Shh! Is this really good for asynchronous events?

The story background

At the beginning of this year, I wrote an article 《 gather and watch ： The process of internal decoupling based on event mechanism 》. This article is mainly about using ES Heterogeneous data scenarios . Program subscription Mysql Binlog Changes , Then the program uses Spring Event To distribute specific events , Because the data change of a table may need to update more than one ES Indexes .

In order to make it easier for you to understand, I copied the pictures of the previous scheme , as follows ：

There is a problem with the scheme in the figure above , That's what we're going to talk about today .

The problem is when MQ Consumer After receiving the message , Just post it directly Event 了 , If it's synchronous , No problem . If a EventListener Failed to process in , So this message will not ACK.

If it's an asynchronous release Event Scene , As soon as the news is released ACK 了 . Even if someone EventListener Failed to process in ,MQ I can't feel it , There will be no redelivery of messages , That's the problem .

Solution

Scheme 1

Now that the news has ACK 了 , Then don't use MQ The retrying function of , Can users try again by themselves ？

It's certainly possible , Whether the internal processing is successful or not is certain to be known , If the processing fails, you can try again by default , Or try again with a certain strategy . I can't, but I can drop it in the library , Keep records .

The problem is that it's too boring , Every place you use it has to do it , And for the program brother who will take over your code in the future , This is likely to make the little brother's hair fall off slowly ....

It doesn't matter if you fall off , The point is that he doesn't know what to do with it , Maybe one day I'll be carrying the pot , Pathetic ....

Option two

To ensure the consistency of messages and business processing , You can't do it right away ACK operation . It's about waiting for the business to complete before deciding whether to ACK.

If there is a failure to handle, it should not ACK, So you can reuse MQ The retrial mechanism of .

Analyze it , This is a typical asynchronous to synchronous scenario . image Dubbo There's also this scene in , So we can learn from Dubbo In the realization of ideas .

Create a DefaultFuture It is used to synchronously wait to get the result of task execution . And then in MQ Where to consume DefaultFuture.

  

   
@Service
   
@RocketMQMessageListener(topic = "${rocketmq.topic.data_change}", consumerGroup = "${rocketmq.group.data_change_consumer}")
   
public class DataChangeConsume implements RocketMQListener<DataChangeRequest> {
   
    @Autowired
   
    private ApplicationContext applicationContext;
   
    @Autowired
   
    private CustomApplicationContextAware customApplicationContextAware;
   
    @Override
   
    public void onMessage(DataChangeRequest dataChangeRequest) {
   
        log.info("received message {} , Thread {}", dataChangeRequest, Thread.currentThread().getName());
   
        DataChangeEvent event = new DataChangeEvent(this);
   
        event.setChangeType(dataChangeRequest.getChangeType());
   
        event.setTable(dataChangeRequest.getTable());
   
        event.setMessageId(dataChangeRequest.getMessageId());
   
        DefaultFuture defaultFuture = DefaultFuture.newFuture(dataChangeRequest, customApplicationContextAware.getTaskCount(), 6000 * 10);
   
        applicationContext.publishEvent(event);
   
        Boolean result = defaultFuture.get();
   
        log.info("MessageId {}  Processing results  {}", dataChangeRequest.getMessageId(), result);
   
        if (!result) {
   
            throw new RuntimeException(" Processing failed , No news ACK, Wait for the next time to try again ");
   
        }
   
    }
   
}
  

newFuture() Event parameters are passed in , Timeout time , Number of tasks several parameters . The number of tasks is used to judge all EventListener Is it all done .

defaultFuture.get(); It doesn't block , Wait for all tasks to complete before returning the result , If all the business is successful , So it will return true, End of the process , Message automatically ACK.

If you return false Prove that there are processing failures or timeouts , There is no need to ACK 了 , Throw an exception and wait to try again .

  

   
public Boolean get() {
   
    if (isDone()) {
   
        return true;
   
    }
   
    long start = System.currentTimeMillis();
   
    lock.lock();
   
    try {
   
        while (!isDone()) {
   
            done.await(timeout, TimeUnit.MILLISECONDS);
   
            //  There is feedback from failed tasks 
   
            if (!isSuccessDone()) {
   
                return false;
   
            }
   
            //  All executed successfully 
   
            if (isDone()) {
   
                return true;
   
            }
   
            //  Overtime 
   
            if (System.currentTimeMillis() - start > timeout) {
   
                return false;
   
            }
   
        }
   
    } catch (InterruptedException e) {
   
        throw new RuntimeException(e);
   
    } finally {
   
        lock.unlock();
   
    }
   
    return true;
   
}
  

isDone() Can judge whether the number of tasks with feedback results is consistent with the total number , If it is consistent, it means that the whole execution is completed .

  

   
public boolean isDone() {
   
    return feedbackResultCount.get() == taskCount;
   
}
  

How to give feedback after the task is completed ? It's impossible for every method used to care about , So we define a facet to do this .

  

   
@Aspect
   
@Component
   
public class EventListenerAspect {
   
    @Around(value = "@annotation(eventListener)")
   
    public Object aroundAdvice(ProceedingJoinPoint joinpoint, EventListener eventListener) throws Throwable {
   
      DataChangeEvent event = null;
   
      boolean executeResult = true;
   
       try {
   
         event = (DataChangeEvent)joinpoint.getArgs()[0];
   
         Object result = joinpoint.proceed();
   
         return result;
   
      } catch (Exception e) {
   
         executeResult = false;
   
          throw e;
   
      } finally {
   
         DefaultFuture.received(event.getMessageId(), executeResult);
   
      }
   
    }
   
}
  

adopt DefaultFuture.received() Feedback on execution results .

  

   
public static void received(String id, boolean result) {
   
    DefaultFuture future = FUTURES.get(id);
   
    if (future != null) {
   
        //  Add up the number of failed tasks 
   
        if (!result) {
   
            future.feedbackFailResultCount.incrementAndGet();
   
        }
   
        //  Add up the number of completed tasks 
   
        future.feedbackResultCount.incrementAndGet();
   
        if (future.isDone()) {
   
            FUTURES.remove(id);
   
            future.doReceived();
   
        }
   
    }
   
}
   
private void doReceived() {
   
    lock.lock();
   
    try {
   
        if (done != null) {
   
            //  Wake up blocked threads 
   
            done.signal();
   
        }
   
    } finally {
   
        lock.unlock();
   
    }
   
}
  

Let's summarize the whole process ：

• received MQ news , Assemble into DefaultFuture, adopt get Method to get the execution result , This method blocks when the execution is not finished .
• Cut through the section and add EventListener Methods , Judge whether there is an exception to judge the execution result of the task .
• adopt DefaultFuture.received() Feedback results .
• Whether the calculation is complete in feedback , When all is done, the blocked thread is awakened .DefaultFuture.get() You can get the results .
• Is it necessary to ACK operation .

It's important to note that each EventListener The logic of internal consumption should be idempotent control .

Source code address ：https://github.com/yinjihuan/kitty-cloud/tree/master/kitty-cloud-mqconsume

About author ： Yin Jihuan , Simple technology enthusiasts ,《Spring Cloud Microservices - Full stack technology and case analysis 》, 《Spring Cloud Microservices introduction Actual combat and advanced 》 author , official account Ape world Originator . Personal wechat jihuan900, Welcome to hook up with .

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