted during the quiet period, fallback to 15 seconds.
     */
    public static final String SHUTDOWN_TIMEOUT = "reactor.netty.ioShutdownTimeout";

    /**
     * Default value whether the native transport (epoll, kqueue) will be preferred,
     * fallback it will be preferred when available
     */
    public static final String NATIVE = "reactor.netty.native";

If you need changes to the these settings, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/eventloop/Application.java

    import reactor.netty.Connection;
    import reactor.netty.resources.LoopResources;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		LoopResources loop = LoopResources.create("event-loop", 1, 4, true);

    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .runOn(loop)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

### 4.7. Connection Pool

By default, the `TCP` client uses a “fixed” connection pool with `500` as the maximum number of channels and `1000` as the maximum number of the registered requests for acquire to keep in the pending queue (for the rest of the configurations check the system properties below). This means that the implementation creates a new channel if someone tries to acquire a channel but none is in the pool. When the maximum number of the channels in the pool is reached, new tries to acquire a channel are delayed until a channel is returned to the pool again.

./../../reactor-netty-core/src/main/java/reactor/netty/ReactorNetty.java

    /**
     * Default max connections. Fallback to
     * available number of processors (but with a minimum value of 16)
     */
    public static final String POOL_MAX_CONNECTIONS = "reactor.netty.pool.maxConnections";
    /**
     * Default acquisition timeout (milliseconds) before error. If -1 will never wait to
     * acquire before opening a new
     * connection in an unbounded fashion. Fallback 45 seconds
     */
    public static final String POOL_ACQUIRE_TIMEOUT = "reactor.netty.pool.acquireTimeout";
    /**
     * Default max idle time, fallback - max idle time is not specified.
     */
    public static final String POOL_MAX_IDLE_TIME = "reactor.netty.pool.maxIdleTime";
    /**
     * Default max life time, fallback - max life time is not specified.
     */
    public static final String POOL_MAX_LIFE_TIME = "reactor.netty.pool.maxLifeTime";
    /**
     * Default leasing strategy (fifo, lifo), fallback to fifo.
     * <ul>
     *     <li>fifo - The connection selection is first in, first out</li>
     *     <li>lifo - The connection selection is last in, first out</li>
     * </ul>
     */
    public static final String POOL_LEASING_STRATEGY = "reactor.netty.pool.leasingStrategy";
    /**
     * Default {@code getPermitsSamplingRate} (between 0d and 1d (percentage))
     * to be used with a {@link SamplingAllocationStrategy}.
     * This strategy wraps a {@link PoolBuilder#sizeBetween(int, int) sizeBetween} {@link AllocationStrategy}
     * and samples calls to {@link AllocationStrategy#getPermits(int)}.
     * Fallback - sampling is not enabled.
     */
    public static final String POOL_GET_PERMITS_SAMPLING_RATE = "reactor.netty.pool.getPermitsSamplingRate";
    /**
     * Default {@code returnPermitsSamplingRate} (between 0d and 1d (percentage))
     * to be used with a {@link SamplingAllocationStrategy}.
     * This strategy wraps a {@link PoolBuilder#sizeBetween(int, int) sizeBetween} {@link AllocationStrategy}
     * and samples calls to {@link AllocationStrategy#returnPermits(int)}.
     * Fallback - sampling is not enabled.
     */
    public static final String POOL_RETURN_PERMITS_SAMPLING_RATE = "reactor.netty.pool.returnPermitsSamplingRate";

If you need to disable the connection pool, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/pool/Application.java

    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.newConnection()
    				         .host("example.com")
    				         .port(80)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

If you need to specify an idle time for the channels in the connection pool, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/pool/config/Application.java

    import reactor.netty.Connection;
    import reactor.netty.resources.ConnectionProvider;
    import reactor.netty.tcp.TcpClient;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		ConnectionProvider provider =
    				ConnectionProvider.builder("fixed")
    				                  .maxConnections(50)
    				                  .pendingAcquireTimeout(Duration.ofMillis(30000))
    				                  .maxIdleTime(Duration.ofMillis(60))
    				                  .build();

    		Connection connection =
    				TcpClient.create(provider)
    				         .host("example.com")
    				         .port(80)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

| When you expect a high load, be cautious with a connection pool with a very high value for maximum connections. You might experience `reactor.netty.http.client.PrematureCloseException` exception with a root cause "Connect Timeout" due to too many concurrent connections opened/acquired.

#### 4.7.1. Metrics

The pooled `ConnectionProvider` supports built-in integration with `Micrometer`. It exposes all metrics with a prefix of `reactor.netty.connection.provider`.

Pooled `ConnectionProvider` metrics

metric name | type | description
--- | --- | ---
reactor.netty.connection.provider.total.connections | Gauge | The number of all connections, active or idle
reactor.netty.connection.provider.active.connections | Gauge | The number of the connections that have been successfully acquired and are in active use
reactor.netty.connection.provider.idle.connections | Gauge | The number of the idle connections
reactor.netty.connection.provider.pending.connections | Gauge | The number of requests that are waiting for a connection

The following example enables that integration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/pool/metrics/Application.java

    import reactor.netty.Connection;
    import reactor.netty.resources.ConnectionProvider;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		ConnectionProvider provider =
    				ConnectionProvider.builder("fixed")
    				                  .maxConnections(50)
    				                  .metrics(true) (1)
    				                  .build();

    		Connection connection =
    				TcpClient.create(provider)
    				         .host("example.com")
    				         .port(80)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Enables the built-in integration with Micrometer

### 4.8. SSL and TLS

When you need SSL or TLS, you can apply the following configuration. By default, if `OpenSSL` is available, the SslProvider.OPENSSL provider is used as a provider. Otherwise, the provider is SslProvider.JDK. You can switch the provider by using SslContextBuilder or by setting `-Dio.netty.handler.ssl.noOpenSsl=true`.

The following example uses `SslContextBuilder`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/security/Application.java

    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;
    import reactor.netty.tcp.TcpSslContextSpec;

    public class Application {

    	public static void main(String[] args) {
    		TcpSslContextSpec tcpSslContextSpec = TcpSslContextSpec.forClient();

    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(443)
    				         .secure(spec -> spec.sslContext(tcpSslContextSpec))
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

#### 4.8.1. Server Name Indication

By default, the `TCP` client sends the remote host name as `SNI` server name. When you need to change this default setting, you can configure the `TCP` client as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/sni/Application.java

    import io.netty.handler.ssl.SslContext;
    import io.netty.handler.ssl.SslContextBuilder;
    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    import javax.net.ssl.SNIHostName;

    public class Application {

    	public static void main(String[] args) throws Exception {
    		SslContext sslContext = SslContextBuilder.forClient().build();

    		Connection connection =
    				TcpClient.create()
    				         .host("127.0.0.1")
    				         .port(8080)
    				         .secure(spec -> spec.sslContext(sslContext)
    				                             .serverNames(new SNIHostName("test.com")))
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

### 4.9. Proxy Support

The TCP client supports the proxy functionality provided by Netty and provides a way to specify “non proxy hosts” through the `ProxyProvider` builder. The following example uses `ProxyProvider`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/proxy/Application.java

    import reactor.netty.Connection;
    import reactor.netty.transport.ProxyProvider;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .proxy(spec -> spec.type(ProxyProvider.Proxy.SOCKS4)
    				                            .host("proxy")
    				                            .port(8080)
    				                            .nonProxyHosts("localhost"))
    				        .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

### 4.10. Metrics

The TCP client supports built-in integration with `Micrometer`. It exposes all metrics with a prefix of `reactor.netty.tcp.client`.

The following table provides information for the TCP client metrics:

metric name | type | description
--- | --- | ---
reactor.netty.tcp.client.data.received | DistributionSummary | Amount of the data received, in bytes
reactor.netty.tcp.client.data.sent | DistributionSummary | Amount of the data sent, in bytes
reactor.netty.tcp.client.errors | Counter | Number of errors that occurred
reactor.netty.tcp.client.tls.handshake.time | Timer | Time spent for TLS handshake
reactor.netty.tcp.client.connect.time | Timer | Time spent for connecting to the remote address
reactor.netty.tcp.client.address.resolver | Timer | Time spent for resolving the address

These additional metrics are also available:

Pooled `ConnectionProvider` metrics

metric name | type | description
--- | --- | ---
reactor.netty.connection.provider.total.connections | Gauge | The number of all connections, active or idle
reactor.netty.connection.provider.active.connections | Gauge | The number of the connections that have been successfully acquired and are in active use
reactor.netty.connection.provider.idle.connections | Gauge | The number of the idle connections
reactor.netty.connection.provider.pending.connections | Gauge | The number of requests that are waiting for a connection

`ByteBufAllocator` metrics

metric name | type | description
--- | --- | ---
reactor.netty.bytebuf.allocator.used.heap.memory | Gauge | The number of the bytes of the heap memory
reactor.netty.bytebuf.allocator.used.direct.memory | Gauge | The number of the bytes of the direct memory
reactor.netty.bytebuf.allocator.used.heap.arenas | Gauge | The number of heap arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.direct.arenas | Gauge | The number of direct arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.threadlocal.caches | Gauge | The number of thread local caches (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.tiny.cache.size | Gauge | The size of the tiny cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.small.cache.size | Gauge | The size of the small cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.normal.cache.size | Gauge | The size of the normal cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.chunk.size | Gauge | The chunk size for an arena (when `PooledByteBufAllocator`)

The following example enables that integration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/metrics/Application.java

    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .metrics(true) (1)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Enables the built-in integration with Micrometer

When TCP client metrics are needed for an integration with a system other than `Micrometer` or you want to provide your own integration with `Micrometer`, you can provide your own metrics recorder, as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/metrics/custom/Application.java

    import reactor.netty.Connection;
    import reactor.netty.channel.ChannelMetricsRecorder;
    import reactor.netty.tcp.TcpClient;

    import java.net.SocketAddress;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .metrics(true, CustomChannelMetricsRecorder::new) (1)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}

1 | Enables TCP client metrics and provides `ChannelMetricsRecorder` implementation.

### 4.11. Unix Domain Sockets

The `TCP` client supports Unix Domain Sockets (UDS) when native transport is in use.

The following example shows how to use UDS support:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/uds/Application.java

    import io.netty.channel.unix.DomainSocketAddress;
    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .remoteAddress(() -> new DomainSocketAddress("/tmp/test.sock")) (1)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Specifies `DomainSocketAddress` that will be used

### 4.12. Host Name Resolution

By default, the `TcpClient` uses Netty’s domain name lookup mechanism that resolves a domain name asynchronously. This is as an alternative of the JVM’s built-in blocking resolver.

When you need to change the default settings, you can configure the `TcpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/resolver/Application.java

    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .resolver(spec -> spec.queryTimeout(Duration.ofMillis(500))) (1)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | The timeout of each DNS query performed by this resolver will be 500ms.

The following listing shows the available configurations:

Configuration name | Description
--- | ---
`cacheMaxTimeToLive` | The max time to live of the cached DNS resource records (resolution: seconds). If the time to live of the DNS resource record returned by the DNS server is greater than this max time to live, this resolver ignores the time to live from the DNS server and uses use this max time to live. Default to `Integer.MAX_VALUE`.
`cacheMinTimeToLive` | The min time to live of the cached DNS resource records (resolution: seconds). If the time to live of the DNS resource record returned by the DNS server is less than this min time to live, this resolver ignores the time to live from the DNS server and uses this min time to live. Default: 0.
`cacheNegativeTimeToLive` | The time to live of the cache for the failed DNS queries (resolution: seconds). Default: 0.
`completeOncePreferredResolved` | When this setting is enabled, the resolver notifies as soon as all queries for the preferred address type are complete. When this setting is disabled, the resolver notifies when all possible address types are complete. This configuration is applicable for `DnsNameResolver#resolveAll(String)`. By default, this setting is enabled.
`disableOptionalRecord` | Disables the automatic inclusion of an optional record that tries to give a hint to the remote DNS server about how much data the resolver can read per response. By default, this setting is enabled.
`disableRecursionDesired` | Specifies whether this resolver has to send a DNS query with the recursion desired (RD) flag set. By default, this setting is enabled.
`maxPayloadSize` | Sets the capacity of the datagram packet buffer (in bytes). Default: 4096.
`maxQueriesPerResolve` | Sets the maximum allowed number of DNS queries to send when resolving a host name. Default: 16.
`ndots` | Sets the number of dots that must appear in a name before an initial absolute query is made. Default: -1 (to determine the value from the OS on Unix or use a value of 1 otherwise).
`queryTimeout` | Sets the timeout of each DNS query performed by this resolver (resolution: milliseconds). Default: 5000.
`resolvedAddressTypes` | The list of the protocol families of the resolved address.
`roundRobinSelection` | Enables an `AddressResolverGroup` of `DnsNameResolver` that supports random selection of destination addresses if multiple are provided by the nameserver. See `RoundRobinDnsAddressResolverGroup`. Default: `DnsAddressResolverGroup`
`runOn` | Performs the communication with the DNS servers on the given `LoopResources`. By default, the LoopResources specified on the client level are used.
`searchDomains` | The list of search domains of the resolver. By default, the effective search domain list is populated by using the system DNS search domains.
`trace` | A specific logger and log level to be used by this resolver when generating detailed trace information in case of resolution failure.

