org.apache.jackrabbit.oak.segment

Class SegmentCache

java.lang.Object

org.apache.jackrabbit.oak.segment.SegmentCache

public class SegmentCache
extends Object

A cache for Segment instances by their SegmentId.

Conceptually this cache serves as a 2nd level cache for segments. The 1st level cache is implemented by memoising the segment in its id (see SegmentId.segment. Every time an segment is evicted from this cache the memoised segment is discarded (see SegmentId.unloaded()).

As a consequence this cache is actually only queried for segments it does not contain, which are then loaded through the loader passed to getSegment(SegmentId, Callable). This behaviour is eventually reflected in the cache statistics (see getCacheStats()), which always reports a CacheStats.getHitRate() () miss rate} of 1.

Field Summary

Fields

Modifier and Type	Field and Description
static int	DEFAULT_SEGMENT_CACHE_MB

Constructor Summary

Constructors

Constructor and Description
SegmentCache(long cacheSizeMB) Create a new segment cache of the given size.

Method Summary

Modifier and Type	Method and Description
void	clear() Clear all segment from the cache
org.apache.jackrabbit.oak.cache.CacheStats	getCacheStats() See the class comment regarding some peculiarities of this cache's statistics
Segment	getSegment(SegmentId id, Callable<Segment> loader) Retrieve an segment from the cache or load it and cache it if not yet in the cache.
void	putSegment(Segment segment) Put a segment into the cache

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

DEFAULT_SEGMENT_CACHE_MB

public static final int DEFAULT_SEGMENT_CACHE_MB

See Also:

Constant Field Values

Constructor Detail

SegmentCache

public SegmentCache(long cacheSizeMB)

Create a new segment cache of the given size.

Parameters:

cacheSizeMB - size of the cache in megabytes.

Method Detail

getSegment

@Nonnull
public Segment getSegment(@Nonnull
                                   SegmentId id,
                                   @Nonnull
                                   Callable<Segment> loader)
                            throws ExecutionException

Retrieve an segment from the cache or load it and cache it if not yet in the cache.

Parameters:

id - the id of the segment

loader - the loader to load the segment if not yet in the cache

Returns:

the segment identified by id

Throws:

ExecutionException - when loader failed to load an segment

putSegment

public void putSegment(@Nonnull
                       Segment segment)

Put a segment into the cache

Parameters:

segment - the segment to cache

clear

public void clear()

Clear all segment from the cache

getCacheStats

@Nonnull
public org.apache.jackrabbit.oak.cache.CacheStats getCacheStats()

See the class comment regarding some peculiarities of this cache's statistics

Returns:

statistics for this cache.