// org.elasticsearch.action.search.TransportSearchAction#executeRequest
    private void executeRequest(Task task, SearchRequest searchRequest,
                                SearchAsyncActionProvider searchAsyncActionProvider, ActionListener<SearchResponse> listener) {
        final long relativeStartNanos = System.nanoTime();
        final SearchTimeProvider timeProvider =
            new SearchTimeProvider(searchRequest.getOrCreateAbsoluteStartMillis(), relativeStartNanos, System::nanoTime);
        ActionListener<SearchSourceBuilder> rewriteListener = ActionListener.wrap(source -> {
            if (source != searchRequest.source()) {
                // only set it if it changed - we don't allow null values to be set but it might be already null. this way we catch
                // situations when source is rewritten to null due to a bug
                searchRequest.source(source);
            }
            final ClusterState clusterState = clusterService.state();
            final SearchContextId searchContext;
            final Map<String, OriginalIndices> remoteClusterIndices;
            if (searchRequest.pointInTimeBuilder() != null) {
                searchContext = SearchContextId.decode(namedWriteableRegistry, searchRequest.pointInTimeBuilder().getId());
                remoteClusterIndices = getIndicesFromSearchContexts(searchContext, searchRequest.indicesOptions());
            } else {
                searchContext = null;
                remoteClusterIndices = remoteClusterService.groupIndices(searchRequest.indicesOptions(),
                    searchRequest.indices(), idx -> indexNameExpressionResolver.hasIndexAbstraction(idx, clusterState));
            }
            OriginalIndices localIndices = remoteClusterIndices.remove(RemoteClusterAware.LOCAL_CLUSTER_GROUP_KEY);
            if (remoteClusterIndices.isEmpty()) {
                executeLocalSearch(
                    task, timeProvider, searchRequest, localIndices, clusterState, listener, searchContext, searchAsyncActionProvider);
            } else {
                // 多節(jié)點數(shù)據(jù)請求
                if (shouldMinimizeRoundtrips(searchRequest)) {
                    // 通過 parentTaskId 關(guān)聯(lián)所有子任務(wù)
                    final TaskId parentTaskId = task.taskInfo(clusterService.localNode().getId(), false).getTaskId();
                    ccsRemoteReduce(parentTaskId, searchRequest, localIndices, remoteClusterIndices, timeProvider,
                        searchService.aggReduceContextBuilder(searchRequest),
                        remoteClusterService, threadPool, listener,
                        (r, l) -> executeLocalSearch(
                            task, timeProvider, r, localIndices, clusterState, l, searchContext, searchAsyncActionProvider));
                } else {
                    AtomicInteger skippedClusters = new AtomicInteger(0);
                    // 直接分發(fā)多shard請求到各節(jié)點
                    collectSearchShards(searchRequest.indicesOptions(), searchRequest.preference(), searchRequest.routing(),
                        skippedClusters, remoteClusterIndices, remoteClusterService, threadPool,
                        ActionListener.wrap(
                            searchShardsResponses -> {
                                // 當(dāng)所有節(jié)點都響應(yīng)后，再做后續(xù)邏輯處理，即此處的后置監(jiān)聽
                                final BiFunction<String, String, DiscoveryNode> clusterNodeLookup =
                                    getRemoteClusterNodeLookup(searchShardsResponses);
                                final Map<String, AliasFilter> remoteAliasFilters;
                                final List<SearchShardIterator> remoteShardIterators;
                                if (searchContext != null) {
                                    remoteAliasFilters = searchContext.aliasFilter();
                                    remoteShardIterators = getRemoteShardsIteratorFromPointInTime(searchShardsResponses,
                                        searchContext, searchRequest.pointInTimeBuilder().getKeepAlive(), remoteClusterIndices);
                                } else {
                                    remoteAliasFilters = getRemoteAliasFilters(searchShardsResponses);
                                    remoteShardIterators = getRemoteShardsIterator(searchShardsResponses, remoteClusterIndices,
                                        remoteAliasFilters);
                                }
                                int localClusters = localIndices == null ? 0 : 1;
                                int totalClusters = remoteClusterIndices.size() + localClusters;
                                int successfulClusters = searchShardsResponses.size() + localClusters;
                                // 至于后續(xù)搜索實現(xiàn)如何，不在此間
                                executeSearch((SearchTask) task, timeProvider, searchRequest, localIndices, remoteShardIterators,
                                    clusterNodeLookup, clusterState, remoteAliasFilters, listener,
                                    new SearchResponse.Clusters(totalClusters, successfulClusters, skippedClusters.get()),
                                    searchContext, searchAsyncActionProvider);
                            },
                            listener::onFailure));
                }
            }
        }, listener::onFailure);
        if (searchRequest.source() == null) {
            rewriteListener.onResponse(searchRequest.source());
        } else {
            Rewriteable.rewriteAndFetch(searchRequest.source(), searchService.getRewriteContext(timeProvider::getAbsoluteStartMillis),
                rewriteListener);
        }
    }

