1、消息消费概述
- 消息消费方式
拉取、推送。 - 消费者组与消费模式
多个消费者组成一个消费组，两种模式：集群（消息被其中任何一个消息者消费）、广播模式（全部消费者消费）。 - ConsumeFromWhere consumeFromWhere
从何处开始消费，可选值：
1）CONSUME_FROM_LAST_OFFSET：上一次消费偏移量
2）CONSUME_FROM_FIRST_OFFSET：从头开始
3）CONSUME_FROM_TIMESTAMP：从某个时间点开始 - 消费进度存储
其实现类为：OffsetStore offsetStore。消费者需要记录消息消费的进度：
1）广播模式：广播模式由于每个消费者都需要消费消息，故消息的进度（最后消费的偏移量可以保存在本地）。
2）集群模式：由于集群中的消费者只要一个消费消息即可，故消息的消费进度，需要保存在集中点，故 RocketMQ存储在Broker所在的服务器。
2、消息消费实现
首先看一下消费 Demo。
使用推送模式，设置消费者所属组，订阅主题、定义消息消费回调接口，推送消息后消费方具体业务处理，并返回CONSUME_SUCCESS表示消费成功。
消息消费者具体实现类：org.apache.rocketmq.client.impl.consumer.DefaultMQPushConsumerImpl。
2.1 DefaultMQPushConsumerImpl
2.1.1 消费端初始化（构造方法）
然后开始重点从 star t方法深入研究 DefaultMQPushConsumerImpl 的内部机制。
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; // @1
this.checkConfig(); //@2
this.copySubscription(); //@3
if (this.defaultMQPushConsumer.getMessageModel() == MessageModel.CLUSTERING) { // @4
this.defaultMQPushConsumer.changeInstanceNameToPID();
}
this.mQClientFactory = MQClientManager.getInstance().getAndCreateMQClientInstance(this.defaultMQPushConsumer, this.rpcHook);
this.rebalanceImpl.setConsumerGroup(this.defaultMQPushConsumer.getConsumerGroup()); // @5
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); //@6
if (this.defaultMQPushConsumer.getOffsetStore() != null) { // @7
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.offsetStore.load(); //@8
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(); //@9
boolean registerOK = mQClientFactory.registerConsumer(this.defaultMQPushConsumer.getConsumerGroup(), this); //@10
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(); //@11
this.mQClientFactory.checkClientInBroker(); //@12
this.mQClientFactory.sendHeartbeatToAllBrokerWithLock(); //@13
this.mQClientFactory.rebalanceImmediately(); //@14
代码@2：checkConfig,检查配置信息，主要检查消费者组（consumeGroup）、消息消费方式（messageModel）、消息消费开始偏移量（consumeFromWhere）、消息队列分配算法（AllocateMessageQueueStrategy）、订阅消息主题（Map<topic,sub expression ）,消息回调监听器(MessageListener)、顺序消息模式时是否只有一个消息队列等等。
代码@3：copySubscription 加工订阅信息，将 Map ,同时，如果消息消费模式为集群模式，还需要为该消费组对应一个重试主题。
代码@4：如果消息消费模式为集群模式，并且当前的实例名为 DEFAULT，替换为当前客户端进程的PID。
代码@5：负载均衡相关实现，后文重点关注。
代码@6：pullAPIWrapper，消息拉取API封装类，后文重点关注。
代码@7：消费进度存储，如果是集群模式，使用远程存储 RemoteBrokerOffsetStore，如果是广播模式，则使用本地存储LocalFileOffsetStore，后文重点关注。
代码@8：加载消息消费进度。
代码@9：消息消费服务并启动，后文重点关注。
代码@10：向远程 Broker 服务器注册消费者。
代码@11，更新订阅信息。
代码@12：检测broker状态。
代码@13：发送心跳包。
代码@14：重新负载。
上面包含的信息量巨大，我们一一进行分析。
2、1.1.1 MQClientInstance 概述
this.mQClientFactory = MQClientManager.getInstance().getAndCreateMQClientInstance(this.defaultMQPushConsumer, this.rpcHook);
