内部实现分析

首先通过调用RestController的registerHandler函数注册split接口的handler为RestSplitIndexAction

RestSplitIndexAction

controller.registerHandler(RestRequest.Method.PUT, "/{index}/_split/{target}", this);
controller.registerHandler(RestRequest.Method.POST, "/{index}/_split/{target}", this);

当elasticsearch收到请求时会进入RestController的dispatcher函数

RestController

boolean dispatchRequest(final RestRequest request, final RestChannel channel, final NodeClient client,
                            final Optional<RestHandler> mHandler) throws Exception {
        final int contentLength = request.hasContent() ? request.content().length() : 0;

        RestChannel responseChannel = channel;
        // Indicator of whether a response was sent or not
        boolean requestHandled;

        if (contentLength > 0 && mHandler.map(h -> hasContentType(request, h) == false).orElse(false)) {
           ...
        } else if (contentLength > 0 && mHandler.map(h -> h.supportsContentStream()).orElse(false) &&
            request.getXContentType() != XContentType.JSON && request.getXContentType() != XContentType.SMILE) {
           ...
        } else if (mHandler.isPresent()) {
            //在这个分支里处理
            try {
                if (canTripCircuitBreaker(mHandler)) {
                    inFlightRequestsBreaker(circuitBreakerService).addEstimateBytesAndMaybeBreak(contentLength, "<http_request>");
                } else {
                    inFlightRequestsBreaker(circuitBreakerService).addWithoutBreaking(contentLength);
                }
                // iff we could reserve bytes for the request we need to send the response also over this channel
                responseChannel = new ResourceHandlingHttpChannel(channel, circuitBreakerService, contentLength);

                final RestHandler wrappedHandler = mHandler.map(h -> handlerWrapper.apply(h)).get();
                //wrappedHandler其实就是在restController中注册的handler，也就是RestSplitIndexAction
                wrappedHandler.handleRequest(request, responseChannel, client);
                requestHandled = true;
            } catch (Exception e) {
                responseChannel.sendResponse(new BytesRestResponse(responseChannel, e));
                // We "handled" the request by returning a response, even though it was an error
                requestHandled = true;
            }
        } else {
            ...
        }
        // Return true if the request was handled, false otherwise.
        return requestHandled;
    }

上面代码省略了一些其它的东西，主要是在mHandler.isPresent分支，因为开始的时候在restController中注册了split的handler。所以mHandler一定不为空。handleRequest其实是BaseRestHandler中的一个方法。跟进去看一下。

BaseRestHandler

public final void handleRequest(RestRequest request, RestChannel channel, NodeClient client) throws Exception {
        // prepare the request for execution; has the side effect of touching the request parameters
        final RestChannelConsumer action = prepareRequest(request, client);
        ...
        usageCount.increment();
        // execute the action
        //正真的执行在这里，调用prepareRequset返回的channleConsumer，并将channle传递给它。
        action.accept(channel);
    }

在该方法中首先调用了prepareRequest方法，该方法会返回一个RestChannleConsumer，在elasticsearch中有大量的这种consumer接口。这种接口其实就是java 1.8中的函数式接口。接口中有一个accept函数，该接受一个参数。该consumer是prepareRequest函数返回的。而在RestSplitIndexAction中覆盖了该方法。

RestSplitIndexAction

public RestChannelConsumer prepareRequest(final RestRequest request, final NodeClient client) throws IOException {
        if (request.param("target") == null) {
            throw new IllegalArgumentException("no target index");
        }
        if (request.param("index") == null) {
            throw new IllegalArgumentException("no source index");
        }
        ResizeRequest shrinkIndexRequest = new ResizeRequest(request.param("target"), request.param("index"));
        shrinkIndexRequest.setResizeType(ResizeType.SPLIT);
        request.applyContentParser(parser -> ResizeRequest.PARSER.parse(parser, shrinkIndexRequest, null));
        shrinkIndexRequest.timeout(request.paramAsTime("timeout", shrinkIndexRequest.timeout()));
        shrinkIndexRequest.masterNodeTimeout(request.paramAsTime("master_timeout", shrinkIndexRequest.masterNodeTimeout()));
        shrinkIndexRequest.setWaitForActiveShards(ActiveShardCount.parseString(request.param("wait_for_active_shards")));
        return channel -> client.admin().indices().resizeIndex(shrinkIndexRequest, new AcknowledgedRestListener<ResizeResponse>(channel) {
            @Override
            public void addCustomFields(XContentBuilder builder, ResizeResponse response) throws IOException {
                response.addCustomFields(builder);
            }
        });
    }