// org.elasticsearch.action.search.TransportSearchAction#collectSearchShards
static void collectSearchShards(IndicesOptions indicesOptions, String preference, String routing, AtomicInteger skippedClusters,
                                Map<String, OriginalIndices> remoteIndicesByCluster, RemoteClusterService remoteClusterService,
                                ThreadPool threadPool, ActionListener<Map<String, ClusterSearchShardsResponse>> listener) {
    // 使用該計數(shù)器進行結(jié)果控制
    final CountDown responsesCountDown = new CountDown(remoteIndicesByCluster.size());
    final Map<String, ClusterSearchShardsResponse> searchShardsResponses = new ConcurrentHashMap<>();
    final AtomicReference<Exception> exceptions = new AtomicReference<>();
    // 迭代各節(jié)點，依次發(fā)送請求
    for (Map.Entry<String, OriginalIndices> entry : remoteIndicesByCluster.entrySet()) {
        final String clusterAlias = entry.getKey();
        boolean skipUnavailable = remoteClusterService.isSkipUnavailable(clusterAlias);
        Client clusterClient = remoteClusterService.getRemoteClusterClient(threadPool, clusterAlias);
        final String[] indices = entry.getValue().indices();
        ClusterSearchShardsRequest searchShardsRequest = new ClusterSearchShardsRequest(indices)
            .indicesOptions(indicesOptions).local(true).preference(preference).routing(routing);
        // 向集群中 clusterAlias 異步發(fā)起請求處理 search
        clusterClient.admin().cluster().searchShards(searchShardsRequest,
            new CCSActionListener<ClusterSearchShardsResponse, Map<String, ClusterSearchShardsResponse>>(
                clusterAlias, skipUnavailable, responsesCountDown, skippedClusters, exceptions, listener) {
                @Override
                void innerOnResponse(ClusterSearchShardsResponse clusterSearchShardsResponse) {
                    // 每次單節(jié)點響應(yīng)時，將結(jié)果存放到 searchShardsResponses 中
                    searchShardsResponses.put(clusterAlias, clusterSearchShardsResponse);
                }

                @Override
                Map<String, ClusterSearchShardsResponse> createFinalResponse() {
                    // 所有節(jié)點都返回時，將結(jié)果集返回
                    return searchShardsResponses;
                }
            }
        );
    }
}
// org.elasticsearch.client.support.AbstractClient.ClusterAdmin#searchShards
@Override
public void searchShards(final ClusterSearchShardsRequest request, final ActionListener<ClusterSearchShardsResponse> listener) {
    // 發(fā)起請求 indices:admin/shards/search_shards, 其對應(yīng)處理器為 TransportClusterSearchShardsAction
    execute(ClusterSearchShardsAction.INSTANCE, request, listener);
}

// org.elasticsearch.action.search.TransportSearchAction.CCSActionListener#CCSActionListener
CCSActionListener(String clusterAlias, boolean skipUnavailable, CountDown countDown, AtomicInteger skippedClusters,
                    AtomicReference<Exception> exceptions, ActionListener<FinalResponse> originalListener) {
    this.clusterAlias = clusterAlias;
    this.skipUnavailable = skipUnavailable;
    this.countDown = countDown;
    this.skippedClusters = skippedClusters;
    this.exceptions = exceptions;
    this.originalListener = originalListener;
}

