RocketMQ源码 — 四、 Consumer 接收消息过程

Consumer

consumer pull message

订阅

在Consumer启动之前先将自己放到一个本地的集合中，再以后获取消费者的时候会用到，同时会将自己订阅的信息告诉broker

接收消息

consumer启动的时候会启动两个service：

RebalanceService：主要实现consumer的负载均衡，但是并不会直接发送获取消息的请求，而是构造request之后放到PullMessageService中，等待PullMessageService的线程取出执行

PullMessageService：主要负责从broker获取message，包含一个需要获取消息的请求队列（是阻塞的），并不断依次从队列中取出请求向broker send Request

执行时序图太大，截屏截不全，所以放在git上，在这里:RocketMQ.asta

从Broker pullMessage

在PullMessageService中通过netty发送pull消息的请求之后，Broker的remoteServer会收到request，然后在PullMessageProcessor中的processRequest处理，先会解析requestHeader，request中带了读取MessageStore的参数：

consumerGroup

topic

queueId

queueOffset

MaxMsgNums

subscriptionData（ConsumerManager中获取）

processRequest处理流程

判断Broker当前是否允许接收消息

找到subscriptionGroupConfig，subscriptionGroupTable，如果不存在当前的group则新增一个

是否subscriptionGroupConfig.consumeEnable

获取从TopicConfigManager.topicConfigTable获取topicConfig

topicConfig是否有读权限

校验queueId是否在范围内

ConsumerManager.getConsumerGroupInfo获取ConsumerGroupInfo

consumerGroupInfo.findSubscriptionData查找subscriptionData

MessageStore.getMessage读取消息，从文件中读取消息，属于pullMessage的核心方法

判断brokerRole，master和slave工作模式的哪一种

判断MessageResult.status

如果responseCode是SUSCESS，判断是使用heap还是非heap方式传输数据

使用netty序列化response返回netty客户端

负载均衡

// AllocateMessageQueueAveragely
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);
for (int i = 0; i < range; i++) {
result.add(mqAll.get((startIndex + i) % mqAll.size()));
}
return result;
}

// RebalanceImpl
private boolean updateProcessQueueTableInRebalance(final String topic, final Set<MessageQueue> mqSet, final boolean isOrder) {
boolean changed = false;

// 去掉topic对应的无用MessageQueue（不包含在processQueueTable或者pullExpired）
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)) {
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;
}
}
}
}

List<PullRequest> pullRequestList = new ArrayList<PullRequest>();
for (MessageQueue mq : mqSet) {
// 如果MessageQueue不在processQueueTable
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);
// 如果message还有数据需要读
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);
}
}
}

// 将pullRequest放在pullRequestQueue中等待去取数据
this.dispatchPullRequest(pullRequestList);

return changed;
}

issue

多个consumer读取同一个consumerQueue的时候怎么记录每个读取的进度

每个consumer只要记住自己读取哪一个队列，以及offset就可以了

pull消息过程中的关键类

DefaultMQPushConsumerImpl

供DefaultMQPushConsumer调用，作为consumer的默认实现：

AllocateMessageQueueAveragely

这个类主要是consumer消费的负载均衡算法

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;
}

// 基本原则，每个队列只能被一个consumer消费
// 当messageQueue个数小于等于consume的时候，排在前面（在list中的顺序）的consumer消费一个queue，index大于messageQueue之后的consumer消费不到queue，也就是为0
// 当messageQueue个数大于consumer的时候，分两种情况
//     当有余数（mod > 0）并且index < mod的时候，当前comsumer可以消费的队列个数是 mqAll.size() / cidAll.size() + 1
//     可以整除或者index 大于余数的时候，队列数为：mqAll.size() / cidAll.size()
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);
for (int i = 0; i < range; i++) {
result.add(mqAll.get((startIndex + i) % mqAll.size()));
}
return result;
}