Sometimes, you may want to switch to the JVM built-in resolver. To do so, you can configure the `TcpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/tcp/client/resolver/custom/Application.java

    import io.netty.resolver.DefaultAddressResolverGroup;
    import reactor.netty.Connection;
    import reactor.netty.tcp.TcpClient;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				TcpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .resolver(DefaultAddressResolverGroup.INSTANCE) (1)
    				         .connectNow();

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Sets the JVM built-in resolver.

Suggest Edit to "TCP Client"

## 5. HTTP Server

`Reactor Netty` provides the easy-to-use and easy-to-configure `HttpServer` class. It hides most of the `Netty` functionality that is needed in order to create a `HTTP` server and adds `Reactive Streams` backpressure.

### 5.1. Starting and Stopping

To start an HTTP server, you must create and configure a HttpServer instance. By default, the `host` is configured for any local address, and the system picks up an ephemeral port when the `bind` operation is invoked. The following example shows how to create an `HttpServer` instance:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/create/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()   (1)
    				          .bindNow(); (2)

    		server.onDispose()
    		      .block();
    	}
    }

1 | Creates an HttpServer instance ready for configuring.
2 | Starts the server in a blocking fashion and waits for it to finish initializing.

The returned `DisposableServer` offers a simple server API, including `disposeNow()`, which shuts the server down in a blocking fashion.

#### 5.1.1. Host and Port

To serve on a specific `host` and `port`, you can apply the following configuration to the `HTTP` server:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/address/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .host("localhost") (1)
    				          .port(8080)        (2)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Configures the `HTTP` server host
2 | Configures the `HTTP` server port

### 5.2. Eager Initialization

By default, the initialization of the `HttpServer` resources happens on demand. This means that the `bind operation` absorbs the extra time needed to initialize and load:

- the event loop groups

- the native transport libraries (when native transport is used)

- the native libraries for the security (in case of `OpenSsl`)

When you need to preload these resources, you can configure the `HttpServer` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/warmup/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		HttpServer httpServer =
    				HttpServer.create()
    				          .handle((request, response) -> request.receive().then());

    		httpServer.warmup() (1)
    		          .block();

    		DisposableServer server = httpServer.bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Initialize and load the event loop groups, the native transport libraries and the native libraries for the security

### 5.3. Routing HTTP

Defining routes for the `HTTP` server requires configuring the provided `HttpServerRoutes` builder. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/routing/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .route(routes ->
    				              routes.get("/hello",        (1)
    				                        (request, response) -> response.sendString(Mono.just("Hello World!")))
    				                    .post("/echo",        (2)
    				                        (request, response) -> response.send(request.receive().retain()))
    				                    .get("/path/{param}", (3)
    				                        (request, response) -> response.sendString(Mono.just(request.param("param"))))
    				                    .ws("/ws",            (4)
    				                        (wsInbound, wsOutbound) -> wsOutbound.send(wsInbound.receive().retain())))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Serves a `GET` request to `/hello` and returns `Hello World!`
2 | Serves a `POST` request to `/echo` and returns the received request body as a response.
3 | Serves a `GET` request to `/path/{param}` and returns the value of the path parameter.
4 | Serves websocket to `/ws` and returns the received incoming data as outgoing data.

| The server routes are unique and only the first matching in order of declaration is invoked.

#### 5.3.1. SSE

The following code shows how you can configure the `HTTP` server to serve `Server-Sent Events`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/sse/Application.java

    import com.fasterxml.jackson.databind.ObjectMapper;
    import io.netty.buffer.ByteBuf;
    import io.netty.buffer.ByteBufAllocator;
    import org.reactivestreams.Publisher;
    import reactor.core.publisher.Flux;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;
    import reactor.netty.http.server.HttpServerRequest;
    import reactor.netty.http.server.HttpServerResponse;

    import java.io.ByteArrayOutputStream;
    import java.nio.charset.Charset;
    import java.time.Duration;
    import java.util.function.BiFunction;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .route(routes -> routes.get("/sse", serveSse()))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}

    	/**
    	 * Prepares SSE response
    	 * The "Content-Type" is "text/event-stream"
    	 * The flushing strategy is "flush after every element" emitted by the provided Publisher
    	 */
    	private static BiFunction<HttpServerRequest, HttpServerResponse, Publisher<Void>> serveSse() {
    		Flux<Long> flux = Flux.interval(Duration.ofSeconds(10));
    		return (request, response) ->
    		        response.sse()
    		                .send(flux.map(Application::toByteBuf), b -> true);
    	}

    	/**
    	 * Transforms the Object to ByteBuf following the expected SSE format.
    	 */
    	private static ByteBuf toByteBuf(Object any) {
    		ByteArrayOutputStream out = new ByteArrayOutputStream();
    		try {
    			out.write("data: ".getBytes(Charset.defaultCharset()));
    			MAPPER.writeValue(out, any);
    			out.write("\n\n".getBytes(Charset.defaultCharset()));
    		}
    		catch (Exception e) {
    			throw new RuntimeException(e);
    		}
    		return ByteBufAllocator.DEFAULT
    		                       .buffer()
    		                       .writeBytes(out.toByteArray());
    	}

    	private static final ObjectMapper MAPPER = new ObjectMapper();
    }

#### 5.3.2. Static Resources

The following code shows how you can configure the `HTTP` server to serve static resources:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/staticresources/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    import java.net.URISyntaxException;
    import java.nio.file.Path;
    import java.nio.file.Paths;

    public class Application {

    	public static void main(String[] args) throws URISyntaxException {
    		Path file = Paths.get(Application.class.getResource("/logback.xml").toURI());
    		DisposableServer server =
    				HttpServer.create()
    				          .route(routes -> routes.file("/index.html", file))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

### 5.4. Writing Data

To send data to a connected client, you must attach an I/O handler by using either `handle(…​)` or `route(…​)`. The I/O handler has access to `HttpServerResponse`, to be able to write data. The following example uses the `handle(…​)` method:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/send/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .handle((request, response) -> response.sendString(Mono.just("hello"))) (1)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Sends `hello` string to the connected clients

#### 5.4.1. Adding Headers and Other Metadata

When you send data to the connected clients, you may need to send additional headers, cookies, status code, and other metadata. You can provide this additional metadata by using `HttpServerResponse`. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/send/headers/Application.java

    import io.netty.handler.codec.http.HttpHeaderNames;
    import io.netty.handler.codec.http.HttpResponseStatus;
    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .route(routes ->
    				              routes.get("/hello",
    				                  (request, response) ->
    				                      response.status(HttpResponseStatus.OK)
    				                              .header(HttpHeaderNames.CONTENT_LENGTH, "12")
    				                              .sendString(Mono.just("Hello World!"))))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

#### 5.4.2. Compression

You can configure the `HTTP` server to send a compressed response, depending on the request header `Accept-Encoding`.

`Reactor Netty` provides three different strategies for compressing the outgoing data:

- `compress(boolean)`: Depending on the boolean that is provided, the compression is enabled (`true`) or disabled (`false`).

- `compress(int)`: The compression is performed once the response size exceeds the given value (in bytes).

- `compress(BiPredicate<HttpServerRequest, HttpServerResponse>)`: The compression is performed if the predicate returns `true`.

The following example uses the `compress` method (set to `true`) to enable compression:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/compression/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    import java.net.URISyntaxException;
    import java.nio.file.Path;
    import java.nio.file.Paths;

    public class Application {

    	public static void main(String[] args) throws URISyntaxException {
    		Path file = Paths.get(Application.class.getResource("/logback.xml").toURI());
    		DisposableServer server =
    				HttpServer.create()
    				          .compress(true)
    				          .route(routes -> routes.file("/index.html", file))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

### 5.5. Consuming Data

To receive data from a connected client, you must attach an I/O handler by using either `handle(…​)` or `route(…​)`. The I/O handler has access to `HttpServerRequest`, to be able to read data.

The following example uses the `handle(…​)` method:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/read/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .handle((request, response) -> request.receive().then()) (1)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Receives data from the connected clients

#### 5.5.1. Reading Headers, URI Params, and other Metadata

When you receive data from the connected clients, you might need to check request headers, parameters, and other metadata. You can obtain this additional metadata by using `HttpServerRequest`. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/read/headers/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .route(routes ->
    				              routes.get("/{param}",
    				                  (request, response) -> {
    				                      if (request.requestHeaders().contains("Some-Header")) {
    				                          return response.sendString(Mono.just(request.param("param")));
    				                      }
    				                      return response.sendNotFound();
    				                  }))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

##### Obtaining the Remote (Client) Address

In addition to the metadata that you can obtain from the request, you can also receive the `host (server)` address, the `remote (client)` address and the `scheme`. Depending on the chosen factory method, you can retrieve the information directly from the channel or by using the `Forwarded` or `X-Forwarded-*` `HTTP` request headers. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/clientaddress/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .forwarded(true) (1)
    				          .route(routes ->
    				              routes.get("/clientip",
    				                  (request, response) ->
    				                      response.sendString(Mono.just(request.remoteAddress() (2)
    				                                                           .getHostString()))))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Specifies that the information about the connection is to be obtained from the `Forwarded` and `X-Forwarded-*` `HTTP` request headers, if possible.
2 | Returns the address of the remote (client) peer.

It is also possible to customize the behavior of the `Forwarded` or `X-Forwarded-*` header handler. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/clientaddress/CustomForwardedHeaderHandlerApplication.java

    import java.net.InetSocketAddress;

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;
    import reactor.netty.transport.AddressUtils;

    public class CustomForwardedHeaderHandlerApplication {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .forwarded((connectionInfo, request) -> {  (1)
    				              String hostHeader = request.headers().get("X-Forwarded-Host");
    				              if (hostHeader != null) {
    				                  String[] hosts = hostHeader.split(",", 2);
    				                  InetSocketAddress hostAddress = AddressUtils.createUnresolved(
    				                      hosts[hosts.length - 1].trim(),
    				                      connectionInfo.getHostAddress().getPort());
    				                  connectionInfo = connectionInfo.withHostAddress(hostAddress);
    				              }
    				              return connectionInfo;
    				          })
    				          .route(routes ->
    				              routes.get("/clientip",
    				                  (request, response) ->
    				                      response.sendString(Mono.just(request.remoteAddress() (2)
    				                                                           .getHostString()))))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Add a custom header handler.
2 | Returns the address of the remote (client) peer.

#### 5.5.2. HTTP Request Decoder

By default, `Netty` configures some restrictions for the incoming requests, such as:

- The maximum length of the initial line.

- The maximum length of all headers.

- The maximum length of the content or each chunk.