在该方法中，构造了一个ResizeRequest对象，并将源索引和目标索引传递进去。同时设置resize的方式为spilt。elasticsearch支持两种resize操作。一种是split，将shard 分裂。另一种是shrink。也就是将shard合并。设置了一些控制超时的参数后返回了一个匿名函数。在BaseRestHandler对象中的handleRequest函数最终会调用该函数。然后进入了IndicesAdmin中的resizeIndex函数。

IndicesAdmin

public void resizeIndex(ResizeRequest request, ActionListener<ResizeResponse> listener) {
            execute(ResizeAction.INSTANCE, request, listener);
        }

在该函数中调用execute函数，传入了一个ResezeAction对象。在elasticsearch中，对外暴露的接口都是通过内部的action对象来处理的。因为elasticsearch本身提供了restful的接口和rpc接口（传输层客户端）。所以有两套action。通过restful调的接口首先会被以rest开头的action处理（rest接口和对应的处理action关系由RestController维护），然后再在中间做一层转换，找到相应的以transport开头的action来处理。比如在split接口中，首先会被RestSplitIndexAction处理。处理完后进入了IndicesAdmin中的resizeIndex函数。而该函数中就直接去执行ResizeAction了。

IndicesAdmin

public <Request extends ActionRequest, Response extends ActionResponse, RequestBuilder extends ActionRequestBuilder<Request, Response, RequestBuilder>> void execute(
                Action<Request, Response, RequestBuilder> action, Request request, ActionListener<Response> listener) {
            client.execute(action, request, listener);
        }

然后会调用client的execute方法来执行

AbstractClient

public final <Request extends ActionRequest, Response extends ActionResponse, RequestBuilder extends ActionRequestBuilder<Request, Response, RequestBuilder>> void execute(
            Action<Request, Response, RequestBuilder> action, Request request, ActionListener<Response> listener) {
        listener = threadedWrapper.wrap(listener);
        doExecute(action, request, listener);
    }

随后再调用doExecute方法，因为在IndicesAdmin中的client是NodeClient，所以直接进入NodeClient的doExecute方法。

NodeClient

public <    Request extends ActionRequest,
                Response extends ActionResponse,
                RequestBuilder extends ActionRequestBuilder<Request, Response, RequestBuilder>
            > void doExecute(Action<Request, Response, RequestBuilder> action, Request request, ActionListener<Response> listener) {
        // Discard the task because the Client interface doesn't use it.
        executeLocally(action, request, listener);
    }

然后再调用executeLocally函数

public <    Request extends ActionRequest,
                Response extends ActionResponse
            > Task executeLocally(GenericAction<Request, Response> action, Request request, ActionListener<Response> listener) {
        return transportAction(action).execute(request, listener);
    }

这里首先调用了transportAction函数，并把ResizeAction传了进去。这一步其实就是解析传输层的handler。

private <    Request extends ActionRequest,
                Response extends ActionResponse
            > TransportAction<Request, Response> transportAction(GenericAction<Request, Response> action) {
        if (actions == null) {
            throw new IllegalStateException("NodeClient has not been initialized");
        }
        //根据传入的action去actions里找到对应的传输层action来处理
        TransportAction<Request, Response> transportAction = actions.get(action);
        if (transportAction == null) {
            throw new IllegalStateException("failed to find action [" + action + "] to execute");
        }
        return transportAction;
    }

