事务消息发送流程

半消息实现了分布式环境下的数据一致性的处理，生产者发送事务消息的流程如上图所示，通过对源码的学习，我们可以弄清楚下面几点，也是半消息机制的核心：

(资料图片)

1.为什么prepare消息不会被Consumer消费？

2.事务消息是如何提交和回滚的？

3.定时回查本地事务状态的实现细节。

发送事务消息源码分析

发送事务消息方法TransactionMQProducer.sendMessageInTransaction：

msg：消息tranExecuter：本地事务执行器arg：本地事务执行器参数

public TransactionSendResult sendMessageInTransaction(final Message msg,
        final LocalTransactionExecuter localTransactionExecuter, final Object arg)
        throws MQClientException {
        TransactionListener transactionListener = getCheckListener();
        if (null == localTransactionExecuter && null == transactionListener) {
            throw new MQClientException("tranExecutor is null", null);
        }

        // 忽视消息延迟的属性
        if (msg.getDelayTimeLevel() != 0) {
            MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_DELAY_TIME_LEVEL);
        }

        Validators.checkMessage(msg, this.defaultMQProducer);
		
		// 发送半消息
        SendResult sendResult = null;
        MessageAccessor.putProperty(msg, MessageConst.PROPERTY_TRANSACTION_PREPARED, "true");
        MessageAccessor.putProperty(msg, MessageConst.PROPERTY_PRODUCER_GROUP, this.defaultMQProducer.getProducerGroup());
        try {
            sendResult = this.send(msg);
        } catch (Exception e) {
            throw new MQClientException("send message Exception", e);
        }
		
		// 处理发送半消息的结果
        LocalTransactionState localTransactionState = LocalTransactionState.UNKNOW;
        Throwable localException = null;
        switch (sendResult.getSendStatus()) {
        	// 发送半消息成功，执行本地事务逻辑
            case SEND_OK: {
                try {
                    if (sendResult.getTransactionId() != null) {
                        msg.putUserProperty("__transactionId__", sendResult.getTransactionId());
                    }
                    String transactionId = msg.getProperty(MessageConst.PROPERTY_UNIQ_CLIENT_MESSAGE_ID_KEYIDX);
                    if (null != transactionId && !"".equals(transactionId)) {
                        msg.setTransactionId(transactionId);
                    }
                    // 执行本地事务逻辑
                    if (null != localTransactionExecuter) {
                        localTransactionState = localTransactionExecuter.executeLocalTransactionBranch(msg, arg);
                    } else if (transactionListener != null) {
                        log.debug("Used new transaction API");
                        localTransactionState = transactionListener.executeLocalTransaction(msg, arg);
                    }
                    if (null == localTransactionState) {
                        localTransactionState = LocalTransactionState.UNKNOW;
                    }

                    if (localTransactionState != LocalTransactionState.COMMIT_MESSAGE) {
                        log.info("executeLocalTransactionBranch return {}", localTransactionState);
                        log.info(msg.toString());
                    }
                } catch (Throwable e) {
                    log.info("executeLocalTransactionBranch exception", e);
                    log.info(msg.toString());
                    localException = e;
                }
            }
            break;
            // 发送半消息失败，标记本地事务状态为回滚
            case FLUSH_DISK_TIMEOUT:
            case FLUSH_SLAVE_TIMEOUT:
            case SLAVE_NOT_AVAILABLE:
                localTransactionState = LocalTransactionState.ROLLBACK_MESSAGE;
                break;
            default:
                break;
        }
		
		// 结束事务，设置消息 COMMIT / ROLLBACK
        try {
            this.endTransaction(msg, sendResult, localTransactionState, localException);
        } catch (Exception e) {
            log.warn("local transaction execute " + localTransactionState + ", but end broker transaction failed", e);
        }
		
		// 返回事务发送结果
        TransactionSendResult transactionSendResult = new TransactionSendResult();
        transactionSendResult.setSendStatus(sendResult.getSendStatus());
        transactionSendResult.setMessageQueue(sendResult.getMessageQueue());
        
