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https://issues.apache.org/jira/browse/FLINK-7143?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16082619#comment-16082619
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Steven Zhen Wu commented on FLINK-7143:
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[~aljoscha] [~tzulitai] agree that {{partitionId % parallelism}} is probably
not a good idea for multiple topics case. what about the old sorting way? Sort
the list by (topic, partition) tuple first. Then do a simple round-robin
assignment (mod). Advantage is that it is much easier to see the assignment
pattern (comparing to hashCode). That usually can help with debugging: easier
to figure out expected assignment.
Here is an example of 3 topics and parallelism of 4
{code}
[subtask-id] topic, partition
[0] t1, p0
[1] t1, p1
[2] t1, p2
[3] t2, p0
[0] t3, p0
[1] t3, p1
{code}
> 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: 1.3.2
>
>
> 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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