RocketMQ的前提回顾
RocketMQ是一款分布式、队列模型的消息中间件，具有以下特点：
- 能够保证严格的消息顺序
- 提供丰富的消息拉取模式
- 高效的订阅者水平扩展能力
- 实时的消息订阅机制
- 亿级消息堆积能力
为什么使用RocketMQ
- 强调集群无单点，可扩展，任意一点高可用、水平可扩展
- 海量消息堆积能力，消息堆积后写入低延迟
- 支持上万个队列
- 消息失败重试机制
- 消息可查询
- 开源社区活跃
- 成熟度已经经过淘宝双十一的考验
RocketMQ的发展变化
RocketMQ开源是使用文件作为持久化工具，阿里内部未开源的性能会更高，使用oceanBase作为持久化工具。
在RocketMQ1.x和2.x使用zookeeper管理集群，3.x开始使用nameserver代替zk，更轻量级，此外RocketMQ的客户端拥有两种的操作方式：DefaultMQPushConsumer和DefaultMQPullConsumer。
DefaultMQPushConsumer的Maven配置
org.apache.rocketmq rocketmq-client 4.3.0 DefaultMQPushConsumer使用示例
- CONSUME_FROM_LAST_OFFSET：第一次启动从队列最后位置消费，后续再启动接着上次消费的进度开始消费
- CONSUME_FROM_FIRST_OFFSET：第一次启动从队列初始位置消费，后续再启动接着上次消费的进度开始消费
- CONSUME_FROM_TIMESTAMP：第一次启动从指定时间点位置消费，后续再启动接着上次消费的进度开始消费
以上所说的第一次启动是指从来没有消费过的消费者，如果该消费者消费过，那么会在broker端记录该消费者的消费位置，如果该消费者挂了再启动，那么自动从上次消费的进度开始
public class MQPushConsumer { public static void main(String[] args) throws MQClientException { String groupName = "rocketMqGroup1"; // 用于把多个Consumer组织到一起，提高并发处理能力 DefaultMQPushConsumer consumer = new DefaultMQPushConsumer(groupName); // 设置nameServer地址，多个以;分隔 consumer.setNamesrvAddr("name-serverl-ip:9876;name-server2-ip:9876"); consumer.setConsumeFromWhere(ConsumeFromWhere.CONSUME_FROM_FIRST_OFFSET); consumer.setMessageModel(MessageModel.BROADCASTING); // 订阅topic，可以对指定消息进行过滤，例如："TopicTest","tagl||tag2||tag3",*或null表示topic所有消息 consumer.subscribe("order-topic", "*"); consumer.registerMessageListener(new MessageListenerConcurrently() { @Override public ConsumeConcurrentlyStatus consumeMessage(List mgs, ConsumeConcurrentlyContext consumeconcurrentlycontext) { System.out.println(Thread.currentThread().getName()+"Receive New Messages:"+mgs); return ConsumeConcurrentlyStatus.CONSUME_SUCCESS; } }); consumer.start(); }} - CLUSTERING：默认模式，同一个ConsumerGroup(groupName相同)每个consumer只消费所订阅消息的一部分内容，同一个ConsumerGroup里所有的Consumer消息加起来才是所
- 订阅topic整体，从而达到负载均衡的目的
- BROADCASTING：同一个ConsumerGroup每个consumer都消费到所订阅topic所有消息，也就是一个消费会被多次分发，被多个consumer消费。
ConsumeConcurrentlyStatus.RECONSUME_LATER boker会根据设置的messageDelayLevel发起重试，默认16次。
DefaultMQPushConsumerImpl中各个对象的主要功能如下：
RebalancePushImpl：主要负责决定，当前的consumer应该从哪些Queue中消费消息；
- 1）PullAPIWrapper：长连接，负责从broker处拉取消息，然后利用ConsumeMessageService回调用户的Listener执行消息消费逻辑；
- 2）ConsumeMessageService：实现所谓的"Push-被动"消费机制；从Broker拉取的消息后，封装成ConsumeRequest提交给ConsumeMessageSerivce，此service负责回调用户的Listener消费消息；
- 3）OffsetStore：维护当前consumer的消费记录（offset）；有两种实现，Local和Rmote，Local存储在本地磁盘上，适用于BROADCASTING广播消费模式；而Remote则将消费进度存储在Broker上，适用于CLUSTERING集群消费模式；
- 4）MQClientFactory：负责管理client（consumer、producer），并提供多中功能接口供各个Service（Rebalance、PullMessage等）调用；大部分逻辑均在这个类中完成；
consumer.registerMessageListener执行过程：
/** * Register a callback to execute on message arrival for concurrent consuming. * @param messageListener message handling callback. */ @Override public void registerMessageListener(MessageListenerConcurrently messageListener) { this.messageListener = messageListener; this.defaultMQPushConsumerImpl.registerMessageListener(messageListener); }通过源码可以看出主要实现过程在DefaultMQPushConsumerImpl类中consumer.start后调用DefaultMQPushConsumerImpl的同步start方法