For more information, see `HttpRequestDecoder` and `HttpServerUpgradeHandler`

By default, the `HTTP` server is configured with the following settings:

./../../reactor-netty-http/src/main/java/reactor/netty/http/HttpDecoderSpec.java

    public static final int DEFAULT_MAX_INITIAL_LINE_LENGTH = 4096;
    public static final int DEFAULT_MAX_HEADER_SIZE         = 8192;
    public static final int DEFAULT_MAX_CHUNK_SIZE          = 8192;
    public static final boolean DEFAULT_VALIDATE_HEADERS    = true;
    public static final int DEFAULT_INITIAL_BUFFER_SIZE     = 128;

./../../reactor-netty-http/src/main/java/reactor/netty/http/server/HttpRequestDecoderSpec.java

    /**
     * The maximum length of the content of the HTTP/2.0 clear-text upgrade request.
     * By default the server will reject an upgrade request with non-empty content,
     * because the upgrade request is most likely a GET request.
     */
    public static final int DEFAULT_H2C_MAX_CONTENT_LENGTH = 0;

When you need to change these default settings, you can configure the `HTTP` server as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/requestdecoder/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .httpRequestDecoder(spec -> spec.maxHeaderSize(16384)) (1)
    				          .handle((request, response) -> response.sendString(Mono.just("hello")))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | The maximum length of all headers will be `16384`. When this value is exceeded, a TooLongFrameException is raised.

### 5.6. Lifecycle Callbacks

The following lifecycle callbacks are provided to let you extend the `HttpServer`:

Callback | Description
--- | ---
`doOnBind` | Invoked when the server channel is about to bind.
`doOnBound` | Invoked when the server channel is bound.
`doOnChannelInit` | Invoked when initializing the channel.
`doOnConnection` | Invoked when a remote client is connected
`doOnUnbound` | Invoked when the server channel is unbound.

The following example uses the `doOnConnection` and `doOnChannelInit` callbacks:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/lifecycle/Application.java

    import io.netty.handler.logging.LoggingHandler;
    import io.netty.handler.timeout.ReadTimeoutHandler;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;
    import java.util.concurrent.TimeUnit;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .doOnConnection(conn ->
    				              conn.addHandler(new ReadTimeoutHandler(10, TimeUnit.SECONDS))) (1)
    				          .doOnChannelInit((observer, channel, remoteAddress) ->
    				              channel.pipeline()
    				                     .addFirst(new LoggingHandler("reactor.netty.examples")))(2)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | `Netty` pipeline is extended with `ReadTimeoutHandler` when a remote client is connected.
2 | `Netty` pipeline is extended with `LoggingHandler` when initializing the channel.

### 5.7. TCP-level Configuration

When you need to change configuration on the TCP level, you can use the following snippet to extend the default `TCP` server configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/channeloptions/Application.java

    import io.netty.channel.ChannelOption;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

See TCP Server for more detail about TCP-level configuration.

#### 5.7.1. Wire Logger

`Reactor Netty` provides wire logging for when you need to inspect the traffic between the peers. By default, wire logging is disabled. To enable it, you must set the logger `reactor.netty.http.server.HttpServer` level to `DEBUG` and apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/wiretap/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .wiretap(true) (1)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Enables the wire logging

By default, the wire logging uses AdvancedByteBufFormat#HEX_DUMP when printing the content. When you need to change this to AdvancedByteBufFormat#SIMPLE or AdvancedByteBufFormat#TEXTUAL, you can configure the `HttpServer` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/wiretap/custom/Application.java

    import io.netty.handler.logging.LogLevel;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;
    import reactor.netty.transport.logging.AdvancedByteBufFormat;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .wiretap("logger-name", LogLevel.DEBUG, AdvancedByteBufFormat.TEXTUAL) (1)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Enables the wire logging, AdvancedByteBufFormat#TEXTUAL is used for printing the content.

### 5.8. SSL and TLS

When you need SSL or TLS, you can apply the configuration shown in the next example. By default, if `OpenSSL` is available, `SslProvider.OPENSSL` provider is used as a provider. Otherwise `SslProvider.JDK` is used. You can switch the provider by using `SslContextBuilder` or by setting `-Dio.netty.handler.ssl.noOpenSsl=true`.

The following example uses `SslContextBuilder`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/security/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.Http11SslContextSpec;
    import reactor.netty.http.server.HttpServer;
    import java.io.File;

    public class Application {

    	public static void main(String[] args) {
    		File cert = new File("certificate.crt");
    		File key = new File("private.key");

    		Http11SslContextSpec http11SslContextSpec = Http11SslContextSpec.forServer(cert, key);

    		DisposableServer server =
    				HttpServer.create()
    				          .secure(spec -> spec.sslContext(http11SslContextSpec))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

#### 5.8.1. Server Name Indication

You can configure the `HTTP` server with multiple `SslContext` mapped to a specific domain. An exact domain name or a domain name containing a wildcard can be used when configuring the `SNI` mapping.

The following example uses a domain name containing a wildcard:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/sni/Application.java

    import io.netty.handler.ssl.SslContext;
    import io.netty.handler.ssl.SslContextBuilder;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    import java.io.File;

    public class Application {

    	public static void main(String[] args) throws Exception {
    		File defaultCert = new File("default_certificate.crt");
    		File defaultKey = new File("default_private.key");

    		File testDomainCert = new File("default_certificate.crt");
    		File testDomainKey = new File("default_private.key");

    		SslContext defaultSslContext = SslContextBuilder.forServer(defaultCert, defaultKey).build();
    		SslContext testDomainSslContext = SslContextBuilder.forServer(testDomainCert, testDomainKey).build();

    		DisposableServer server =
    				HttpServer.create()
    				          .secure(spec -> spec.sslContext(defaultSslContext)
    				                              .addSniMapping("*.test.com",
    				                                      testDomainSpec -> testDomainSpec.sslContext(testDomainSslContext)))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

### 5.9. HTTP Access Log

You can enable the `HTTP` access log either programmatically or by configuration. By default, it is disabled.

You can use `-Dreactor.netty.http.server.accessLogEnabled=true` to enable the `HTTP` access log by configuration.

You can use the following configuration (for Logback or similar logging frameworks) to have a separate `HTTP` access log file:

    <appender name="accessLog" class="ch.qos.logback.core.FileAppender">
        <file>access_log.log</file>
        <encoder>
            <pattern>%msg%n</pattern>
        </encoder>
    </appender>
    <appender name="async" class="ch.qos.logback.classic.AsyncAppender">
        <appender-ref ref="accessLog" />
    </appender>

    <logger name="reactor.netty.http.server.AccessLog" level="INFO" additivity="false">
        <appender-ref ref="async"/>
    </logger>

The following example enables it programmatically:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/accessLog/Application.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .accessLog(true)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

Calling this method takes precedence over the system property configuration.

By default, the logging format is Common Log Format, but you can specify a custom one as a parameter, as in the following example:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/accessLog/CustomLogAccessFormatApplication.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;
    import reactor.netty.http.server.logging.AccessLog;

    public class CustomLogAccessFormatApplication {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .accessLog(true, x -> AccessLog.create("method={}, uri={}", x.method(), x.uri()))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

You can also filter `HTTP` access logs by using the `AccessLogFactory#createFilter` method, as in the following example:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/accessLog/FilterLogAccessApplication.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;
    import reactor.netty.http.server.logging.AccessLogFactory;

    public class FilterLogAccessApplication {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .accessLog(true, AccessLogFactory.createFilter(p -> !String.valueOf(p.uri()).startsWith("/health/")))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

Note that this method can take a custom format parameter too, as in this example:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/accessLog/CustomFormatAndFilterAccessLogApplication.java.java

    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;
    import reactor.netty.http.server.logging.AccessLog;
    import reactor.netty.http.server.logging.AccessLogFactory;

    public class CustomFormatAndFilterAccessLogApplication {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .accessLog(true, AccessLogFactory.createFilter(p -> !String.valueOf(p.uri()).startsWith("/health/"), (1)
    						          x -> AccessLog.create("method={}, uri={}", x.method(), x.uri()))) (2)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Specifies the filter predicate to use
2 | Specifies the custom format to apply

### 5.10. HTTP/2

By default, the `HTTP` server supports `HTTP/1.1`. If you need `HTTP/2`, you can get it through configuration. In addition to the protocol configuration, if you need `H2` but not `H2C (cleartext)`, you must also configure SSL.

| As Application-Layer Protocol Negotiation (ALPN) is not supported “out-of-the-box” by JDK8 (although some vendors backported ALPN to JDK8), you need an additional dependency to a native library that supports it — for example, `netty-tcnative-boringssl-static`.

The following listing presents a simple `H2` example:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/http2/H2Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.Http2SslContextSpec;
    import reactor.netty.http.HttpProtocol;
    import reactor.netty.http.server.HttpServer;
    import java.io.File;

    public class H2Application {

    	public static void main(String[] args) {
    		File cert = new File("certificate.crt");
    		File key = new File("private.key");

    		Http2SslContextSpec http2SslContextSpec = Http2SslContextSpec.forServer(cert, key);

    		DisposableServer server =
    				HttpServer.create()
    				          .port(8080)
    				          .protocol(HttpProtocol.H2)                            (1)
    				          .secure(spec -> spec.sslContext(http2SslContextSpec)) (2)
    				          .handle((request, response) -> response.sendString(Mono.just("hello")))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Configures the server to support only `HTTP/2`
2 | Configures `SSL`

The application should now behave as follows:

    $ curl --http2 https://localhost:8080 -i
    HTTP/2 200

    hello

The following listing presents a simple `H2C` example:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/http2/H2CApplication.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.HttpProtocol;
    import reactor.netty.http.server.HttpServer;

    public class H2CApplication {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .port(8080)
    				          .protocol(HttpProtocol.H2C)
    				          .handle((request, response) -> response.sendString(Mono.just("hello")))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

The application should now behave as follows:

    $ curl --http2-prior-knowledge http://localhost:8080 -i
    HTTP/2 200

    hello

#### 5.10.1. Protocol Selection

./../../reactor-netty-http/src/main/java/reactor/netty/http/HttpProtocol.java

    public enum HttpProtocol {

    	/**
    	 * The default supported HTTP protocol by HttpServer and HttpClient
    	 */
    	HTTP11,

    	/**
    	 * HTTP/2.0 support with TLS
    	 * <p>If used along with HTTP/1.1 protocol, HTTP/2.0 will be the preferred protocol.
    	 * While negotiating the application level protocol, HTTP/2.0 or HTTP/1.1 can be chosen.
    	 * <p>If used without HTTP/1.1 protocol, HTTP/2.0 will always be offered as a protocol
    	 * for communication with no fallback to HTTP/1.1.
    	 */
    	H2,

    	/**
    	 * HTTP/2.0 support with clear-text.
    	 * <p>If used along with HTTP/1.1 protocol, will support H2C "upgrade":
    	 * Request or consume requests as HTTP/1.1 first, looking for HTTP/2.0 headers
    	 * and {@literal Connection: Upgrade}. A server will typically reply a successful
    	 * 101 status if upgrade is successful or a fallback HTTP/1.1 response. When
    	 * successful the client will start sending HTTP/2.0 traffic.
    	 * <p>If used without HTTP/1.1 protocol, will support H2C "prior-knowledge": Doesn't
    	 * require {@literal Connection: Upgrade} handshake between a client and server but
    	 * fallback to HTTP/1.1 will not be supported.
    	 */
    	H2C
    }

### 5.11. Metrics

The HTTP server supports built-in integration with `Micrometer`. It exposes all metrics with a prefix of `reactor.netty.http.server`.

