RocketMQ(八)：高效能探祕之執行緒池

　　上一篇文章講了如何設計和實現高併發高效能的應用，從根本上說明了一些道理。且以rocketmq的mappedFile的實現作為一個突破點，講解了rocketmq是如何具體實現高效能的。從中我們也知道，mappedFile只是其利用的作業系統的一個特性小點。

　　今天，我們就來說說，rockmq實現高效能的第二個小：執行緒池的應用。

 

1. rocketmq的執行緒模型概述

　　談到多執行緒，一般我們都會談到鎖和執行緒模型。

　　鎖我們就不多說了，基本原理都差不多，大家可以參考網上資料或檢視我之前的文章。

　　而執行緒模型，則可能各有不同，它會根據各自應用的特性，玩出自己的花樣。下面我們就來數數rocketmq中的執行緒模型吧！

　　事實上，rocketmq中，存在多種不同的角色元件，而執行緒模型也往往會根據元件的特性變得不一樣。最好我們都能瞭解一點，以不至於一葉障目。

 

2. 通用的netty執行緒模型

　　首先，整個rockmq都是基於netty進行的網路通訊，所以其最基礎的執行緒模型既是netty的執行緒模型，即reactor模型，（注：以下netty執行緒模型來源於網路）

　　rocketmq中具體應用的通用的netty執行緒如下：

 

 

 　　1個acceptor -> n個event.processor.selector -> serverWorkerThreads.worker -> 業務處理執行緒

　　其呼叫netty的應用實現程式碼如下:

    // org.apache.rocketmq.remoting.netty.NettyRemotingServer#start
    @Override
    public void start() {
        this.defaultEventExecutorGroup = new DefaultEventExecutorGroup(
            nettyServerConfig.getServerWorkerThreads(),
            new ThreadFactory() {

                private AtomicInteger threadIndex = new AtomicInteger(0);

                @Override
                public Thread newThread(Runnable r) {
                    return new Thread(r, "NettyServerCodecThread_" + this.threadIndex.incrementAndGet());
                }
            });

        prepareSharableHandlers();
        // netty 核心引數 eventLoopGroupBoss, eventLoopGroupSelector
        ServerBootstrap childHandler =
            this.serverBootstrap.group(this.eventLoopGroupBoss, this.eventLoopGroupSelector)
                .channel(useEpoll() ? EpollServerSocketChannel.class : NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 1024)
                .option(ChannelOption.SO_REUSEADDR, true)
                .option(ChannelOption.SO_KEEPALIVE, false)
                .childOption(ChannelOption.TCP_NODELAY, true)
                .childOption(ChannelOption.SO_SNDBUF, nettyServerConfig.getServerSocketSndBufSize())
                .childOption(ChannelOption.SO_RCVBUF, nettyServerConfig.getServerSocketRcvBufSize())
                .localAddress(new InetSocketAddress(this.nettyServerConfig.getListenPort()))
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    public void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline()
                            .addLast(defaultEventExecutorGroup, HANDSHAKE_HANDLER_NAME, handshakeHandler)
                            .addLast(defaultEventExecutorGroup,
                                encoder,
                                new NettyDecoder(),
                                new IdleStateHandler(0, 0, nettyServerConfig.getServerChannelMaxIdleTimeSeconds()),
                                connectionManageHandler,
                                serverHandler
                            );
                    }
                });

        if (nettyServerConfig.isServerPooledByteBufAllocatorEnable()) {
            childHandler.childOption(ChannelOption.ALLOCATOR, PooledByteBufAllocator.DEFAULT);
        }

        try {
            ChannelFuture sync = this.serverBootstrap.bind().sync();
            InetSocketAddress addr = (InetSocketAddress) sync.channel().localAddress();
            this.port = addr.getPort();
        } catch (InterruptedException e1) {
            throw new RuntimeException("this.serverBootstrap.bind().sync() InterruptedException", e1);
        }

        if (this.channelEventListener != null) {
            this.nettyEventExecutor.start();
        }

        this.timer.scheduleAtFixedRate(new TimerTask() {

            @Override
            public void run() {
                try {
                    NettyRemotingServer.this.scanResponseTable();
                } catch (Throwable e) {
                    log.error("scanResponseTable exception", e);
                }
            }
        }, 1000 * 3, 1000);
    }

　　這個每個server端的所有角色網路io處理執行緒模型，client端就不用說了吧（同樣遵循netty程式設計模型）。

　　但是到具體的角色，分工是不一樣的。下面我們就來細分幾個角色詳細分解下。

 

3. nameserver的執行緒模型

　　nameserver的業務執行緒模型非常簡單，因為它只有一個 defaultProcessor, 其執行緒模型如下：

 

 

 

　　1個acceptor -> serverSelectorThreads個io.processor -> serverWorkerThreads.worker(defaultEventExecutorGroup) -> serverWorkerThreads.executors(remotingExecutor)

　　之所以nameserver的執行緒模型如此簡單，可能是因為其業務處理本身就非常輕量級，基本都是一些純記憶體操作，只需幾個簡單的執行緒就可以處理了。

 

4. broker的執行緒模型

　　broker的業務執行緒模型則相對複雜了，它需要處理各種不同型別的業務，所以它劃分出了許多的業務執行緒池，供各型別業務處理使用。

 

　　這可以在broker初始化程式碼中看到具體實現：

    // org.apache.rocketmq.broker.BrokerController#initialize
    public boolean initialize() throws CloneNotSupportedException {
        boolean result = this.topicConfigManager.load();

        result = result && this.consumerOffsetManager.load();
        result = result && this.subscriptionGroupManager.load();
        result = result && this.consumerFilterManager.load();

        if (result) {
            try {
                this.messageStore =
                    new DefaultMessageStore(this.messageStoreConfig, this.brokerStatsManager, this.messageArrivingListener,
                        this.brokerConfig);
                if (messageStoreConfig.isEnableDLegerCommitLog()) {
                    DLedgerRoleChangeHandler roleChangeHandler = new DLedgerRoleChangeHandler(this, (DefaultMessageStore) messageStore);
                    ((DLedgerCommitLog)((DefaultMessageStore) messageStore).getCommitLog()).getdLedgerServer().getdLedgerLeaderElector().addRoleChangeHandler(roleChangeHandler);
                }
                this.brokerStats = new BrokerStats((DefaultMessageStore) this.messageStore);
                //load plugin
                MessageStorePluginContext context = new MessageStorePluginContext(messageStoreConfig, brokerStatsManager, messageArrivingListener, brokerConfig);
                this.messageStore = MessageStoreFactory.build(context, this.messageStore);
                this.messageStore.getDispatcherList().addFirst(new CommitLogDispatcherCalcBitMap(this.brokerConfig, this.consumerFilterManager));
            } catch (IOException e) {
                result = false;
                log.error("Failed to initialize", e);
            }
        }

        result = result && this.messageStore.load();

        if (result) {
            this.remotingServer = new NettyRemotingServer(this.nettyServerConfig, this.clientHousekeepingService);
            NettyServerConfig fastConfig = (NettyServerConfig) this.nettyServerConfig.clone();
            fastConfig.setListenPort(nettyServerConfig.getListenPort() - 2);
            this.fastRemotingServer = new NettyRemotingServer(fastConfig, this.clientHousekeepingService);
            this.sendMessageExecutor = new BrokerFixedThreadPoolExecutor(
                this.brokerConfig.getSendMessageThreadPoolNums(),
                this.brokerConfig.getSendMessageThreadPoolNums(),
                1000 * 60,
                TimeUnit.MILLISECONDS,
                this.sendThreadPoolQueue,
                new ThreadFactoryImpl("SendMessageThread_"));

            this.pullMessageExecutor = new BrokerFixedThreadPoolExecutor(
                this.brokerConfig.getPullMessageThreadPoolNums(),
                this.brokerConfig.getPullMessageThreadPoolNums(),
                1000 * 60,
                TimeUnit.MILLISECONDS,
                this.pullThreadPoolQueue,
                new ThreadFactoryImpl("PullMessageThread_"));

            this.replyMessageExecutor = new BrokerFixedThreadPoolExecutor(
                this.brokerConfig.getProcessReplyMessageThreadPoolNums(),
                this.brokerConfig.getProcessReplyMessageThreadPoolNums(),
                1000 * 60,
                TimeUnit.MILLISECONDS,
                this.replyThreadPoolQueue,
                new ThreadFactoryImpl("ProcessReplyMessageThread_"));

            this.queryMessageExecutor = new BrokerFixedThreadPoolExecutor(
                this.brokerConfig.getQueryMessageThreadPoolNums(),
                this.brokerConfig.getQueryMessageThreadPoolNums(),
                1000 * 60,
                TimeUnit.MILLISECONDS,
                this.queryThreadPoolQueue,
                new ThreadFactoryImpl("QueryMessageThread_"));

            this.adminBrokerExecutor =
                Executors.newFixedThreadPool(this.brokerConfig.getAdminBrokerThreadPoolNums(), new ThreadFactoryImpl(
                    "AdminBrokerThread_"));

            this.clientManageExecutor = new ThreadPoolExecutor(
                this.brokerConfig.getClientManageThreadPoolNums(),
                this.brokerConfig.getClientManageThreadPoolNums(),
                1000 * 60,
                TimeUnit.MILLISECONDS,
                this.clientManagerThreadPoolQueue,
                new ThreadFactoryImpl("ClientManageThread_"));

            this.heartbeatExecutor = new BrokerFixedThreadPoolExecutor(
                this.brokerConfig.getHeartbeatThreadPoolNums(),
                this.brokerConfig.getHeartbeatThreadPoolNums(),
                1000 * 60,
                TimeUnit.MILLISECONDS,
                this.heartbeatThreadPoolQueue,
                new ThreadFactoryImpl("HeartbeatThread_", true));

            this.endTransactionExecutor = new BrokerFixedThreadPoolExecutor(
                this.brokerConfig.getEndTransactionThreadPoolNums(),
                this.brokerConfig.getEndTransactionThreadPoolNums(),
                1000 * 60,
                TimeUnit.MILLISECONDS,
                this.endTransactionThreadPoolQueue,
                new ThreadFactoryImpl("EndTransactionThread_"));

            this.consumerManageExecutor =
                Executors.newFixedThreadPool(this.brokerConfig.getConsumerManageThreadPoolNums(), new ThreadFactoryImpl(
                    "ConsumerManageThread_"));

            this.registerProcessor();

            final long initialDelay = UtilAll.computeNextMorningTimeMillis() - System.currentTimeMillis();
            final long period = 1000 * 60 * 60 * 24;
            this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                @Override
                public void run() {
                    try {
                        BrokerController.this.getBrokerStats().record();
                    } catch (Throwable e) {
                        log.error("schedule record error.", e);
                    }
                }
            }, initialDelay, period, TimeUnit.MILLISECONDS);

            this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                @Override
                public void run() {
                    try {
                        BrokerController.this.consumerOffsetManager.persist();
                    } catch (Throwable e) {
                        log.error("schedule persist consumerOffset error.", e);
                    }
                }
            }, 1000 * 10, this.brokerConfig.getFlushConsumerOffsetInterval(), TimeUnit.MILLISECONDS);

            this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                @Override
                public void run() {
                    try {
                        BrokerController.this.consumerFilterManager.persist();
                    } catch (Throwable e) {
                        log.error("schedule persist consumer filter error.", e);
                    }
                }
            }, 1000 * 10, 1000 * 10, TimeUnit.MILLISECONDS);

            this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                @Override
                public void run() {
                    try {
                        BrokerController.this.protectBroker();
                    } catch (Throwable e) {
                        log.error("protectBroker error.", e);
                    }
                }
            }, 3, 3, TimeUnit.MINUTES);

            this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                @Override
                public void run() {
                    try {
                        BrokerController.this.printWaterMark();
                    } catch (Throwable e) {
                        log.error("printWaterMark error.", e);
                    }
                }
            }, 10, 1, TimeUnit.SECONDS);

            this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

                @Override
                public void run() {
                    try {
                        log.info("dispatch behind commit log {} bytes", BrokerController.this.getMessageStore().dispatchBehindBytes());
                    } catch (Throwable e) {
                        log.error("schedule dispatchBehindBytes error.", e);
                    }
                }
            }, 1000 * 10, 1000 * 60, TimeUnit.MILLISECONDS);

            if (this.brokerConfig.getNamesrvAddr() != null) {
                this.brokerOuterAPI.updateNameServerAddressList(this.brokerConfig.getNamesrvAddr());
                log.info("Set user specified name server address: {}", this.brokerConfig.getNamesrvAddr());
            } else if (this.brokerConfig.isFetchNamesrvAddrByAddressServer()) {
                this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

