背景
????基于一次fielddata_cache(容量還沒到閾值)被逐出后轧飞，想具體了解fielddata_cache的實現(xiàn)來判斷fielddata數(shù)據(jù)是否是常駐內(nèi)存亦或是只是個軟惑畴、弱引用臊旭，本文基于v1.0.0版本考廉。
實現(xiàn)
????我們直接從Elasticsearch.java這個啟動類開始往下看：
Elasticsearch.java {
public static void main(String[] args) {
Bootstrap.main(args);
}
}
????Elasticsearch通過Bootstrap類來啟動掏膏，具體再看Bootstrap的實現(xiàn)拜轨，忽略一些代碼盯拱，我們來Bootstrap的實例化和初始化：
Bootstrap.java {
public static void main(String[] args) {
bootstrap = new Bootstrap();
Tuple<Settings, Environment> tuple = null; //我們的一些配置
try {
tuple = initialSettings();
setupLogging(tuple);
} catch (Exception e) {
...
}
try {
bootstrap.setup(true, tuple);
...
} catch (Throwable e) {
...
}
}
}
????Bootstrap的setup()會創(chuàng)建我們的Elasticsearch的節(jié)點實例：
Bootstrap.java {
private Node node;
private void setup(boolean addShutdownHook, Tuple<Settings, Environment> tuple) throws Exception {
NodeBuilder nodeBuilder = NodeBuilder.nodeBuilder().settings(tuple.v1()).loadConfigSettings(false);
node = nodeBuilder.build();
...
}
}
????NodeBuilder會創(chuàng)建一個InternalNode實例，我們InternalNode的初始化郁竟，重點看到我們會添加一個IndicesModule：
InternalNode.java {
public InternalNode(Settings pSettings, boolean loadConfigSettings) throws ElasticsearchException {
logger.info("initializing ...");
...
ModulesBuilder modules = new ModulesBuilder();
modules.add(new IndicesModule(settings));
...
logger.info("initialized");
}
}
????再接著看IndicesModule的實現(xiàn)玛迄，我們通過綁定IndicesFieldDataCache類來實現(xiàn)索引級別的fielddata_cache：
IndicesModule.java {
protected void configure() {
...
bind(IndicesFieldDataCache.class).asEagerSingleton();
...
}
}
????重點來看IndicesFieldDataCache的實現(xiàn)，從下面代碼可以看到Elasticsearch通過guava的CacheBuilder來實現(xiàn)索引級別的fielddata_cache棚亩，具體的CacheBuilder介紹可以自行查閱一下：
IndicesFieldDataCache.java {
Cache<Key, AtomicFieldData> cache;
private volatile String size;
private volatile long sizeInBytes;
private volatile TimeValue expire;
@Inject
public IndicesFieldDataCache(Settings settings) {
super(settings);
this.size = componentSettings.get("size", "-1"); //indices.fielddata.cache.size的大小
this.sizeInBytes = componentSettings.getAsMemory("size", "-1").bytes(); //indices.fielddata.cache.size的大小
this.expire = componentSettings.getAsTime("expire", null); //indices.fielddata.cache.expire的大小
buildCache();
}
private void buildCache() {
CacheBuilder<Key, AtomicFieldData> cacheBuilder = CacheBuilder.newBuilder()
.removalListener(this);
if (sizeInBytes > 0) { //設(shè)置LRU的閾值
cacheBuilder.maximumWeight(sizeInBytes).weigher(new FieldDataWeigher());
}
cacheBuilder.concurrencyLevel(16);
if (expire != null && expire.millis() > 0) { //設(shè)置Cache的過期時間
cacheBuilder.expireAfterAccess(expire.millis(), TimeUnit.MILLISECONDS);
}
logger.debug("using size [{}] [{}], expire [{}]", size, new ByteSizeValue(sizeInBytes), expire);
cache = cacheBuilder.build();
}
...
}
????最后再看CacheBuilder是怎么被使用的(默認(rèn)情況下CacheBuilder的key和value都是強引用的)蓖议，IndicesFieldDataCache在給上層提供實現(xiàn)時是返回了一個IndexFieldCache虏杰，可以看到在需要load索引的fielddata_cache時通過CacheBuilder在get時候的原則"獲取緩存-如果沒有-則計算"實現(xiàn)：
IndexFieldCache.java {
@Nullable
private final IndexService indexService;
final Index index;
final FieldMapper.Names fieldNames;
final FieldDataType fieldDataType;
IndexFieldCache(@Nullable IndexService indexService, Index index, FieldMapper.Names fieldNames, FieldDataType fieldDataType) {
this.indexService = indexService;
this.index = index;
this.fieldNames = fieldNames;
this.fieldDataType = fieldDataType;
}
@Override
public <FD extends AtomicFieldData, IFD extends IndexFieldData<FD>> FD load(final AtomicReaderContext context, final IFD indexFieldData) throws Exception {
final Key key = new Key(this, context.reader().getCoreCacheKey());
return (FD) cache.get(key, new Callable<AtomicFieldData>() {
@Override
public AtomicFieldData call() throws Exception {
SegmentReaderUtils.registerCoreListener(context.reader(), IndexFieldCache.this);
AtomicFieldData fieldData = indexFieldData.loadDirect(context);
...
return fieldData;
}
});
}
}
總結(jié)
????簡單介紹了Elasticsearch-1.0.0版本fielddata_cache的實現(xiàn)，經(jīng)過分析知道fielddata_cache默認(rèn)是強引用對象勒虾，所以只存在LRU并不會被GC掉纺阔，至于為啥會被逐出還需要再看看指標(biāo)怎么統(tǒng)計的。
(個人分析修然，有錯誤請指正)