The following table provides information for the HTTP server metrics:

metric name | type | description
--- | --- | ---
reactor.netty.http.server.data.received | DistributionSummary | Amount of the data received, in bytes
reactor.netty.http.server.data.sent | DistributionSummary | Amount of the data sent, in bytes
reactor.netty.http.server.errors | Counter | Number of errors that occurred
reactor.netty.http.server.data.received.time | Timer | Time spent in consuming incoming data
reactor.netty.http.server.data.sent.time | Timer | Time spent in sending outgoing data
reactor.netty.http.server.response.time | Timer | Total time for the request/response

These additional metrics are also available:

`ByteBufAllocator` metrics

metric name | type | description
--- | --- | ---
reactor.netty.bytebuf.allocator.used.heap.memory | Gauge | The number of the bytes of the heap memory
reactor.netty.bytebuf.allocator.used.direct.memory | Gauge | The number of the bytes of the direct memory
reactor.netty.bytebuf.allocator.used.heap.arenas | Gauge | The number of heap arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.direct.arenas | Gauge | The number of direct arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.threadlocal.caches | Gauge | The number of thread local caches (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.tiny.cache.size | Gauge | The size of the tiny cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.small.cache.size | Gauge | The size of the small cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.normal.cache.size | Gauge | The size of the normal cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.chunk.size | Gauge | The chunk size for an arena (when `PooledByteBufAllocator`)

The following example enables that integration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/metrics/Application.java

    import io.micrometer.core.instrument.Metrics;
    import io.micrometer.core.instrument.config.MeterFilter;
    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		Metrics.globalRegistry (1)
    		       .config()
    		       .meterFilter(MeterFilter.maximumAllowableTags("reactor.netty.http.server", "URI", 100, MeterFilter.deny()));

    		DisposableServer server =
    				HttpServer.create()
    				          .metrics(true, s -> {
    				              if (s.startsWith("/stream/")) { (2)
    				                  return "/stream/{n}";
    				              }
    				              else if (s.startsWith("/bytes/")) {
    				                  return "/bytes/{n}";
    				              }
    				              return s;
    				          }) (3)
    				          .route(r ->
    				              r.get("/stream/{n}",
    				                   (req, res) -> res.sendString(Mono.just(req.param("n"))))
    				               .get("/bytes/{n}",
    				                   (req, res) -> res.sendString(Mono.just(req.param("n")))))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Applies upper limit for the meters with `URI` tag
2 | Templated URIs will be used as an URI tag value when possible
3 | Enables the built-in integration with Micrometer

| In order to avoid a memory and CPU overhead of the enabled metrics, it is important to convert the real URIs to templated URIs when possible. Without a conversion to a template-like form, each distinct URI leads to the creation of a distinct tag, which takes a lot of memory for the metrics.

| Always apply an upper limit for the meters with URI tags. Configuring an upper limit on the number of meters can help in cases when the real URIs cannot be templated. You can find more information at `maximumAllowableTags`.

When HTTP server metrics are needed for an integration with a system other than `Micrometer` or you want to provide your own integration with `Micrometer`, you can provide your own metrics recorder, as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/metrics/custom/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.DisposableServer;
    import reactor.netty.channel.ChannelMetricsRecorder;
    import reactor.netty.http.server.HttpServer;

    import java.net.SocketAddress;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .metrics(true, CustomHttpServerMetricsRecorder::new) (1)
    				          .route(r ->
    				              r.get("/stream/{n}",
    				                   (req, res) -> res.sendString(Mono.just(req.param("n"))))
    				               .get("/bytes/{n}",
    				                   (req, res) -> res.sendString(Mono.just(req.param("n")))))
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}

1 | Enables HTTP server metrics and provides `HttpServerMetricsRecorder` implementation.

### 5.12. Unix Domain Sockets

The `HTTP` server supports Unix Domain Sockets (UDS) when native transport is in use.

The following example shows how to use UDS support:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/server/uds/Application.java

    import io.netty.channel.unix.DomainSocketAddress;
    import reactor.netty.DisposableServer;
    import reactor.netty.http.server.HttpServer;

    public class Application {

    	public static void main(String[] args) {
    		DisposableServer server =
    				HttpServer.create()
    				          .bindAddress(() -> new DomainSocketAddress("/tmp/test.sock")) (1)
    				          .bindNow();

    		server.onDispose()
    		      .block();
    	}
    }

1 | Specifies `DomainSocketAddress` that will be used

Suggest Edit to "HTTP Server"

## 6. HTTP Client

Reactor Netty provides the easy-to-use and easy-to-configure `HttpClient`. It hides most of the Netty functionality that is required to create an `HTTP` client and adds Reactive Streams backpressure.

### 6.1. Connect

To connect the `HTTP` client to a given `HTTP` endpoint, you must create and configure a `HttpClient` instance. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/connect/Application.java

    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client = HttpClient.create();  (1)

    		client.get()                       (2)
    		      .uri("https://example.com/") (3)
    		      .response()                  (4)
    		      .block();
    	}
    }

1 | Creates a HttpClient instance ready for configuring.
2 | Specifies that `GET` method will be used.
3 | Specifies the path.
4 | Obtains the response HttpClientResponse

The following example uses `WebSocket`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/websocket/Application.java

    import io.netty.buffer.Unpooled;
    import io.netty.util.CharsetUtil;
    import reactor.core.publisher.Flux;
    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client = HttpClient.create();

    		client.websocket()
    		      .uri("wss://echo.websocket.org")
    		      .handle((inbound, outbound) -> {
    		          inbound.receive()
    		                 .asString()
    		                 .take(1)
    		                 .subscribe(System.out::println);

    		          final byte[] msgBytes = "hello".getBytes(CharsetUtil.ISO_8859_1);
    		          return outbound.send(Flux.just(Unpooled.wrappedBuffer(msgBytes), Unpooled.wrappedBuffer(msgBytes)))
    		                         .neverComplete();
    		      })
    		      .blockLast();
    	}
    }

#### 6.1.1. Host and Port

In order to connect to a specific host and port, you can apply the following configuration to the `HTTP` client:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/address/Application.java

    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .host("example.com") (1)
    				          .port(80);           (2)

    		client.get()
    		      .uri("/")
    		      .response()
    		      .block();
    	}
    }

1 | Configures the `HTTP` host
2 | Configures the `HTTP` port

### 6.2. Eager Initialization

By default, the initialization of the `HttpClient` resources happens on demand. This means that the `first request` absorbs the extra time needed to initialize and load:

- the event loop group

- the host name resolver

- the native transport libraries (when native transport is used)

- the native libraries for the security (in case of `OpenSsl`)

When you need to preload these resources, you can configure the `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/warmup/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.ByteBufFlux;
    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client = HttpClient.create();

    		client.warmup() (1)
    		      .block();

    		client.post()
    		      .uri("https://example.com/")
    		      .send(ByteBufFlux.fromString(Mono.just("hello")))
    		      .response()
    		      .block(); (2)
    	}
    }

1 | Initialize and load the event loop group, the host name resolver, the native transport libraries and the native libraries for the security
2 | Host name resolution happens with the first request. In this example, a connection pool is used, so with the first request the connection to the URL is established, the subsequent requests to the same URL reuse the connections from the pool.

### 6.3. Writing Data

To send data to a given `HTTP` endpoint, you can provide a `Publisher` by using the `send(Publisher)` method. By default, `Transfer-Encoding: chunked` is applied for those `HTTP` methods for which a request body is expected. `Content-Length` provided through request headers disables `Transfer-Encoding: chunked`, if necessary. The following example sends `hello`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/send/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.ByteBufFlux;
    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client = HttpClient.create();

    		client.post()
    		      .uri("https://example.com/")
    		      .send(ByteBufFlux.fromString(Mono.just("hello"))) (1)
    		      .response()
    		      .block();
    	}
    }

1 | Sends a `hello` string to the given `HTTP` endpoint

#### 6.3.1. Adding Headers and Other Metadata

When sending data to a given `HTTP` endpoint, you may need to send additional headers, cookies and other metadata. You can use the following configuration to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/send/headers/Application.java

    import io.netty.handler.codec.http.HttpHeaderNames;
    import reactor.core.publisher.Mono;
    import reactor.netty.ByteBufFlux;
    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .headers(h -> h.set(HttpHeaderNames.CONTENT_LENGTH, 5)); (1)

    		client.post()
    		      .uri("https://example.com/")
    		      .send(ByteBufFlux.fromString(Mono.just("hello")))
    		      .response()
    		      .block();
    	}
    }

1 | Disables `Transfer-Encoding: chunked` and provides `Content-Length` header.

##### Compression

You can enable compression on the `HTTP` client, which means the request header `Accept-Encoding` is added to the request headers. The following example shows how to do so:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/compression/Application.java

    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .compress(true);

    		client.get()
    		      .uri("https://example.com/")
    		      .response()
    		      .block();
    	}
    }

##### Auto-Redirect Support

You can configure the `HTTP` client to enable auto-redirect support.

Reactor Netty provides two different strategies for auto-redirect support:

- `followRedirect(boolean)`: Specifies whether HTTP auto-redirect support is enabled for statuses `301|302|307|308`.

- `followRedirect(BiPredicate<HttpClientRequest, HttpClientResponse>)`: Enables auto-redirect support if the supplied predicate matches.

The following example uses `followRedirect(true)`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/redirect/Application.java

    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .followRedirect(true);

    		client.get()
    		      .uri("https://example.com/")
    		      .response()
    		      .block();
    	}
    }

### 6.4. Consuming Data

To receive data from a given `HTTP` endpoint, you can use one of the methods from `HttpClient.ResponseReceiver`. The following example uses the `responseContent` method:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/read/Application.java

    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client = HttpClient.create();

    		client.get()
    		      .uri("https://example.com/")
    		      .responseContent() (1)
    		      .aggregate()       (2)
    		      .asString()        (3)
    		      .block();
    	}
    }

1 | Receives data from a given `HTTP` endpoint
2 | Aggregates the data
3 | Transforms the data as string

#### 6.4.1. Reading Headers and Other Metadata

When receiving data from a given `HTTP` endpoint, you can check response headers, status code, and other metadata. You can obtain this additional metadata by using `HttpClientResponse`. The following example shows how to do so.

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/read/status/Application.java

    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client = HttpClient.create();

    		client.get()
    		      .uri("https://example.com/")
    		      .responseSingle((resp, bytes) -> {
    		          System.out.println(resp.status()); (1)
    		          return bytes.asString();
    		      })
    		      .block();
    	}
    }

1 | Obtains the status code.

#### 6.4.2. HTTP Response Decoder

By default, `Netty` configures some restrictions for the incoming responses, such as:

- The maximum length of the initial line.

- The maximum length of all headers.

- The maximum length of the content or each chunk.

For more information, see HttpResponseDecoder

By default, the `HTTP` client is configured with the following settings:

./../../reactor-netty-http/src/main/java/reactor/netty/http/HttpDecoderSpec.java

    public static final int DEFAULT_MAX_INITIAL_LINE_LENGTH = 4096;
    public static final int DEFAULT_MAX_HEADER_SIZE         = 8192;
    public static final int DEFAULT_MAX_CHUNK_SIZE          = 8192;
    public static final boolean DEFAULT_VALIDATE_HEADERS    = true;
    public static final int DEFAULT_INITIAL_BUFFER_SIZE     = 128;

./../../reactor-netty-http/src/main/java/reactor/netty/http/client/HttpResponseDecoderSpec.java

    public static final boolean DEFAULT_FAIL_ON_MISSING_RESPONSE         = false;
    public static final boolean DEFAULT_PARSE_HTTP_AFTER_CONNECT_REQUEST = false;

    /**
     * The maximum length of the content of the HTTP/2.0 clear-text upgrade request.
     * By default the client will allow an upgrade request with up to 65536 as
     * the maximum length of the aggregated content.
     */
    public static final int DEFAULT_H2C_MAX_CONTENT_LENGTH = 65536;

When you need to change these default settings, you can configure the `HTTP` client as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/responsedecoder/Application.java

    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .httpResponseDecoder(spec -> spec.maxHeaderSize(16384)); (1)

    		client.get()
    		      .uri("https://example.com/")
    		      .responseContent()
    		      .aggregate()
    		      .asString()
    		      .block();
    	}
    }

1 | The maximum length of all headers will be `16384`. When this value is exceeded, a TooLongFrameException is raised.

### 6.5. Lifecycle Callbacks

The following lifecycle callbacks are provided to let you extend the `HttpClient`.

Callback | Description
--- | ---
`doAfterRequest` | Invoked when the request has been sent.
`doAfterResolve` | Invoked after the remote address has been resolved successfully.
`doAfterResponseSuccess` | Invoked after the response has been fully received.
`doOnChannelInit` | Invoked when initializing the channel.
`doOnConnect` | Invoked when the channel is about to connect.
`doOnConnected` | Invoked after the channel has been connected.
`doOnDisconnected` | Invoked after the channel has been disconnected.
`doOnError` | Invoked when the request has not been sent and when the response has not been fully received.
`doOnRedirect` | Invoked when the response headers have been received, and the request is about to be redirected.
`doOnRequest` | Invoked when the request is about to be sent.
`doOnRequestError` | Invoked when the request has not been sent.
`doOnResolve` | Invoked when the remote address is about to be resolved.
`doOnResolveError` | Invoked in case the remote address hasn’t been resolved successfully.
`doOnResponse` | Invoked after the response headers have been received.
`doOnResponseError` | Invoked when the response has not been fully received.