                    @Override
                    public void run() {
                        try {
                            BrokerController.this.brokerOuterAPI.fetchNameServerAddr();
                        } catch (Throwable e) {
                            log.error("ScheduledTask fetchNameServerAddr exception", e);
                        }
                    }
                }, 1000 * 10, 1000 * 60 * 2, TimeUnit.MILLISECONDS);
            }

            if (!messageStoreConfig.isEnableDLegerCommitLog()) {
                if (BrokerRole.SLAVE == this.messageStoreConfig.getBrokerRole()) {
                    if (this.messageStoreConfig.getHaMasterAddress() != null && this.messageStoreConfig.getHaMasterAddress().length() >= 6) {
                        this.messageStore.updateHaMasterAddress(this.messageStoreConfig.getHaMasterAddress());
                        this.updateMasterHAServerAddrPeriodically = false;
                    } else {
                        this.updateMasterHAServerAddrPeriodically = true;
                    }
                } else {
                    this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                        @Override
                        public void run() {
                            try {
                                BrokerController.this.printMasterAndSlaveDiff();
                            } catch (Throwable e) {
                                log.error("schedule printMasterAndSlaveDiff error.", e);
                            }
                        }
                    }, 1000 * 10, 1000 * 60, TimeUnit.MILLISECONDS);
                }
            }

            if (TlsSystemConfig.tlsMode != TlsMode.DISABLED) {
                // Register a listener to reload SslContext
                try {
                    fileWatchService = new FileWatchService(
                        new String[] {
                            TlsSystemConfig.tlsServerCertPath,
                            TlsSystemConfig.tlsServerKeyPath,
                            TlsSystemConfig.tlsServerTrustCertPath
                        },
                        new FileWatchService.Listener() {
                            boolean certChanged, keyChanged = false;

                            @Override
                            public void onChanged(String path) {
                                if (path.equals(TlsSystemConfig.tlsServerTrustCertPath)) {
                                    log.info("The trust certificate changed, reload the ssl context");
                                    reloadServerSslContext();
                                }
                                if (path.equals(TlsSystemConfig.tlsServerCertPath)) {
                                    certChanged = true;
                                }
                                if (path.equals(TlsSystemConfig.tlsServerKeyPath)) {
                                    keyChanged = true;
                                }
                                if (certChanged && keyChanged) {
                                    log.info("The certificate and private key changed, reload the ssl context");
                                    certChanged = keyChanged = false;
                                    reloadServerSslContext();
                                }
                            }

                            private void reloadServerSslContext() {
                                ((NettyRemotingServer) remotingServer).loadSslContext();
                                ((NettyRemotingServer) fastRemotingServer).loadSslContext();
                            }
                        });
                } catch (Exception e) {
                    log.warn("FileWatchService created error, can't load the certificate dynamically");
                }
            }
            initialTransaction();
            initialAcl();
            initialRpcHooks();
        }
        return result;
    }

　　可以說broker的執行緒模型是最複雜的，當然也是因為其業務是最複雜和最核心的。整個rocketmq的資料儲存與讀取都是在broker端，而nameserver只是一個輔助管理角色。所以，broker需要自行保證各種高可用性，各種隔離性，以及各種有效性維護。其實現就是通過分配不同的執行緒池，來隔離各自型別的業務處理。其使用各執行緒池的方式也是固定不變的，使用一個 Pair<T1, T2> 的包裝類，將executor和proccor包裝起來，使用時就可以一起繫結使用了。前面我們也說過，它是通過一個登錄檔的方式，將所有可處理的code對映出去，再使用時取出對應executor.submit()即可。

    // org.apache.rocketmq.remoting.netty.NettyRemotingAbstract#processRequestCommand
    /**
     * Process incoming request command issued by remote peer.
     *
     * @param ctx channel handler context.
     * @param cmd request command.
     */
    public void processRequestCommand(final ChannelHandlerContext ctx, final RemotingCommand cmd) {
        // 找到對應的code處理器，使用 Pair<NettyRequestProcessor, ExecutorService> 進行包裝
        final Pair<NettyRequestProcessor, ExecutorService> matched = this.processorTable.get(cmd.getCode());
        final Pair<NettyRequestProcessor, ExecutorService> pair = null == matched ? this.defaultRequestProcessor : matched;
        final int opaque = cmd.getOpaque();

        if (pair != null) {
            Runnable run = new Runnable() {
                @Override
                public void run() {
                    try {
                        doBeforeRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd);
                        final RemotingResponseCallback callback = new RemotingResponseCallback() {
                            @Override
                            public void callback(RemotingCommand response) {
                                doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd, response);
                                if (!cmd.isOnewayRPC()) {
                                    if (response != null) {
                                        response.setOpaque(opaque);
                                        response.markResponseType();
                                        try {
                                            ctx.writeAndFlush(response);
                                        } catch (Throwable e) {
                                            log.error("process request over, but response failed", e);
                                            log.error(cmd.toString());
                                            log.error(response.toString());
                                        }
                                    } else {
                                    }
                                }
                            }
                        };
                        if (pair.getObject1() instanceof AsyncNettyRequestProcessor) {
                            AsyncNettyRequestProcessor processor = (AsyncNettyRequestProcessor)pair.getObject1();
                            processor.asyncProcessRequest(ctx, cmd, callback);
                        } else {
                            NettyRequestProcessor processor = pair.getObject1();
                            RemotingCommand response = processor.processRequest(ctx, cmd);
                            callback.callback(response);
                        }
                    } catch (Throwable e) {
                        log.error("process request exception", e);
                        log.error(cmd.toString());

                        if (!cmd.isOnewayRPC()) {
                            final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_ERROR,
                                RemotingHelper.exceptionSimpleDesc(e));
                            response.setOpaque(opaque);
                            ctx.writeAndFlush(response);
                        }
                    }
                }
            };

            if (pair.getObject1().rejectRequest()) {
                final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_BUSY,
                    "[REJECTREQUEST]system busy, start flow control for a while");
                response.setOpaque(opaque);
                ctx.writeAndFlush(response);
                return;
            }

            try {
                final RequestTask requestTask = new RequestTask(run, ctx.channel(), cmd);
                // object2 為executor執行緒池, 直接向其submit()任務即可
                pair.getObject2().submit(requestTask);
            } catch (RejectedExecutionException e) {
                if ((System.currentTimeMillis() % 10000) == 0) {
                    log.warn(RemotingHelper.parseChannelRemoteAddr(ctx.channel())
                        + ", too many requests and system thread pool busy, RejectedExecutionException "
                        + pair.getObject2().toString()
                        + " request code: " + cmd.getCode());
                }

                if (!cmd.isOnewayRPC()) {
                    final RemotingCommand response = RemotingCommand.createResponseCommand(RemotingSysResponseCode.SYSTEM_BUSY,
                        "[OVERLOAD]system busy, start flow control for a while");
                    response.setOpaque(opaque);
                    ctx.writeAndFlush(response);
                }
            }
        } else {
            String error = " request type " + cmd.getCode() + " not supported";
            final RemotingCommand response =
                RemotingCommand.createResponseCommand(RemotingSysResponseCode.REQUEST_CODE_NOT_SUPPORTED, error);
            response.setOpaque(opaque);
            ctx.writeAndFlush(response);
            log.error(RemotingHelper.parseChannelRemoteAddr(ctx.channel()) + error);
        }
    }

　　所以，看起來rocketmq的broker的執行緒模型非常複雜，但在這種統一的框架管理之下，並沒有給業務程式碼帶來許多的困難。這是優秀的也是應該的。

 

5. producer執行緒模型

　　producer作為生產客戶端，更多的只是一個使用者的一個角色。所以，他不可能也不應該承受太多的複雜性。所以，理論上，其執行緒模型也會是非常簡單的。

　　事實上，它確實只是在netty框架之上，簡單的使用了其功能而已。稍微多說一點是，它也存在同步傳送和非同步傳送的差異。而這其中的差別，僅是一個回撥方法解決的事而已，並不影響其執行緒模型。

　　客戶端使用netty的樣例如下：

    @Override
    public void start() {
        this.defaultEventExecutorGroup = new DefaultEventExecutorGroup(
            nettyClientConfig.getClientWorkerThreads(),
            new ThreadFactory() {

                private AtomicInteger threadIndex = new AtomicInteger(0);

                @Override
                public Thread newThread(Runnable r) {
                    return new Thread(r, "NettyClientWorkerThread_" + this.threadIndex.incrementAndGet());
                }
            });

        Bootstrap handler = this.bootstrap.group(this.eventLoopGroupWorker).channel(NioSocketChannel.class)
            .option(ChannelOption.TCP_NODELAY, true)
            .option(ChannelOption.SO_KEEPALIVE, false)
            .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, nettyClientConfig.getConnectTimeoutMillis())
            .option(ChannelOption.SO_SNDBUF, nettyClientConfig.getClientSocketSndBufSize())
            .option(ChannelOption.SO_RCVBUF, nettyClientConfig.getClientSocketRcvBufSize())
            .handler(new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel ch) throws Exception {
                    ChannelPipeline pipeline = ch.pipeline();
                    if (nettyClientConfig.isUseTLS()) {
                        if (null != sslContext) {
                            pipeline.addFirst(defaultEventExecutorGroup, "sslHandler", sslContext.newHandler(ch.alloc()));
                            log.info("Prepend SSL handler");
                        } else {
                            log.warn("Connections are insecure as SSLContext is null!");
                        }
                    }
                    // 同樣使用 各種編解碼器處理業務即可
                    pipeline.addLast(
                        defaultEventExecutorGroup,
                        new NettyEncoder(),
                        new NettyDecoder(),
                        new IdleStateHandler(0, 0, nettyClientConfig.getClientChannelMaxIdleTimeSeconds()),
                        new NettyConnectManageHandler(),
                        // 該處理器主要用於接收server端的響應訊息
                        new NettyClientHandler());
                }
            });

        this.timer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                try {
                    NettyRemotingClient.this.scanResponseTable();
                } catch (Throwable e) {
                    log.error("scanResponseTable exception", e);
                }
            }
        }, 1000 * 3, 1000);

        if (this.channelEventListener != null) {
            this.nettyEventExecutor.start();
        }
    }
    // 以上僅僅是初始化了netty的使用環境，但到真正使用時，需要各自建立對應channel
    // 建立channel完成後會放入 channelTables 中，以便可複用
    private Channel createChannel(final String addr) throws InterruptedException {
        ChannelWrapper cw = this.channelTables.get(addr);
        if (cw != null && cw.isOK()) {
            return cw.getChannel();
        }

        if (this.lockChannelTables.tryLock(LOCK_TIMEOUT_MILLIS, TimeUnit.MILLISECONDS)) {
            try {
                boolean createNewConnection;
                cw = this.channelTables.get(addr);
                if (cw != null) {

                    if (cw.isOK()) {
                        return cw.getChannel();
                    } else if (!cw.getChannelFuture().isDone()) {
                        createNewConnection = false;
                    } else {
                        this.channelTables.remove(addr);
                        createNewConnection = true;
                    }
                } else {
                    createNewConnection = true;
                }
                // 通過呼叫 connect 方法，建立連線遠端的tcp通道
                // 放入 channelTables 中
                if (createNewConnection) {
                    ChannelFuture channelFuture = this.bootstrap.connect(RemotingHelper.string2SocketAddress(addr));
                    log.info("createChannel: begin to connect remote host[{}] asynchronously", addr);
                    cw = new ChannelWrapper(channelFuture);
                    this.channelTables.put(addr, cw);
                }
            } catch (Exception e) {
                log.error("createChannel: create channel exception", e);
            } finally {
                this.lockChannelTables.unlock();
            }
        } else {
            log.warn("createChannel: try to lock channel table, but timeout, {}ms", LOCK_TIMEOUT_MILLIS);
        }

        if (cw != null) {
            // 測試連線可用性後返回
            ChannelFuture channelFuture = cw.getChannelFuture();
            if (channelFuture.awaitUninterruptibly(this.nettyClientConfig.getConnectTimeoutMillis())) {
                if (cw.isOK()) {
                    log.info("createChannel: connect remote host[{}] success, {}", addr, channelFuture.toString());
                    return cw.getChannel();
                } else {
                    log.warn("createChannel: connect remote host[" + addr + "] failed, " + channelFuture.toString(), channelFuture.cause());
                }
            } else {
                log.warn("createChannel: connect remote host[{}] timeout {}ms, {}", addr, this.nettyClientConfig.getConnectTimeoutMillis(),
                    channelFuture.toString());
            }
        }

        return null;
    }

    // server端資料響應處理
    // org.apache.rocketmq.remoting.netty.NettyRemotingAbstract#processResponseCommand
    /**
     * Process response from remote peer to the previous issued requests.
     *
     * @param ctx channel handler context.
     * @param cmd response command instance.
     */
    public void processResponseCommand(ChannelHandlerContext ctx, RemotingCommand cmd) {
        final int opaque = cmd.getOpaque();
        // 根據 opaque 獲取請求時的上下文
        final ResponseFuture responseFuture = responseTable.get(opaque);
        if (responseFuture != null) {
            responseFuture.setResponseCommand(cmd);

            responseTable.remove(opaque);

            if (responseFuture.getInvokeCallback() != null) {
                executeInvokeCallback(responseFuture);
            } else {
                responseFuture.putResponse(cmd);
                responseFuture.release();
            }
        } else {
            log.warn("receive response, but not matched any request, " + RemotingHelper.parseChannelRemoteAddr(ctx.channel()));
            log.warn(cmd.toString());
        }
    }

