[GitHub] [kafka] ocadaruma commented on pull request #7539: KAFKA-6968: Adds calls to listener on rebalance of MockConsumer

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ocadaruma commented on pull request #7539:
URL: https://github.com/apache/kafka/pull/7539#issuecomment-626127671


   Yeah, rebalance listener is called upon `onJoinComplete/onJoinPrepare`, and 
it happens as part of `KafkaConsumer#poll` as stated in 
`ConsumerRebalanceListener`'s javadoc. 
(https://github.com/apache/kafka/blob/2.5.0/clients/src/main/java/org/apache/kafka/clients/consumer/ConsumerRebalanceListener.java#L48)
   
   as like below:
   ```
   KafkaConsumer#poll() =>
     KafkaConsumer#updateAssignmentMetadataIfNeeded() =>
       ConsumerCoordinator#poll() =>
         AbstractCoordinator#ensureActiveGroup() =>
           AbstractCoordinator#joinGroupIfNeeded() =>
             ConsumerCoordinator#onJoinPrepare()
   ```
   
   So, if we respect real KafkaConsumer behavior, rebalance listener invocation 
should be happen in `poll()` rather than `MockConsumer#rebalance()`, which 
might be called separately from poll().
   
   However, after reconsideration, though my initial intention was like above, 
I have different thought about this now.
   
   Even if we call rebalance listener in `MockConsumer#rebalance()` (as 
@efgpinto 's original patch), still we can simulate poll()-driven rebalance 
invocation (described above) as like below:
   ```java
   @Test
   public void testRebalance() {
       MockConsumer<String, String> consumer = new MockConsumer<>();
   
       consumer.subscribe(Arrays.asList("topic"), new SomeRebalanceListener());
   
       // simulate rebalance happens after first poll()
       consumer.scheduleNopPollTask();
       consumer.schedulePollTask(() => {
           consumer.rebalance(newAssignment);
       });
   
       consumer.poll(100L);
       consumer.poll(100L); // => rebalance will be triggered
   }
   ```
   
   So reverting my patch 
(https://github.com/apache/kafka/pull/7539/commits/4b62aa09eacfea70daa55151ee0928720c772e48)
 would be fine.


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