public MQClientInstance getAndCreateMQClientInstance(final ClientConfig clientConfig, RPCHook rpcHook) {
String clientId = clientConfig.buildMQClientId();
MQClientInstance instance = this.factoryTable.get(clientId);
if (null == instance) {
instance =
new MQClientInstance(clientConfig.cloneClientConfig(),
this.factoryIndexGenerator.getAndIncrement(), clientId, rpcHook);
MQClientInstance prev = this.factoryTable.putIfAbsent(clientId, instance);
if (prev != null) {
instance = prev;
log.warn("Returned Previous MQClientInstance for clientId:[{}]", clientId);
} else {
log.info("Created new MQClientInstance for clientId:[{}]", clientId);
}
}
return instance;
}
public String buildMQClientId() {
StringBuilder sb = new StringBuilder();
sb.append(this.getClientIP());
sb.append("@");
sb.append(this.getInstanceName());
if (!UtilAll.isBlank(this.unitName)) {
sb.append("@");
sb.append(this.unitName);
}
return sb.toString();
}
从这段代码可以看成，一个客户端 IP@InstanceName 只会持有一个 MQClientInstance 对象，MQClientInstance 无论是消费者还是生产者，都在应用程序这一端。
有了这一层认识，我们就重点关注一下该类的属性：
- ClientConfig clientConfig
配置信息。 - int instanceIndex
MQClientInstance在同一台机器上的创建序号。 - String clientId
客户端id。 - ConcurrentMap
<String/\* group \*/, MQProducerInner>producerTable
生产组–》消息生产者，也就是在应用程序一端，每个生产者组在同一台应用服务器只需要初始化一个生产者实例。 - ConcurrentMap
<String/\* group \*/, MQConsumerInner>consumerTable
消费组–》消费者，也就是在应用程序一 端 ，每个消费组，在同一台应用服务器只需要初始化一个消费者即可。 - ConcurrentMap
<String/\* group \*/, MQAdminExtInner>adminExtTable
主要是处理运维命令的。 - NettyClientConfig nettyClientConfig
网络配置。 - MQClientAPIImpl mQClientAPIImpl
MQ 客户端实现类。 - MQAdminImpl mQAdminImpl
MQ 管理命令实现类。 - ConcurrentMap
<String/\* Topic \*/, TopicRouteData>topicRouteTable
topic 路由信息。 - ConcurrentMap
<String/\* Broker Name \*/, HashMap<Long/* brokerId */, String/* address */>>` brokerAddrTable
broker信息，这些信息存在于NameServer,但缓存在本地客户端，供生产者、消费者共同使用。 - ClientRemotingProcessor clientRemotingProcessor
客户端命令处理器。 - PullMessageService pullMessageService
消息拉取线程，一个MQClientInstance 只会启动一个消息拉取线程。 - RebalanceService rebalanceService
队列动态负载线程。 - DefaultMQProducer defaultMQProducer
默认的消息生产者。 - ConsumerStatsManager consumerStatsManager
消费端统计。 - AtomicLong sendHeartbeatTimesTotal = new AtomicLong(0)
心跳包发送次数。 - ServiceState serviceState = ServiceState.CREATE_JUST
状态。
MQClientInstance mq客户端实例，每台应用服务器将持有一个MQClientInstance对象，供该应用服务器的消费者，生产者使用。该类是消费者，生产者网络处理的核心类。
2、1.1.2 RebalanceImpl
同样在这里我们重点先理解该类的作用。