The following example uses the `doOnConnected` and `doOnChannelInit` callbacks:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/lifecycle/Application.java

    import io.netty.handler.logging.LoggingHandler;
    import io.netty.handler.timeout.ReadTimeoutHandler;
    import reactor.netty.http.client.HttpClient;
    import java.util.concurrent.TimeUnit;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .doOnConnected(conn ->
    				              conn.addHandler(new ReadTimeoutHandler(10, TimeUnit.SECONDS)))   (1)
    				          .doOnChannelInit((observer, channel, remoteAddress) ->
    				              channel.pipeline()
    				                     .addFirst(new LoggingHandler("reactor.netty.examples"))); (2)

    		client.get()
    		      .uri("https://example.com/")
    		      .response()
    		      .block();
    	}
    }

1 | `Netty` pipeline is extended with `ReadTimeoutHandler` when the channel has been connected.
2 | `Netty` pipeline is extended with `LoggingHandler` when initializing the channel.

### 6.6. TCP-level Configuration

When you need configurations on a TCP level, you can use the following snippet to extend the default `TCP` client configuration (add an option, bind address etc.):

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/channeloptions/Application.java

    import io.netty.channel.ChannelOption;
    import io.netty.channel.epoll.EpollChannelOption;
    import reactor.netty.http.client.HttpClient;
    import java.net.InetSocketAddress;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .bindAddress(() -> new InetSocketAddress("host", 1234))
    				          .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000) (1)
    				          .option(ChannelOption.SO_KEEPALIVE, true)            (2)
    				          // The options below are available only when Epoll transport is used
    				          .option(EpollChannelOption.TCP_KEEPIDLE, 300)        (3)
    				          .option(EpollChannelOption.TCP_KEEPINTVL, 60)        (4)
    				          .option(EpollChannelOption.TCP_KEEPCNT, 8);          (5)

    		String response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response);
    	}
    }

1 | Configures the connection establishment timeout to 10 seconds.
2 | Enables TCP `keepalive`. This means that TCP starts sending `keepalive` probes when a connection is idle for some time.
3 | The connection needs to remain idle for 5 minutes before TCP starts sending `keepalive` probes.
4 | Configures the time between individual `keepalive` probes to 1 minute.
5 | Configures the maximum number of TCP `keepalive` probes to 8.

See TCP Client for more about `TCP` level configurations.

#### 6.6.1. Wire Logger

Reactor Netty provides wire logging for when the traffic between the peers needs to be inspected. By default, wire logging is disabled. To enable it, you must set the logger `reactor.netty.http.client.HttpClient` level to `DEBUG` and apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/wiretap/Application.java

    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .wiretap(true); (1)

    		client.get()
    		      .uri("https://example.com/")
    		      .response()
    		      .block();
    	}
    }

1 | Enables the wire logging

By default, the wire logging uses AdvancedByteBufFormat#HEX_DUMP when printing the content. When you need to change this to AdvancedByteBufFormat#SIMPLE or AdvancedByteBufFormat#TEXTUAL, you can configure the `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/wiretap/custom/Application.java

    import io.netty.handler.logging.LogLevel;
    import reactor.netty.http.client.HttpClient;
    import reactor.netty.transport.logging.AdvancedByteBufFormat;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .wiretap("logger-name", LogLevel.DEBUG, AdvancedByteBufFormat.TEXTUAL); (1)

    		client.get()
    		      .uri("https://example.com/")
    		      .response()
    		      .block();
    	}
    }

1 | Enables the wire logging, AdvancedByteBufFormat#TEXTUAL is used for printing the content.

### 6.7. SSL and TLS

When you need SSL or TLS, you can apply the configuration shown in the next example. By default, if `OpenSSL` is available, a SslProvider.OPENSSL provider is used as a provider. Otherwise, a SslProvider.JDK provider is used You can switch the provider by using SslContextBuilder or by setting `-Dio.netty.handler.ssl.noOpenSsl=true`. The following example uses `SslContextBuilder`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/security/Application.java

    import reactor.netty.http.Http11SslContextSpec;
    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		Http11SslContextSpec http11SslContextSpec = Http11SslContextSpec.forClient();

    		HttpClient client =
    				HttpClient.create()
    				          .secure(spec -> spec.sslContext(http11SslContextSpec));

    		client.get()
    		      .uri("https://example.com/")
    		      .response()
    		      .block();
    	}
    }

#### 6.7.1. Server Name Indication

By default, the `HTTP` client sends the remote host name as `SNI` server name. When you need to change this default setting, you can configure the `HTTP` client as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/sni/Application.java

    import io.netty.handler.ssl.SslContext;
    import io.netty.handler.ssl.SslContextBuilder;
    import reactor.netty.http.client.HttpClient;

    import javax.net.ssl.SNIHostName;

    public class Application {

    	public static void main(String[] args) throws Exception {
    		SslContext sslContext = SslContextBuilder.forClient().build();

    		HttpClient client =
    				HttpClient.create()
    				          .secure(spec -> spec.sslContext(sslContext)
    				                              .serverNames(new SNIHostName("test.com")));

    		client.get()
    		      .uri("https://127.0.0.1:8080/")
    		      .response()
    		      .block();
    	}
    }

### 6.8. Retry Strategies

By default, the `HTTP` client retries the request once if it was aborted on the `TCP` level.

### 6.9. HTTP/2

By default, the `HTTP` client supports `HTTP/1.1`. If you need `HTTP/2`, you can get it through configuration. In addition to the protocol configuration, if you need `H2` but not `H2C (cleartext)`, you must also configure SSL.

| As Application-Layer Protocol Negotiation (ALPN) is not supported “out-of-the-box” by JDK8 (although some vendors backported ALPN to JDK8), you need an additional dependency to a native library that supports it — for example, `netty-tcnative-boringssl-static`.

The following listing presents a simple `H2` example:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/http2/H2Application.java

    import io.netty.handler.codec.http.HttpHeaders;
    import reactor.core.publisher.Mono;
    import reactor.netty.http.HttpProtocol;
    import reactor.netty.http.client.HttpClient;
    import reactor.util.function.Tuple2;

    public class H2Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .protocol(HttpProtocol.H2) (1)
    				          .secure();                 (2)

    		Tuple2<String, HttpHeaders> response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseSingle((res, bytes) -> bytes.asString()
    				                                           .zipWith(Mono.just(res.responseHeaders())))
    				      .block();

    		System.out.println("Used stream ID: " + response.getT2().get("x-http2-stream-id"));
    		System.out.println("Response: " + response.getT1());
    	}
    }

1 | Configures the client to support only `HTTP/2`
2 | Configures `SSL`

The following listing presents a simple `H2C` example:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/http2/H2CApplication.java

    import io.netty.handler.codec.http.HttpHeaders;
    import reactor.core.publisher.Mono;
    import reactor.netty.http.HttpProtocol;
    import reactor.netty.http.client.HttpClient;
    import reactor.util.function.Tuple2;

    public class H2CApplication {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .protocol(HttpProtocol.H2C);

    		Tuple2<String, HttpHeaders> response =
    				client.get()
    				      .uri("http://localhost:8080/")
    				      .responseSingle((res, bytes) -> bytes.asString()
    				                                           .zipWith(Mono.just(res.responseHeaders())))
    				      .block();

    		System.out.println("Used stream ID: " + response.getT2().get("x-http2-stream-id"));
    		System.out.println("Response: " + response.getT1());
    	}
    }

#### 6.9.1. Protocol Selection

./../../reactor-netty-http/src/main/java/reactor/netty/http/HttpProtocol.java

    public enum HttpProtocol {

    	/**
    	 * The default supported HTTP protocol by HttpServer and HttpClient
    	 */
    	HTTP11,

    	/**
    	 * HTTP/2.0 support with TLS
    	 * <p>If used along with HTTP/1.1 protocol, HTTP/2.0 will be the preferred protocol.
    	 * While negotiating the application level protocol, HTTP/2.0 or HTTP/1.1 can be chosen.
    	 * <p>If used without HTTP/1.1 protocol, HTTP/2.0 will always be offered as a protocol
    	 * for communication with no fallback to HTTP/1.1.
    	 */
    	H2,

    	/**
    	 * HTTP/2.0 support with clear-text.
    	 * <p>If used along with HTTP/1.1 protocol, will support H2C "upgrade":
    	 * Request or consume requests as HTTP/1.1 first, looking for HTTP/2.0 headers
    	 * and {@literal Connection: Upgrade}. A server will typically reply a successful
    	 * 101 status if upgrade is successful or a fallback HTTP/1.1 response. When
    	 * successful the client will start sending HTTP/2.0 traffic.
    	 * <p>If used without HTTP/1.1 protocol, will support H2C "prior-knowledge": Doesn't
    	 * require {@literal Connection: Upgrade} handshake between a client and server but
    	 * fallback to HTTP/1.1 will not be supported.
    	 */
    	H2C
    }

### 6.10. Metrics

The HTTP client supports built-in integration with `Micrometer`. It exposes all metrics with a prefix of `reactor.netty.http.client`.

The following table provides information for the HTTP client metrics:

metric name | type | description
--- | --- | ---
reactor.netty.http.client.data.received | DistributionSummary | Amount of the data received, in bytes
reactor.netty.http.client.data.sent | DistributionSummary | Amount of the data sent, in bytes
reactor.netty.http.client.errors | Counter | Number of errors that occurred
reactor.netty.http.client.tls.handshake.time | Timer | Time spent for TLS handshake
reactor.netty.http.client.connect.time | Timer | Time spent for connecting to the remote address
reactor.netty.http.client.address.resolver | Timer | Time spent for resolving the address
reactor.netty.http.client.data.received.time | Timer | Time spent in consuming incoming data
reactor.netty.http.client.data.sent.time | Timer | Time spent in sending outgoing data
reactor.netty.http.client.response.time | Timer | Total time for the request/response

These additional metrics are also available:

Pooled `ConnectionProvider` metrics

metric name | type | description
--- | --- | ---
reactor.netty.connection.provider.total.connections | Gauge | The number of all connections, active or idle
reactor.netty.connection.provider.active.connections | Gauge | The number of the connections that have been successfully acquired and are in active use
reactor.netty.connection.provider.idle.connections | Gauge | The number of the idle connections
reactor.netty.connection.provider.pending.connections | Gauge | The number of requests that are waiting for a connection

`ByteBufAllocator` metrics

metric name | type | description
--- | --- | ---
reactor.netty.bytebuf.allocator.used.heap.memory | Gauge | The number of the bytes of the heap memory
reactor.netty.bytebuf.allocator.used.direct.memory | Gauge | The number of the bytes of the direct memory
reactor.netty.bytebuf.allocator.used.heap.arenas | Gauge | The number of heap arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.direct.arenas | Gauge | The number of direct arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.threadlocal.caches | Gauge | The number of thread local caches (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.tiny.cache.size | Gauge | The size of the tiny cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.small.cache.size | Gauge | The size of the small cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.normal.cache.size | Gauge | The size of the normal cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.chunk.size | Gauge | The chunk size for an arena (when `PooledByteBufAllocator`)

The following example enables that integration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/metrics/Application.java

    import io.micrometer.core.instrument.Metrics;
    import io.micrometer.core.instrument.config.MeterFilter;
    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		Metrics.globalRegistry (1)
    		       .config()
    		       .meterFilter(MeterFilter.maximumAllowableTags("reactor.netty.http.client", "URI", 100, MeterFilter.deny()));

    		HttpClient client =
    				HttpClient.create()
    				          .metrics(true, s -> {
    				              if (s.startsWith("/stream/")) { (2)
    				                  return "/stream/{n}";
    				              }
    				              else if (s.startsWith("/bytes/")) {
    				                  return "/bytes/{n}";
    				              }
    				              return s;
    				          }); (3)

    		client.get()
    		      .uri("https://httpbin.org/stream/2")
    		      .responseContent()
    		      .blockLast();

    		client.get()
    		      .uri("https://httpbin.org/bytes/1024")
    		      .responseContent()
    		      .blockLast();
    	}
    }

1 | Applies upper limit for the meters with `URI` tag
2 | Templated URIs will be used as a URI tag value when possible
3 | Enables the built-in integration with Micrometer

| In order to avoid a memory and CPU overhead of the enabled metrics, it is important to convert the real URIs to templated URIs when possible. Without a conversion to a template-like form, each distinct URI leads to the creation of a distinct tag, which takes a lot of memory for the metrics.

| Always apply an upper limit for the meters with URI tags. Configuring an upper limit on the number of meters can help in cases when the real URIs cannot be templated. You can find more information at `maximumAllowableTags`.