其中actions就是一个map，在ActionModule中的setupActions中会向actions中注册所有的传输层action。

ActionModule

static Map<String, ActionHandler<?, ?>> setupActions(List<ActionPlugin> actionPlugins) {
        // Subclass NamedRegistry for easy registration
        class ActionRegistry extends NamedRegistry<ActionHandler<?, ?>> {
            ActionRegistry() {
                super("action");
            }

            public void register(ActionHandler<?, ?> handler) {
                register(handler.getAction().name(), handler);
            }

            public <Request extends ActionRequest, Response extends ActionResponse> void register(
                    GenericAction<Request, Response> action, Class<? extends TransportAction<Request, Response>> transportAction,
                    Class<?>... supportTransportActions) {
                register(new ActionHandler<>(action, transportAction, supportTransportActions));
            }
        }
        ActionRegistry actions = new ActionRegistry();

        ...
        //此处省略了很多action的注册
        actions.register(ResizeAction.INSTANCE, TransportResizeAction.class);
        ...

        actionPlugins.stream().flatMap(p -> p.getActions().stream()).forEach(actions::register);

        return unmodifiableMap(actions.getRegistry());
    }

可以看到，ResizeAction对应的处理对象为TransportResizeAction。回到NodeClient中的executeLocally函数，在找到对应的action处理后，调用其execute方法。

TransportAction

public final Task execute(Request request, ActionListener<Response> listener) {
        /*
         * While this version of execute could delegate to the TaskListener
         * version of execute that'd add yet another layer of wrapping on the
         * listener and prevent us from using the listener bare if there isn't a
         * task. That just seems like too many objects. Thus the two versions of
         * this method.k
         */
        Task task = taskManager.register("transport", actionName, request);
        if (task == null) {
            execute(null, request, listener);
        } else {
            execute(task, request, new ActionListener<Response>() {
                @Override
                public void onResponse(Response response) {
                    taskManager.unregister(task);
                    listener.onResponse(response);
                }

                @Override
                public void onFailure(Exception e) {
                    taskManager.unregister(task);
                    listener.onFailure(e);
                }
            });
        }
        return task;
    }

这里首先向taskManager注册一个task，task就是一个任务的包装，包括该任务的类型、创建的时间、执行的action、task id及父task信息。生成task后调用了另一个重载的execute函数，同时对listener重新包装了一下，这里之所以重新包装主要是为了在listener中调用taskManager的unregister函数，把该task去掉。

public final void execute(Task task, Request request, ActionListener<Response> listener) {
        ActionRequestValidationException validationException = request.validate();
        if (validationException != null) {
            listener.onFailure(validationException);
            return;
        }

        if (task != null && request.getShouldStoreResult()) {
            listener = new TaskResultStoringActionListener<>(taskManager, task, listener);
        }

        RequestFilterChain<Request, Response> requestFilterChain = new RequestFilterChain<>(this, logger);
        requestFilterChain.proceed(task, actionName, request, listener);
    }

execute函数中首先调用request.validate验证该请求是否有效，如果通过后会构造一个RequestFilterChain对象。

RequestFilterChain

public void proceed(Task task, String actionName, Request request, ActionListener<Response> listener) {
            int i = index.getAndIncrement();
            try {
                if (i < this.action.filters.length) {
                    this.action.filters[i].apply(task, actionName, request, listener, this);
                } else if (i == this.action.filters.length) {
                    this.action.doExecute(task, request, listener);
                } else {
                    listener.onFailure(new IllegalStateException("proceed was called too many times"));
                }
            } catch(Exception e) {
                logger.trace("Error during transport action execution.", e);
                listener.onFailure(e);
            }
        }

action中有一个filter数组，维护着所有的filter。如果有filter的话会逐个的调用filter来处理。直到最后调用action的doExecute方法。TransportResizeAction没有设置filter，所以会直接调用action.doExecute方法。因为TransportResizeAction继承了TransportMasterNodeAction，最终进入了TransportMasterNodeAction的doExecute方法。

TransportMasterNodeAction

protected void doExecute(Task task, final Request request, ActionListener<Response> listener) {
        new AsyncSingleAction(task, request, listener).start();
    }

这里创建了一个AsyncSingleAction对象，并调用了期start方法。

AsyncSingleAction

public void start() {
            ClusterState state = clusterService.state();
            this.observer = new ClusterStateObserver(state, clusterService, request.masterNodeTimeout(), logger, threadPool.getThreadContext());
            doStart(state);
        }

