RocketMQ实现分布式事务原理
RocketMQ是一种最终一致性的分布式事务,就是说它保证的是消息最终一致性,而不是像2PC、3PC、TCC那样强一致分布式事务.
Half Message(半消息)
Half Message是指暂不能被Consumer消费的消息。Producer 已经把消息成功发送到了 Broker 端,但此消息被标记为暂不能投递状态,处于该种状态下的消息称为半消息。需要 Producer对消息的二次确认后,Consumer才能去消费它。
消息回查
由于网络闪断,生产者应用重启等原因。导致 Producer 端一直没有对 Half Message(半消息) 进行 二次确认。这是Brock服务器会定时扫描长期处于半消息的消息,会主动询问 Producer端 该消息的最终状态(Commit或者Rollback),该消息即为消息回查。
流程
- A服务先发送Half Message给Brock端,消息中携带 B服务 即将要+100元的信息。
- 当A服务知道Half Message发送成功后,那么开始第3步执行本地事务。
- 执行本地事务,会有三种情况执行成功、执行失败、网络等原因导致没有响应。
如果本地事务成功,那么Product向Brock服务器发送Commit,这样B服务就可以消费该message。
如果本地事务失败,那么Product像Brock服务器发送Rollback,那么就会直接删除上面这条半消息。
如果因为网络等原因迟迟没有返回失败还是成功,那么会执行RocketMQ的回调接口,来进行事务的回查。
实例
public class TransactionListenerImpl implements TransactionListener {
private AtomicInteger transactionIndex = new AtomicInteger(0);
private ConcurrentHashMap<String, Integer> localTrans = new ConcurrentHashMap<>();
@Override
public LocalTransactionState executeLocalTransaction(Message msg, Object arg) {
int value = transactionIndex.getAndIncrement();
int status = value % 3;
localTrans.put(msg.getTransactionId(), status);
return LocalTransactionState.UNKNOW;
}
@Override
public LocalTransactionState checkLocalTransaction(MessageExt msg) {
Integer status = localTrans.get(msg.getTransactionId());
if (null != status) {
switch (status) {
case 0:
return LocalTransactionState.UNKNOW;
case 1:
return LocalTransactionState.COMMIT_MESSAGE;
case 2:
return LocalTransactionState.ROLLBACK_MESSAGE;
}
}
return LocalTransactionState.COMMIT_MESSAGE;
}
}
public class TransactionProducer {
public static void main(String[] args) throws MQClientException, InterruptedException {
TransactionListener transactionListener = new TransactionListenerImpl();
TransactionMQProducer producer = new TransactionMQProducer("please_rename_unique_group_name");
ExecutorService executorService = new ThreadPoolExecutor(2, 5, 100, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2000), new ThreadFactory() {
@Override
public Thread newThread(Runnable r) {
Thread thread = new Thread(r);
thread.setName("client-transaction-msg-check-thread");
return thread;
}
});
producer.setExecutorService(executorService);
producer.setTransactionListener(transactionListener);
producer.start();
String[] tags = new String[] {"TagA", "TagB", "TagC", "TagD", "TagE"};
for (int i = 0; i < 10; i++) {
try {
Message msg =
new Message("TopicTest1234", tags[i % tags.length], "KEY" + i,
("Hello RocketMQ " + i).getBytes(RemotingHelper.DEFAULT_CHARSET));
SendResult sendResult = producer.sendMessageInTransaction(msg, null);
System.out.printf("%s%n", sendResult);
Thread.sleep(10);
} catch (MQClientException | UnsupportedEncodingException e) {
e.printStackTrace();
}
}
for (int i = 0; i < 100000; i++) {
Thread.sleep(1000);
}
producer.shutdown();
}
}
发布于 2020-06-26 22:04