[GitHub] [rocketmq] mypgether opened a new issue #1092: why use SingleThreadScheduledExecutor, may cause bug?

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mypgether opened a new issue #1092: why use SingleThreadScheduledExecutor, may cause bug?
URL: https://github.com/apache/rocketmq/issues/1092
 
 
   Hello, When I read rocketmq source code, I found In MQClientInstance class, scheduledExecutorService variable is instance of **SingleThreadScheduledExecutor**. And In startScheduledTask method it submit almost _4 fixedRate scheduledTasks_. 
   
   I had a test on SingleThreadScheduledExecutor, it show **if one task is timeout or blocking, may cause other tasks delay, and not working as fixedRate writting in code.** So  I wonder why use SingleThreadScheduledExecutor?
   
   Here is the source code and log.
   
   ```
   import java.util.concurrent.Executors;
   import java.util.concurrent.ScheduledExecutorService;
   import java.util.concurrent.TimeUnit;
   import lombok.extern.slf4j.Slf4j;
   import org.junit.Test;
   
   @Slf4j
   public class SingleThreadExecutorTest {
   
     private final ScheduledExecutorService scheduledExecutorService = Executors
         .newSingleThreadScheduledExecutor(r -> new Thread(r, "singleThread"));
   
     @Test
     public void testSingle() throws InterruptedException {
       scheduledExecutorService.scheduleAtFixedRate(() -> {
         log.info("start working a ,threadId: {},threadName: {}", Thread.currentThread().getId(),
             Thread.currentThread().getName());
       }, 10, 2000 * 1, TimeUnit.MILLISECONDS);
   
       scheduledExecutorService.scheduleAtFixedRate(() -> {
         log.info("start working b ,threadId: {},threadName: {}", Thread.currentThread().getId(),
             Thread.currentThread().getName());
   
         try {
           TimeUnit.SECONDS.sleep(2);
         } catch (InterruptedException e) {
           e.printStackTrace();
         }
       }, 10, 1000 * 1, TimeUnit.MILLISECONDS);
   
       scheduledExecutorService.scheduleAtFixedRate(() -> {
         log.info("start working c ,threadId: {},threadName: {}", Thread.currentThread().getId(),
             Thread.currentThread().getName());
       }, 10, 2000 * 1, TimeUnit.MILLISECONDS);
   
       Thread.sleep(10000);
     }
   }
   ```
   ```
   2019-03-17 11:32:52 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working a ,threadId: 14,threadName: singleThread
   2019-03-17 11:32:52 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working b ,threadId: 14,threadName: singleThread
   2019-03-17 11:32:54 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working c ,threadId: 14,threadName: singleThread
   2019-03-17 11:32:54 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working b ,threadId: 14,threadName: singleThread
   2019-03-17 11:32:56 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working a ,threadId: 14,threadName: singleThread
   2019-03-17 11:32:56 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working b ,threadId: 14,threadName: singleThread
   2019-03-17 11:32:58 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working c ,threadId: 14,threadName: singleThread
   2019-03-17 11:32:58 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working b ,threadId: 14,threadName: singleThread
   2019-03-17 11:33:00 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working a ,threadId: 14,threadName: singleThread
   2019-03-17 11:33:00 [singleThread] INFO  c.g.r.t.r.SingleThreadExecutorTest - start working b ,threadId: 14,threadName: singleThread
   Disconnected from the target VM, address: '127.0.0.1:61138', transport: 'socket'
   Process finished with exit code 0
   ```

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