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https://issues.apache.org/jira/browse/FLINK-7143?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16081386#comment-16081386
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Tzu-Li (Gordon) Tai commented on FLINK-7143:
--------------------------------------------
IIRC, sorting the fetched partition list was removed in favor of using the
hashCode of {{KafkaTopicPartition}}s for the mod operation. This must have been
a remnant from that change ...
Apparently, the current partition assignment tests do not have enough coverage.
We also need a test that verifies assignment stability in the case of different
fetched partitions ordering.
> 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
> 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 mode 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 partition id to do the mod {{if
> (kafkaTopicPartitions.get\(i\).getPartition() % numParallelSubtasks ==
> indexOfThisSubtask)}}. 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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