        // 提取Prepared消息的uniqID
        transactionSendResult.setMsgId(sendResult.getMsgId());
        transactionSendResult.setQueueOffset(sendResult.getQueueOffset());
        transactionSendResult.setTransactionId(sendResult.getTransactionId());
        transactionSendResult.setLocalTransactionState(localTransactionState);
        return transactionSendResult;
    }

该方法的入参包含有一个需要用户实现本地事务的LocalTransactionExecuter executer，executer中会进行事务操作以保证本地事务和消息发送这两个操作的原子性。

由上面的源码可知：

Producer会首先发送一个半消息到Broker中：

半消息发送成功，执行事务半消息发送失败，不执行事务

半消息发送到Broker后不会被Consumer消费掉的原因有以下两点：

Broker在将消息写入CommitLog时会判断消息类型，如果是prepare或者rollback消息，ConsumeQueue的offset不变Broker在构造ConsumeQueue时会判断是否是处于prepare或者rollback状态的消息，如果是则不会将该消息放入ConsumeQueue里，Consumer在拉取消息时也就不会拉取到这条消息

Producer会根据半消息的发送结果和本地任务执行结果来决定如何处理事务（commit或rollback），方法最后调用了endTransaction来处理事务的执行结果，源码如下：

sendResult：发送半消息的结果localTransactionState：本地事务状态localException：执行本地事务逻辑产生的异常RemotingException：远程调用异常MQBrokerException：Broker异常InterruptedException：当线程中断异常UnknownHostException：未知host异常

public void endTransaction(
        final Message msg,
        final SendResult sendResult,
        final LocalTransactionState localTransactionState,
        final Throwable localException) throws RemotingException, MQBrokerException, InterruptedException, UnknownHostException {
        // 解码消息id
        final MessageId id;
        if (sendResult.getOffsetMsgId() != null) {
            id = MessageDecoder.decodeMessageId(sendResult.getOffsetMsgId());
        } else {
            id = MessageDecoder.decodeMessageId(sendResult.getMsgId());
        }

		// 创建请求
        String transactionId = sendResult.getTransactionId();
        final String brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(sendResult.getMessageQueue().getBrokerName());
        EndTransactionRequestHeader requestHeader = new EndTransactionRequestHeader();
        requestHeader.setTransactionId(transactionId);
        requestHeader.setCommitLogOffset(id.getOffset());
        switch (localTransactionState) {
            case COMMIT_MESSAGE:
                requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_COMMIT_TYPE);
                break;
            case ROLLBACK_MESSAGE:
                requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_ROLLBACK_TYPE);
                break;
            case UNKNOW:
                requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_NOT_TYPE);
                break;
            default:
                break;
        }

        doExecuteEndTransactionHook(msg, sendResult.getMsgId(), brokerAddr, localTransactionState, false);
        requestHeader.setProducerGroup(this.defaultMQProducer.getProducerGroup());
        requestHeader.setTranStateTableOffset(sendResult.getQueueOffset());
        requestHeader.setMsgId(sendResult.getMsgId());
        String remark = localException != null ? ("executeLocalTransactionBranch exception: " + localException.toString()) : null;

		// 提交 commit / rollback 消息 
        this.mQClientFactory.getMQClientAPIImpl().endTransactionOneway(brokerAddr, requestHeader, remark,
            this.defaultMQProducer.getSendMsgTimeout());
    }

该方法是将事务执行的结果发送给Broker，再由Broker决定是否进行消息投递，执行步骤如下：

1.收到消息后先检查是否是事务消息，如果不是事务消息则直接返回

2.根据请求头里的offset查询半消息，如果查询结果为空则直接返回

3.根据半消息构造新消息，新构造的消息会被重新写入到CommitLog里，rollback消息的消息体为空

4.如果是rollback消息，则该消息不会被投递

具体原因上文中已经分析过：只有commit消息才会被Broker投递给consumer

RocketMQ会将commit消息和rollback消息都写入到commitLog里，但rollback消息的消息体为空且不会被投递，CommitLog在删除过期消息时才会将其删除。当事务commit成功之后，RocketMQ会重新封装半消息并将其投递给Consumer端消费。