When HTTP client metrics are needed for an integration with a system other than `Micrometer` or you want to provide your own integration with `Micrometer`, you can provide your own metrics recorder, as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/metrics/custom/Application.java

    import reactor.netty.channel.ChannelMetricsRecorder;
    import reactor.netty.http.client.HttpClient;

    import java.net.SocketAddress;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .metrics(true, CustomHttpClientMetricsRecorder::new); (1)

    		client.get()
    		      .uri("https://httpbin.org/stream/2")
    		      .response()
    		      .block();
    	}

1 | Enables HTTP client metrics and provides `HttpClientMetricsRecorder` implementation.

### 6.11. Unix Domain Sockets

The `HTTP` client supports Unix Domain Sockets (UDS) when native transport is in use.

The following example shows how to use UDS support:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/uds/Application.java

    import io.netty.channel.unix.DomainSocketAddress;
    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .remoteAddress(() -> new DomainSocketAddress("/tmp/test.sock")); (1)

    		client.get()
    		      .uri("/")
    		      .response()
    		      .block();
    	}
    }

1 | Specifies `DomainSocketAddress` that will be used

### 6.12. Host Name Resolution

By default, the `HttpClient` uses Netty’s domain name lookup mechanism that resolves a domain name asynchronously. This is as an alternative of the JVM’s built-in blocking resolver.

When you need to change the default settings, you can configure the `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/resolver/Application.java

    import reactor.netty.http.client.HttpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .resolver(spec -> spec.queryTimeout(Duration.ofMillis(500))); (1)

    		String response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response);
    	}
    }

1 | The timeout of each DNS query performed by this resolver will be 500ms.

The following listing shows the available configurations:

Configuration name | Description
--- | ---
`cacheMaxTimeToLive` | The max time to live of the cached DNS resource records (resolution: seconds). If the time to live of the DNS resource record returned by the DNS server is greater than this max time to live, this resolver ignores the time to live from the DNS server and uses this max time to live. Default to `Integer.MAX_VALUE`.
`cacheMinTimeToLive` | The min time to live of the cached DNS resource records (resolution: seconds). If the time to live of the DNS resource record returned by the DNS server is less than this min time to live, this resolver ignores the time to live from the DNS server and uses this min time to live. Default: 0.
`cacheNegativeTimeToLive` | The time to live of the cache for the failed DNS queries (resolution: seconds). Default: 0.
`completeOncePreferredResolved` | When this setting is enabled, the resolver notifies as soon as all queries for the preferred address type are complete. When this setting is disabled, the resolver notifies when all possible address types are complete. This configuration is applicable for `DnsNameResolver#resolveAll(String)`. By default, this setting is enabled.
`disableOptionalRecord` | Disables the automatic inclusion of an optional record that tries to give a hint to the remote DNS server about how much data the resolver can read per response. By default, this setting is enabled.
`disableRecursionDesired` | Specifies whether this resolver has to send a DNS query with the recursion desired (RD) flag set. By default, this setting is enabled.
`maxPayloadSize` | Sets the capacity of the datagram packet buffer (in bytes). Default: 4096.
`maxQueriesPerResolve` | Sets the maximum allowed number of DNS queries to send when resolving a host name. Default: 16.
`ndots` | Sets the number of dots that must appear in a name before an initial absolute query is made. Default: -1 (to determine the value from the OS on Unix or use a value of 1 otherwise).
`queryTimeout` | Sets the timeout of each DNS query performed by this resolver (resolution: milliseconds). Default: 5000.
`resolvedAddressTypes` | The list of the protocol families of the resolved address.
`roundRobinSelection` | Enables an `AddressResolverGroup` of `DnsNameResolver` that supports random selection of destination addresses if multiple are provided by the nameserver. See `RoundRobinDnsAddressResolverGroup`. Default: `DnsAddressResolverGroup`
`runOn` | Performs the communication with the DNS servers on the given `LoopResources`. By default, the LoopResources specified on the client level are used.
`searchDomains` | The list of search domains of the resolver. By default, the effective search domain list is populated by using the system DNS search domains.
`trace` | A specific logger and log level to be used by this resolver when generating detailed trace information in case of resolution failure.

Sometimes, you may want to switch to the JVM built-in resolver. To do so, you can configure the `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/resolver/custom/Application.java

    import io.netty.resolver.DefaultAddressResolverGroup;
    import reactor.netty.http.client.HttpClient;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .resolver(DefaultAddressResolverGroup.INSTANCE); (1)

    		String response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response);
    	}
    }

1 | Sets the JVM built-in resolver.

### 6.13. Timeout Configuration

This section describes various timeout configuration options that can be used in `HttpClient`. Configuring a proper timeout may improve or solve issues in the communication process. The configuration options can be grouped as follows:

- Connection Pool Timeout

- HttpClient Timeout

  - Response Timeout

  - Connection Timeout

  - SSL/TLS Timeout

  - Proxy Timeout

  - Host Name Resolution Timeout

#### 6.13.1. Connection Pool Timeout

By default, `HttpClient` uses a connection pool. When a request is completed successfully and if the connection is not scheduled for closing, the connection is returned to the connection pool and can thus be reused for processing another request. The connection may be reused immediately for another request or may stay idle in the connection pool for some time.

The following list describes the available timeout configuration options:

- `maxIdleTime` - The maximum time (resolution: ms) that this connection stays idle in the connection pool. By default, `maxIdleTime` is not specified.

| When you configure `maxIdleTime`, you should consider the idle timeout configuration on the target server. Choose a configuration that is equal to or less than the one on the target server. By doing so, you can reduce the I/O issues caused by a connection closed by the target server.

- `maxLifeTime` - The maximum time (resolution: ms) that this connection stays alive. By default, `maxLifeTime` is not specified.

- `pendingAcquireTimeout` - The maximum time (resolution: ms) after which a pending acquire operation must complete, or a `PoolAcquireTimeoutException` is thrown. Default: 45s.

By default, these timeouts are checked on connection `release` or `acquire` operations and, if some timeout is reached, the connection is closed and removed from the connection pool. However, you can also configure the connection pool, by setting `evictInBackground`, to perform periodic checks on connections.

To customize the default settings, you can configure `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/pool/Application.java

    import reactor.netty.http.client.HttpClient;
    import reactor.netty.resources.ConnectionProvider;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		ConnectionProvider provider =
    				ConnectionProvider.builder("custom")
    				                  .maxConnections(50)
    				                  .maxIdleTime(Duration.ofSeconds(20))           (1)
    				                  .maxLifeTime(Duration.ofSeconds(60))           (2)
    				                  .pendingAcquireTimeout(Duration.ofSeconds(60)) (3)
    				                  .evictInBackground(Duration.ofSeconds(120))    (4)
    				                  .build();

    		HttpClient client = HttpClient.create(provider);

    		String response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response);

    		provider.disposeLater()
    		        .block();
    	}
    }

1 | Configures the maximum time for a connection to stay idle to 20 seconds.
2 | Configures the maximum time for a connection to stay alive to 60 seconds.
3 | Configures the maximum time for the pending acquire operation to 60 seconds.
4 | Every two minutes, the connection pool is regularly checked for connections that are applicable for removal.

#### 6.13.2. HttpClient Timeout

This section provides information for the various timeout configuration options at the `HttpClient` level.

| Reactor Netty uses Reactor Core as its Reactive Streams implementation, and you may want to use the `timeout` operator that `Mono` and `Flux` provide. Keep in mind, however, that it is better to use the more specific timeout configuration options available in Reactor Netty, since they provide more control for a specific purpose and use case. By contrast, the `timeout` operator can only apply to the operation as a whole, from establishing the connection to the remote peer to receiving the response.

##### Response Timeout

`HttpClient` provides an API for configuring a default response timeout for all requests. You can change this default response timeout through an API for a specific request. By default, `responseTimeout` is not specified.

| It is always a good practice to configure a response timeout.

To customize the default settings, you can configure `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/read/timeout/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.http.client.HttpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .responseTimeout(Duration.ofSeconds(1));    (1)

    		String response1 =
    				client.post()
    				      .uri("https://example.com/")
    				      .send((req, out) -> {
    				          req.responseTimeout(Duration.ofSeconds(2)); (2)
    				          return out.sendString(Mono.just("body1"));
    				      })
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response1);

    		String response2 =
    				client.post()
    				      .uri("https://example.com/")
    				      .send((req, out) -> out.sendString(Mono.just("body2")))
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response2);
    	}
    }

1 | Configures the default response timeout to 1 second.
2 | Configures a response timeout for a specific request to 2 seconds.

##### Connection Timeout

The following listing shows all available connection timeout configuration options, but some of them may apply only to a specific transport.

- `CONNECT_TIMEOUT_MILLIS` - If the connection establishment attempt to the remote peer does not finish within the configured connect timeout (resolution: ms), the connection establishment attempt fails. Default: 30s.

- `SO_KEEPALIVE` - When the connection stays idle for some time (the time is implementation dependent, but the default is typically two hours), TCP automatically sends a `keepalive` probe to the remote peer. By default, `SO_KEEPALIVE` is not enabled. When you run with `Epoll` transport, you may also configure:

  - `TCP_KEEPIDLE` - The maximum time (resolution: seconds) that this connection stays idle before TCP starts sending `keepalive` probes, if `SO_KEEPALIVE` has been set. The maximum time is implementation dependent, but the default is typically two hours.

  - `TCP_KEEPINTVL` - The time (resolution: seconds) between individual `keepalive` probes.

  - `TCP_KEEPCNT` - The maximum number of `keepalive` probes TCP should send before dropping the connection.

| Sometimes, between the client and the server, you may have a network component that silently drops the idle connections without sending a response. From the Reactor Netty point of view, in this use case, the remote peer just does not respond. To be able to handle such a use case you may consider configuring `SO_KEEPALIVE`.

To customize the default settings, you can configure `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/channeloptions/Application.java

    import io.netty.channel.ChannelOption;
    import io.netty.channel.epoll.EpollChannelOption;
    import reactor.netty.http.client.HttpClient;
    import java.net.InetSocketAddress;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .bindAddress(() -> new InetSocketAddress("host", 1234))
    				          .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000) (1)
    				          .option(ChannelOption.SO_KEEPALIVE, true)            (2)
    				          // The options below are available only when Epoll transport is used
    				          .option(EpollChannelOption.TCP_KEEPIDLE, 300)        (3)
    				          .option(EpollChannelOption.TCP_KEEPINTVL, 60)        (4)
    				          .option(EpollChannelOption.TCP_KEEPCNT, 8);          (5)

    		String response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response);
    	}
    }

1 | Configures the connection establishment timeout to 10 seconds.
2 | Enables TCP `keepalive`. This means that TCP starts sending `keepalive` probes when a connection is idle for some time.
3 | The connection needs to remain idle for 5 minutes before TCP starts sending `keepalive` probes.
4 | Configures the time between individual `keepalive` probes to 1 minute.
5 | Configures the maximum number of TCP `keepalive` probes to 8.

##### SSL/TLS Timeout

`HttpClient` supports the SSL/TLS functionality provided by Netty.

The following list describes the available timeout configuration options:

- `handshakeTimeout` - Use this option to configure the SSL handshake timeout (resolution: ms). Default: 10s.

| You should consider increasing the SSL handshake timeout when expecting slow network connections.

- `closeNotifyFlushTimeout` - Use this option to configure the SSL `close_notify` flush timeout (resolution: ms). Default: 3s.

- `closeNotifyReadTimeout` - Use this option to configure the SSL `close_notify` read timeout (resolution: ms). Default: 0s.