// 成功時的響應(yīng)
@Override
public final void onResponse(Response response) {
    // inner響應(yīng)為將結(jié)果放入 searchShardsResponses 中
    innerOnResponse(response);
    // maybeFinish 則進行結(jié)果是否完成判定，如果完成，則調(diào)用回調(diào)方法，構(gòu)造結(jié)果
    maybeFinish();
}

private void maybeFinish() {
    // 使用一個 AtomicInteger 進行控制
    if (countDown.countDown()) {
        Exception exception = exceptions.get();
        if (exception == null) {
            FinalResponse response;
            try {
                // 創(chuàng)建響應(yīng)結(jié)果，此處 search 即為 searchShardsResponses
                response = createFinalResponse();
            } catch(Exception e) {
                originalListener.onFailure(e);
                return;
            }
            // 成功響應(yīng)回調(diào)，實現(xiàn)結(jié)果收集后的其他業(yè)務(wù)處理
            originalListener.onResponse(response);
        } else {
            originalListener.onFailure(exceptions.get());
        }
    }
}
// CountDown 實現(xiàn)比較簡單，只有最后一個返回true, 其他皆為false, 即實現(xiàn)了 At Most Once 語義
/**
* Decrements the count-down and returns <code>true</code> iff this call
* reached zero otherwise <code>false</code>
*/
public boolean countDown() {
    assert originalCount > 0;
    for (;;) {
        final int current = countDown.get();
        assert current >= 0;
        if (current == 0) {
            return false;
        }
        if (countDown.compareAndSet(current, current - 1)) {
            return current == 1;
        }
    }
}

// org.elasticsearch.action.admin.cluster.shards.TransportClusterSearchShardsAction
public class TransportClusterSearchShardsAction extends
    TransportMasterNodeReadAction<ClusterSearchShardsRequest, ClusterSearchShardsResponse> {

    private final IndicesService indicesService;

    @Inject
    public TransportClusterSearchShardsAction(TransportService transportService, ClusterService clusterService,
                                              IndicesService indicesService, ThreadPool threadPool, ActionFilters actionFilters,
                                              IndexNameExpressionResolver indexNameExpressionResolver) {
        super(ClusterSearchShardsAction.NAME, transportService, clusterService, threadPool, actionFilters,
            ClusterSearchShardsRequest::new, indexNameExpressionResolver, ClusterSearchShardsResponse::new, ThreadPool.Names.SAME);
        this.indicesService = indicesService;
    }

    @Override
    protected ClusterBlockException checkBlock(ClusterSearchShardsRequest request, ClusterState state) {
        return state.blocks().indicesBlockedException(ClusterBlockLevel.METADATA_READ,
                indexNameExpressionResolver.concreteIndexNames(state, request));
    }

    @Override
    protected void masterOperation(final ClusterSearchShardsRequest request, final ClusterState state,
                                   final ActionListener<ClusterSearchShardsResponse> listener) {
        ClusterState clusterState = clusterService.state();
        String[] concreteIndices = indexNameExpressionResolver.concreteIndexNames(clusterState, request);
        Map<String, Set<String>> routingMap = indexNameExpressionResolver.resolveSearchRouting(state, request.routing(), request.indices());
        Map<String, AliasFilter> indicesAndFilters = new HashMap<>();
        Set<String> indicesAndAliases = indexNameExpressionResolver.resolveExpressions(clusterState, request.indices());
        for (String index : concreteIndices) {
            final AliasFilter aliasFilter = indicesService.buildAliasFilter(clusterState, index, indicesAndAliases);
            final String[] aliases = indexNameExpressionResolver.indexAliases(clusterState, index, aliasMetadata -> true, true,
                indicesAndAliases);
            indicesAndFilters.put(index, new AliasFilter(aliasFilter.getQueryBuilder(), aliases));
        }

        Set<String> nodeIds = new HashSet<>();
        GroupShardsIterator<ShardIterator> groupShardsIterator = clusterService.operationRouting()
            .searchShards(clusterState, concreteIndices, routingMap, request.preference());
        ShardRouting shard;
        ClusterSearchShardsGroup[] groupResponses = new ClusterSearchShardsGroup[groupShardsIterator.size()];
        int currentGroup = 0;
        for (ShardIterator shardIt : groupShardsIterator) {
            ShardId shardId = shardIt.shardId();
            ShardRouting[] shardRoutings = new ShardRouting[shardIt.size()];
            int currentShard = 0;
            shardIt.reset();
            while ((shard = shardIt.nextOrNull()) != null) {
                shardRoutings[currentShard++] = shard;
                nodeIds.add(shard.currentNodeId());
            }
            groupResponses[currentGroup++] = new ClusterSearchShardsGroup(shardId, shardRoutings);
        }
        DiscoveryNode[] nodes = new DiscoveryNode[nodeIds.size()];
        int currentNode = 0;
        for (String nodeId : nodeIds) {
            nodes[currentNode++] = clusterState.getNodes().get(nodeId);
        }
        listener.onResponse(new ClusterSearchShardsResponse(groupResponses, nodes, indicesAndFilters));
    }
}
// doExecute 在父類中完成
// org.elasticsearch.action.support.master.TransportMasterNodeAction#doExecute
@Override
protected void doExecute(Task task, final Request request, ActionListener<Response> listener) {
    ClusterState state = clusterService.state();
    logger.trace("starting processing request [{}] with cluster state version [{}]", request, state.version());
    if (task != null) {
        request.setParentTask(clusterService.localNode().getId(), task.getId());
    }
    new AsyncSingleAction(task, request, listener).doStart(state);
}

