rocket源码 consumer端根据offset获取消息

提出疑问

第一次pullMessage时是不是根据offset去获取呢

offset从远程获取到还是存在本地呢

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consumer端在启动时会开启负载均衡服务RebalanceService，一系列调用后会执行RebalanceImpl.doRebalance()方法，内部调用方法rebalanceByTopic()，这里主要是为了找到可供消费的队列，如果存在，则获取这个队列可供消费的消息偏移量

// RebalanceImpl

    private boolean updateProcessQueueTableInRebalance(final String topic, final Set mqSet,
        final boolean isOrder) {
        boolean changed = false;

        ...
        
        List pullRequestList = new ArrayList();
        for (MessageQueue mq : mqSet) {
            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();
                // 获取nextOffset
                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);
                }
            }
        }
        // 构造请求发送至PullService，不断的拉取消息
        this.dispatchPullRequest(pullRequestList);

        return changed;
    }

可以看到开始的偏移量是从computePullFromWhere(mq)获取到的。主要看一下PushImpl的内容

// RebalancePushImpl 部分代码

    @Override
    public long computePullFromWhere(MessageQueue mq) {
        long result = -1;

        final OffsetStore offsetStore = this.defaultMQPushConsumerImpl.getOffsetStore();

        long lastOffset = offsetStore.readOffset(mq, ReadOffsetType.READ_FROM_STORE);
        if (lastOffset >= 0) {
            result = lastOffset;
        }
        // First start,no offset
        else if (-1 == lastOffset) {
            if (mq.getTopic().startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                result = 0L;
            } else {
                try {
                    result = this.mQClientFactory.getMQAdminImpl().maxOffset(mq);
                } catch (MQClientException e) {
                    result = -1;
                }
            }
        } else {
            result = -1;
        }
    
        return result;
    }

可以看到是从offsetStore中获取的。

在DefaultMQPushConsumerImpl的start方法中可知默认的集群模式下使用的是RemoteBrokerOffsetStore。

    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;
    }

去查看RemoteBrokerOffsetStore类代码，type为READ_FROM_STORE

    @Override
    public long readOffset(final MessageQueue mq, final ReadOffsetType type) {
        if (mq != null) {
            switch (type) {
                case MEMORY_FIRST_THEN_STORE:
                case READ_FROM_MEMORY: {
                    AtomicLong offset = this.offsetTable.get(mq);
                    if (offset != null) {
                        return offset.get();
                    } else if (ReadOffsetType.READ_FROM_MEMORY == type) {
                        return -1;
                    }
                }
                case READ_FROM_STORE: {
                    try {
                        long brokerOffset = this.fetchConsumeOffsetFromBroker(mq);
                        AtomicLong offset = new AtomicLong(brokerOffset);
                        this.updateOffset(mq, offset.get(), false);
                        return brokerOffset;
                    }
                    // No offset in broker
                    catch (MQBrokerException e) {
                        return -1;
                    }
                    //Other exceptions
                    catch (Exception e) {
                        log.warn("fetchConsumeOffsetFromBroker exception, " + mq, e);
                        return -2;
                    }
                }
                default:
                    break;
            }
        }

        return -1;
    }

看一下fetchConsumeOffsetFromBroker方法

    private long fetchConsumeOffsetFromBroker(MessageQueue mq) throws RemotingException, MQBrokerException,
        InterruptedException, MQClientException {
        FindBrokerResult findBrokerResult = this.mQClientFactory.findBrokerAddressInAdmin(mq.getBrokerName());
        if (null == findBrokerResult) {

            this.mQClientFactory.updateTopicRouteInfoFromNameServer(mq.getTopic());
            findBrokerResult = this.mQClientFactory.findBrokerAddressInAdmin(mq.getBrokerName());
        }

        if (findBrokerResult != null) {
            QueryConsumerOffsetRequestHeader requestHeader = new QueryConsumerOffsetRequestHeader();
            requestHeader.setTopic(mq.getTopic());
            requestHeader.setConsumerGroup(this.groupName);
            requestHeader.setQueueId(mq.getQueueId());

            return this.mQClientFactory.getMQClientAPIImpl().queryConsumerOffset(
                findBrokerResult.getBrokerAddr(), requestHeader, 1000 * 5);
        } else {
            throw new MQClientException("The broker[" + mq.getBrokerName() + "] not exist", null);
        }
    }

可以发现是去broker端查询offset。

这里要注意，提到的offset对应的是ConsumeQueue的偏移量，ConsumeQueue中存储的为定长20字节的数据。根据offset找到要消费的消息在CommitLog中的offset和消息长度，再去CommitLog中获取真正的消息。

第一次如果找到消息，broker会返回下次要消费的消息偏移量，在DefaultMQPushConsumerImpl.pullMessage方法中会将该偏移量设置到原request，下次直接使用该偏移量拉取即可。

// DefaultMQPushConsumerImpl.pullMessage pullCallback部分代码
    case FOUND:
        long prevRequestOffset = pullRequest.getNextOffset();
        pullRequest.setNextOffset(pullResult.getNextBeginOffset());