Elasticsearch中fielddata_cache的实现

简书地址

背景

    基于一次fielddata_cache(容量还没到阈值)被逐出后，想具体了解fielddata_cache的实现来判断fielddata数据是否是常驻内存亦或是只是个软、弱引用，本文基于v1.0.0版本。

---

实现

    我们直接从Elasticsearch.java这个启动类开始往下看：

Elasticsearch.java {    public static void main(String[] args) {        Bootstrap.main(args);    }}

    Elasticsearch通过Bootstrap类来启动，具体再看Bootstrap的实现，忽略一些代码，我们来Bootstrap的实例化和初始化：

Bootstrap.java {    public static void main(String[] args) {        bootstrap = new Bootstrap();        Tuple<Settings, Environment> tuple = null; //我们的一些配置        try {            tuple = initialSettings();            setupLogging(tuple);        } catch (Exception e) {            ...        }        try {            bootstrap.setup(true, tuple);            ...        } catch (Throwable e) {                ...        }    }}

    Bootstrap的setup()会创建我们的Elasticsearch的节点实例：

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会创建一个InternalNode实例，我们InternalNode的初始化，重点看到我们会添加一个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");    }}

    再接着看IndicesModule的实现，我们通过绑定IndicesFieldDataCache类来实现索引级别的fielddata_cache：

IndicesModule.java {    protected void configure() {        ...        bind(IndicesFieldDataCache.class).asEagerSingleton();        ...    }}

    重点来看IndicesFieldDataCache的实现，从下面代码可以看到Elasticsearch通过guava的CacheBuilder来实现索引级别的fielddata_cache，具体的CacheBuilder介绍可以自行查阅一下：

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) { //设置LRU的阈值            cacheBuilder.maximumWeight(sizeInBytes).weigher(new FieldDataWeigher());        }        cacheBuilder.concurrencyLevel(16);        if (expire != null && expire.millis() > 0) { //设置Cache的过期时间            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(默认情况下CacheBuilder的key和value都是强引用的)，IndicesFieldDataCache在给上层提供实现时是返回了一个IndexFieldCache，可以看到在需要load索引的fielddata_cache时通过CacheBuilder在get时候的原则”获取缓存-如果没有-则计算”实现:

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;            }        });    }}

---

总结

    简单介绍了Elasticsearch-1.0.0版本fielddata_cache的实现，经过分析知道fielddata_cache默认是强引用对象，所以只存在LRU并不会被GC掉。

---

(个人分析，有错误请指正)