To customize the default settings, you can configure `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/security/custom/Application.java

    import reactor.netty.http.Http11SslContextSpec;
    import reactor.netty.http.client.HttpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Http11SslContextSpec http11SslContextSpec = Http11SslContextSpec.forClient();

    		HttpClient client =
    				HttpClient.create()
    				          .secure(spec -> spec.sslContext(http11SslContextSpec)
    				                              .handshakeTimeout(Duration.ofSeconds(30))         (1)
    				                              .closeNotifyFlushTimeout(Duration.ofSeconds(10))  (2)
    				                              .closeNotifyReadTimeout(Duration.ofSeconds(10))); (3)

    		String response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response);
    	}
    }

1 | Configures the SSL handshake timeout to 30 seconds.
2 | Configures the SSL `close_notify` flush timeout to 10 seconds.
3 | Configures the SSL `close_notify` read timeout to 10 seconds.

##### Proxy Timeout

`HttpClient` supports the proxy functionality provided by Netty and provides a way to specify the connection establishment timeout. If the connection establishment attempt to the remote peer does not finish within the timeout, the connection establishment attempt fails. Default: 10s.

To customize the default settings, you can configure `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/proxy/Application.java

    import reactor.netty.http.client.HttpClient;
    import reactor.netty.transport.ProxyProvider;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .proxy(spec -> spec.type(ProxyProvider.Proxy.HTTP)
    				                             .host("proxy")
    				                             .port(8080)
    				                             .connectTimeoutMillis(20_000)); (1)

    		String response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response);
    	}
    }

1 | Configures the connection establishment timeout to 20 seconds.

##### Host Name Resolution Timeout

By default, the `HttpClient` uses Netty’s domain name lookup mechanism to resolve a domain name asynchronously.

The following list describes the available timeout configuration options:

- `cacheMaxTimeToLive` - The maximum time to live of the cached DNS resource records (resolution: seconds). If the time to live of the DNS resource record returned by the DNS server is greater than this maximum time to live, this resolver ignores the time to live from the DNS server and uses this maximum time to live. Default: `Integer.MAX_VALUE`.

- `cacheMinTimeToLive` - The minimum time to live of the cached DNS resource records (resolution: seconds). If the time to live of the DNS resource record returned by the DNS server is less than this minimum time to live, this resolver ignores the time to live from the DNS server and uses this minimum time to live. Default: 0s.

- `cacheNegativeTimeToLive` - The time to live of the cache for the failed DNS queries (resolution: seconds). Default: 0s.

- `queryTimeout` - Sets the timeout of each DNS query performed by this resolver (resolution: milliseconds). Default: 5s.

To customize the default settings, you can configure `HttpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/resolver/Application.java

    import reactor.netty.http.client.HttpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		HttpClient client =
    				HttpClient.create()
    				          .resolver(spec -> spec.queryTimeout(Duration.ofMillis(500))); (1)

    		String response =
    				client.get()
    				      .uri("https://example.com/")
    				      .responseContent()
    				      .aggregate()
    				      .asString()
    				      .block();

    		System.out.println("Response " + response);
    	}
    }

1 | The timeout of each DNS query performed by this resolver will be 500ms.

Suggest Edit to "HTTP Client"

## 7. UDP Server

Reactor Netty provides the easy-to-use and easy-to-configure `UdpServer`. It hides most of the Netty functionality that is required to create a `UDP` server and adds `Reactive Streams` backpressure.

### 7.1. Starting and Stopping

To start a UDP server, a UdpServer instance has to be created and configured. By default, the host is configured to be `localhost` and the port is `12012`. The following example shows how to create and start a UDP server:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/create/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()                         (1)
    				         .bindNow(Duration.ofSeconds(30)); (2)

    		server.onDispose()
    		      .block();
    	}
    }

1 | Creates a `UdpServer` instance that is ready for configuring.
2 | Starts the server in a blocking fashion and waits for it to finish initializing.

The returned `Connection` offers a simple server API, including `disposeNow()`, which shuts the server down in a blocking fashion.

#### 7.1.1. Host and Port

In order to serve on a specific host and port, you can apply the following configuration to the `UDP` server:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/address/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .host("localhost") (1)
    				         .port(8080)        (2)
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

1 | Configures the `UDP` server host
2 | Configures the `UDP` server port

### 7.2. Eager Initialization

By default, the initialization of the `UdpServer` resources happens on demand. This means that the `bind operation` absorbs the extra time needed to initialize and load:

- the event loop group

- the native transport libraries (when native transport is used)

When you need to preload these resources, you can configure the `UdpServer` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/warmup/Application.java

    import io.netty.channel.socket.DatagramPacket;
    import reactor.core.publisher.Mono;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		UdpServer udpServer =
    				UdpServer.create()
    				         .handle((in, out) ->
    				             out.sendObject(
    				                 in.receiveObject()
    				                   .map(o -> {
    				                       if (o instanceof DatagramPacket) {
    				                           DatagramPacket p = (DatagramPacket) o;
    				                           return new DatagramPacket(p.content().retain(), p.sender());
    				                       }
    				                       else {
    				                           return Mono.error(new Exception("Unexpected type of the message: " + o));
    				                       }
    				                   })));

    		udpServer.warmup() (1)
    		         .block();

    		Connection server = udpServer.bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

1 | Initialize and load the event loop group and the native transport libraries

### 7.3. Writing Data

To send data to the remote peer, you must attach an I/O handler. The I/O handler has access to `UdpOutbound`, to be able to write data. The following example shows how to send `hello`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/send/Application.java

    import io.netty.buffer.ByteBuf;
    import io.netty.buffer.Unpooled;
    import io.netty.channel.socket.DatagramPacket;
    import io.netty.util.CharsetUtil;
    import reactor.core.publisher.Mono;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .handle((in, out) ->
    				             out.sendObject(
    				                 in.receiveObject()
    				                   .map(o -> {
    				                       if (o instanceof DatagramPacket) {
    				                           DatagramPacket p = (DatagramPacket) o;
    				                           ByteBuf buf = Unpooled.copiedBuffer("hello", CharsetUtil.UTF_8);
    				                           return new DatagramPacket(buf, p.sender()); (1)
    				                       }
    				                       else {
    				                           return Mono.error(new Exception("Unexpected type of the message: " + o));
    				                       }
    				                   })))
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

1 | Sends a `hello` string to the remote peer

### 7.4. Consuming Data

To receive data from a remote peer, you must attach an I/O handler. The I/O handler has access to `UdpInbound`, to be able to read data. The following example shows how to consume data:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/read/Application.java

    import io.netty.channel.socket.DatagramPacket;
    import reactor.core.publisher.Mono;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .handle((in, out) ->
    				             out.sendObject(
    				                 in.receiveObject()
    				                   .map(o -> {
    				                       if (o instanceof DatagramPacket) {
    				                           DatagramPacket p = (DatagramPacket) o;
    				                           return new DatagramPacket(p.content().retain(), p.sender()); (1)
    				                       }
    				                       else {
    				                           return Mono.error(new Exception("Unexpected type of the message: " + o));
    				                       }
    				                   })))
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

1 | Receives data from the remote peer

### 7.5. Lifecycle Callbacks

The following lifecycle callbacks are provided to let you extend the `UdpServer`:

Callback | Description
--- | ---
`doOnBind` | Invoked when the server channel is about to bind.
`doOnBound` | Invoked when the server channel is bound.
`doOnChannelInit` | Invoked when initializing the channel.
`doOnUnbound` | Invoked when the server channel is unbound.

The following example uses the `doOnBound` and `doOnChannelInit` callbacks:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/lifecycle/Application.java

    import io.netty.handler.codec.LineBasedFrameDecoder;
    import io.netty.handler.logging.LoggingHandler;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .doOnBound(conn -> conn.addHandler(new LineBasedFrameDecoder(8192))) (1)
    				         .doOnChannelInit((observer, channel, remoteAddress) ->
    				             channel.pipeline()
    				                    .addFirst(new LoggingHandler("reactor.netty.examples")))  (2)
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

1 | `Netty` pipeline is extended with `LineBasedFrameDecoder` when the server channel is bound.
2 | `Netty` pipeline is extended with `LoggingHandler` when initializing the channel.

### 7.6. Connection Configuration

This section describes three kinds of configuration that you can use at the UDP level:

- Channel Options

- Wire Logger

- Event Loop Group

#### 7.6.1. Channel Options

By default, the `UDP` server is configured with the following options:

./../../reactor-netty-core/src/main/java/reactor/netty/udp/UdpServerBind.java

    UdpServerBind() {
    	this.config = new UdpServerConfig(
    			Collections.singletonMap(ChannelOption.AUTO_READ, false),
    			() -> new InetSocketAddress(NetUtil.LOCALHOST, DEFAULT_PORT));
    }

If you need additional options or need to change the current options, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/channeloptions/Application.java

    import io.netty.channel.ChannelOption;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000)
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

For more information about Netty channel options, see the following links:

- Common ChannelOption

- Epoll ChannelOption

- KQueue ChannelOption

- Socket Options

#### 7.6.2. Wire Logger

Reactor Netty provides wire logging for when the traffic between the peers has to be inspected. By default, wire logging is disabled. To enable it, you must set the logger `reactor.netty.udp.UdpServer` level to `DEBUG` and apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/wiretap/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .wiretap(true) (1)
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

1 | Enables the wire logging

By default, the wire logging uses AdvancedByteBufFormat#HEX_DUMP when printing the content. When you need to change this to AdvancedByteBufFormat#SIMPLE or AdvancedByteBufFormat#TEXTUAL, you can configure the `UdpServer` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/wiretap/custom/Application.java

    import io.netty.handler.logging.LogLevel;
    import reactor.netty.Connection;
    import reactor.netty.transport.logging.AdvancedByteBufFormat;
    import reactor.netty.udp.UdpServer;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .wiretap("logger-name", LogLevel.DEBUG, AdvancedByteBufFormat.TEXTUAL) (1)
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

1 | Enables the wire logging, AdvancedByteBufFormat#TEXTUAL is used for printing the content.

#### 7.6.3. Event Loop Group

By default, the UDP server uses “Event Loop Group,” where the number of the worker threads equals the number of processors available to the runtime on initialization (but with a minimum value of 4). When you need a different configuration, you can use one of the LoopResource`#create` methods.

The default configuration for the “Event Loop Group” is the following:

./../../reactor-netty-core/src/main/java/reactor/netty/ReactorNetty.java

    /**
     * Default worker thread count, fallback to available processor
     * (but with a minimum value of 4)
     */
    public static final String IO_WORKER_COUNT = "reactor.netty.ioWorkerCount";
    /**
     * Default selector thread count, fallback to -1 (no selector thread)
     */
    public static final String IO_SELECT_COUNT = "reactor.netty.ioSelectCount";
    /**
     * Default worker thread count for UDP, fallback to available processor
     * (but with a minimum value of 4)
     */
    public static final String UDP_IO_THREAD_COUNT = "reactor.netty.udp.ioThreadCount";
    /**
     * Default quiet period that guarantees that the disposal of the underlying LoopResources
     * will not happen, fallback to 2 seconds.
     */
    public static final String SHUTDOWN_QUIET_PERIOD = "reactor.netty.ioShutdownQuietPeriod";
    /**
     * Default maximum amount of time to wait until the disposal of the underlying LoopResources
     * regardless if a task was submitted during the quiet period, fallback to 15 seconds.
     */
    public static final String SHUTDOWN_TIMEOUT = "reactor.netty.ioShutdownTimeout";

    /**
     * Default value whether the native transport (epoll, kqueue) will be preferred,
     * fallback it will be preferred when available
     */
    public static final String NATIVE = "reactor.netty.native";

If you need changes to these settings, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/eventloop/Application.java

    import reactor.netty.Connection;
    import reactor.netty.resources.LoopResources;
    import reactor.netty.udp.UdpServer;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		LoopResources loop = LoopResources.create("event-loop", 1, 4, true);

    		Connection server =
    				UdpServer.create()
    				         .runOn(loop)
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

### 7.7. Metrics

The UDP server supports built-in integration with `Micrometer`. It exposes all metrics with a prefix of `reactor.netty.udp.server`.