首先获取了集群当前的状态，然后调用doStart方法

protected void doStart(ClusterState clusterState) {
            final Predicate<ClusterState> masterChangePredicate = MasterNodeChangePredicate.build(clusterState);
            final DiscoveryNodes nodes = clusterState.nodes();
            if (nodes.isLocalNodeElectedMaster() || localExecute(request)) {
                // check for block, if blocked, retry, else, execute locally
                final ClusterBlockException blockException = checkBlock(request, clusterState);
                if (blockException != null) {
                    if (!blockException.retryable()) {
                        listener.onFailure(blockException);
                    } else {
                        logger.trace("can't execute due to a cluster block, retrying", blockException);
                        retry(blockException, newState -> {
                            ClusterBlockException newException = checkBlock(request, newState);
                            return (newException == null || !newException.retryable());
                        });
                    }
                } else {
                    ActionListener<Response> delegate = new ActionListener<Response>() {
                        @Override
                        public void onResponse(Response response) {
                            listener.onResponse(response);
                        }

                        @Override
                        public void onFailure(Exception t) {
                            if (t instanceof Discovery.FailedToCommitClusterStateException
                                    || (t instanceof NotMasterException)) {
                                logger.debug((org.apache.logging.log4j.util.Supplier<?>) () -> new ParameterizedMessage("master could not publish cluster state or stepped down before publishing action [{}], scheduling a retry", actionName), t);
                                retry(t, masterChangePredicate);
                            } else {
                                listener.onFailure(t);
                            }
                        }
                    };
                    threadPool.executor(executor).execute(new ActionRunnable(delegate) {
                        @Override
                        protected void doRun() throws Exception {
                            masterOperation(task, request, clusterState, delegate);
                        }
                    });
                }
            } else {
                if (nodes.getMasterNode() == null) {
                    logger.debug("no known master node, scheduling a retry");
                    retry(null, masterChangePredicate);
                } else {
                    DiscoveryNode masterNode = nodes.getMasterNode();
                    final String actionName = getMasterActionName(masterNode);
                    transportService.sendRequest(masterNode, actionName, request, new ActionListenerResponseHandler<Response>(listener,
                        TransportMasterNodeAction.this::newResponse) {
                        @Override
                        public void handleException(final TransportException exp) {
                            Throwable cause = exp.unwrapCause();
                            if (cause instanceof ConnectTransportException) {
                                // we want to retry here a bit to see if a new master is elected
                                logger.debug("connection exception while trying to forward request with action name [{}] to master node [{}], scheduling a retry. Error: [{}]",
                                        actionName, nodes.getMasterNode(), exp.getDetailedMessage());
                                retry(cause, masterChangePredicate);
                            } else {
                                listener.onFailure(exp);
                            }
                        }
                    });
                }
            }
        }

这里先判断该节点是不是master节点。如果不是master节点需要获取master节点并且把请求转发到master上去执行。否则就在本地执行。如果是本地执行，又将listener包装了一次，这次包装主要是为了在失败的时候能重试。准备工作做完后就获取生成一个ActionRunnable对象，并执行起run方法。注意，这里还是同步执行的。

EsExecutor

public void execute(Runnable command) {
            command.run();
        }

AbstractRunnable

public final void run() {
        try {
            doRun();
        } catch (Exception t) {
            onFailure(t);
        } finally {
            onAfter();
        }
    }

ActionRunnable

protected void doRun() throws Exception {
          masterOperation(task, request, clusterState, delegate);
   }

TransportMasterNodeAction

protected void masterOperation(Task task, Request request, ClusterState state, ActionListener<Response> listener) throws Exception {
        masterOperation(request, state, listener);
    }

在这里task信息就直接背忽略了，最终调用了TransportResizeAction的masterOperation方法

TransportResizeAction

protected void masterOperation(final ResizeRequest resizeRequest, final ClusterState state,
                                   final ActionListener<ResizeResponse> listener) {

