面试中经常会问到比如RocketMQ的事务是如何实现的呢?学习框架,我们不仅要熟练使用,更要掌握设计及原理,才算熟悉一个框架。
public class CreateOrderService {
@Autowired
private OrderDao orderDao;
@Autowired
private ExecutorService executorService;
private TransactionMQProducer producer;
// 初始化transactionListener 和 producer @Init
public void init() throws MQClientException {
TransactionListener transactionListener = createTransactionListener();
producer = new TransactionMQProducer("myGroup");
producer.setExecutorService(executorService);
producer.setTransactionListener(transactionListener);
producer.start();
}
// 创建订单服务的请求入口 @PUT
@RequestMapping(...)
public boolean createOrder(@RequestBody CreateOrderRequest request) {
// 根据创建订单请求创建一条消息 Message msg = createMessage(request);
// 发送事务消息 SendResult sendResult = producer.sendMessageInTransaction(msg, request);
// 返回:事务是否成功 return sendResult.getSendStatus() == SendStatus.SEND_OK;
}
private TransactionListener createTransactionListener() {
return new TransactionListener() {
@Override
public LocalTransactionState executeLocalTransaction(Message msg, Object arg) {
CreateOrderRequest request = (CreateOrderRequest ) arg;
try {
// 执行本地事务创建订单 orderDao.createOrderInDB(request);
// 如果没抛异常说明执行成功,提交事务消息 return LocalTransactionState.COMMIT_MESSAGE;
} catch (Throwable t) {
// 失败则直接回滚事务消息 return LocalTransactionState.ROLLBACK_MESSAGE;
}
}
// 反查本地事务 @Override
public LocalTransactionState checkLocalTransaction(MessageExt msg) {
// 从消息中获得订单ID String orderId = msg.getUserProperty("orderId");
// 去db查询订单号是否存在,若存在则提交事务 // 若不存在,可能是本地事务失败了,也可能是本地事务还在执行,所以返回UNKNOW return orderDao.isOrderIdExistsInDB(orderId)?
LocalTransactionState.COMMIT_MESSAGE: LocalTransactionState.UNKNOW;
}
};
}}
如上案例展示了一个订单创建服务,即往db插一条订单记录,并发一条创建订单的消息,要求写db和发消息俩个操作在一个事务内执行。 首先在init()方法中初始化了transactionListener和发生RocketMQ事务消息的变量producer。 - createOrder() 真正提供创建订单服务的方法,根据请求的参数创建一条消息,然后调用 producer发事务消息,并返回事务执行结果。
这样便使用RocketMQ的事务简单实现了一个创建订单的分布式事务。
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);
}
// ignore DelayTimeLevel parameter if (msg.getDelayTimeLevel() != 0) {
MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_DELAY_TIME_LEVEL);
}
Validators.checkMessage(msg, this.defaultMQProducer);
SendResult sendResult = null;
// 给待发送消息添加属性,表明是一个事务消息(即半消息) MessageAccessor.putProperty(msg, MessageConst.PROPERTY_TRANSACTION_PREPARED, "true");
MessageAccessor.putProperty(msg, MessageConst.PROPERTY_PRODUCER_GROUP, this.defaultMQProducer.getProducerGroup());
// 像发送普通消息一样,把这条事务消息发往Broker try {
sendResult = this.send(msg);
} catch (Exception e) {
throw new MQClientException("send message Exception", e);
}
LocalTransactionState localTransactionState = LocalTransactionState.UNKNOW;
Throwable localException = null;
switch (sendResult.getSendStatus()) {
// 事务消息发送成功 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);
}
if (null != localTransactionExecuter) {
localTransactionState = localTransactionExecuter.executeLocalTransactionBranch(msg, arg);
} else if (transactionListener != null) {
log.debug("Used new transaction API");
// 开始执行本地事务 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;
case FLUSH_DISK_TIMEOUT:
case FLUSH_SLAVE_TIMEOUT:
case SLAVE_NOT_AVAILABLE:
localTransactionState = LocalTransactionState.ROLLBACK_MESSAGE;
break;
default:
break;
}
// 事务过程的最后,给Broker发送提交或回滚事务的RPC请求。 try {