The following table provides information for the UDP server metrics:

metric name | type | description
--- | --- | ---
reactor.netty.udp.server.data.received | DistributionSummary | Amount of the data received, in bytes
reactor.netty.udp.server.data.sent | DistributionSummary | Amount of the data sent, in bytes
reactor.netty.udp.server.errors | Counter | Number of errors that occurred

These additional metrics are also available:

`ByteBufAllocator` metrics

metric name | type | description
--- | --- | ---
reactor.netty.bytebuf.allocator.used.heap.memory | Gauge | The number of the bytes of the heap memory
reactor.netty.bytebuf.allocator.used.direct.memory | Gauge | The number of the bytes of the direct memory
reactor.netty.bytebuf.allocator.used.heap.arenas | Gauge | The number of heap arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.direct.arenas | Gauge | The number of direct arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.threadlocal.caches | Gauge | The number of thread local caches (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.tiny.cache.size | Gauge | The size of the tiny cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.small.cache.size | Gauge | The size of the small cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.normal.cache.size | Gauge | The size of the normal cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.chunk.size | Gauge | The chunk size for an arena (when `PooledByteBufAllocator`)

The following example enables that integration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/metrics/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpServer;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .metrics(true) (1)
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}
    }

1 | Enables the built-in integration with Micrometer

When UDP server metrics are needed for an integration with a system other than `Micrometer` or you want to provide your own integration with `Micrometer`, you can provide your own metrics recorder, as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/server/metrics/custom/Application.java

    import reactor.netty.Connection;
    import reactor.netty.channel.ChannelMetricsRecorder;
    import reactor.netty.udp.UdpServer;

    import java.net.SocketAddress;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection server =
    				UdpServer.create()
    				         .metrics(true, CustomChannelMetricsRecorder::new) (1)
    				         .bindNow(Duration.ofSeconds(30));

    		server.onDispose()
    		      .block();
    	}

1 | Enables UDP server metrics and provides `ChannelMetricsRecorder` implementation.

Suggest Edit to "UDP Server"

## 8. UDP Client

Reactor Netty provides the easy-to-use and easy-to-configure `UdpClient`. It hides most of the Netty functionality that is required to create a `UDP` client and adds Reactive Streams backpressure.

### 8.1. Connecting and Disconnecting

To connect the UDP client to a given endpoint, you must create and configure a UdpClient instance. By default, the host is configured for `localhost` and the port is `12012`. The following example shows how to create and connect a UDP client:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/create/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()                            (1)
    				         .connectNow(Duration.ofSeconds(30)); (2)

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Creates a `UdpClient` instance that is ready for configuring.
2 | Connects the client in a blocking fashion and waits for it to finish initializing.

The returned `Connection` offers a simple connection API, including `disposeNow()`, which shuts the client down in a blocking fashion.

#### 8.1.1. Host and Port

To connect to a specific `host` and `port`, you can apply the following configuration to the `UDP` client:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/address/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com") (1)
    				         .port(80)            (2)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Configures the `host` to which this client should connect
2 | Configures the `port` to which this client should connect

### 8.2. Eager Initialization

By default, the initialization of the `UdpClient` resources happens on demand. This means that the `connect operation` absorbs the extra time needed to initialize and load:

- the event loop group

- the host name resolver

- the native transport libraries (when native transport is used)

When you need to preload these resources, you can configure the `UdpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/warmup/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		UdpClient udpClient = UdpClient.create()
    		                               .host("example.com")
    		                               .port(80)
    		                               .handle((udpInbound, udpOutbound) -> udpOutbound.sendString(Mono.just("hello")));

    		udpClient.warmup() (1)
    		         .block();

    		Connection connection = udpClient.connectNow(Duration.ofSeconds(30)); (2)

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Initialize and load the event loop group, the host name resolver, and the native transport libraries
2 | Host name resolution happens when connecting to the remote peer

### 8.3. Writing Data

To send data to a given peer, you must attach an I/O handler. The I/O handler has access to `UdpOutbound`, to be able to write data.

The following example shows how to send `hello`:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/send/Application.java

    import reactor.core.publisher.Mono;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .handle((udpInbound, udpOutbound) -> udpOutbound.sendString(Mono.just("hello"))) (1)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Sends `hello` string to the remote peer.

### 8.4. Consuming Data

To receive data from a given peer, you must attach an I/O handler. The I/O handler has access to `UdpInbound`, to be able to read data. The following example shows how to consume data:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/read/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .handle((udpInbound, udpOutbound) -> udpInbound.receive().then()) (1)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Receives data from a given peer

### 8.5. Lifecycle Callbacks

The following lifecycle callbacks are provided to let you extend the `UdpClient`:

Callback | Description
--- | ---
`doAfterResolve` | Invoked after the remote address has been resolved successfully.
`doOnChannelInit` | Invoked when initializing the channel.
`doOnConnect` | Invoked when the channel is about to connect.
`doOnConnected` | Invoked after the channel has been connected.
`doOnDisconnected` | Invoked after the channel has been disconnected.
`doOnResolve` | Invoked when the remote address is about to be resolved.
`doOnResolveError` | Invoked in case the remote address hasn’t been resolved successfully.

The following example uses the `doOnConnected` and `doOnChannelInit` callbacks:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/lifecycle/Application.java

    import io.netty.handler.codec.LineBasedFrameDecoder;
    import io.netty.handler.logging.LoggingHandler;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .doOnConnected(conn -> conn.addHandler(new LineBasedFrameDecoder(8192))) (1)
    				         .doOnChannelInit((observer, channel, remoteAddress) ->
    				             channel.pipeline()
    				                    .addFirst(new LoggingHandler("reactor.netty.examples")))      (2)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

1 | `Netty` pipeline is extended with `LineBasedFrameDecoder` when the channel has been connected.
2 | `Netty` pipeline is extended with `LoggingHandler` when initializing the channel.

### 8.6. Connection Configuration

This section describes three kinds of configuration that you can use at the UDP level:

- Channel Options

- Wire Logger

- Event Loop Group

#### 8.6.1. Channel Options

By default, the `UDP` client is configured with the following options:

./../../reactor-netty-core/src/main/java/reactor/netty/udp/UdpClientConnect.java

    UdpClientConnect() {
    	this.config = new UdpClientConfig(
    			ConnectionProvider.newConnection(),
    			Collections.singletonMap(ChannelOption.AUTO_READ, false),
    			() -> new InetSocketAddress(NetUtil.LOCALHOST, DEFAULT_PORT));
    }

If you need additional options or need to change the current options, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/channeloptions/Application.java

    import io.netty.channel.ChannelOption;
    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

You can find more about Netty channel options at the following links:

- Common ChannelOption

- Epoll ChannelOption

- KQueue ChannelOption

- Socket Options

#### 8.6.2. Wire Logger

Reactor Netty provides wire logging for when the traffic between the peers has to be inspected. By default, wire logging is disabled. To enable it, you must set the logger `reactor.netty.udp.UdpClient` level to `DEBUG` and apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/wiretap/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .wiretap(true) (1)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Enables the wire logging

By default, the wire logging uses AdvancedByteBufFormat#HEX_DUMP when printing the content. When you need to change this to AdvancedByteBufFormat#SIMPLE or AdvancedByteBufFormat#TEXTUAL, you can configure the `UdpClient` as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/wiretap/custom/Application.java

    import io.netty.handler.logging.LogLevel;
    import reactor.netty.Connection;
    import reactor.netty.transport.logging.AdvancedByteBufFormat;
    import reactor.netty.udp.UdpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .wiretap("logger-name", LogLevel.DEBUG, AdvancedByteBufFormat.TEXTUAL) (1)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Enables the wire logging, AdvancedByteBufFormat#TEXTUAL is used for printing the content.

#### 8.6.3. Event Loop Group

By default, the UDP client uses “Event Loop Group,” where the number of the worker threads equals the number of processors available to the runtime on initialization (but with a minimum value of 4). When you need a different configuration, you can use one of the `LoopResources``#create` methods.

The following listing shows the default configuration for the “Event Loop Group”:

./../../reactor-netty-core/src/main/java/reactor/netty/ReactorNetty.java

    /**
     * Default worker thread count, fallback to available processor
     * (but with a minimum value of 4)
     */
    public static final String IO_WORKER_COUNT = "reactor.netty.ioWorkerCount";
    /**
     * Default selector thread count, fallback to -1 (no selector thread)
     */
    public static final String IO_SELECT_COUNT = "reactor.netty.ioSelectCount";
    /**
     * Default worker thread count for UDP, fallback to available processor
     * (but with a minimum value of 4)
     */
    public static final String UDP_IO_THREAD_COUNT = "reactor.netty.udp.ioThreadCount";
    /**
     * Default quiet period that guarantees that the disposal of the underlying LoopResources
     * will not happen, fallback to 2 seconds.
     */
    public static final String SHUTDOWN_QUIET_PERIOD = "reactor.netty.ioShutdownQuietPeriod";
    /**
     * Default maximum amount of time to wait until the disposal of the underlying LoopResources
     * regardless if a task was submitted during the quiet period, fallback to 15 seconds.
     */
    public static final String SHUTDOWN_TIMEOUT = "reactor.netty.ioShutdownTimeout";

    /**
     * Default value whether the native transport (epoll, kqueue) will be preferred,
     * fallback it will be preferred when available
     */
    public static final String NATIVE = "reactor.netty.native";

If you need changes to the these settings, you can apply the following configuration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/eventloop/Application.java

    import reactor.netty.Connection;
    import reactor.netty.resources.LoopResources;
    import reactor.netty.udp.UdpClient;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		LoopResources loop = LoopResources.create("event-loop", 1, 4, true);

    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .runOn(loop)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

### 8.7. Metrics

The UDP client supports built-in integration with `Micrometer`. It exposes all metrics with a prefix of `reactor.netty.udp.client`.

The following table provides information for the UDP client metrics:

metric name | type | description
--- | --- | ---
reactor.netty.udp.client.data.received | DistributionSummary | Amount of the data received, in bytes
reactor.netty.udp.client.data.sent | DistributionSummary | Amount of the data sent, in bytes
reactor.netty.udp.client.errors | Counter | Number of errors that occurred
reactor.netty.udp.client.connect.time | Timer | Time spent for connecting to the remote address
reactor.netty.udp.client.address.resolver | Timer | Time spent for resolving the address

These additional metrics are also available:

`ByteBufAllocator` metrics

metric name | type | description
--- | --- | ---
reactor.netty.bytebuf.allocator.used.heap.memory | Gauge | The number of the bytes of the heap memory
reactor.netty.bytebuf.allocator.used.direct.memory | Gauge | The number of the bytes of the direct memory
reactor.netty.bytebuf.allocator.used.heap.arenas | Gauge | The number of heap arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.direct.arenas | Gauge | The number of direct arenas (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.threadlocal.caches | Gauge | The number of thread local caches (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.tiny.cache.size | Gauge | The size of the tiny cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.small.cache.size | Gauge | The size of the small cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.normal.cache.size | Gauge | The size of the normal cache (when `PooledByteBufAllocator`)
reactor.netty.bytebuf.allocator.used.chunk.size | Gauge | The chunk size for an arena (when `PooledByteBufAllocator`)

The following example enables that integration:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/metrics/Application.java

    import reactor.netty.Connection;
    import reactor.netty.udp.UdpClient;

    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .metrics(true) (1)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}
    }

1 | Enables the built-in integration with Micrometer

When UDP client metrics are needed for an integration with a system other than `Micrometer` or you want to provide your own integration with `Micrometer`, you can provide your own metrics recorder, as follows:

./../../reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/udp/client/metrics/custom/Application.java

    import reactor.netty.Connection;
    import reactor.netty.channel.ChannelMetricsRecorder;
    import reactor.netty.udp.UdpClient;

    import java.net.SocketAddress;
    import java.time.Duration;

    public class Application {

    	public static void main(String[] args) {
    		Connection connection =
    				UdpClient.create()
    				         .host("example.com")
    				         .port(80)
    				         .metrics(true, CustomChannelMetricsRecorder::new) (1)
    				         .connectNow(Duration.ofSeconds(30));

    		connection.onDispose()
    		          .block();
    	}

1 | Enables UDP client metrics and provides `ChannelMetricsRecorder` implementation.

Suggest Edit to "UDP Client"