　　核心傳送訊息的程式碼如下：

    // org.apache.rocketmq.client.impl.producer.DefaultMQProducerImpl#sendDefaultImpl
    private SendResult sendDefaultImpl(
        Message msg,
        final CommunicationMode communicationMode,
        final SendCallback sendCallback,
        final long timeout
    ) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
        this.makeSureStateOK();
        Validators.checkMessage(msg, this.defaultMQProducer);
        final long invokeID = random.nextLong();
        long beginTimestampFirst = System.currentTimeMillis();
        long beginTimestampPrev = beginTimestampFirst;
        long endTimestamp = beginTimestampFirst;
        TopicPublishInfo topicPublishInfo = this.tryToFindTopicPublishInfo(msg.getTopic());
        if (topicPublishInfo != null && topicPublishInfo.ok()) {
            boolean callTimeout = false;
            MessageQueue mq = null;
            Exception exception = null;
            SendResult sendResult = null;
            int timesTotal = communicationMode == CommunicationMode.SYNC ? 1 + this.defaultMQProducer.getRetryTimesWhenSendFailed() : 1;
            int times = 0;
            String[] brokersSent = new String[timesTotal];
            for (; times < timesTotal; times++) {
                String lastBrokerName = null == mq ? null : mq.getBrokerName();
                MessageQueue mqSelected = this.selectOneMessageQueue(topicPublishInfo, lastBrokerName);
                if (mqSelected != null) {
                    mq = mqSelected;
                    brokersSent[times] = mq.getBrokerName();
                    try {
                        beginTimestampPrev = System.currentTimeMillis();
                        if (times > 0) {
                            //Reset topic with namespace during resend.
                            msg.setTopic(this.defaultMQProducer.withNamespace(msg.getTopic()));
                        }
                        long costTime = beginTimestampPrev - beginTimestampFirst;
                        if (timeout < costTime) {
                            callTimeout = true;
                            break;
                        }

                        sendResult = this.sendKernelImpl(msg, mq, communicationMode, sendCallback, topicPublishInfo, timeout - costTime);
                        endTimestamp = System.currentTimeMillis();
                        this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, false);
                        switch (communicationMode) {
                            case ASYNC:
                                return null;
                            case ONEWAY:
                                return null;
                            case SYNC:
                                if (sendResult.getSendStatus() != SendStatus.SEND_OK) {
                                    if (this.defaultMQProducer.isRetryAnotherBrokerWhenNotStoreOK()) {
                                        continue;
                                    }
                                }

                                return sendResult;
                            default:
                                break;
                        }
                    } catch (RemotingException e) {
                        endTimestamp = System.currentTimeMillis();
                        this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, true);
                        log.warn(String.format("sendKernelImpl exception, resend at once, InvokeID: %s, RT: %sms, Broker: %s", invokeID, endTimestamp - beginTimestampPrev, mq), e);
                        log.warn(msg.toString());
                        exception = e;
                        continue;
                    } catch (MQClientException e) {
                        endTimestamp = System.currentTimeMillis();
                        this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, true);
                        log.warn(String.format("sendKernelImpl exception, resend at once, InvokeID: %s, RT: %sms, Broker: %s", invokeID, endTimestamp - beginTimestampPrev, mq), e);
                        log.warn(msg.toString());
                        exception = e;
                        continue;
                    } catch (MQBrokerException e) {
                        endTimestamp = System.currentTimeMillis();
                        this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, true);
                        log.warn(String.format("sendKernelImpl exception, resend at once, InvokeID: %s, RT: %sms, Broker: %s", invokeID, endTimestamp - beginTimestampPrev, mq), e);
                        log.warn(msg.toString());
                        exception = e;
                        switch (e.getResponseCode()) {
                            case ResponseCode.TOPIC_NOT_EXIST:
                            case ResponseCode.SERVICE_NOT_AVAILABLE:
                            case ResponseCode.SYSTEM_ERROR:
                            case ResponseCode.NO_PERMISSION:
                            case ResponseCode.NO_BUYER_ID:
                            case ResponseCode.NOT_IN_CURRENT_UNIT:
                                continue;
                            default:
                                if (sendResult != null) {
                                    return sendResult;
                                }

                                throw e;
                        }
                    } catch (InterruptedException e) {
                        endTimestamp = System.currentTimeMillis();
                        this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, false);
                        log.warn(String.format("sendKernelImpl exception, throw exception, InvokeID: %s, RT: %sms, Broker: %s", invokeID, endTimestamp - beginTimestampPrev, mq), e);
                        log.warn(msg.toString());

                        log.warn("sendKernelImpl exception", e);
                        log.warn(msg.toString());
                        throw e;
                    }
                } else {
                    break;
                }
            }

            if (sendResult != null) {
                return sendResult;
            }

            String info = String.format("Send [%d] times, still failed, cost [%d]ms, Topic: %s, BrokersSent: %s",
                times,
                System.currentTimeMillis() - beginTimestampFirst,
                msg.getTopic(),
                Arrays.toString(brokersSent));

            info += FAQUrl.suggestTodo(FAQUrl.SEND_MSG_FAILED);

            MQClientException mqClientException = new MQClientException(info, exception);
            if (callTimeout) {
                throw new RemotingTooMuchRequestException("sendDefaultImpl call timeout");
            }

            if (exception instanceof MQBrokerException) {
                mqClientException.setResponseCode(((MQBrokerException) exception).getResponseCode());
            } else if (exception instanceof RemotingConnectException) {
                mqClientException.setResponseCode(ClientErrorCode.CONNECT_BROKER_EXCEPTION);
            } else if (exception instanceof RemotingTimeoutException) {
                mqClientException.setResponseCode(ClientErrorCode.ACCESS_BROKER_TIMEOUT);
            } else if (exception instanceof MQClientException) {
                mqClientException.setResponseCode(ClientErrorCode.BROKER_NOT_EXIST_EXCEPTION);
            }

            throw mqClientException;
        }

        validateNameServerSetting();

        throw new MQClientException("No route info of this topic: " + msg.getTopic() + FAQUrl.suggestTodo(FAQUrl.NO_TOPIC_ROUTE_INFO),
            null).setResponseCode(ClientErrorCode.NOT_FOUND_TOPIC_EXCEPTION);
    }
    // org.apache.rocketmq.client.impl.producer.DefaultMQProducerImpl#sendKernelImpl
    private SendResult sendKernelImpl(final Message msg,
        final MessageQueue mq,
        final CommunicationMode communicationMode,
        final SendCallback sendCallback,
        final TopicPublishInfo topicPublishInfo,
        final long timeout) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
        long beginStartTime = System.currentTimeMillis();
        String brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(mq.getBrokerName());
        if (null == brokerAddr) {
            tryToFindTopicPublishInfo(mq.getTopic());
            brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(mq.getBrokerName());
        }

        SendMessageContext context = null;
        if (brokerAddr != null) {
            brokerAddr = MixAll.brokerVIPChannel(this.defaultMQProducer.isSendMessageWithVIPChannel(), brokerAddr);

            byte[] prevBody = msg.getBody();
            try {
                //for MessageBatch,ID has been set in the generating process
                if (!(msg instanceof MessageBatch)) {
                    MessageClientIDSetter.setUniqID(msg);
                }

                boolean topicWithNamespace = false;
                if (null != this.mQClientFactory.getClientConfig().getNamespace()) {
                    msg.setInstanceId(this.mQClientFactory.getClientConfig().getNamespace());
                    topicWithNamespace = true;
                }

                int sysFlag = 0;
                boolean msgBodyCompressed = false;
                if (this.tryToCompressMessage(msg)) {
                    sysFlag |= MessageSysFlag.COMPRESSED_FLAG;
                    msgBodyCompressed = true;
                }

                final String tranMsg = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED);
                if (tranMsg != null && Boolean.parseBoolean(tranMsg)) {
                    sysFlag |= MessageSysFlag.TRANSACTION_PREPARED_TYPE;
                }

                if (hasCheckForbiddenHook()) {
                    CheckForbiddenContext checkForbiddenContext = new CheckForbiddenContext();
                    checkForbiddenContext.setNameSrvAddr(this.defaultMQProducer.getNamesrvAddr());
                    checkForbiddenContext.setGroup(this.defaultMQProducer.getProducerGroup());
                    checkForbiddenContext.setCommunicationMode(communicationMode);
                    checkForbiddenContext.setBrokerAddr(brokerAddr);
                    checkForbiddenContext.setMessage(msg);
                    checkForbiddenContext.setMq(mq);
                    checkForbiddenContext.setUnitMode(this.isUnitMode());
                    this.executeCheckForbiddenHook(checkForbiddenContext);
                }

                if (this.hasSendMessageHook()) {
                    context = new SendMessageContext();
                    context.setProducer(this);
                    context.setProducerGroup(this.defaultMQProducer.getProducerGroup());
                    context.setCommunicationMode(communicationMode);
                    context.setBornHost(this.defaultMQProducer.getClientIP());
                    context.setBrokerAddr(brokerAddr);
                    context.setMessage(msg);
                    context.setMq(mq);
                    context.setNamespace(this.defaultMQProducer.getNamespace());
                    String isTrans = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED);
                    if (isTrans != null && isTrans.equals("true")) {
                        context.setMsgType(MessageType.Trans_Msg_Half);
                    }

                    if (msg.getProperty("__STARTDELIVERTIME") != null || msg.getProperty(MessageConst.PROPERTY_DELAY_TIME_LEVEL) != null) {
                        context.setMsgType(MessageType.Delay_Msg);
                    }
                    this.executeSendMessageHookBefore(context);
                }

                SendMessageRequestHeader requestHeader = new SendMessageRequestHeader();
                requestHeader.setProducerGroup(this.defaultMQProducer.getProducerGroup());
                requestHeader.setTopic(msg.getTopic());
                requestHeader.setDefaultTopic(this.defaultMQProducer.getCreateTopicKey());
                requestHeader.setDefaultTopicQueueNums(this.defaultMQProducer.getDefaultTopicQueueNums());
                requestHeader.setQueueId(mq.getQueueId());
                requestHeader.setSysFlag(sysFlag);
                requestHeader.setBornTimestamp(System.currentTimeMillis());
                requestHeader.setFlag(msg.getFlag());
                requestHeader.setProperties(MessageDecoder.messageProperties2String(msg.getProperties()));
                requestHeader.setReconsumeTimes(0);
                requestHeader.setUnitMode(this.isUnitMode());
                requestHeader.setBatch(msg instanceof MessageBatch);
                if (requestHeader.getTopic().startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                    String reconsumeTimes = MessageAccessor.getReconsumeTime(msg);
                    if (reconsumeTimes != null) {
                        requestHeader.setReconsumeTimes(Integer.valueOf(reconsumeTimes));
                        MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_RECONSUME_TIME);
                    }

                    String maxReconsumeTimes = MessageAccessor.getMaxReconsumeTimes(msg);
                    if (maxReconsumeTimes != null) {
                        requestHeader.setMaxReconsumeTimes(Integer.valueOf(maxReconsumeTimes));
                        MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_MAX_RECONSUME_TIMES);
                    }
                }

                SendResult sendResult = null;
                switch (communicationMode) {
                    case ASYNC:
                        Message tmpMessage = msg;
                        boolean messageCloned = false;
                        if (msgBodyCompressed) {
                            //If msg body was compressed, msgbody should be reset using prevBody.
                            //Clone new message using commpressed message body and recover origin massage.
                            //Fix bug:https://github.com/apache/rocketmq-externals/issues/66
                            tmpMessage = MessageAccessor.cloneMessage(msg);
                            messageCloned = true;
                            msg.setBody(prevBody);
                        }

                        if (topicWithNamespace) {
                            if (!messageCloned) {
                                tmpMessage = MessageAccessor.cloneMessage(msg);
                                messageCloned = true;
                            }
                            msg.setTopic(NamespaceUtil.withoutNamespace(msg.getTopic(), this.defaultMQProducer.getNamespace()));
                        }