this.endTransaction(sendResult, localTransactionState, localException);
} catch (Exception e) {
log.warn("local transaction execute " + localTransactionState + ", but end broker transaction failed", e);
}
TransactionSendResult transactionSendResult = new TransactionSendResult();
transactionSendResult.setSendStatus(sendResult.getSendStatus());
transactionSendResult.setMessageQueue(sendResult.getMessageQueue());
transactionSendResult.setMsgId(sendResult.getMsgId());
transactionSendResult.setQueueOffset(sendResult.getQueueOffset());
transactionSendResult.setTransactionId(sendResult.getTransactionId());
transactionSendResult.setLocalTransactionState(localTransactionState);
return transactionSendResult;
}
public void endTransaction(
final SendResult sendResult,
final LocalTransactionState localTransactionState,
final Throwable localException) throws RemotingException, MQBrokerException, InterruptedException, UnknownHostException {
final MessageId id;
if (sendResult.getOffsetMsgId() != null) {
id = MessageDecoder.decodeMessageId(sendResult.getOffsetMsgId());
} else {
id = MessageDecoder.decodeMessageId(sendResult.getMsgId());
}
String transactionId = sendResult.getTransactionId();
final String brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(sendResult.getMessageQueue().getBrokerName());
EndTransactionRequestHeader requestHeader = new EndTransactionRequestHeader();
requestHeader.setTransactionId(transactionId);
requestHeader.setCommitLogOffset(id.getOffset());
// 根据事务消息和本地事务的执行结果localTransactionState,决定后续行为 // producer就是给Broker发送了一个单向的RPC请求,告知Broker完成事务的提交或者回滚 switch (localTransactionState) {
// 提交事务消息 case COMMIT_MESSAGE:
requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_COMMIT_TYPE);
break;
// 回滚事务消息 case ROLLBACK_MESSAGE:
requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_ROLLBACK_TYPE);
break;
case UNKNOW:
requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_NOT_TYPE);
break;
default:
break;
}
requestHeader.setProducerGroup(this.defaultMQProducer.getProducerGroup());
requestHeader.setTranStateTableOffset(sendResult.getQueueOffset());
requestHeader.setMsgId(sendResult.getMsgId());
String remark = localException != null ? ("executeLocalTransactionBranch exception: " + localException.toString()) : null;
this.mQClientFactory.getMQClientAPIImpl().endTransactionOneway(brokerAddr, requestHeader, remark,
this.defaultMQProducer.getSendMsgTimeout());
}
有事务反查机制作兜底,该RPC请求即使失败或丢失,也不会影响事务最终的结果。最后构建事务消息的发送结果,并返回。
跟进去看看真正处理半消息的业务逻辑,这段处理逻辑在类
RocketMQ并非将事务消息保存至消息中 client 指定的 queue,而是记录了原始的 topic 和 queue 后,把这个事务消息保存在 - 特殊的内部 topic:RMQ_SYS_TRANS_HALF_TOPIC
- 序号为 0
的 queue
这套 topic 和 queue 对消费者不可见,因此里面的消息也永远不会被消费。这就保证在事务提交成功之前,这个事务消息对 Consumer 是消费不到的。
在Broker的TransactionalMessageCheckService服务中启动了一个定时器,定时从事务消息queue中读出所有待反查的事务消息。
AbstractTransactionalMessageCheckListener#resolveHalfMsg - 针对每个需要反查的半消息,Broker会给对应的Producer发一个要求执行事务状态反查的RPC请求
- AbstractTransactionalMessageCheckListener#sendCheckMessage
- Broker2Client#checkProducerTransactionState
根据RPC返回响应中的反查结果,来决定这个半消息是需要提交还是回滚,或者后续继续来反查。
最后,提交或者回滚事务。首先把半消息标记为已处理 1. 如果是提交事务,就把半消息从半消息队列中复制到该消息真正的topic和queue中 2. 如果是回滚事务,什么都不做 - EndTransactionProcessor#processRequest
最后结束该事务。
RocketMQ是基于两阶段提交来实现的事务,把这些事务消息暂存在一个特殊的queue中,待事务提交后再移动到业务队列中。最后,RocketMQ的事务适用于解决本地事务和发消息的数据一致性问题。
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