public synchronized void start() throws MQClientException { switch (this.serviceState) { 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().getAndCreateMQClientInstance(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()); } this.consumeMessageService.start(); boolean registerOK = mQClientFactory.registerConsumer(this.defaultMQPushConsumer.getConsumerGroup(), this); if (!registerOK) { this.serviceState = ServiceState.CREATE_JUST; this.consumeMessageService.shutdown(); 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); } 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(); }通过mQClientFactory.start();发我们发现他调用
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 RUNNING: break; case SHUTDOWN_ALREADY: break; case START_FAILED: throw new MQClientException("The Factory object[" + this.getClientId() + "] has been created before, and failed.", null); default: break; } } }在这个方法中有多个start，我们主要看pullMessageService.start();通过这里我们发现RocketMQ的Push模式底层其实也是通过pull实现的，下面我们来看下pullMessageService处理了哪些逻辑：
private void pullMessage(final PullRequest pullRequest) { final MQConsumerInner consumer = this.mQClientFactory.selectConsumer(pullRequest.getConsumerGroup()); if (consumer != null) { DefaultMQPushConsumerImpl impl = (DefaultMQPushConsumerImpl) consumer; impl.pullMessage(pullRequest); } else { log.warn("No matched consumer for the PullRequest {}, drop it", pullRequest); } }我们发现其实他还是通过DefaultMQPushConsumerImpl类的pullMessage方法来进行消息的逻辑处理.
pullRequest拉取方式
PullRequest这里说明一下，上面我们已经提了一下rocketmq的push模式其实是通过pull模式封装实现的，pullrequest这里是通过长轮询的方式达到push效果。
长轮询方式既有pull的优点又有push模式的实时性有点。
push方式是server端接收到消息后，主动把消息推送给client端，实时性高。弊端是server端工作量大，影响性能，其次是client端处理能力不同且client端的状态不受server端的控制，如果client端不能及时处理消息容易导致消息堆积已经影响正常业务等。
- pull方式是client循环从server端拉取消息，主动权在client端，自己处理完一个消息再去拉取下一个，缺点是循环的时间不好设定，时间太短容易忙等，浪费CPU资源，时间间隔太长client的处理能力会下降，有时候有些消息会处理不及时。
长轮询的方式可以结合两者优点
- 检查PullRequest对象中的ProcessQueue对象的dropped是否为true（在RebalanceService线程中为topic下的MessageQueue创建拉取消息请求时要维护对应的ProcessQueue对象，若Consumer不再订阅该topic则会将该对象的dropped置为true）；若是则认为该请求是已经取消的，则直接跳出该方法；
- 更新PullRequest对象中的ProcessQueue对象的时间戳（ProcessQueue.lastPullTimestamp）为当前时间戳；
- 检查该Consumer是否运行中，即DefaultMQPushConsumerImpl.serviceState是否为RUNNING;若不是运行状态或者是暂停状态（DefaultMQPushConsumerImpl.pause=true），则调用PullMessageService.executePullRequestLater(PullRequest pullRequest, long timeDelay)方法延迟再拉取消息，其中timeDelay=3000；该方法的目的是在3秒之后再次将该PullRequest对象放入PullMessageService. pullRequestQueue队列中；并跳出该方法；
- 进行流控。若ProcessQueue对象的msgCount大于了消费端的流控阈值（DefaultMQPushConsumer.pullThresholdForQueue，默认值为1000），则调用PullMessageService.executePullRequestLater方法，在50毫秒之后重新该PullRequest请求放入PullMessageService.pullRequestQueue队列中；并跳出该方法；
- 若不是顺序消费（即DefaultMQPushConsumerImpl.consumeOrderly等于false），则检查ProcessQueue对象的msgTreeMap:TreeMap
- 以PullRequest.messageQueue对象的topic值为参数从RebalanceImpl.subscriptionInner: ConcurrentHashMap, SubscriptionData>中获取对应的SubscriptionData对象，若该对象为null，考虑到并发的关系，调用executePullRequestLater方法，稍后重试；并跳出该方法；