                        long costTimeAsync = System.currentTimeMillis() - beginStartTime;
                        if (timeout < costTimeAsync) {
                            throw new RemotingTooMuchRequestException("sendKernelImpl call timeout");
                        }
                        sendResult = this.mQClientFactory.getMQClientAPIImpl().sendMessage(
                            brokerAddr,
                            mq.getBrokerName(),
                            tmpMessage,
                            requestHeader,
                            timeout - costTimeAsync,
                            communicationMode,
                            sendCallback,
                            topicPublishInfo,
                            this.mQClientFactory,
                            this.defaultMQProducer.getRetryTimesWhenSendAsyncFailed(),
                            context,
                            this);
                        break;
                    case ONEWAY:
                    case SYNC:
                        long costTimeSync = System.currentTimeMillis() - beginStartTime;
                        if (timeout < costTimeSync) {
                            throw new RemotingTooMuchRequestException("sendKernelImpl call timeout");
                        }
                        sendResult = this.mQClientFactory.getMQClientAPIImpl().sendMessage(
                            brokerAddr,
                            mq.getBrokerName(),
                            msg,
                            requestHeader,
                            timeout - costTimeSync,
                            communicationMode,
                            context,
                            this);
                        break;
                    default:
                        assert false;
                        break;
                }

                if (this.hasSendMessageHook()) {
                    context.setSendResult(sendResult);
                    this.executeSendMessageHookAfter(context);
                }

                return sendResult;
            } catch (RemotingException e) {
                if (this.hasSendMessageHook()) {
                    context.setException(e);
                    this.executeSendMessageHookAfter(context);
                }
                throw e;
            } catch (MQBrokerException e) {
                if (this.hasSendMessageHook()) {
                    context.setException(e);
                    this.executeSendMessageHookAfter(context);
                }
                throw e;
            } catch (InterruptedException e) {
                if (this.hasSendMessageHook()) {
                    context.setException(e);
                    this.executeSendMessageHookAfter(context);
                }
                throw e;
            } finally {
                msg.setBody(prevBody);
                msg.setTopic(NamespaceUtil.withoutNamespace(msg.getTopic(), this.defaultMQProducer.getNamespace()));
            }
        }

        throw new MQClientException("The broker[" + mq.getBrokerName() + "] not exist", null);
    }

    public SendResult sendMessage(
        final String addr,
        final String brokerName,
        final Message msg,
        final SendMessageRequestHeader requestHeader,
        final long timeoutMillis,
        final CommunicationMode communicationMode,
        final SendMessageContext context,
        final DefaultMQProducerImpl producer
    ) throws RemotingException, MQBrokerException, InterruptedException {
        return sendMessage(addr, brokerName, msg, requestHeader, timeoutMillis, communicationMode, null, null, null, 0, context, producer);
    }

    public SendResult sendMessage(
        final String addr,
        final String brokerName,
        final Message msg,
        final SendMessageRequestHeader requestHeader,
        final long timeoutMillis,
        final CommunicationMode communicationMode,
        final SendCallback sendCallback,
        final TopicPublishInfo topicPublishInfo,
        final MQClientInstance instance,
        final int retryTimesWhenSendFailed,
        final SendMessageContext context,
        final DefaultMQProducerImpl producer
    ) throws RemotingException, MQBrokerException, InterruptedException {
        long beginStartTime = System.currentTimeMillis();
        RemotingCommand request = null;
        String msgType = msg.getProperty(MessageConst.PROPERTY_MESSAGE_TYPE);
        boolean isReply = msgType != null && msgType.equals(MixAll.REPLY_MESSAGE_FLAG);
        if (isReply) {
            if (sendSmartMsg) {
                SendMessageRequestHeaderV2 requestHeaderV2 = SendMessageRequestHeaderV2.createSendMessageRequestHeaderV2(requestHeader);
                request = RemotingCommand.createRequestCommand(RequestCode.SEND_REPLY_MESSAGE_V2, requestHeaderV2);
            } else {
                request = RemotingCommand.createRequestCommand(RequestCode.SEND_REPLY_MESSAGE, requestHeader);
            }
        } else {
            if (sendSmartMsg || msg instanceof MessageBatch) {
                SendMessageRequestHeaderV2 requestHeaderV2 = SendMessageRequestHeaderV2.createSendMessageRequestHeaderV2(requestHeader);
                request = RemotingCommand.createRequestCommand(msg instanceof MessageBatch ? RequestCode.SEND_BATCH_MESSAGE : RequestCode.SEND_MESSAGE_V2, requestHeaderV2);
            } else {
                request = RemotingCommand.createRequestCommand(RequestCode.SEND_MESSAGE, requestHeader);
            }
        }
        request.setBody(msg.getBody());

        switch (communicationMode) {
            case ONEWAY:
                this.remotingClient.invokeOneway(addr, request, timeoutMillis);
                return null;
            case ASYNC:
                final AtomicInteger times = new AtomicInteger();
                long costTimeAsync = System.currentTimeMillis() - beginStartTime;
                if (timeoutMillis < costTimeAsync) {
                    throw new RemotingTooMuchRequestException("sendMessage call timeout");
                }
                this.sendMessageAsync(addr, brokerName, msg, timeoutMillis - costTimeAsync, request, sendCallback, topicPublishInfo, instance,
                    retryTimesWhenSendFailed, times, context, producer);
                return null;
            case SYNC:
                long costTimeSync = System.currentTimeMillis() - beginStartTime;
                if (timeoutMillis < costTimeSync) {
                    throw new RemotingTooMuchRequestException("sendMessage call timeout");
                }
                return this.sendMessageSync(addr, brokerName, msg, timeoutMillis - costTimeSync, request);
            default:
                assert false;
                break;
        }

        return null;
    }

 

　　同步傳送訊息實現如下：（直接獲取寫入結果）

    // org.apache.rocketmq.client.impl.MQClientAPIImpl#sendMessageSync
    private SendResult sendMessageSync(
        final String addr,
        final String brokerName,
        final Message msg,
        final long timeoutMillis,
        final RemotingCommand request
    ) throws RemotingException, MQBrokerException, InterruptedException {
        RemotingCommand response = this.remotingClient.invokeSync(addr, request, timeoutMillis);
        assert response != null;
        return this.processSendResponse(brokerName, msg, response);
    }

    @Override
    public RemotingCommand invokeSync(String addr, final RemotingCommand request, long timeoutMillis)
        throws InterruptedException, RemotingConnectException, RemotingSendRequestException, RemotingTimeoutException {
        long beginStartTime = System.currentTimeMillis();
        final Channel channel = this.getAndCreateChannel(addr);
        if (channel != null && channel.isActive()) {
            try {
                doBeforeRpcHooks(addr, request);
                long costTime = System.currentTimeMillis() - beginStartTime;
                if (timeoutMillis < costTime) {
                    throw new RemotingTimeoutException("invokeSync call timeout");
                }
                RemotingCommand response = this.invokeSyncImpl(channel, request, timeoutMillis - costTime);
                doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(channel), request, response);
                return response;
            } catch (RemotingSendRequestException e) {
                log.warn("invokeSync: send request exception, so close the channel[{}]", addr);
                this.closeChannel(addr, channel);
                throw e;
            } catch (RemotingTimeoutException e) {
                if (nettyClientConfig.isClientCloseSocketIfTimeout()) {
                    this.closeChannel(addr, channel);
                    log.warn("invokeSync: close socket because of timeout, {}ms, {}", timeoutMillis, addr);
                }
                log.warn("invokeSync: wait response timeout exception, the channel[{}]", addr);
                throw e;
            }
        } else {
            this.closeChannel(addr, channel);
            throw new RemotingConnectException(addr);
        }
    }
    // org.apache.rocketmq.remoting.netty.NettyRemotingAbstract#invokeSyncImpl
    public RemotingCommand invokeSyncImpl(final Channel channel, final RemotingCommand request,
        final long timeoutMillis)
        throws InterruptedException, RemotingSendRequestException, RemotingTimeoutException {
        final int opaque = request.getOpaque();

        try {
            final ResponseFuture responseFuture = new ResponseFuture(channel, opaque, timeoutMillis, null, null);
            this.responseTable.put(opaque, responseFuture);
            final SocketAddress addr = channel.remoteAddress();
            // 核心: 向channel中寫入資料，即是往遠端伺服器傳送資料
            channel.writeAndFlush(request).addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture f) throws Exception {
                    if (f.isSuccess()) {
                        responseFuture.setSendRequestOK(true);
                        return;
                    } else {
                        responseFuture.setSendRequestOK(false);
                    }
                    // 出現異常情況下，主動處理 responseTable
                    // 在 putResponse() 會觸發鎖釋放
                    responseTable.remove(opaque);
                    responseFuture.setCause(f.cause());
                    responseFuture.putResponse(null);
                    log.warn("send a request command to channel <" + addr + "> failed.");
                }
            });
            // 同步阻塞等待寫入結果響應（使用 CountDownLatch 實現）
            RemotingCommand responseCommand = responseFuture.waitResponse(timeoutMillis);
            if (null == responseCommand) {
                if (responseFuture.isSendRequestOK()) {
                    throw new RemotingTimeoutException(RemotingHelper.parseSocketAddressAddr(addr), timeoutMillis,
                        responseFuture.getCause());
                } else {
                    throw new RemotingSendRequestException(RemotingHelper.parseSocketAddressAddr(addr), responseFuture.getCause());
                }
            }

            return responseCommand;
        } finally {
            this.responseTable.remove(opaque);
        }
    }

　　非同步傳送訊息核心實現如下：（等待非同步回撥或者忽略結果）

    // org.apache.rocketmq.client.impl.MQClientAPIImpl#sendMessageAsync
    private void sendMessageAsync(
        final String addr,
        final String brokerName,
        final Message msg,
        final long timeoutMillis,
        final RemotingCommand request,
        final SendCallback sendCallback,
        final TopicPublishInfo topicPublishInfo,
        final MQClientInstance instance,
        final int retryTimesWhenSendFailed,
        final AtomicInteger times,
        final SendMessageContext context,
        final DefaultMQProducerImpl producer
    ) throws InterruptedException, RemotingException {
        final long beginStartTime = System.currentTimeMillis();
        this.remotingClient.invokeAsync(addr, request, timeoutMillis, new InvokeCallback() {
            @Override
            public void operationComplete(ResponseFuture responseFuture) {
                long cost = System.currentTimeMillis() - beginStartTime;
                RemotingCommand response = responseFuture.getResponseCommand();
                if (null == sendCallback && response != null) {

                    try {
                        SendResult sendResult = MQClientAPIImpl.this.processSendResponse(brokerName, msg, response);
                        if (context != null && sendResult != null) {
                            context.setSendResult(sendResult);
                            context.getProducer().executeSendMessageHookAfter(context);
                        }
                    } catch (Throwable e) {
                    }

                    producer.updateFaultItem(brokerName, System.currentTimeMillis() - responseFuture.getBeginTimestamp(), false);
                    return;
                }

                if (response != null) {
                    try {
                        SendResult sendResult = MQClientAPIImpl.this.processSendResponse(brokerName, msg, response);
                        assert sendResult != null;
                        if (context != null) {
                            context.setSendResult(sendResult);
                            context.getProducer().executeSendMessageHookAfter(context);
                        }

                        try {
                            sendCallback.onSuccess(sendResult);
                        } catch (Throwable e) {
                        }

                        producer.updateFaultItem(brokerName, System.currentTimeMillis() - responseFuture.getBeginTimestamp(), false);
                    } catch (Exception e) {
                        producer.updateFaultItem(brokerName, System.currentTimeMillis() - responseFuture.getBeginTimestamp(), true);
                        onExceptionImpl(brokerName, msg, timeoutMillis - cost, request, sendCallback, topicPublishInfo, instance,
                            retryTimesWhenSendFailed, times, e, context, false, producer);
                    }
                } else {
                    producer.updateFaultItem(brokerName, System.currentTimeMillis() - responseFuture.getBeginTimestamp(), true);
                    if (!responseFuture.isSendRequestOK()) {
                        MQClientException ex = new MQClientException("send request failed", responseFuture.getCause());
                        onExceptionImpl(brokerName, msg, timeoutMillis - cost, request, sendCallback, topicPublishInfo, instance,
                            retryTimesWhenSendFailed, times, ex, context, true, producer);
                    } else if (responseFuture.isTimeout()) {
                        MQClientException ex = new MQClientException("wait response timeout " + responseFuture.getTimeoutMillis() + "ms",
                            responseFuture.getCause());
                        onExceptionImpl(brokerName, msg, timeoutMillis - cost, request, sendCallback, topicPublishInfo, instance,
                            retryTimesWhenSendFailed, times, ex, context, true, producer);
                    } else {
                        MQClientException ex = new MQClientException("unknow reseaon", responseFuture.getCause());
                        onExceptionImpl(brokerName, msg, timeoutMillis - cost, request, sendCallback, topicPublishInfo, instance,
                            retryTimesWhenSendFailed, times, ex, context, true, producer);
                    }
                }
            }
        });
    }
    // org.apache.rocketmq.remoting.netty.NettyRemotingClient#invokeAsync
    @Override
    public void invokeAsync(String addr, RemotingCommand request, long timeoutMillis, InvokeCallback invokeCallback)
        throws InterruptedException, RemotingConnectException, RemotingTooMuchRequestException, RemotingTimeoutException,
        RemotingSendRequestException {
        long beginStartTime = System.currentTimeMillis();
        final Channel channel = this.getAndCreateChannel(addr);
        if (channel != null && channel.isActive()) {
            try {
                doBeforeRpcHooks(addr, request);
                long costTime = System.currentTimeMillis() - beginStartTime;
                if (timeoutMillis < costTime) {
                    throw new RemotingTooMuchRequestException("invokeAsync call timeout");
                }
                this.invokeAsyncImpl(channel, request, timeoutMillis - costTime, invokeCallback);
            } catch (RemotingSendRequestException e) {
                log.warn("invokeAsync: send request exception, so close the channel[{}]", addr);
                this.closeChannel(addr, channel);
                throw e;
            }
        } else {
            this.closeChannel(addr, channel);
            throw new RemotingConnectException(addr);
        }
    }
    // org.apache.rocketmq.remoting.netty.NettyRemotingAbstract#invokeAsyncImpl
    public void invokeAsyncImpl(final Channel channel, final RemotingCommand request, final long timeoutMillis,
        final InvokeCallback invokeCallback)
        throws InterruptedException, RemotingTooMuchRequestException, RemotingTimeoutException, RemotingSendRequestException {
        long beginStartTime = System.currentTimeMillis();
        final int opaque = request.getOpaque();
        // 使使用者可控非同步請求數，預設為無限 65535
        boolean acquired = this.semaphoreAsync.tryAcquire(timeoutMillis, TimeUnit.MILLISECONDS);
        if (acquired) {
            final SemaphoreReleaseOnlyOnce once = new SemaphoreReleaseOnlyOnce(this.semaphoreAsync);
            long costTime = System.currentTimeMillis() - beginStartTime;
            if (timeoutMillis < costTime) {
                once.release();
                throw new RemotingTimeoutException("invokeAsyncImpl call timeout");
            }