        // there is no need to fetch docs stats for split but we keep it simple and do it anyway for simplicity of the code
        final String sourceIndex = indexNameExpressionResolver.resolveDateMathExpression(resizeRequest.getSourceIndex());
        final String targetIndex = indexNameExpressionResolver.resolveDateMathExpression(resizeRequest.getTargetIndexRequest().index());
        client.admin().indices().prepareStats(sourceIndex).clear().setDocs(true).execute(new ActionListener<IndicesStatsResponse>() {
            @Override
            public void onResponse(IndicesStatsResponse indicesStatsResponse) {
                CreateIndexClusterStateUpdateRequest updateRequest = prepareCreateIndexRequest(resizeRequest, state,
                    (i) -> {
                        IndexShardStats shard = indicesStatsResponse.getIndex(sourceIndex).getIndexShards().get(i);
                        return shard == null ? null : shard.getPrimary().getDocs();
                    }, sourceIndex, targetIndex);
                createIndexService.createIndex(
                    updateRequest,
                    ActionListener.wrap(response ->
                            listener.onResponse(new ResizeResponse(response.isAcknowledged(), response.isShardsAcked(),
                                updateRequest.index())), listener::onFailure
                    )
                );
            }

            @Override
            public void onFailure(Exception e) {
                listener.onFailure(e);
            }
        });

    }

这里似乎又执行了一个task，其实该task是为了获取索引的状态信息的。但貌似这个索引状态只有在调用shrink api的时候才会有用，这里暂时不分析。当索引状态获取完毕后，会调用listener的onResponse函数。注意，这里进入到onResponse函数里其实已经是在另外的线程里了。在onResponse函数中紧接着调用了prepareCreateIndexRequest函数。

static CreateIndexClusterStateUpdateRequest prepareCreateIndexRequest(final ResizeRequest resizeRequest, final ClusterState state
       , final IntFunction<DocsStats> perShardDocStats, String sourceIndexName, String targetIndexName) {
     //此处省略了一些代码
       ...
       //对目标索引中的每个shard
       for (int i = 0; i < numShards; i++) {
           if (resizeRequest.getResizeType() == ResizeType.SHRINK) {
               Set<ShardId> shardIds = IndexMetaData.selectShrinkShards(i, metaData, numShards);
               long count = 0;
               for (ShardId id : shardIds) {
                   DocsStats docsStats = perShardDocStats.apply(id.id());
                   if (docsStats != null) {
                       count += docsStats.getCount();
                   }
                   if (count > IndexWriter.MAX_DOCS) {
                       throw new IllegalStateException("Can't merge index with more than [" + IndexWriter.MAX_DOCS
                           + "] docs - too many documents in shards " + shardIds);
                   }
               }
           } else {
               //在这里对目标索引中的每个shard都选择一个源shard。判断源shard是否为空，如果为空则抛异常
               Objects.requireNonNull(IndexMetaData.selectSplitShard(i, metaData, numShards));
               // we just execute this to ensure we get the right exceptions if the number of shards is wrong or less then etc.
           }
       }

       if (IndexMetaData.INDEX_ROUTING_PARTITION_SIZE_SETTING.exists(targetIndexSettings)) {
           throw new IllegalArgumentException("cannot provide a routing partition size value when resizing an index");
       }
       if (IndexMetaData.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.exists(targetIndexSettings)) {
           throw new IllegalArgumentException("cannot provide index.number_of_routing_shards on resize");
       }
       String cause = resizeRequest.getResizeType().name().toLowerCase(Locale.ROOT) + "_index";
       targetIndex.cause(cause);
       Settings.Builder settingsBuilder = Settings.builder().put(targetIndexSettings);
       settingsBuilder.put("index.number_of_shards", numShards);
       targetIndex.settings(settingsBuilder);

       return new CreateIndexClusterStateUpdateRequest(targetIndex,
           cause, targetIndex.index(), targetIndexName, true)
           // mappings are updated on the node when creating in the shards, this prevents race-conditions since all mapping must be
           // applied once we took the snapshot and if somebody messes things up and switches the index read/write and adds docs we miss
           // the mappings for everything is corrupted and hard to debug
           .ackTimeout(targetIndex.timeout())
           .masterNodeTimeout(targetIndex.masterNodeTimeout())
           .settings(targetIndex.settings())
           .aliases(targetIndex.aliases())
           .customs(targetIndex.customs())
           .waitForActiveShards(targetIndex.waitForActiveShards())
           .recoverFrom(metaData.getIndex())
           .resizeType(resizeRequest.getResizeType());
   }