- 若消息模型为集群模式（RebalanceImpl.messageModel等于CLUSTERING），则以PullRequest对象的MessageQueue变量值、type =READ_FROM_MEMORY（从内存中获取消费进度offset值）为参数调用DefaultMQPushConsumerImpl. offsetStore对象（初始化为RemoteBrokerOffsetStore对象）的readOffset(MessageQueue mq, ReadOffsetType type)方法从本地内存中获取消费进度offset值。若该offset值大于0 则置临时变量commitOffsetEnable等于true否则为false；该offset值作为pullKernelImpl方法中的commitOffset参数，在Broker端拉取消息之后根据commitOffsetEnable参数值决定是否用该offset更新消息进度。该readOffset方法的逻辑是：以入参MessageQueue对象从RemoteBrokerOffsetStore.offsetTable:ConcurrentHashMap
- 当每次拉取消息之后需要更新订阅关系（由DefaultMQPushConsumer. postSubscriptionWhenPull参数表示，默认为false）并且以topic值参数从RebalanceImpl.subscriptionInner获取的SubscriptionData对象的classFilterMode等于false（默认为false），则将sysFlag标记的第3个字节置为1，否则该字节置为0；
- 该sysFlag标记的第1个字节置为commitOffsetEnable的值；第2个字节（suspend标记）置为1；第4个字节置为classFilterMode的值；
- 初始化匿名内部类PullCallback，实现了onSucess/onException方法； 该方法只有在异步请求的情况下才会回调；
- 调用底层的拉取消息API接口：
PullAPIWrapper.pullKernelImpl
PullAPIWrapper.pullKernelImpl(MessageQueue mq, String subExpression, long subVersion,long offset, int maxNums, int sysFlag,long commitOffset,long brokerSuspendMaxTimeMillis, long timeoutMillis, CommunicationMode communicationMode, PullCallback pullCallback)方法进行消息拉取操作。
将回调类PullCallback传入该方法中，当采用异步方式拉取消息时，在收到响应之后会回调该回调类的方法。
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, PULL_TIME_DELAY_MILLS_WHEN_EXCEPTION); 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 queues 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, PULL_TIME_DELAY_MILLS_WHEN_EXCEPTION); 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, PULL_TIME_DELAY_MILLS_WHEN_EXCEPTION); log.warn("find the consumers 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, PULL_TIME_DELAY_MILLS_WHEN_EXCEPTION); } }; 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 { // 下面我们看继续跟进这个方法，这个方法已经就是客户端如何拉取消息 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, // 消息的通信方式为异步 CommunicationMode.ASYNC, pullCallback ); } catch (Exception e) { log.error("pullKernelImpl exception", e); this.executePullRequestLater(pullRequest, PULL_TIME_DELAY_MILLS_WHEN_EXCEPTION); } }发送远程请求拉取消息
在MQClientAPIImpl.pullMessage方法中，根据入参communicationMode的值分为异步拉取和同步拉取方式两种。
无论是异步方式拉取还是同步方式拉取，在发送拉取请求之前都会构造一个ResponseFuture对象，以请求消息的序列号为key值，存入NettyRemotingAbstract.responseTable:ConcurrentHashMap, ResponseFuture>变量中，对该变量有几种情况会处理：
- 发送失败后直接删掉responseTable变量中的相应记录；
- 收到响应消息之后，会以响应消息中的序列号（由服务端根据请求消息的序列号原样返回）从responseTable中查找ResponseFuture对象，并设置该对象的responseCommand变量。若是同步发送会唤醒等待响应的ResponseFuture.waitResponse方法；若是异步发送会调用ResponseFuture.executeInvokeCallback()方法完成回调逻辑处理；
- 在NettyRemotingClient.start()启动时，也会初始化定时任务，该定时任务每隔1秒定期扫描responseTable列表，遍历该列表中的ResponseFuture对象，检查等待响应是否超时，若超时，则调用ResponseFuture. executeInvokeCallback()方法，并将该对象从responseTable列表中删除；
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) { 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; }同步拉取
对于同步发送方式，调用MQClientAPIImpl.pullMessageSync(String addr, RemotingCommand request, long timeoutMillis)方法,大致步骤如下：