            final ResponseFuture responseFuture = new ResponseFuture(channel, opaque, timeoutMillis - costTime, invokeCallback, once);
            this.responseTable.put(opaque, responseFuture);
            try {
                // 向channel中寫入資料後即返回，從而達到非同步執行的效果
                channel.writeAndFlush(request).addListener(new ChannelFutureListener() {
                    @Override
                    public void operationComplete(ChannelFuture f) throws Exception {
                        if (f.isSuccess()) {
                            responseFuture.setSendRequestOK(true);
                            return;
                        }
                        requestFail(opaque);
                        log.warn("send a request command to channel <{}> failed.", RemotingHelper.parseChannelRemoteAddr(channel));
                    }
                });
            } catch (Exception e) {
                responseFuture.release();
                log.warn("send a request command to channel <" + RemotingHelper.parseChannelRemoteAddr(channel) + "> Exception", e);
                throw new RemotingSendRequestException(RemotingHelper.parseChannelRemoteAddr(channel), e);
            }
        } else {
            if (timeoutMillis <= 0) {
                throw new RemotingTooMuchRequestException("invokeAsyncImpl invoke too fast");
            } else {
                String info =
                    String.format("invokeAsyncImpl tryAcquire semaphore timeout, %dms, waiting thread nums: %d semaphoreAsyncValue: %d",
                        timeoutMillis,
                        this.semaphoreAsync.getQueueLength(),
                        this.semaphoreAsync.availablePermits()
                    );
                log.warn(info);
                throw new RemotingTimeoutException(info);
            }
        }
    }

　　本身producer的執行緒模型是非常簡單的，只不過我們順便了解了下producer如何實現同步、非同步傳送訊息的過程而已。

 

6. consumer的執行緒模型

　　consumer與producer同屬於客戶端，理論上其執行緒模型都會非常簡單，且應基本一致。但是，因各自業務不同，特性自然不同。我們就其中一個探討下：producer傳送訊息時，通過重試的方式，向broker傳送訊息，它的訊息可能落到其中任意一臺broker中。那麼，consumer在做訊息消費時，又當如何呢？有兩個點我們需要得到保證：消費的安全性和完整性。換個名詞說就是：At Least Once/At Most Once/Exactly Once。另外，rocketmq的消費模型中，提供了所謂的推送模型和拉取模型，所以又會存在同步和非同步的問題了可能。（實際上都拉取模型的不同偽裝而已）

　　下面我們可以簡單看看其訂閱消費基本過程：

    // 1. 訂閱配置
    // org.apache.rocketmq.client.impl.consumer.DefaultMQPushConsumerImpl#subscribe(java.lang.String, java.lang.String)
    public void subscribe(String topic, String subExpression) throws MQClientException {
        try {
            SubscriptionData subscriptionData = FilterAPI.buildSubscriptionData(this.defaultMQPushConsumer.getConsumerGroup(),
                topic, subExpression);
            this.rebalanceImpl.getSubscriptionInner().put(topic, subscriptionData);
            if (this.mQClientFactory != null) {
                this.mQClientFactory.sendHeartbeatToAllBrokerWithLock();
            }
        } catch (Exception e) {
            throw new MQClientException("subscription exception", e);
        }
    }
    // 2. 開啟消費執行緒
    // org.apache.rocketmq.client.consumer.DefaultMQPushConsumer#start
    /**
     * This method gets internal infrastructure readily to serve. Instances must call this method after configuration.
     *
     * @throws MQClientException if there is any client error.
     */
    @Override
    public void start() throws MQClientException {
        setConsumerGroup(NamespaceUtil.wrapNamespace(this.getNamespace(), this.consumerGroup));
        this.defaultMQPushConsumerImpl.start();
        if (null != traceDispatcher) {
            try {
                traceDispatcher.start(this.getNamesrvAddr(), this.getAccessChannel());
            } catch (MQClientException e) {
                log.warn("trace dispatcher start failed ", e);
            }
        }
    }
    // org.apache.rocketmq.client.impl.consumer.DefaultMQPushConsumerImpl#start
    public synchronized void start() throws MQClientException {
        switch (this.serviceState) {
            // 一開始就是這個狀態
            case CREATE_JUST:
                log.info("the consumer [{}] start beginning. messageModel={}, isUnitMode={}", this.defaultMQPushConsumer.getConsumerGroup(),
                    this.defaultMQPushConsumer.getMessageModel(), this.defaultMQPushConsumer.isUnitMode());
                this.serviceState = ServiceState.START_FAILED;

                this.checkConfig();

                this.copySubscription();

                if (this.defaultMQPushConsumer.getMessageModel() == MessageModel.CLUSTERING) {
                    this.defaultMQPushConsumer.changeInstanceNameToPID();
                }

                this.mQClientFactory = MQClientManager.getInstance().getOrCreateMQClientInstance(this.defaultMQPushConsumer, this.rpcHook);

                this.rebalanceImpl.setConsumerGroup(this.defaultMQPushConsumer.getConsumerGroup());
                this.rebalanceImpl.setMessageModel(this.defaultMQPushConsumer.getMessageModel());
                this.rebalanceImpl.setAllocateMessageQueueStrategy(this.defaultMQPushConsumer.getAllocateMessageQueueStrategy());
                this.rebalanceImpl.setmQClientFactory(this.mQClientFactory);

                this.pullAPIWrapper = new PullAPIWrapper(
                    mQClientFactory,
                    this.defaultMQPushConsumer.getConsumerGroup(), isUnitMode());
                this.pullAPIWrapper.registerFilterMessageHook(filterMessageHookList);

                if (this.defaultMQPushConsumer.getOffsetStore() != null) {
                    this.offsetStore = this.defaultMQPushConsumer.getOffsetStore();
                } else {
                    switch (this.defaultMQPushConsumer.getMessageModel()) {
                        case BROADCASTING:
                            this.offsetStore = new LocalFileOffsetStore(this.mQClientFactory, this.defaultMQPushConsumer.getConsumerGroup());
                            break;
                        case CLUSTERING:
                            this.offsetStore = new RemoteBrokerOffsetStore(this.mQClientFactory, this.defaultMQPushConsumer.getConsumerGroup());
                            break;
                        default:
                            break;
                    }
                    this.defaultMQPushConsumer.setOffsetStore(this.offsetStore);
                }
                this.offsetStore.load();

                if (this.getMessageListenerInner() instanceof MessageListenerOrderly) {
                    this.consumeOrderly = true;
                    this.consumeMessageService =
                        new ConsumeMessageOrderlyService(this, (MessageListenerOrderly) this.getMessageListenerInner());
                } else if (this.getMessageListenerInner() instanceof MessageListenerConcurrently) {
                    this.consumeOrderly = false;
                    this.consumeMessageService =
                        new ConsumeMessageConcurrentlyService(this, (MessageListenerConcurrently) this.getMessageListenerInner());
                }
                // 開啟消費後臺執行緒服務
                this.consumeMessageService.start();

                boolean registerOK = mQClientFactory.registerConsumer(this.defaultMQPushConsumer.getConsumerGroup(), this);
                if (!registerOK) {
                    this.serviceState = ServiceState.CREATE_JUST;
                    this.consumeMessageService.shutdown(defaultMQPushConsumer.getAwaitTerminationMillisWhenShutdown());
                    throw new MQClientException("The consumer group[" + this.defaultMQPushConsumer.getConsumerGroup()
                        + "] has been created before, specify another name please." + FAQUrl.suggestTodo(FAQUrl.GROUP_NAME_DUPLICATE_URL),
                        null);
                }
                // 開啟消費執行緒
                mQClientFactory.start();
                log.info("the consumer [{}] start OK.", this.defaultMQPushConsumer.getConsumerGroup());
                this.serviceState = ServiceState.RUNNING;
                break;
            case RUNNING:
            case START_FAILED:
            case SHUTDOWN_ALREADY:
                throw new MQClientException("The PushConsumer service state not OK, maybe started once, "
                    + this.serviceState
                    + FAQUrl.suggestTodo(FAQUrl.CLIENT_SERVICE_NOT_OK),
                    null);
            default:
                break;
        }

        this.updateTopicSubscribeInfoWhenSubscriptionChanged();
        this.mQClientFactory.checkClientInBroker();
        this.mQClientFactory.sendHeartbeatToAllBrokerWithLock();
        this.mQClientFactory.rebalanceImmediately();
    }
    // org.apache.rocketmq.client.impl.consumer.ConsumeMessageConcurrentlyService#ConsumeMessageConcurrentlyService
    public ConsumeMessageConcurrentlyService(DefaultMQPushConsumerImpl defaultMQPushConsumerImpl,
        MessageListenerConcurrently messageListener) {
        this.defaultMQPushConsumerImpl = defaultMQPushConsumerImpl;
        this.messageListener = messageListener;

        this.defaultMQPushConsumer = this.defaultMQPushConsumerImpl.getDefaultMQPushConsumer();
        this.consumerGroup = this.defaultMQPushConsumer.getConsumerGroup();
        this.consumeRequestQueue = new LinkedBlockingQueue<Runnable>();

        this.consumeExecutor = new ThreadPoolExecutor(
            this.defaultMQPushConsumer.getConsumeThreadMin(),
            this.defaultMQPushConsumer.getConsumeThreadMax(),
            1000 * 60,
            TimeUnit.MILLISECONDS,
            this.consumeRequestQueue,
            new ThreadFactoryImpl("ConsumeMessageThread_"));

        this.scheduledExecutorService = Executors.newSingleThreadScheduledExecutor(new ThreadFactoryImpl("ConsumeMessageScheduledThread_"));
        this.cleanExpireMsgExecutors = Executors.newSingleThreadScheduledExecutor(new ThreadFactoryImpl("CleanExpireMsgScheduledThread_"));
    }

    public void start() {
        this.cleanExpireMsgExecutors.scheduleAtFixedRate(new Runnable() {

            @Override
            public void run() {
                cleanExpireMsg();
            }

        }, this.defaultMQPushConsumer.getConsumeTimeout(), this.defaultMQPushConsumer.getConsumeTimeout(), TimeUnit.MINUTES);
    }
    // org.apache.rocketmq.client.impl.factory.MQClientInstance#start
    public void start() throws MQClientException {

        synchronized (this) {
            switch (this.serviceState) {
                case CREATE_JUST:
                    this.serviceState = ServiceState.START_FAILED;
                    // If not specified,looking address from name server
                    if (null == this.clientConfig.getNamesrvAddr()) {
                        this.mQClientAPIImpl.fetchNameServerAddr();
                    }
                    // Start request-response channel
                    this.mQClientAPIImpl.start();
                    // Start various schedule tasks
                    this.startScheduledTask();
                    // Start pull service
                    this.pullMessageService.start();
                    // Start rebalance service
                    this.rebalanceService.start();
                    // Start push service
                    this.defaultMQProducer.getDefaultMQProducerImpl().start(false);
                    log.info("the client factory [{}] start OK", this.clientId);
                    this.serviceState = ServiceState.RUNNING;
                    break;
                case START_FAILED:
                    throw new MQClientException("The Factory object[" + this.getClientId() + "] has been created before, and failed.", null);
                default:
                    break;
            }
        }
    }

