聊一下 RocketMQ 的顺序消息
rocketmq 里有一种比较特殊的用法，就是顺序消息，比如订单的生命周期里，在创建，支付，签收等状态轮转中，会发出来对应的消息，这里面就比较需要去保证他们的顺序，当然在处理的业务代码也可以做对应的处理，结合消息重投，但是如果这里消息就能保证顺序性了，那么业务代码就能更好的关注业务代码的处理。
首先有一种情况是全局的有序，比如对于一个 topic 里就得发送线程保证只有一个，内部的 queue 也只有一个，消费线程也只有一个，这样就能比较容易的保证全局顺序性了，但是这里的问题就是完全限制了性能，不是很现实，在真实场景里很多都是比如对于同一个订单，需要去保证状态的轮转是按照预期的顺序来，而不必要全局的有序性。
对于这类的有序性，需要在发送和接收方都有对应的处理，在发送消息中，需要去指定 selector，即MessageQueueSelector，能够以固定的方式是分配到对应的 MessageQueue
比如像 RocketMQ 中的示例
SendResult sendResult = producer.send(msg, new MessageQueueSelector() {
@Override
public MessageQueue select(List<MessageQueue> mqs, Message msg, Object arg) {
Long id = (Long) arg; //message queue is selected by #salesOrderID
long index = id % mqs.size();
return mqs.get((int) index);
}
}, orderList.get(i).getOrderId());而在消费侧有几个点比较重要，首先我们要保证一个 MessageQueue只被一个消费者消费，消费队列存在broker端，要保证 MessageQueue 只被一个消费者消费，那么消费者在进行消息拉取消费时就必须向mq服务器申请队列锁，消费者申请队列锁的代码存在于RebalanceService消息队列负载的实现代码中。
List<PullRequest> pullRequestList = new ArrayList<PullRequest>();
for (MessageQueue mq : mqSet) {
if (!this.processQueueTable.containsKey(mq)) {
// 判断是否顺序，如果是顺序消费的，则需要加锁
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);
}
}
}在申请到锁之后会创建 pullRequest 进行消息拉取，消息拉取部分的代码实现在PullMessageService中，
@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");
}消息拉取完后，需要提交到ConsumeMessageService中进行消费，顺序消费的实现为ConsumeMessageOrderlyService，提交消息进行消费的方法为ConsumeMessageOrderlyService#submitConsumeRequest，具体实现如下：
@Override
public void submitConsumeRequest(
final List<MessageExt> msgs,
final ProcessQueue processQueue,
final MessageQueue messageQueue,
final boolean dispathToConsume) {
if (dispathToConsume) {
ConsumeRequest consumeRequest = new ConsumeRequest(processQueue, messageQueue);
this.consumeExecutor.submit(consumeRequest);
}
}构建了一个ConsumeRequest对象，它是个实现了 runnable 接口的类，并提交给了线程池来并行消费，看下顺序消费的ConsumeRequest的run方法实现：
@Override
public void run() {
if (this.processQueue.isDropped()) {
log.warn("run, the message queue not be able to consume, because it's dropped. {}", this.messageQueue);
return;
}
// 获得 Consumer 消息队列锁，即单个线程独占
final Object objLock = messageQueueLock.fetchLockObject(this.messageQueue);
synchronized (objLock) {
// (广播模式) 或者 (集群模式 && Broker消息队列锁有效)
if (MessageModel.BROADCASTING.equals(ConsumeMessageOrderlyService.this.defaultMQPushConsumerImpl.messageModel())
|| (this.processQueue.isLocked() && !this.processQueue.isLockExpired())) {
final long beginTime = System.currentTimeMillis();
// 循环
for (boolean continueConsume = true; continueConsume; ) {
if (this.processQueue.isDropped()) {
log.warn("the message queue not be able to consume, because it's dropped. {}", this.messageQueue);
break;
}
// 消息队列分布式锁未锁定，提交延迟获得锁并消费请求
if (MessageModel.CLUSTERING.equals(ConsumeMessageOrderlyService.this.defaultMQPushConsumerImpl.messageModel())
&& !this.processQueue.isLocked()) {
log.warn("the message queue not locked, so consume later, {}", this.messageQueue);
ConsumeMessageOrderlyService.this.tryLockLaterAndReconsume(this.messageQueue, this.processQueue, 10);
break;
}
// 消息队列分布式锁已经过期，提交延迟获得锁并消费请求
if (MessageModel.CLUSTERING.equals(ConsumeMessageOrderlyService.this.defaultMQPushConsumerImpl.messageModel())
&& this.processQueue.isLockExpired()) {
log.warn("the message queue lock expired, so consume later, {}", this.messageQueue);
ConsumeMessageOrderlyService.this.tryLockLaterAndReconsume(this.messageQueue, this.processQueue, 10);
break;
}
// 当前周期消费时间超过连续时长，默认：60s，提交延迟消费请求。默认情况下，每消费1分钟休息10ms。
long interval = System.currentTimeMillis() - beginTime;
if (interval > MAX_TIME_CONSUME_CONTINUOUSLY) {
ConsumeMessageOrderlyService.this.submitConsumeRequestLater(processQueue, messageQueue, 10);
break;
}
// 获取消费消息。此处和并发消息请求不同，并发消息请求已经带了消费哪些消息。
final int consumeBatchSize =
ConsumeMessageOrderlyService.this.defaultMQPushConsumer.getConsumeMessageBatchMaxSize();
List<MessageExt> msgs = this.processQueue.takeMessags(consumeBatchSize);
defaultMQPushConsumerImpl.resetRetryAndNamespace(msgs, defaultMQPushConsumer.getConsumerGroup());
if (!msgs.isEmpty()) {
