RocketMQ事務(wù)消息（Transactional?Message）是指應(yīng)用本地事務(wù)和發(fā)送消息操作可以被定義到全局事務(wù)中，要么同時(shí)成功，要么同時(shí)失敗。本文主要介紹了RocketMQ事務(wù)消息的相關(guān)知識，需要的可以參考一下

事務(wù)消息發(fā)送流程

半消息實(shí)現(xiàn)了分布式環(huán)境下的數(shù)據(jù)一致性的處理，生產(chǎn)者發(fā)送事務(wù)消息的流程如上圖所示，通過對源碼的學(xué)習(xí)，我們可以弄清楚下面幾點(diǎn)，也是半消息機(jī)制的核心：

1.為什么prepare消息不會被Consumer消費(fèi)？

2.事務(wù)消息是如何提交和回滾的？

3.定時(shí)回查本地事務(wù)狀態(tài)的實(shí)現(xiàn)細(xì)節(jié)。

發(fā)送事務(wù)消息源碼分析

發(fā)送事務(wù)消息方法TransactionMQProducer.sendMessageInTransaction：

msg：消息
tranExecuter：本地事務(wù)執(zhí)行器
arg：本地事務(wù)執(zhí)行器參數(shù)

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);
        }

        // 忽視消息延遲的屬性
        if (msg.getDelayTimeLevel() != 0) {
            MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_DELAY_TIME_LEVEL);
        }

        Validators.checkMessage(msg, this.defaultMQProducer);
		
		// 發(fā)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);
        }
		
		// 處理發(fā)送半消息的結(jié)果
        LocalTransactionState localTransactionState = LocalTransactionState.UNKNOW;
        Throwable localException = null;
        switch (sendResult.getSendStatus()) {
        	// 發(fā)送半消息成功，執(zhí)行本地事務(wù)邏輯
            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);
                    }
                    // 執(zhí)行本地事務(wù)邏輯
                    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;
            // 發(fā)送半消息失敗，標(biāo)記本地事務(wù)狀態(tài)為回滾
            case FLUSH_DISK_TIMEOUT:
            case FLUSH_SLAVE_TIMEOUT:
            case SLAVE_NOT_AVAILABLE:
                localTransactionState = LocalTransactionState.ROLLBACK_MESSAGE;
                break;
            default:
                break;
        }
		
		// 結(jié)束事務(wù)，設(shè)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);
        }
		
		// 返回事務(wù)發(fā)送結(jié)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;
    }

該方法的入?yún)幸粋€(gè)需要用戶實(shí)現(xiàn)本地事務(wù)的LocalTransactionExecuter executer，executer中會進(jìn)行事務(wù)操作以保證本地事務(wù)和消息發(fā)送這兩個(gè)操作的原子性。

由上面的源碼可知：

Producer會首先發(fā)送一個(gè)半消息到Broker中：

半消息發(fā)送成功，執(zhí)行事務(wù)
半消息發(fā)送失敗，不執(zhí)行事務(wù)

半消息發(fā)送到Broker后不會被Consumer消費(fèi)掉的原因有以下兩點(diǎn)：

Broker在將消息寫入CommitLog時(shí)會判斷消息類型，如果是prepare或者rollback消息，ConsumeQueue的offset不變
Broker在構(gòu)造ConsumeQueue時(shí)會判斷是否是處于prepare或者rollback狀態(tài)的消息，如果是則不會將該消息放入ConsumeQueue里，Consumer在拉取消息時(shí)也就不會拉取到這條消息

Producer會根據(jù)半消息的發(fā)送結(jié)果和本地任務(wù)執(zhí)行結(jié)果來決定如何處理事務(wù)（commit或rollback），方法最后調(diào)用了endTransaction來處理事務(wù)的執(zhí)行結(jié)果，源碼如下：

sendResult：發(fā)送半消息的結(jié)果
localTransactionState：本地事務(wù)狀態(tài)
localException：執(zhí)行本地事務(wù)邏輯產(chǎn)生的異常
RemotingException：遠(yuǎn)程調(diào)用異常
MQBrokerException：Broker異常
InterruptedException：當(dāng)線程中斷異常
UnknownHostException：未知host異常

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

		// 創(chuàng)建請求
        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());
    }

該方法是將事務(wù)執(zhí)行的結(jié)果發(fā)送給Broker，再由Broker決定是否進(jìn)行消息投遞，執(zhí)行步驟如下：

1.收到消息后先檢查是否是事務(wù)消息，如果不是事務(wù)消息則直接返回

2.根據(jù)請求頭里的offset查詢半消息，如果查詢結(jié)果為空則直接返回

3.根據(jù)半消息構(gòu)造新消息，新構(gòu)造的消息會被重新寫入到CommitLog里，rollback消息的消息體為空

4.如果是rollback消息，則該消息不會被投遞

具體原因上文中已經(jīng)分析過：只有commit消息才會被Broker投遞給consumer

RocketMQ會將commit消息和rollback消息都寫入到commitLog里，但rollback消息的消息體為空且不會被投遞，CommitLog在刪除過期消息時(shí)才會將其刪除。當(dāng)事務(wù)commit成功之后，RocketMQ會重新封裝半消息并將其投遞給Consumer端消費(fèi)。