　　更多後臺執行緒執行情況請展開閱讀：

    // org.apache.rocketmq.client.impl.consumer.PullMessageService#run
    @Override
    public void run() {
        log.info(this.getServiceName() + " service started");

        while (!this.isStopped()) {
            try {
                PullRequest pullRequest = this.pullRequestQueue.take();
                this.pullMessage(pullRequest);
            } catch (InterruptedException ignored) {
            } catch (Exception e) {
                log.error("Pull Message Service Run Method exception", e);
            }
        }

        log.info(this.getServiceName() + " service end");
    }
    // org.apache.rocketmq.client.impl.consumer.DefaultMQPushConsumerImpl#executePullRequestImmediately
    public void executePullRequestImmediately(final PullRequest pullRequest) {
        this.mQClientFactory.getPullMessageService().executePullRequestImmediately(pullRequest);
    }
    // 從serveru端拉取指定messageQueue的訊息
    public void pullMessage(final PullRequest pullRequest) {
        final ProcessQueue processQueue = pullRequest.getProcessQueue();
        if (processQueue.isDropped()) {
            log.info("the pull request[{}] is dropped.", pullRequest.toString());
            return;
        }

        pullRequest.getProcessQueue().setLastPullTimestamp(System.currentTimeMillis());

        try {
            this.makeSureStateOK();
        } catch (MQClientException e) {
            log.warn("pullMessage exception, consumer state not ok", e);
            this.executePullRequestLater(pullRequest, pullTimeDelayMillsWhenException);
            return;
        }

        if (this.isPause()) {
            log.warn("consumer was paused, execute pull request later. instanceName={}, group={}", this.defaultMQPushConsumer.getInstanceName(), this.defaultMQPushConsumer.getConsumerGroup());
            this.executePullRequestLater(pullRequest, PULL_TIME_DELAY_MILLS_WHEN_SUSPEND);
            return;
        }

        long cachedMessageCount = processQueue.getMsgCount().get();
        long cachedMessageSizeInMiB = processQueue.getMsgSize().get() / (1024 * 1024);

        if (cachedMessageCount > this.defaultMQPushConsumer.getPullThresholdForQueue()) {
            this.executePullRequestLater(pullRequest, PULL_TIME_DELAY_MILLS_WHEN_FLOW_CONTROL);
            if ((queueFlowControlTimes++ % 1000) == 0) {
                log.warn(
                    "the cached message count exceeds the threshold {}, so do flow control, minOffset={}, maxOffset={}, count={}, size={} MiB, pullRequest={}, flowControlTimes={}",
                    this.defaultMQPushConsumer.getPullThresholdForQueue(), processQueue.getMsgTreeMap().firstKey(), processQueue.getMsgTreeMap().lastKey(), cachedMessageCount, cachedMessageSizeInMiB, pullRequest, queueFlowControlTimes);
            }
            return;
        }

        if (cachedMessageSizeInMiB > this.defaultMQPushConsumer.getPullThresholdSizeForQueue()) {
            this.executePullRequestLater(pullRequest, PULL_TIME_DELAY_MILLS_WHEN_FLOW_CONTROL);
            if ((queueFlowControlTimes++ % 1000) == 0) {
                log.warn(
                    "the cached message size exceeds the threshold {} MiB, so do flow control, minOffset={}, maxOffset={}, count={}, size={} MiB, pullRequest={}, flowControlTimes={}",
                    this.defaultMQPushConsumer.getPullThresholdSizeForQueue(), processQueue.getMsgTreeMap().firstKey(), processQueue.getMsgTreeMap().lastKey(), cachedMessageCount, cachedMessageSizeInMiB, pullRequest, queueFlowControlTimes);
            }
            return;
        }

        if (!this.consumeOrderly) {
            if (processQueue.getMaxSpan() > this.defaultMQPushConsumer.getConsumeConcurrentlyMaxSpan()) {
                this.executePullRequestLater(pullRequest, PULL_TIME_DELAY_MILLS_WHEN_FLOW_CONTROL);
                if ((queueMaxSpanFlowControlTimes++ % 1000) == 0) {
                    log.warn(
                        "the queue's messages, span too long, so do flow control, minOffset={}, maxOffset={}, maxSpan={}, pullRequest={}, flowControlTimes={}",
                        processQueue.getMsgTreeMap().firstKey(), processQueue.getMsgTreeMap().lastKey(), processQueue.getMaxSpan(),
                        pullRequest, queueMaxSpanFlowControlTimes);
                }
                return;
            }
        } else {
            if (processQueue.isLocked()) {
                if (!pullRequest.isLockedFirst()) {
                    final long offset = this.rebalanceImpl.computePullFromWhere(pullRequest.getMessageQueue());
                    boolean brokerBusy = offset < pullRequest.getNextOffset();
                    log.info("the first time to pull message, so fix offset from broker. pullRequest: {} NewOffset: {} brokerBusy: {}",
                        pullRequest, offset, brokerBusy);
                    if (brokerBusy) {
                        log.info("[NOTIFYME]the first time to pull message, but pull request offset larger than broker consume offset. pullRequest: {} NewOffset: {}",
                            pullRequest, offset);
                    }

                    pullRequest.setLockedFirst(true);
                    pullRequest.setNextOffset(offset);
                }
            } else {
                this.executePullRequestLater(pullRequest, pullTimeDelayMillsWhenException);
                log.info("pull message later because not locked in broker, {}", pullRequest);
                return;
            }
        }

        final SubscriptionData subscriptionData = this.rebalanceImpl.getSubscriptionInner().get(pullRequest.getMessageQueue().getTopic());
        if (null == subscriptionData) {
            this.executePullRequestLater(pullRequest, pullTimeDelayMillsWhenException);
            log.warn("find the consumer's subscription failed, {}", pullRequest);
            return;
        }

        final long beginTimestamp = System.currentTimeMillis();

        PullCallback pullCallback = new PullCallback() {
            @Override
            public void onSuccess(PullResult pullResult) {
                if (pullResult != null) {
                    pullResult = DefaultMQPushConsumerImpl.this.pullAPIWrapper.processPullResult(pullRequest.getMessageQueue(), pullResult,
                        subscriptionData);

                    switch (pullResult.getPullStatus()) {
                        case FOUND:
                            long prevRequestOffset = pullRequest.getNextOffset();
                            pullRequest.setNextOffset(pullResult.getNextBeginOffset());
                            long pullRT = System.currentTimeMillis() - beginTimestamp;
                            DefaultMQPushConsumerImpl.this.getConsumerStatsManager().incPullRT(pullRequest.getConsumerGroup(),
                                pullRequest.getMessageQueue().getTopic(), pullRT);

                            long firstMsgOffset = Long.MAX_VALUE;
                            if (pullResult.getMsgFoundList() == null || pullResult.getMsgFoundList().isEmpty()) {
                                DefaultMQPushConsumerImpl.this.executePullRequestImmediately(pullRequest);
                            } else {
                                firstMsgOffset = pullResult.getMsgFoundList().get(0).getQueueOffset();

                                DefaultMQPushConsumerImpl.this.getConsumerStatsManager().incPullTPS(pullRequest.getConsumerGroup(),
                                    pullRequest.getMessageQueue().getTopic(), pullResult.getMsgFoundList().size());

                                boolean dispatchToConsume = processQueue.putMessage(pullResult.getMsgFoundList());
                                DefaultMQPushConsumerImpl.this.consumeMessageService.submitConsumeRequest(
                                    pullResult.getMsgFoundList(),
                                    processQueue,
                                    pullRequest.getMessageQueue(),
                                    dispatchToConsume);

                                if (DefaultMQPushConsumerImpl.this.defaultMQPushConsumer.getPullInterval() > 0) {
                                    DefaultMQPushConsumerImpl.this.executePullRequestLater(pullRequest,
                                        DefaultMQPushConsumerImpl.this.defaultMQPushConsumer.getPullInterval());
                                } else {
                                    DefaultMQPushConsumerImpl.this.executePullRequestImmediately(pullRequest);
                                }
                            }

                            if (pullResult.getNextBeginOffset() < prevRequestOffset
                                || firstMsgOffset < prevRequestOffset) {
                                log.warn(
                                    "[BUG] pull message result maybe data wrong, nextBeginOffset: {} firstMsgOffset: {} prevRequestOffset: {}",
                                    pullResult.getNextBeginOffset(),
                                    firstMsgOffset,
                                    prevRequestOffset);
                            }

                            break;
                        case NO_NEW_MSG:
                            pullRequest.setNextOffset(pullResult.getNextBeginOffset());

                            DefaultMQPushConsumerImpl.this.correctTagsOffset(pullRequest);

                            DefaultMQPushConsumerImpl.this.executePullRequestImmediately(pullRequest);
                            break;
                        case NO_MATCHED_MSG:
                            pullRequest.setNextOffset(pullResult.getNextBeginOffset());

                            DefaultMQPushConsumerImpl.this.correctTagsOffset(pullRequest);

                            DefaultMQPushConsumerImpl.this.executePullRequestImmediately(pullRequest);
                            break;
                        case OFFSET_ILLEGAL:
                            log.warn("the pull request offset illegal, {} {}",
                                pullRequest.toString(), pullResult.toString());
                            pullRequest.setNextOffset(pullResult.getNextBeginOffset());

                            pullRequest.getProcessQueue().setDropped(true);
                            DefaultMQPushConsumerImpl.this.executeTaskLater(new Runnable() {

                                @Override
                                public void run() {
                                    try {
                                        DefaultMQPushConsumerImpl.this.offsetStore.updateOffset(pullRequest.getMessageQueue(),
                                            pullRequest.getNextOffset(), false);

                                        DefaultMQPushConsumerImpl.this.offsetStore.persist(pullRequest.getMessageQueue());

                                        DefaultMQPushConsumerImpl.this.rebalanceImpl.removeProcessQueue(pullRequest.getMessageQueue());

                                        log.warn("fix the pull request offset, {}", pullRequest);
                                    } catch (Throwable e) {
                                        log.error("executeTaskLater Exception", e);
                                    }
                                }
                            }, 10000);
                            break;
                        default:
                            break;
                    }
                }
            }

            @Override
            public void onException(Throwable e) {
                if (!pullRequest.getMessageQueue().getTopic().startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                    log.warn("execute the pull request exception", e);
                }

                DefaultMQPushConsumerImpl.this.executePullRequestLater(pullRequest, pullTimeDelayMillsWhenException);
            }
        };

        boolean commitOffsetEnable = false;
        long commitOffsetValue = 0L;
        if (MessageModel.CLUSTERING == this.defaultMQPushConsumer.getMessageModel()) {
            commitOffsetValue = this.offsetStore.readOffset(pullRequest.getMessageQueue(), ReadOffsetType.READ_FROM_MEMORY);
            if (commitOffsetValue > 0) {
                commitOffsetEnable = true;
            }
        }

        String subExpression = null;
        boolean classFilter = false;
        SubscriptionData sd = this.rebalanceImpl.getSubscriptionInner().get(pullRequest.getMessageQueue().getTopic());
        if (sd != null) {
            if (this.defaultMQPushConsumer.isPostSubscriptionWhenPull() && !sd.isClassFilterMode()) {
                subExpression = sd.getSubString();
            }

            classFilter = sd.isClassFilterMode();
        }

        int sysFlag = PullSysFlag.buildSysFlag(
            commitOffsetEnable, // commitOffset
            true, // suspend
            subExpression != null, // subscription
            classFilter // class filter
        );
        try {
            this.pullAPIWrapper.pullKernelImpl(
                pullRequest.getMessageQueue(),
                subExpression,
                subscriptionData.getExpressionType(),
                subscriptionData.getSubVersion(),
                pullRequest.getNextOffset(),
                this.defaultMQPushConsumer.getPullBatchSize(),
                sysFlag,
                commitOffsetValue,
                BROKER_SUSPEND_MAX_TIME_MILLIS,
                CONSUMER_TIMEOUT_MILLIS_WHEN_SUSPEND,
                CommunicationMode.ASYNC,
                pullCallback
            );
        } catch (Exception e) {
            log.error("pullKernelImpl exception", e);
            this.executePullRequestLater(pullRequest, pullTimeDelayMillsWhenException);
        }
    }
    
    public PullResult pullKernelImpl(
        final MessageQueue mq,
        final String subExpression,
        final String expressionType,
        final long subVersion,
        final long offset,
        final int maxNums,
        final int sysFlag,
        final long commitOffset,
        final long brokerSuspendMaxTimeMillis,
        final long timeoutMillis,
        final CommunicationMode communicationMode,
        final PullCallback pullCallback
    ) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
        FindBrokerResult findBrokerResult =
            this.mQClientFactory.findBrokerAddressInSubscribe(mq.getBrokerName(),
                this.recalculatePullFromWhichNode(mq), false);
        if (null == findBrokerResult) {
            this.mQClientFactory.updateTopicRouteInfoFromNameServer(mq.getTopic());
            findBrokerResult =
                this.mQClientFactory.findBrokerAddressInSubscribe(mq.getBrokerName(),
                    this.recalculatePullFromWhichNode(mq), false);
        }

        if (findBrokerResult != null) {
            {
                // check version
                if (!ExpressionType.isTagType(expressionType)
                    && findBrokerResult.getBrokerVersion() < MQVersion.Version.V4_1_0_SNAPSHOT.ordinal()) {
                    throw new MQClientException("The broker[" + mq.getBrokerName() + ", "
                        + findBrokerResult.getBrokerVersion() + "] does not upgrade to support for filter message by " + expressionType, null);
                }
            }
            int sysFlagInner = sysFlag;

            if (findBrokerResult.isSlave()) {
                sysFlagInner = PullSysFlag.clearCommitOffsetFlag(sysFlagInner);
            }