这个函数比较长，其中最重要的一步可以看

Objects.requireNonNull(IndexMetaData.selectSplitShard(i, metaData, numShards));

这一行代码内部其实体现了elasticsearch在split的时候是怎么分裂的。即目标索引的shard是从源索引的哪个shard split得到的。

IndexMetaData

public static ShardId selectSplitShard(int shardId, IndexMetaData sourceIndexMetadata, int numTargetShards) {
        if (shardId >= numTargetShards) {
            throw new IllegalArgumentException("the number of target shards (" + numTargetShards + ") must be greater than the shard id: "
                + shardId);
        }
        int numSourceShards = sourceIndexMetadata.getNumberOfShards();
        if (numSourceShards > numTargetShards) {
            throw new IllegalArgumentException("the number of source shards [" + numSourceShards
                 + "] must be less that the number of target shards [" + numTargetShards + "]");
        }
        int routingFactor = getRoutingFactor(numSourceShards, numTargetShards);
        // now we verify that the numRoutingShards is valid in the source index
        int routingNumShards = sourceIndexMetadata.getRoutingNumShards();
        if (routingNumShards % numTargetShards != 0) {
            throw new IllegalStateException("the number of routing shards ["
                + routingNumShards + "] must be a multiple of the target shards [" + numTargetShards + "]");
        }
        // this is just an additional assertion that ensures we are a factor of the routing num shards.
        assert getRoutingFactor(numTargetShards, sourceIndexMetadata.getRoutingNumShards()) >= 0;
        return new ShardId(sourceIndexMetadata.getIndex(), shardId/routingFactor);
    }

这个函数接受三个参数，分别为目标索引中的某个shard， 源索引的metadata，目标索引中总共有多少个shard。先计算出routingFactor，这里的routingFactor其实是指split 扩大了多少倍。也就是用numTargetShards / numSourceShards。到后面还有有个地方有计算routingFactor，但其实和这里的概念不一样。从最后的返回值可以看出，最终的计算表达式为：

  sourceShardId = targetShardId / (numTargetShards / numSourceShards)

比如源索引有两个shard，想要分裂为四个shard。那么目标索引的shard id 和源索引的shard id关系为：

源shard	目标shard
0	0
0	1
1	2
1	3

然后回到prepareCreateIndexRequest函数，验证通过后，创建了一个CreateIndexClusterStateUpdateRequest对象。从名字也可以看出这是一个集群状态变更对象，而且是一次创建索引的集群变更。创建后设置了一些属性，最重要的我觉得是recoverFrom属性，该属性用于决定目标索引数据怎么获取。然后继续回退到masterOperation函数。这里将prepareCreateIndexRequest对象复制给updateRequest后，传递到了createIndexService的createIndex函数，同时对listener又包装了一次，这次包装主要是为了替换response对象，在这里将其替换成了ResizeResponse对象。createIndexService其实是一个MetaDataCreateIndexService对象，负责创建索引的请求。

MetaDataCreateIndexService

public void createIndex(final CreateIndexClusterStateUpdateRequest request,
                            final ActionListener<CreateIndexClusterStateUpdateResponse> listener) {
        onlyCreateIndex(request, ActionListener.wrap(response -> {
            if (response.isAcknowledged()) {
                activeShardsObserver.waitForActiveShards(new String[]{request.index()}, request.waitForActiveShards(), request.ackTimeout(),
                    shardsAcked -> {
                        if (shardsAcked == false) {
                            logger.debug("[{}] index created, but the operation timed out while waiting for " +
                                             "enough shards to be started.", request.index());
                        }
                        listener.onResponse(new CreateIndexClusterStateUpdateResponse(response.isAcknowledged(), shardsAcked));
                    }, listener::onFailure);
            } else {
                listener.onResponse(new CreateIndexClusterStateUpdateResponse(false, false));
            }
        }, listener::onFailure));
    }