// org.elasticsearch.action.support.master.TransportMasterNodeAction.AsyncSingleAction#doStart
AsyncSingleAction(Task task, Request request, ActionListener<Response> listener) {
    this.task = task;
    this.request = request;
    this.listener = listener;
    this.startTime = threadPool.relativeTimeInMillis();
}

protected void doStart(ClusterState clusterState) {
    try {
        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.debug("can't execute due to a cluster block, retrying", blockException);
                    // 重試處理
                    retry(clusterState, blockException, newState -> {
                        try {
                            ClusterBlockException newException = checkBlock(request, newState);
                            return (newException == null || !newException.retryable());
                        } catch (Exception e) {
                            // accept state as block will be rechecked by doStart() and listener.onFailure() then called
                            logger.trace("exception occurred during cluster block checking, accepting state", e);
                            return true;
                        }
                    });
                }
            } else {
                ActionListener<Response> delegate = ActionListener.delegateResponse(listener, (delegatedListener, t) -> {
                    if (t instanceof FailedToCommitClusterStateException || t instanceof NotMasterException) {
                        logger.debug(() -> new ParameterizedMessage("master could not publish cluster state or " +
                            "stepped down before publishing action [{}], scheduling a retry", actionName), t);
                        retryOnMasterChange(clusterState, t);
                    } else {
                        delegatedListener.onFailure(t);
                    }
                });
                // 本地節(jié)點執(zhí)行結(jié)果，直接以異步線程處理即可
                threadPool.executor(executor)
                    .execute(ActionRunnable.wrap(delegate, l -> masterOperation(task, request, clusterState, l)));
            }
        } else {
            if (nodes.getMasterNode() == null) {
                logger.debug("no known master node, scheduling a retry");
                retryOnMasterChange(clusterState, null);
            } else {
                DiscoveryNode masterNode = nodes.getMasterNode();
                final String actionName = getMasterActionName(masterNode);
                // 發(fā)送到master節(jié)點，以netty作為通訊工具，完成后回調(diào) 當(dāng)前l(fā)istner
                transportService.sendRequest(masterNode, actionName, request,
                    new ActionListenerResponseHandler<Response>(listener, responseReader) {
                        @Override
                        public void handleException(final TransportException exp) {
                            Throwable cause = exp.unwrapCause();
                            if (cause instanceof ConnectTransportException ||
                                (exp instanceof RemoteTransportException && cause instanceof NodeClosedException)) {
                                // 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());
                                retryOnMasterChange(clusterState, cause);
                            } else {
                                listener.onFailure(exp);
                            }
                        }
                });
            }
        }
    } catch (Exception e) {
        listener.onFailure(e);
    }
}

// org.elasticsearch.transport.TransportService#sendRequest
public final <T extends TransportResponse> void sendRequest(final DiscoveryNode node, final String action,
                                                            final TransportRequest request,
                                                            final TransportRequestOptions options,
                                                            TransportResponseHandler<T> handler) {
    final Transport.Connection connection;
    try {
        // 假設(shè)不是本節(jié)點，則獲取遠程的一個 connection, channel
        connection = getConnection(node);
    } catch (final NodeNotConnectedException ex) {
        // the caller might not handle this so we invoke the handler
        handler.handleException(ex);
        return;
    }
    sendRequest(connection, action, request, options, handler);
}
// org.elasticsearch.transport.TransportService#getConnection
/**
    * Returns either a real transport connection or a local node connection if we are using the local node optimization.
    * @throws NodeNotConnectedException if the given node is not connected
    */
public Transport.Connection getConnection(DiscoveryNode node) {
    if (isLocalNode(node)) {
        return localNodeConnection;
    } else {
        return connectionManager.getConnection(node);
    }
}