            PullMessageRequestHeader requestHeader = new PullMessageRequestHeader();
            requestHeader.setConsumerGroup(this.consumerGroup);
            requestHeader.setTopic(mq.getTopic());
            requestHeader.setQueueId(mq.getQueueId());
            requestHeader.setQueueOffset(offset);
            requestHeader.setMaxMsgNums(maxNums);
            requestHeader.setSysFlag(sysFlagInner);
            requestHeader.setCommitOffset(commitOffset);
            requestHeader.setSuspendTimeoutMillis(brokerSuspendMaxTimeMillis);
            requestHeader.setSubscription(subExpression);
            requestHeader.setSubVersion(subVersion);
            requestHeader.setExpressionType(expressionType);

            String brokerAddr = findBrokerResult.getBrokerAddr();
            if (PullSysFlag.hasClassFilterFlag(sysFlagInner)) {
                brokerAddr = computPullFromWhichFilterServer(mq.getTopic(), brokerAddr);
            }

            PullResult pullResult = this.mQClientFactory.getMQClientAPIImpl().pullMessage(
                brokerAddr,
                requestHeader,
                timeoutMillis,
                communicationMode,
                pullCallback);

            return pullResult;
        }

        throw new MQClientException("The broker[" + mq.getBrokerName() + "] not exist", null);
    }
    // org.apache.rocketmq.client.impl.MQClientAPIImpl#pullMessage
    public PullResult pullMessage(
        final String addr,
        final PullMessageRequestHeader requestHeader,
        final long timeoutMillis,
        final CommunicationMode communicationMode,
        final PullCallback pullCallback
    ) throws RemotingException, MQBrokerException, InterruptedException {
        RemotingCommand request = RemotingCommand.createRequestCommand(RequestCode.PULL_MESSAGE, requestHeader);

        switch (communicationMode) {
            // ONEWAY是不會支援的
            case ONEWAY:
                assert false;
                return null;
            case ASYNC:
                this.pullMessageAsync(addr, request, timeoutMillis, pullCallback);
                return null;
            case SYNC:
                return this.pullMessageSync(addr, request, timeoutMillis);
            default:
                assert false;
                break;
        }

        return null;
    }
    
    // 我們只看非同步處理
    // org.apache.rocketmq.client.impl.MQClientAPIImpl#pullMessageAsync
    private void pullMessageAsync(
        final String addr,
        final RemotingCommand request,
        final long timeoutMillis,
        final PullCallback pullCallback
    ) throws RemotingException, InterruptedException {
        this.remotingClient.invokeAsync(addr, request, timeoutMillis, new InvokeCallback() {
            @Override
            public void operationComplete(ResponseFuture responseFuture) {
                RemotingCommand response = responseFuture.getResponseCommand();
                if (response != null) {
                    try {
                        PullResult pullResult = MQClientAPIImpl.this.processPullResponse(response);
                        assert pullResult != null;
                        pullCallback.onSuccess(pullResult);
                    } catch (Exception e) {
                        pullCallback.onException(e);
                    }
                } else {
                    // 消費異常回撥處理
                    if (!responseFuture.isSendRequestOK()) {
                        pullCallback.onException(new MQClientException("send request failed to " + addr + ". Request: " + request, responseFuture.getCause()));
                    } else if (responseFuture.isTimeout()) {
                        pullCallback.onException(new MQClientException("wait response from " + addr + " timeout :" + responseFuture.getTimeoutMillis() + "ms" + ". Request: " + request,
                            responseFuture.getCause()));
                    } else {
                        pullCallback.onException(new MQClientException("unknown reason. addr: " + addr + ", timeoutMillis: " + timeoutMillis + ". Request: " + request, responseFuture.getCause()));
                    }
                }
            }
        });
    }
    // 與producer一樣
    // org.apache.rocketmq.remoting.netty.NettyRemotingClient#invokeAsync
    @Override
    public void invokeAsync(String addr, RemotingCommand request, long timeoutMillis, InvokeCallback invokeCallback)
        throws InterruptedException, RemotingConnectException, RemotingTooMuchRequestException, RemotingTimeoutException,
        RemotingSendRequestException {
        long beginStartTime = System.currentTimeMillis();
        final Channel channel = this.getAndCreateChannel(addr);
        if (channel != null && channel.isActive()) {
            try {
                doBeforeRpcHooks(addr, request);
                long costTime = System.currentTimeMillis() - beginStartTime;
                if (timeoutMillis < costTime) {
                    throw new RemotingTooMuchRequestException("invokeAsync call timeout");
                }
                // 與producer一樣, channel.writeAndFlush() 後直接返回
                this.invokeAsyncImpl(channel, request, timeoutMillis - costTime, invokeCallback);
            } catch (RemotingSendRequestException e) {
                log.warn("invokeAsync: send request exception, so close the channel[{}]", addr);
                this.closeChannel(addr, channel);
                throw e;
            }
        } else {
            this.closeChannel(addr, channel);
            throw new RemotingConnectException(addr);
        }
    }
    

　　

　　至於每個節點消費哪些broker的資料，邏輯有點複雜，核心主要是依賴於消費策略的配置。感興趣的同學可展開閱讀。

    // 訊息消費的處理
    // org.apache.rocketmq.client.impl.consumer.RebalanceService#run
    @Override
    public void run() {
        log.info(this.getServiceName() + " service started");

        while (!this.isStopped()) {
            this.waitForRunning(waitInterval);
            this.mqClientFactory.doRebalance();
        }

        log.info(this.getServiceName() + " service end");
    }
    // org.apache.rocketmq.client.impl.factory.MQClientInstance#doRebalance
    public void doRebalance() {
        for (Map.Entry<String, MQConsumerInner> entry : this.consumerTable.entrySet()) {
            MQConsumerInner impl = entry.getValue();
            if (impl != null) {
                try {
                    impl.doRebalance();
                } catch (Throwable e) {
                    log.error("doRebalance exception", e);
                }
            }
        }
    }
    // org.apache.rocketmq.client.impl.consumer.DefaultMQPushConsumerImpl#doRebalance
    @Override
    public void doRebalance() {
        if (!this.pause) {
            this.rebalanceImpl.doRebalance(this.isConsumeOrderly());
        }
    }
    // org.apache.rocketmq.client.impl.consumer.RebalanceImpl#doRebalance
    public void doRebalance(final boolean isOrder) {
        Map<String, SubscriptionData> subTable = this.getSubscriptionInner();
        if (subTable != null) {
            for (final Map.Entry<String, SubscriptionData> entry : subTable.entrySet()) {
                final String topic = entry.getKey();
                try {
                    this.rebalanceByTopic(topic, isOrder);
                } catch (Throwable e) {
                    if (!topic.startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                        log.warn("rebalanceByTopic Exception", e);
                    }
                }
            }
        }

        this.truncateMessageQueueNotMyTopic();
    }
    // org.apache.rocketmq.client.impl.consumer.RebalanceImpl#rebalanceByTopic
    private void rebalanceByTopic(final String topic, final boolean isOrder) {
        switch (messageModel) {
            // 廣播訊息，所有節點都消費
            case BROADCASTING: {
                Set<MessageQueue> mqSet = this.topicSubscribeInfoTable.get(topic);
                if (mqSet != null) {
                    boolean changed = this.updateProcessQueueTableInRebalance(topic, mqSet, isOrder);
                    if (changed) {
                        this.messageQueueChanged(topic, mqSet, mqSet);
                        log.info("messageQueueChanged {} {} {} {}",
                            consumerGroup,
                            topic,
                            mqSet,
                            mqSet);
                    }
                } else {
                    log.warn("doRebalance, {}, but the topic[{}] not exist.", consumerGroup, topic);
                }
                break;
            }
            // 叢集訊息，同一級訊息分組只有一個節點消費同一訊息
            case CLUSTERING: {
                Set<MessageQueue> mqSet = this.topicSubscribeInfoTable.get(topic);
                List<String> cidAll = this.mQClientFactory.findConsumerIdList(topic, consumerGroup);
                if (null == mqSet) {
                    if (!topic.startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                        log.warn("doRebalance, {}, but the topic[{}] not exist.", consumerGroup, topic);
                    }
                }

                if (null == cidAll) {
                    log.warn("doRebalance, {} {}, get consumer id list failed", consumerGroup, topic);
                }

                if (mqSet != null && cidAll != null) {
                    List<MessageQueue> mqAll = new ArrayList<MessageQueue>();
                    mqAll.addAll(mqSet);

                    Collections.sort(mqAll);
                    Collections.sort(cidAll);

                    AllocateMessageQueueStrategy strategy = this.allocateMessageQueueStrategy;

                    List<MessageQueue> allocateResult =  null;
                    try {
                        // 根據再均衡策略分配消費節點
                        allocateResult = strategy.allocate(
                            this.consumerGroup,
                            this.mQClientFactory.getClientId(),
                            mqAll,
                            cidAll);
                    } catch (Throwable e) {
                        log.error("AllocateMessageQueueStrategy.allocate Exception. allocateMessageQueueStrategyName={}", strategy.getName(),
                            e);
                        return;
                    }

                    Set<MessageQueue> allocateResultSet = new HashSet<MessageQueue>();
                    if (allocateResult != null) {
                        allocateResultSet.addAll(allocateResult);
                    }
                    // 如果發生了變更，則重新分配消費節點
                    boolean changed = this.updateProcessQueueTableInRebalance(topic, allocateResultSet, isOrder);
                    if (changed) {
                        log.info(
                            "rebalanced result changed. allocateMessageQueueStrategyName={}, group={}, topic={}, clientId={}, mqAllSize={}, cidAllSize={}, rebalanceResultSize={}, rebalanceResultSet={}",
                            strategy.getName(), consumerGroup, topic, this.mQClientFactory.getClientId(), mqSet.size(), cidAll.size(),
                            allocateResultSet.size(), allocateResultSet);
                        this.messageQueueChanged(topic, mqSet, allocateResultSet);
                    }
                }
                break;
            }
            default:
                break;
        }
    }
    // 樣例：分配消費節點策略之平均分配策略
    // org.apache.rocketmq.client.consumer.rebalance.AllocateMessageQueueAveragely#allocate
    @Override
    public List<MessageQueue> allocate(String consumerGroup, String currentCID, List<MessageQueue> mqAll,
        List<String> cidAll) {
        if (currentCID == null || currentCID.length() < 1) {
            throw new IllegalArgumentException("currentCID is empty");
        }
        if (mqAll == null || mqAll.isEmpty()) {
            throw new IllegalArgumentException("mqAll is null or mqAll empty");
        }
        if (cidAll == null || cidAll.isEmpty()) {
            throw new IllegalArgumentException("cidAll is null or cidAll empty");
        }

        List<MessageQueue> result = new ArrayList<MessageQueue>();
        if (!cidAll.contains(currentCID)) {
            log.info("[BUG] ConsumerGroup: {} The consumerId: {} not in cidAll: {}",
                consumerGroup,
                currentCID,
                cidAll);
            return result;
        }

        int index = cidAll.indexOf(currentCID);
        int mod = mqAll.size() % cidAll.size();
        int averageSize =
            mqAll.size() <= cidAll.size() ? 1 : (mod > 0 && index < mod ? mqAll.size() / cidAll.size()
                + 1 : mqAll.size() / cidAll.size());
        int startIndex = (mod > 0 && index < mod) ? index * averageSize : index * averageSize + mod;
        int range = Math.min(averageSize, mqAll.size() - startIndex);
        // 依次分配節點
        for (int i = 0; i < range; i++) {
            result.add(mqAll.get((startIndex + i) % mqAll.size()));
        }
        return result;
    }
    // org.apache.rocketmq.client.impl.consumer.RebalanceImpl#updateProcessQueueTableInRebalance
    private boolean updateProcessQueueTableInRebalance(final String topic, final Set<MessageQueue> mqSet,
        final boolean isOrder) {
        boolean changed = false;

        Iterator<Entry<MessageQueue, ProcessQueue>> it = this.processQueueTable.entrySet().iterator();
        while (it.hasNext()) {
            Entry<MessageQueue, ProcessQueue> next = it.next();
            MessageQueue mq = next.getKey();
            ProcessQueue pq = next.getValue();

            if (mq.getTopic().equals(topic)) {
                // 上鎖成功說明該佇列還沒有消費者，方才可以進行消費
                // 安全性由 broker 保證
                if (!mqSet.contains(mq)) {
                    pq.setDropped(true);
                    if (this.removeUnnecessaryMessageQueue(mq, pq)) {
                        it.remove();
                        changed = true;
                        log.info("doRebalance, {}, remove unnecessary mq, {}", consumerGroup, mq);
                    }
                } else if (pq.isPullExpired()) {
                    switch (this.consumeType()) {
                        case CONSUME_ACTIVELY:
                            break;
                        case CONSUME_PASSIVELY:
                            pq.setDropped(true);
                            if (this.removeUnnecessaryMessageQueue(mq, pq)) {
                                it.remove();
                                changed = true;
                                log.error("[BUG]doRebalance, {}, remove unnecessary mq, {}, because pull is pause, so try to fixed it",
                                    consumerGroup, mq);
                            }
                            break;
                        default:
                            break;
                    }
                }
            }
        }