这里只是对listener又包装了一次，用于判断集群状态是否被正确变更，如果是，则等待相应的shard个数被激活。可以看到elasticsearch里边采用了大量的异步方式，大量的listener包装、使用导致很容易跟丢代码，并且相应的注释也比较少。所以elasticsearch的代码还是比较难阅读的。废话不多说，再次对listener包装后进入了onlyCreateIndex函数，从名字上也可以看出这个函数仅仅只创建索引。所以创建索引和等待相应的shard被激活这是异步的。有可能索引创建成功，但shard并没有被创建。

private void onlyCreateIndex(final CreateIndexClusterStateUpdateRequest request,
                                 final ActionListener<ClusterStateUpdateResponse> listener) {
        Settings.Builder updatedSettingsBuilder = Settings.builder();
        Settings build = updatedSettingsBuilder.put(request.settings()).normalizePrefix(IndexMetaData.INDEX_SETTING_PREFIX).build();
        indexScopedSettings.validate(build, true); // we do validate here - index setting must be consistent
        request.settings(build);
        clusterService.submitStateUpdateTask("create-index [" + request.index() + "], cause [" + request.cause() + "]",
            new IndexCreationTask(logger, allocationService, request, listener, indicesService, aliasValidator, xContentRegistry, settings,
                this::validate));
    }

到这一步就向集群服务提交了一个状态更新任务，并指命令操作原因，及封装了一个IndexCreationTask任务。

ClusterService

public <T extends ClusterStateTaskConfig & ClusterStateTaskExecutor<T> & ClusterStateTaskListener>
        void submitStateUpdateTask(String source, T updateTask) {
        submitStateUpdateTask(source, updateTask, updateTask, updateTask, updateTask);
    }

public <T> void submitStateUpdateTask(String source, T task,
                                          ClusterStateTaskConfig config,
                                          ClusterStateTaskExecutor<T> executor,
                                          ClusterStateTaskListener listener) {
        submitStateUpdateTasks(source, Collections.singletonMap(task, listener), config, executor);
    }

public <T> void submitStateUpdateTasks(final String source,
                                           final Map<T, ClusterStateTaskListener> tasks, final ClusterStateTaskConfig config,
                                           final ClusterStateTaskExecutor<T> executor) {
        masterService.submitStateUpdateTasks(source, tasks, config, executor);
    }

到这里变成了调用MasterService的服务。

MasterService

public <T> void submitStateUpdateTasks(final String source,
                                           final Map<T, ClusterStateTaskListener> tasks, final ClusterStateTaskConfig config,
                                           final ClusterStateTaskExecutor<T> executor) {
        if (!lifecycle.started()) {
            return;
        }
        try {
            List<Batcher.UpdateTask> safeTasks = tasks.entrySet().stream()
                .map(e -> taskBatcher.new UpdateTask(config.priority(), source, e.getKey(), safe(e.getValue()), executor))
                .collect(Collectors.toList());
            taskBatcher.submitTasks(safeTasks, config.timeout());
        } catch (EsRejectedExecutionException e) {
            // ignore cases where we are shutting down..., there is really nothing interesting
            // to be done here...
            if (!lifecycle.stoppedOrClosed()) {
                throw e;
            }
        }
    }

这里用到了java1.8中的语法，其实就是对task做了一次封装。封装成UpdateTask对象，并调用TaskBatcher提交task。

UpdateTask

UpdateTask(Priority priority, String source, Object task, ClusterStateTaskListener listener,
                       ClusterStateTaskExecutor<?> executor) {
                super(priority, source, executor, task);
                this.listener = listener;
            }

TaskBatcher

public void submitTasks(List<? extends BatchedTask> tasks, @Nullable TimeValue timeout) throws EsRejectedExecutionException {
        if (tasks.isEmpty()) {
            return;
        }
        final BatchedTask firstTask = tasks.get(0);
        assert tasks.stream().allMatch(t -> t.batchingKey == firstTask.batchingKey) :
            "tasks submitted in a batch should share the same batching key: " + tasks;
        // convert to an identity map to check for dups based on task identity
        final Map<Object, BatchedTask> tasksIdentity = tasks.stream().collect(Collectors.toMap(
            BatchedTask::getTask,
            Function.identity(),
            (a, b) -> { throw new IllegalStateException("cannot add duplicate task: " + a); },
            IdentityHashMap::new));

