[jira] [Created] (FLINK-7143) Partition assignment for Kafka consumer is not stable

["Steven Zhen Wu (JIRA)" <ji...@apache.org>]

Steven Zhen Wu created FLINK-7143:
-------------------------------------

             Summary: Partition assignment for Kafka consumer is not stable
                 Key: FLINK-7143
                 URL: https://issues.apache.org/jira/browse/FLINK-7143
             Project: Flink
          Issue Type: Bug
          Components: Kafka Connector
    Affects Versions: 1.3.1
            Reporter: Steven Zhen Wu
            Priority: Blocker


while deploying Flink 1.3 release to hundreds of routing jobs, we found some issues with partition assignment for Kafka consumer. some partitions weren't assigned and some partitions got assigned more than once.

Here is the bug introduced in Flink 1.3. 
{code}
	protected static void initializeSubscribedPartitionsToStartOffsets(...) {
                ...
		for (int i = 0; i < kafkaTopicPartitions.size(); i++) {
			if (i % numParallelSubtasks == indexOfThisSubtask) {
				if (startupMode != StartupMode.SPECIFIC_OFFSETS) {
					subscribedPartitionsToStartOffsets.put(kafkaTopicPartitions.get(i), startupMode.getStateSentinel());
				}
                ...
         }
{code}

The bug is using array index {code}i{code} to mode against {code}numParallelSubtasks{code}. if the {code}kafkaTopicPartitions{code} has different order among different subtasks, assignment is not stable cross subtasks and creates the assignment issue mentioned earlier. fix is also very simple, we should use partition id to do the mod {code}if (kafkaTopicPartitions.get(i).getPartition() % numParallelSubtasks == indexOfThisSubtask){code}. That would result in stable assignment cross subtasks that independent of ordering in the array.

marking it as blocker because of its impact.



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