        // 按MessageQueue進行消費
        List<PullRequest> pullRequestList = new ArrayList<PullRequest>();
        for (MessageQueue mq : mqSet) {
            if (!this.processQueueTable.containsKey(mq)) {
                if (isOrder && !this.lock(mq)) {
                    log.warn("doRebalance, {}, add a new mq failed, {}, because lock failed", consumerGroup, mq);
                    continue;
                }

                this.removeDirtyOffset(mq);
                ProcessQueue pq = new ProcessQueue();
                long nextOffset = this.computePullFromWhere(mq);
                if (nextOffset >= 0) {
                    ProcessQueue pre = this.processQueueTable.putIfAbsent(mq, pq);
                    if (pre != null) {
                        log.info("doRebalance, {}, mq already exists, {}", consumerGroup, mq);
                    } else {
                        log.info("doRebalance, {}, add a new mq, {}", consumerGroup, mq);
                        PullRequest pullRequest = new PullRequest();
                        pullRequest.setConsumerGroup(consumerGroup);
                        pullRequest.setNextOffset(nextOffset);
                        pullRequest.setMessageQueue(mq);
                        pullRequest.setProcessQueue(pq);
                        pullRequestList.add(pullRequest);
                        changed = true;
                    }
                } else {
                    log.warn("doRebalance, {}, add new mq failed, {}", consumerGroup, mq);
                }
            }
        }
        // 依次消費資料
        this.dispatchPullRequest(pullRequestList);

        return changed;
    }
    // org.apache.rocketmq.client.impl.consumer.RebalancePushImpl#dispatchPullRequest
    @Override
    public void dispatchPullRequest(List<PullRequest> pullRequestList) {
        for (PullRequest pullRequest : pullRequestList) {
            this.defaultMQPushConsumerImpl.executePullRequestImmediately(pullRequest);
            log.info("doRebalance, {}, add a new pull request {}", consumerGroup, pullRequest);
        }
    }

　　確定了從哪些broker上消費資料之後，就只需向傳送 PULL_MESSAGE 的請求即可，拿到資料之後再執行回撥方法。然後再根據使用者使用的消費模型，將資料給到使用者端即可。即對消費者而言，模型總是簡單的，複雜的點已由框架部分處理。消費者只管接收資料，處理業務即可。

　　但對消費者而言，還有兩個指標可能需要關注：訊息資料的及時性和主動或被動消費資料（即或推或拉模型）。不過，這看似兩個點，實際上在某些情況下可能會變成一個點。因為對於主動拉取訊息的實現，使用者可以自定義控制訊息的及時性，從而變成同一個問題。所以，我們主要考慮下推送消費的情況下，訊息的延遲是怎麼樣的？

        // 事實上，rocketmq是通過回撥響應時設定定時間隔來實現消費的及時性的
        // org.apache.rocketmq.client.impl.consumer.DefaultMQPushConsumerImpl#pullMessage
        // 該回撥函式會在拉取訊息時註冊進去，拉取成功或失敗後被回撥
        ...
        PullCallback pullCallback = new PullCallback() {
            @Override
            public void onSuccess(PullResult pullResult) {
                if (pullResult != null) {
                    pullResult = DefaultMQPushConsumerImpl.this.pullAPIWrapper.processPullResult(pullRequest.getMessageQueue(), pullResult,
                        subscriptionData);

                    switch (pullResult.getPullStatus()) {
                        case FOUND:
                            long prevRequestOffset = pullRequest.getNextOffset();
                            pullRequest.setNextOffset(pullResult.getNextBeginOffset());
                            long pullRT = System.currentTimeMillis() - beginTimestamp;
                            DefaultMQPushConsumerImpl.this.getConsumerStatsManager().incPullRT(pullRequest.getConsumerGroup(),
                                pullRequest.getMessageQueue().getTopic(), pullRT);

                            long firstMsgOffset = Long.MAX_VALUE;
                            // 拉取到的訊息為空時，會立即再提交一個pull請求
                            // 也就是說在沒有訊息的時候，broker將會收到非常多的空請求，所以這需要broker端做點什麼
                            if (pullResult.getMsgFoundList() == null || pullResult.getMsgFoundList().isEmpty()) {
                                DefaultMQPushConsumerImpl.this.executePullRequestImmediately(pullRequest);
                            } else {
                                firstMsgOffset = pullResult.getMsgFoundList().get(0).getQueueOffset();

                                DefaultMQPushConsumerImpl.this.getConsumerStatsManager().incPullTPS(pullRequest.getConsumerGroup(),
                                    pullRequest.getMessageQueue().getTopic(), pullResult.getMsgFoundList().size());

                                boolean dispatchToConsume = processQueue.putMessage(pullResult.getMsgFoundList());
                                // 先將本次拉取到的訊息提交給使用者程式，然後繼續拉取訊息
                                DefaultMQPushConsumerImpl.this.consumeMessageService.submitConsumeRequest(
                                    pullResult.getMsgFoundList(),
                                    processQueue,
                                    pullRequest.getMessageQueue(),
                                    dispatchToConsume);
                                // 拉取的頻率由 pullInterval 這個值來決定
                                // 從而使用者可以自行決定消費的頻率, 該間隔預設是0
                                if (DefaultMQPushConsumerImpl.this.defaultMQPushConsumer.getPullInterval() > 0) {
                                    DefaultMQPushConsumerImpl.this.executePullRequestLater(pullRequest,
                                        DefaultMQPushConsumerImpl.this.defaultMQPushConsumer.getPullInterval());
                                } else {
                                    DefaultMQPushConsumerImpl.this.executePullRequestImmediately(pullRequest);
                                }
                            }

                            if (pullResult.getNextBeginOffset() < prevRequestOffset
                                || firstMsgOffset < prevRequestOffset) {
                                log.warn(
                                    "[BUG] pull message result maybe data wrong, nextBeginOffset: {} firstMsgOffset: {} prevRequestOffset: {}",
                                    pullResult.getNextBeginOffset(),
                                    firstMsgOffset,
                                    prevRequestOffset);
                            }

                            break;
                        case NO_NEW_MSG:
                            pullRequest.setNextOffset(pullResult.getNextBeginOffset());

                            DefaultMQPushConsumerImpl.this.correctTagsOffset(pullRequest);
                            // 沒有新訊息，立即再提交拉取請求
                            DefaultMQPushConsumerImpl.this.executePullRequestImmediately(pullRequest);
                            break;
                        case NO_MATCHED_MSG:
                            pullRequest.setNextOffset(pullResult.getNextBeginOffset());

                            DefaultMQPushConsumerImpl.this.correctTagsOffset(pullRequest);
                            // 沒有匹配的訊息，立即再提交拉取請求
                            DefaultMQPushConsumerImpl.this.executePullRequestImmediately(pullRequest);
                            break;
                        case OFFSET_ILLEGAL:
                            log.warn("the pull request offset illegal, {} {}",
                                pullRequest.toString(), pullResult.toString());
                            pullRequest.setNextOffset(pullResult.getNextBeginOffset());

                            pullRequest.getProcessQueue().setDropped(true);
                            // 偏移量有問題時處理略不一樣
                            // 會嘗試修復該offset (丟棄以便使用新的)
                            DefaultMQPushConsumerImpl.this.executeTaskLater(new Runnable() {

                                @Override
                                public void run() {
                                    try {
                                        DefaultMQPushConsumerImpl.this.offsetStore.updateOffset(pullRequest.getMessageQueue(),
                                            pullRequest.getNextOffset(), false);

                                        DefaultMQPushConsumerImpl.this.offsetStore.persist(pullRequest.getMessageQueue());

                                        DefaultMQPushConsumerImpl.this.rebalanceImpl.removeProcessQueue(pullRequest.getMessageQueue());

                                        log.warn("fix the pull request offset, {}", pullRequest);
                                    } catch (Throwable e) {
                                        log.error("executeTaskLater Exception", e);
                                    }
                                }
                            }, 10000);
                            break;
                        default:
                            break;
                    }
                }
            }

            @Override
            public void onException(Throwable e) {
                if (!pullRequest.getMessageQueue().getTopic().startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                    log.warn("execute the pull request exception", e);
                }
                // 消費失敗，則間隔另一段時間再嘗試
                DefaultMQPushConsumerImpl.this.executePullRequestLater(pullRequest, pullTimeDelayMillsWhenException);
            }
        };
        ...

　　從上面的實現可以看出，consumer的消費還是非常及時的，它會在上一次消費拉取完成後，幾乎可以立即重新再進行拉取。（只要你的業務程式處理得過來，你完成可以這麼幹）。但有個點得考慮下，因為客戶端及時性非常高，那麼是否會導致broker端壓力非常大呢？這是值得考慮的！而broker通過一個延遲響應，完美解決快速空請求的問題：

        // org.apache.rocketmq.broker.processor.PullMessageProcessor#processRequest
        ...
        // 根據取到的訊息資料情況，判斷如何響應請求
        switch (response.getCode()) {
            case ResponseCode.SUCCESS:
                ...
                if (this.brokerController.getBrokerConfig().isTransferMsgByHeap()) {
                    final long beginTimeMills = this.brokerController.getMessageStore().now();
                    final byte[] r = this.readGetMessageResult(getMessageResult, requestHeader.getConsumerGroup(), requestHeader.getTopic(), requestHeader.getQueueId());
                    this.brokerController.getBrokerStatsManager().incGroupGetLatency(requestHeader.getConsumerGroup(),
                        requestHeader.getTopic(), requestHeader.getQueueId(),
                        (int) (this.brokerController.getMessageStore().now() - beginTimeMills));
                    response.setBody(r);
                } else {
                    try {
                        FileRegion fileRegion =
                            new ManyMessageTransfer(response.encodeHeader(getMessageResult.getBufferTotalSize()), getMessageResult);
                        channel.writeAndFlush(fileRegion).addListener(new ChannelFutureListener() {
                            @Override
                            public void operationComplete(ChannelFuture future) throws Exception {
                                getMessageResult.release();
                                if (!future.isSuccess()) {
                                    log.error("transfer many message by pagecache failed, {}", channel.remoteAddress(), future.cause());
                                }
                            }
                        });
                    } catch (Throwable e) {
                        log.error("transfer many message by pagecache exception", e);
                        getMessageResult.release();
                    }

                    response = null;
                }
                break;
            case ResponseCode.PULL_NOT_FOUND:
                // 當設定了消費掛起時，hang住連線，直到有資料到來或者超時
                if (brokerAllowSuspend && hasSuspendFlag) {
                    // 該掛起時間預設為 brokerSuspendMaxTimeMillis 為 20秒
                    long pollingTimeMills = suspendTimeoutMillisLong;
                    if (!this.brokerController.getBrokerConfig().isLongPollingEnable()) {
                        pollingTimeMills = this.brokerController.getBrokerConfig().getShortPollingTimeMills();
                    }

                    String topic = requestHeader.getTopic();
                    long offset = requestHeader.getQueueOffset();
                    int queueId = requestHeader.getQueueId();
                    PullRequest pullRequest = new PullRequest(request, channel, pollingTimeMills,
                        this.brokerController.getMessageStore().now(), offset, subscriptionData, messageFilter);
                    this.brokerController.getPullRequestHoldService().suspendPullRequest(topic, queueId, pullRequest);
                    // response 為null 將不會被框架響應客戶端請求
                    response = null;
                    break;
                }

            case ResponseCode.PULL_RETRY_IMMEDIATELY:
                break;
            case ResponseCode.PULL_OFFSET_MOVED:
                ...
                break;
            default:
                assert false;
        }
        // org.apache.rocketmq.remoting.netty.NettyRemotingAbstract#processRequestCommand
        // Runnable.RemotingResponseCallback
        ...
        final RemotingResponseCallback callback = new RemotingResponseCallback() {
            @Override
            public void callback(RemotingCommand response) {
                doAfterRpcHooks(RemotingHelper.parseChannelRemoteAddr(ctx.channel()), cmd, response);
                if (!cmd.isOnewayRPC()) {
                    // response 為null 將不會響應客端, 即hang住了連線
                    if (response != null) {
                        response.setOpaque(opaque);
                        response.markResponseType();
                        try {
                            ctx.writeAndFlush(response);
                        } catch (Throwable e) {
                            log.error("process request over, but response failed", e);
                            log.error(cmd.toString());
                            log.error(response.toString());
                        }
                    } else {
                    }
                }
            }
        };
        ...

　　綜上，broker端通過hang住連線的方式，避免客戶頻繁進行空請求，保證了及時性的同時，又避免了客戶端的快速處理。至於broker具體是如何做到hang住請求的，我們可以另詳解。也可以簡單歸結為一句話：就是先放入一張表中，然後定時掃表是否有資料到來，有資料或者超時時，再響應客戶端即可。

 

7. 總結

　　多執行緒的恰當應用，無疑是很多應用效能強悍的原因。本文講解rocketmq的執行緒模型，其作用不言而喻。