        synchronized (tasksPerBatchingKey) {
            LinkedHashSet<BatchedTask> existingTasks = tasksPerBatchingKey.computeIfAbsent(firstTask.batchingKey,
                k -> new LinkedHashSet<>(tasks.size()));
            for (BatchedTask existing : existingTasks) {
                // check that there won't be two tasks with the same identity for the same batching key
                BatchedTask duplicateTask = tasksIdentity.get(existing.getTask());
                if (duplicateTask != null) {
                    throw new IllegalStateException("task [" + duplicateTask.describeTasks(
                        Collections.singletonList(existing)) + "] with source [" + duplicateTask.source + "] is already queued");
                }
            }
            existingTasks.addAll(tasks);
        }

        if (timeout != null) {
            threadExecutor.execute(firstTask, timeout, () -> onTimeoutInternal(tasks, timeout));
        } else {
            threadExecutor.execute(firstTask);
        }
    }

这个函数里有两处查重的步骤，第一个是检查函数传入的tasks中有没有重复的task，第二个是检查本次提交的tasks是否和历史提交的tasks有重复。tasksPerBatchingKey维护了同一个batchingKey对应的所有task。随后就调用executor执行这个task。

PrioritizedEsThreadPoolExecutor

public void execute(Runnable command, final TimeValue timeout, final Runnable timeoutCallback) {
        command = wrapRunnable(command);
        doExecute(command);
        if (timeout.nanos() >= 0) {
            if (command instanceof TieBreakingPrioritizedRunnable) {
                ((TieBreakingPrioritizedRunnable) command).scheduleTimeout(timer, timeoutCallback, timeout);
            } else {
                // We really shouldn't be here. The only way we can get here if somebody created PrioritizedFutureTask
                // and passed it to execute, which doesn't make much sense
                throw new UnsupportedOperationException("Execute with timeout is not supported for future tasks");
            }
        }
    }

这里先会对command包装一下，先看下里边究竟干了啥。

protected Runnable wrapRunnable(Runnable command) {
        if (command instanceof PrioritizedRunnable) {
            if ((command instanceof TieBreakingPrioritizedRunnable)) {
                return command;
            }
            Priority priority = ((PrioritizedRunnable) command).priority();
          //UpdateTask对象最终会被包装成这个对象
            return new TieBreakingPrioritizedRunnable(super.wrapRunnable(command), priority, insertionOrder.incrementAndGet());
        } else if (command instanceof PrioritizedFutureTask) {
            return command;
        } else { // it might be a callable wrapper...
            if (command instanceof TieBreakingPrioritizedRunnable) {
                return command;
            }
            return new TieBreakingPrioritizedRunnable(super.wrapRunnable(command), Priority.NORMAL, insertionOrder.incrementAndGet());
        }
    }

command对象其实是前文所说的UpdateTask对象，该对象是一种优先级执行对象，所以最终会被包装成TieBreakingPrioritizedRunnable对象，这个对象实现了Runnable的run方法：

TieBreakingPrioritizedRunnable

public void run() {
            synchronized (this) {
                // make the task as stared. This is needed for synchronization with the timeout handling
                // see  #scheduleTimeout()
                started = true;
                FutureUtils.cancel(timeoutFuture);
            }
            runAndClean(runnable);
        }

继续回到execute方法中，包装完command后直接调用了doExecute方法，该方法其实是PrioritizedEsThreadPoolExecutor父类EsThreadPoolExecutor的一个方法。

EsThreadPoolExecutor

protected void doExecute(final Runnable command) {
        try {
            super.execute(command);
        } catch (EsRejectedExecutionException ex) {
            if (command instanceof AbstractRunnable) {
                // If we are an abstract runnable we can handle the rejection
                // directly and don't need to rethrow it.
                try {
                    ((AbstractRunnable) command).onRejection(ex);
                } finally {
                    ((AbstractRunnable) command).onAfter();

                }
            } else {
                throw ex;
            }
        }
    }

EsThreadPoolExecutor继承了java中的ThreadPoolExecutor，有关elasticsearch中的executor后面再分析。到这里可以看到在doExecute中调用了父类的execute方法，最终提交了该任务到线程池中执行。提交后回到execute方法中，如果设置了超时时间，则在一段时间后调用超时回调函数。
至此，创建索引的任务已经被提交。在下篇文章中将会分析任务是怎么执行的。