final ConsumeOrderlyContext context = new ConsumeOrderlyContext(this.messageQueue);
ConsumeOrderlyStatus status = null;
ConsumeMessageContext consumeMessageContext = null;
if (ConsumeMessageOrderlyService.this.defaultMQPushConsumerImpl.hasHook()) {
consumeMessageContext = new ConsumeMessageContext();
consumeMessageContext
.setConsumerGroup(ConsumeMessageOrderlyService.this.defaultMQPushConsumer.getConsumerGroup());
consumeMessageContext.setNamespace(defaultMQPushConsumer.getNamespace());
consumeMessageContext.setMq(messageQueue);
consumeMessageContext.setMsgList(msgs);
consumeMessageContext.setSuccess(false);
// init the consume context type
consumeMessageContext.setProps(new HashMap<String, String>());
ConsumeMessageOrderlyService.this.defaultMQPushConsumerImpl.executeHookBefore(consumeMessageContext);
}
// 执行消费
long beginTimestamp = System.currentTimeMillis();
ConsumeReturnType returnType = ConsumeReturnType.SUCCESS;
boolean hasException = false;
try {
this.processQueue.getLockConsume().lock(); // 锁定处理队列
if (this.processQueue.isDropped()) {
log.warn("consumeMessage, the message queue not be able to consume, because it's dropped. {}",
this.messageQueue);
break;
}
status = messageListener.consumeMessage(Collections.unmodifiableList(msgs), context);
} catch (Throwable e) {
log.warn("consumeMessage exception: {} Group: {} Msgs: {} MQ: {}",
RemotingHelper.exceptionSimpleDesc(e),
ConsumeMessageOrderlyService.this.consumerGroup,
msgs,
messageQueue);
hasException = true;
} finally {
this.processQueue.getLockConsume().unlock(); // 解锁
}
if (null == status
|| ConsumeOrderlyStatus.ROLLBACK == status
|| ConsumeOrderlyStatus.SUSPEND_CURRENT_QUEUE_A_MOMENT == status) {
log.warn("consumeMessage Orderly return not OK, Group: {} Msgs: {} MQ: {}",
ConsumeMessageOrderlyService.this.consumerGroup,
msgs,
messageQueue);
}
long consumeRT = System.currentTimeMillis() - beginTimestamp;
if (null == status) {
if (hasException) {
returnType = ConsumeReturnType.EXCEPTION;
} else {
returnType = ConsumeReturnType.RETURNNULL;
}
} else if (consumeRT >= defaultMQPushConsumer.getConsumeTimeout() * 60 * 1000) {
returnType = ConsumeReturnType.TIME_OUT;
} else if (ConsumeOrderlyStatus.SUSPEND_CURRENT_QUEUE_A_MOMENT == status) {
returnType = ConsumeReturnType.FAILED;
} else if (ConsumeOrderlyStatus.SUCCESS == status) {
returnType = ConsumeReturnType.SUCCESS;
}
if (ConsumeMessageOrderlyService.this.defaultMQPushConsumerImpl.hasHook()) {
consumeMessageContext.getProps().put(MixAll.CONSUME_CONTEXT_TYPE, returnType.name());
}
if (null == status) {
status = ConsumeOrderlyStatus.SUSPEND_CURRENT_QUEUE_A_MOMENT;
}
if (ConsumeMessageOrderlyService.this.defaultMQPushConsumerImpl.hasHook()) {
consumeMessageContext.setStatus(status.toString());
consumeMessageContext
.setSuccess(ConsumeOrderlyStatus.SUCCESS == status || ConsumeOrderlyStatus.COMMIT == status);
ConsumeMessageOrderlyService.this.defaultMQPushConsumerImpl.executeHookAfter(consumeMessageContext);
}
ConsumeMessageOrderlyService.this.getConsumerStatsManager()
.incConsumeRT(ConsumeMessageOrderlyService.this.consumerGroup, messageQueue.getTopic(), consumeRT);
continueConsume = ConsumeMessageOrderlyService.this.processConsumeResult(msgs, status, context, this);
} else {
continueConsume = false;
}
}
} else {
if (this.processQueue.isDropped()) {
log.warn("the message queue not be able to consume, because it's dropped. {}", this.messageQueue);
return;
}
ConsumeMessageOrderlyService.this.tryLockLaterAndReconsume(this.messageQueue, this.processQueue, 100);
}
}
}获取到锁对象后，使用synchronized尝试申请线程级独占锁。
如果加锁成功，同一时刻只有一个线程进行消息消费。
如果加锁失败，会延迟100ms重新尝试向broker端申请锁定messageQueue，锁定成功后重新提交消费请求
创建消息拉取任务时，消息客户端向broker端申请锁定MessageQueue，使得一个MessageQueue同一个时刻只能被一个消费客户端消费。
消息消费时，多线程针对同一个消息队列的消费先尝试使用synchronized申请独占锁，加锁成功才能进行消费，使得一个MessageQueue同一个时刻只能被一个消费客户端中一个线程消费。
这里其实还有很重要的一点是对processQueue的加锁，这里其实是保证了在 rebalance的过程中如果 processQueue 被分配给了另一个 consumer，但是当前已经被我这个 consumer 再消费，但是没提交，就有可能出现被两个消费者消费，所以得进行加锁保证不受 rebalance 影响。