出门莫恨无人随，书中车马多如簇。这篇文章主要讲述Alibaba中间件技术「RocketMQ专题」探索DefaultMQPushConsumer相关的知识，希望能为你提供帮助。
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
- 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
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
<
dependency>
<
groupId>
org.apache.rocketmq<
/groupId>
<
artifactId>
rocketmq-client<
/artifactId>
<
version>
4.3.0<
/version>
<
/dependency>
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
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<
MessageExt>
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。
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；
/**
* 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。
- 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< Long,MessageExt> 变量的第一个key值与最后一个key值之间的差额，该key值表示查询的队列偏移量queueoffset；若差额大于阈值（由DefaultMQPushConsumer. consumeConcurrentlyMaxSpan指定，默认是2000），则调用PullMessageService.executePullRequestLater方法，在50毫秒之后重新将该PullRequest请求放入PullMessageService.pullRequestQueue队列中；并跳出该方法；
- 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 < MessageQueue,AtomicLong> 变量中获取消费进度偏移量；若该偏移量不为null则返回该值，否则返回-1；
- 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：
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 = https://www.songbingjia.com/android/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 = https://www.songbingjia.com/android/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< ? extends Future< ? super Void> > 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);
}
}【Alibaba中间件技术「RocketMQ专题」探索DefaultMQPushConsumer】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。
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