// org.elasticsearch.transport.TransportService#sendRequest
/**
    * Sends a request on the specified connection. If there is a failure sending the request, the specified handler is invoked.
    *
    * @param connection the connection to send the request on
    * @param action     the name of the action
    * @param request    the request
    * @param options    the options for this request
    * @param handler    the response handler
    * @param <T>        the type of the transport response
    */
public final <T extends TransportResponse> void sendRequest(final Transport.Connection connection, final String action,
                                                            final TransportRequest request,
                                                            final TransportRequestOptions options,
                                                            final TransportResponseHandler<T> handler) {
    try {
        final TransportResponseHandler<T> delegate;
        if (request.getParentTask().isSet()) {
            // If the connection is a proxy connection, then we will create a cancellable proxy task on the proxy node and an actual
            // child task on the target node of the remote cluster.
            //  ----> a parent task on the local cluster
            //        |
            //         ----> a proxy task on the proxy node on the remote cluster
            //               |
            //                ----> an actual child task on the target node on the remote cluster
            // To cancel the child task on the remote cluster, we must send a cancel request to the proxy node instead of the target
            // node as the parent task of the child task is the proxy task not the parent task on the local cluster. Hence, here we
            // unwrap the connection and keep track of the connection to the proxy node instead of the proxy connection.
            final Transport.Connection unwrappedConn = unwrapConnection(connection);
            final Releasable unregisterChildNode = taskManager.registerChildConnection(request.getParentTask().getId(), unwrappedConn);
            delegate = new TransportResponseHandler<T>() {
                @Override
                public void handleResponse(T response) {
                    unregisterChildNode.close();
                    handler.handleResponse(response);
                }

                @Override
                public void handleException(TransportException exp) {
                    unregisterChildNode.close();
                    handler.handleException(exp);
                }

                @Override
                public String executor() {
                    return handler.executor();
                }

                @Override
                public T read(StreamInput in) throws IOException {
                    return handler.read(in);
                }

                @Override
                public String toString() {
                    return getClass().getName() + "/[" + action + "]:" + handler.toString();
                }
            };
        } else {
            delegate = handler;
        }
        asyncSender.sendRequest(connection, action, request, options, delegate);
    } catch (final Exception ex) {
        // the caller might not handle this so we invoke the handler
        final TransportException te;
        if (ex instanceof TransportException) {
            te = (TransportException) ex;
        } else {
            te = new TransportException("failure to send", ex);
        }
        handler.handleException(te);
    }
}

// org.elasticsearch.transport.TransportService#sendRequestInternal
private <T extends TransportResponse> void sendRequestInternal(final Transport.Connection connection, final String action,
                                                                final TransportRequest request,
                                                                final TransportRequestOptions options,
                                                                TransportResponseHandler<T> handler) {
    if (connection == null) {
        throw new IllegalStateException("can't send request to a null connection");
    }
    DiscoveryNode node = connection.getNode();