- 调用RemotingClient.invokeSync(String addr, RemotingCommand request, long timeoutMillis)方法：
- 获取Broker地址的Channel信息。根据broker地址从RemotingClient.channelTables:ConcurrentHashMap, ChannelWrapper>变量中获取ChannelWrapper对象并返回该对象的Channel变量；若没有ChannelWrapper对象则与broker地址建立新的连接并将连接信息存入channelTables变量中，便于下次使用；
- 若NettyRemotingClient.rpcHook:RPCHook变量不为空（该变量在应用层初始化DefaultMQPushConsumer或者DefaultMQPullConsumer对象传入该值），则调用RPCHook.doBeforeRequest(String remoteAddr, RemotingCommand request)方法；
- 调用NettyRemotingAbstract.invokeSyncImpl(Channel channel, RemotingCommand request, long timeoutMillis)方法，该方法的逻辑如下：
- A）使用请求的序列号（opaue）、超时时间初始化ResponseFuture对象；并将该ResponseFuture对象存入NettyRemotingAbstract.responseTable: ConcurrentHashMap变量中；
- B）调用Channel.writeAndFlush(Object msg)方法将请求对象RemotingCommand发送给Broker；然后调用addListener(GenericFutureListener> listener)方法添加内部匿名类：该内部匿名类实现了ChannelFutureListener接口的operationComplete方法，在发送完成之后回调该监听类的operationComplete方法，在该方法中，首先调用ChannelFuture. isSuccess()方法检查是否发送成功，若成功则置ResponseFuture对象的sendRequestOK等于true并退出此回调方法等待响应结果；若不成功则置ResponseFuture对象的sendRequestOK等于false，然后从NettyRemotingAbstract.responseTable中删除此请求序列号（opaue）的记录，置ResponseFuture对象的responseCommand等于null，并唤醒ResponseFuture.waitResponse(long timeoutMillis)方法的等待；
- C）调用ResponseFuture.waitResponse(long timeoutMillis)方法等待响应结果；在发送失败或者收到响应消息（详见5.10.3小节）或者超时的情况下会唤醒该方法返回ResponseFuture.responseCommand变量值；
- D）若上一步返回的responseCommand值为null，则抛出异常:若ResponseFuture.sendRequestOK为true，则抛出RemotingTimeoutException异常，否则抛出RemotingSendRequestException异常；
- E）若上一步返回的responseCommand值不为null，则返回responseCommand变量值；
- 若NettyRemotingClient.rpcHook: RPCHook变量不为空，则调用RPCHook.doAfterResponse(String remoteAddr, RemotingCommand request)方法；
- 以上一步的返回值RemotingCommand对象为参数调用MQClientAPIImpl. processPullResponse (RemotingCommand response)方法将返回对象解析并封装成PullResultExt对象然后返回给调用者，响应消息的结果状态转换如下：
- 若RemotingCommand对象的Code等于SUCCESS，则PullResultExt.pullStatus=FOUND；
- 若RemotingCommand对象的Code等于PULL_NOT_FOUND，则PullResultExt.pullStatus= NO_NEW_MSG；
- 若RemotingCommand对象的Code等于PULL_RETRY_IMMEDIATELY，则PullResultExt.pullStatus= NO_MATCHED_MSG；
- 若RemotingCommand对象的Code等于PULL_OFFSET_MOVED，则PullResultExt.pullStatus= OFFSET_ILLEGAL；
@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 { if (this.rpcHook != null) { this.rpcHook.doBeforeRequest(addr, request); } long costTime = System.currentTimeMillis() - beginStartTime; if (timeoutMillis < costTime) { throw new RemotingTimeoutException("invokeSync call timeout"); } RemotingCommand response = this.invokeSyncImpl(channel, request, timeoutMillis - costTime); if (this.rpcHook != null) { this.rpcHook.doAfterResponse(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); } }getMQClientAPIImpl().pullMessage最终通过channel写入并刷新队列中。然后在消息服务端大体的处理逻辑是服务端收到新消息请求后，如果队列中没有消息不急于返回，通过一个循环状态，每次waitForRunning一段时间默认5秒，然后再check，如果broker一直没有新新消息，第三次check的时间等到时间超过SuspendMaxTimeMills就返回空，如果在等待过程中收到了新消息直接调用notifyMessageArriving函数返回请求结果。“长轮询”的核心是，Broker端HOLD住客户端过来的请求一小段时间，在这个时间内有新消息到达，就利用现有的连接立刻返回消息给 Consumer 。长轮询的主动权掌握在consumer中，即使broker有大量的消息堆积也不会主动推送给consumer。
