【ES源码分析】强制合并分段（_forcemerge API）源码分析

_forcemerge API 源码分析

源码基于6.7.2

合并方式

RestForceMergeAction中提供了对api的处理入口。在做了简单的参数处理之后，将请求转发给TransportForceMergeAction进行进一步的处理，里面有一个分片级的方法shardOperation。通过indicesService找到对应的shard，再执行InternalEngine.forceMerge。

    public void forceMerge(final boolean flush, int maxNumSegments, boolean onlyExpungeDeletes,
                           final boolean upgrade, final boolean upgradeOnlyAncientSegments) throws EngineException, IOException {
        assert indexWriter.getConfig().getMergePolicy() instanceof ElasticsearchMergePolicy : "MergePolicy is " +
            indexWriter.getConfig().getMergePolicy().getClass().getName();
        // 获取合并策略
        ElasticsearchMergePolicy mp = (ElasticsearchMergePolicy) indexWriter.getConfig().getMergePolicy();
        optimizeLock.lock();
        try {
            ensureOpen();
            if (upgrade) {
                logger.info("starting segment upgrade upgradeOnlyAncientSegments={}", upgradeOnlyAncientSegments);
                mp.setUpgradeInProgress(true, upgradeOnlyAncientSegments);
            }
            store.incRef(); // increment the ref just to ensure nobody closes the store while we optimize
            try {
                // 将合并分为，1.只合并删除文档 2.没有配置最大segment数的合并 3.配置了最大segment数的合并
                if (onlyExpungeDeletes) {
                    assert upgrade == false;
                    indexWriter.forceMergeDeletes(true /* blocks and waits for merges*/);
                } else if (maxNumSegments