    Supplier<ThreadContext.StoredContext> storedContextSupplier = threadPool.getThreadContext().newRestorableContext(true);
    ContextRestoreResponseHandler<T> responseHandler = new ContextRestoreResponseHandler<>(storedContextSupplier, handler);
    // TODO we can probably fold this entire request ID dance into connection.sendReqeust but it will be a bigger refactoring
    final long requestId = responseHandlers.add(new Transport.ResponseContext<>(responseHandler, connection, action));
    final TimeoutHandler timeoutHandler;
    if (options.timeout() != null) {
        timeoutHandler = new TimeoutHandler(requestId, connection.getNode(), action);
        responseHandler.setTimeoutHandler(timeoutHandler);
    } else {
        timeoutHandler = null;
    }
    try {
        if (lifecycle.stoppedOrClosed()) {
            /*
                * If we are not started the exception handling will remove the request holder again and calls the handler to notify the
                * caller. It will only notify if toStop hasn't done the work yet.
                */
            throw new NodeClosedException(localNode);
        }
        if (timeoutHandler != null) {
            assert options.timeout() != null;
            timeoutHandler.scheduleTimeout(options.timeout());
        }
        connection.sendRequest(requestId, action, request, options); // local node optimization happens upstream
    } catch (final Exception e) {
        // usually happen either because we failed to connect to the node
        // or because we failed serializing the message
        final Transport.ResponseContext<? extends TransportResponse> contextToNotify = responseHandlers.remove(requestId);
        // If holderToNotify == null then handler has already been taken care of.
        if (contextToNotify != null) {
            if (timeoutHandler != null) {
                timeoutHandler.cancel();
            }
            // callback that an exception happened, but on a different thread since we don't
            // want handlers to worry about stack overflows. In the special case of running into a closing node we run on the current
            // thread on a best effort basis though.
            final SendRequestTransportException sendRequestException = new SendRequestTransportException(node, action, e);
            final String executor = lifecycle.stoppedOrClosed() ? ThreadPool.Names.SAME : ThreadPool.Names.GENERIC;
            threadPool.executor(executor).execute(new AbstractRunnable() {
                @Override
                public void onRejection(Exception e) {
                    // if we get rejected during node shutdown we don't wanna bubble it up
                    logger.debug(
                        () -> new ParameterizedMessage(
                            "failed to notify response handler on rejection, action: {}",
                            contextToNotify.action()),
                        e);
                }
                @Override
                public void onFailure(Exception e) {
                    logger.warn(
                        () -> new ParameterizedMessage(
                            "failed to notify response handler on exception, action: {}",
                            contextToNotify.action()),
                        e);
                }
                @Override
                protected void doRun() throws Exception {
                    contextToNotify.handler().handleException(sendRequestException);
                }
            });
        } else {
            logger.debug("Exception while sending request, handler likely already notified due to timeout", e);
        }
    }
}
// org.elasticsearch.transport.RemoteConnectionManager.ProxyConnection#sendRequest
@Override
public void sendRequest(long requestId, String action, TransportRequest request, TransportRequestOptions options)
    throws IOException, TransportException {
    connection.sendRequest(requestId, TransportActionProxy.getProxyAction(action),
        TransportActionProxy.wrapRequest(targetNode, request), options);
}
// org.elasticsearch.transport.TcpTransport.NodeChannels#sendRequest
@Override
public void sendRequest(long requestId, String action, TransportRequest request, TransportRequestOptions options)
    throws IOException, TransportException {
    if (isClosing.get()) {
        throw new NodeNotConnectedException(node, "connection already closed");
    }
    TcpChannel channel = channel(options.type());
    outboundHandler.sendRequest(node, channel, requestId, action, request, options, getVersion(), compress, false);
}
// org.elasticsearch.transport.OutboundHandler#sendRequest
/**
    * Sends the request to the given channel. This method should be used to send {@link TransportRequest}
    * objects back to the caller.
    */
void sendRequest(final DiscoveryNode node, final TcpChannel channel, final long requestId, final String action,
                    final TransportRequest request, final TransportRequestOptions options, final Version channelVersion,
                    final boolean compressRequest, final boolean isHandshake) throws IOException, TransportException {
    Version version = Version.min(this.version, channelVersion);
    OutboundMessage.Request message = new OutboundMessage.Request(threadPool.getThreadContext(), features, request, version, action,
        requestId, isHandshake, compressRequest);
    ActionListener<Void> listener = ActionListener.wrap(() ->
        messageListener.onRequestSent(node, requestId, action, request, options));
    sendMessage(channel, message, listener);
}
// org.elasticsearch.transport.OutboundHandler#sendMessage
private void sendMessage(TcpChannel channel, OutboundMessage networkMessage, ActionListener<Void> listener) throws IOException {
    MessageSerializer serializer = new MessageSerializer(networkMessage, bigArrays);
    SendContext sendContext = new SendContext(channel, serializer, listener, serializer);
    internalSend(channel, sendContext);
}
private void internalSend(TcpChannel channel, SendContext sendContext) throws IOException {
    channel.getChannelStats().markAccessed(threadPool.relativeTimeInMillis());
    BytesReference reference = sendContext.get();
    // stash thread context so that channel event loop is not polluted by thread context
    try (ThreadContext.StoredContext existing = threadPool.getThreadContext().stashContext()) {
        channel.sendMessage(reference, sendContext);
    } catch (RuntimeException ex) {
        sendContext.onFailure(ex);
        CloseableChannel.closeChannel(channel);
        throw ex;
    }
}
// org.elasticsearch.transport.netty4.Netty4TcpChannel#sendMessage
@Override
public void sendMessage(BytesReference reference, ActionListener<Void> listener) {
    // netty 發(fā)送數(shù)據(jù)，異步回調(diào)，完成異步請求
    channel.writeAndFlush(Netty4Utils.toByteBuf(reference), addPromise(listener, channel));

    if (channel.eventLoop().isShutdown()) {
        listener.onFailure(new TransportException("Cannot send message, event loop is shutting down."));
    }
}