RebalanceImpl 是 consume 的重新负载，什么意思呢？就是消费者与消费队列的对应关系，我们来思考一个问题，比如现在有4个消息队列（q1,q2,q3,q4），3个消费者(m1,m2,m3)，那么消费者与消息队列的对应关系是什么呢?我们按照一个轮询算法来表示， m1(q1,q4) m2(q2) m3(q3)，如果此时q2消息队列失效（所在的broker挂了），那么消息队列的消费就需要重新分配，RebalanceImpl 就是干这事的，该类的调用轨迹如下：(MQClientInstance start –> (this.rebalanceService.start()) —> RebalanceService.run（this.mqClientFactory.doRebalance()） —> MQConsumerInner.doRebalance(DefaultMQPushConsumerImpl) —>RebalanceImpl.doRebalance。
在这里着重说明一点：消息队列数量与消费者关系：1个消费者可以消费多个队列，但1个消息队列只会被一个消费者消费；如果消费者数量大于消息队列数量，则有的消费者会消费不到消息（集群模式）。
2.1.2 消息消费过程
消息的消费过程，就是从服务器拉取，然后消费者进行消费，再根据业务反馈是否成功消费来推动消费进度，也就是消息的消费进度并不是保存在服务端（比如 commitlog 文件中），而是保存在消费端（可以是本地（广播模式）、broke端（集群模式））具体的代码实现：
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 size = processQueue.getMsgCount().get();
if (size > this.defaultMQPushConsumer.getPullThresholdForQueue()) {
this.executePullRequestLater(pullRequest, PULL_TIME_DELAY_MILLS_WHEN_FLOW_CONTROL);
if ((flowControlTimes1++ % 1000) == 0) {
log.warn(
"the consumer message buffer is full, so do flow control, minOffset={}, maxOffset={}, size={}, pullRequest={}, flowControlTimes={}",
processQueue.getMsgTreeMap().firstKey(), processQueue.getMsgTreeMap().lastKey(), size, pullRequest, flowControlTimes1);
}
return;
}
if (!this.consumeOrderly) {
if (processQueue.getMaxSpan() > this.defaultMQPushConsumer.getConsumeConcurrentlyMaxSpan()) {
this.executePullRequestLater(pullRequest, PULL_TIME_DELAY_MILLS_WHEN_FLOW_CONTROL);
if ((flowControlTimes2++ % 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, flowControlTimes2);
}
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 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 dispathToConsume = processQueue.putMessage(pullResult.getMsgFoundList());
DefaultMQPushConsumerImpl.this.consumeMessageService.submitConsumeRequest(//
pullResult.getMsgFoundList(), //
processQueue, //
pullRequest.getMessageQueue(), //
dispathToConsume);
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 {
this.pullAPIWrapper.pullKernelImpl(//
pullRequest.getMessageQueue(), // 1
subExpression, // 2
subscriptionData.getExpressionType(), // 3
subscriptionData.getSubVersion(), // 4
pullRequest.getNextOffset(), // 5
this.defaultMQPushConsumer.getPullBatchSize(), // 6
sysFlag, // 7
commitOffsetValue, // 8
BROKER_SUSPEND_MAX_TIME_MILLIS, // 9
CONSUMER_TIMEOUT_MILLIS_WHEN_SUSPEND, // 10
CommunicationMode.ASYNC, // 11
pullCallback // 12
);
} catch (Exception e) {
log.error("pullKernelImpl exception", e);
this.executePullRequestLater(pullRequest, PULL_TIME_DELAY_MILLS_WHEN_EXCEPTION);
}
消费过程暂不详细讲解了，后续文章会进一步对消息消费的各个方面做出详细的剖析。
本文旨在梳理出消息的消费过程，消息队列与消费者的负载、消费模式（广播、集群），消息进度等等。
后续文章重点关注：
1、 消费进度反馈源码分析；
2、 消费者消息负载实现原理；
3、 消息过滤与tag机制；
4、 常用问题分析；
1）一个消费者多个线程（线程之间如何协调）
备注：本文是《RocketMQ技术内幕》的前期素材，建议关注笔者的书籍：《RocketMQ技术内幕》。