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

["Aljoscha Krettek (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/FLINK-7143?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Aljoscha Krettek reopened FLINK-7143:
-------------------------------------

Re-open to adapt issue test.

> 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
>            Assignee: Tzu-Li (Gordon) Tai
>            Priority: Blocker
>             Fix For: 1.4.0, 1.3.2
>
>
> 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 {{i}} to mod against {{numParallelSubtasks}}. if the {{kafkaTopicPartitions}} 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 partitionId to do the mod {{if (kafkaTopicPartitions.get\(i\).getPartition() % numParallelSubtasks == indexOfThisSubtask)}}. That would result in stable assignment cross subtasks that is independent of ordering in the array.
> marking it as blocker because of its impact.



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