<Codis><JedisPool><DeadLock>

Overview

• Background: I start a thread [call thread A below]in Spark driver to handle opening files in codis, in which I start six thread [call sub threads below]to handle the files in parallel.

• What happened:

  • I found the codis openingFilePool never update since a given time.

  • So I looked into the Thread Dump tools of spark, and saw that thread A is always been WAITING. As below:
    
    

  • The normal one is as follow:

    

  • See the difference?
    The thread should be TIMED_WAITING(sleep), not be WAITING(wait for some other threads to awake it)!

  • It seems there is a deadlock?

• Thread A get a jedis instance from JedisResourcePool to get all opening files.

• Then, the sub threads handle these opening files and interact with codis in pipeline. [I use try(Pipeline pipeline = CodisPool.getPool().getResource().pipelined())]

Possible causes

• I am not sure this is because 1) there is no enough instance in JedisPool; or 2) there is a deadlock? 

  • try (Pipeline pipeline = CodisPool.getPool().getResource().pipelined())

    • I am not sure whether this code will close jedis and pipeline, or it will just close pipeline?

  • JedisResourcePool is not thread-safe?

    • Some says that we should sync when we getResource() from JedisResourcePool. I am not sure.

    • And also I think should I configure the number of Jedis pool. 

 Final words

• I have been wondering how to check whether the chained resource is closed. Uh... So I raised a question in stackoverflow.
• Show the amzaing answer:

• Try-with-resources close only variable, in your case Pipeline pipeline. You can read more in docs https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

  You can check it with example:

  MyResource class:

  class MyResource implements AutoCloseable {
  
      public SubResource getSubResource() {
          return new SubResource();
      }
  
      @Override
      public void close() throws Exception {
          System.out.println("Resource closed");
      }
  }

  SubResource class:

  class SubResource implements AutoCloseable{
      @Override
      public void close() throws Exception {
          System.out.println("SubResource closed");
      }
  }

  Main class:

  class Main {
  
      public static void main(String[] args) {
          try (SubResource s = new MyResource().getSubResource()) {
              System.out.println("Before closing");
          } catch (Exception e) {
              e.printStackTrace();
          }
  
          System.out.println("After closing");
      }
  }

  Execution result:

  Before closing
  SubResource closed
  After closing