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Posted to dev@kafka.apache.org by "Vahid Hashemian (JIRA)" <ji...@apache.org> on 2017/01/04 22:31:58 UTC
[jira] [Commented] (KAFKA-4547) Consumer.position returns incorrect
results for Kafka 0.10.1.0 client
[ https://issues.apache.org/jira/browse/KAFKA-4547?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15799561#comment-15799561 ]
Vahid Hashemian commented on KAFKA-4547:
----------------------------------------
So here's what I've found so far.
The inconsistency in behavior seems to have started by [this PR|https://github.com/apache/kafka/pull/1720]. Due to refactoring of the method {{KafkaConsumer.updateFetchPositions}} the outcome of this scenario has changed.
To be more specific, I started with a fresh Kafka cluster, and ran these steps:
# Started ZooKeeper and a Kafka broker
# Created a topic {{foo}} with 3 partitions.
# Ran the consumer code provided to check offsets. They are 0,0,0.
# Ran the producer code to produce messages.
# Ran the consumer code again. The offsets are now 0,0,2.
If we consider what's happening in step 5, the consumer code checks for offset position of partitions one by one.
------------------------
So it starts with {{foo-0}}:
This partition has no valid offset position. Therefore, a call is made to [{{updateFetchPositions(Collections.singleton(partition));}}|https://github.com/apache/kafka/blob/be36b322749003581474e2c84a3ec9ba2aaec53c/clients/src/main/java/org/apache/kafka/clients/consumer/KafkaConsumer.java#L1221].
In that [method|https://github.com/apache/kafka/blob/be36b322749003581474e2c84a3ec9ba2aaec53c/clients/src/main/java/org/apache/kafka/clients/consumer/KafkaConsumer.java#L1428]:
* {{fetcher.resetOffsetsIfNeeded(partitions);}} resets the offset of {{foo-0}} and seeks to the latest offset (which is 2)
* {{subscriptions.hasAllFetchPositions()}} is false, because there are two other partitions that still waiting for offset reset, so the if block runs
** {{coordinator.refreshCommittedOffsetsIfNeeded();}} updates committed offsets of all partitions to {{foo-0}}: 0, {{foo-1}}: 0, {{foo-2}}: 0
** {{fetcher.updateFetchPositions(partitions);}} updates fetch position of {{foo-0}} only based on its committed offset, which is 0. So it seeks to offset 0 of {{foo-0}} (which will be the reported offset for that partition)
------------------------
Then it's {{foo-1}}'s turn which has no offset position yet:
Again, a call to {{updateFetchPositions(Collections.singleton(partition));}} is made.
* {{fetcher.resetOffsetsIfNeeded(partitions);}} similar to above, resets the offset of {{foo-1}} and seeks to the latest offset (which is 2)
* {{subscriptions.hasAllFetchPositions()}} is still false (since {{foo-2}} still has no offset position)
** {{coordinator.refreshCommittedOffsetsIfNeeded();}} does nothing, since committed offsets are already set above
** {{fetcher.updateFetchPositions(partitions);}} updates fetch position for {{foo-1}} based on its committed offset, which is 0. so it seeks to offset 0 of {{foo-1}} (which will be the reported offset for that partition)
------------------------
Finally it's {{foo-2}}'s turn which has no offset position yet:
Another call to {{updateFetchPositions(Collections.singleton(partition));}} is made.
* {{fetcher.resetOffsetsIfNeeded(partitions);}} similar to above, resets the offset of {{foo-2}} and seeks to the latest offset (which is 2)
* {{subscriptions.hasAllFetchPositions()}} is now true since all offsets have valid positions now. So the if block does not run and 2 will be the reported offset for foo-2.
------------------------
The significance pausing partitions to reproduce the issue is that the seek in the last call ({{fetcher.updateFetchPositions(partitions);}}) for partitions {{foo-0}} and {{foo-1}} would not occur if those partitions are un-paused (resumed) (due to [this {{if}} block|https://github.com/apache/kafka/blob/be36b322749003581474e2c84a3ec9ba2aaec53c/clients/src/main/java/org/apache/kafka/clients/consumer/internals/Fetcher.java#L177]).
A potential work around would be to modify the {{if}} block condition as in [here|https://github.com/apache/kafka/compare/trunk...vahidhashemian:KAFKA-4547?expand=1] to avoid unnecessary commits and seeks.
It would be great if [~hachikuji] could weigh in here too.
> Consumer.position returns incorrect results for Kafka 0.10.1.0 client
> ---------------------------------------------------------------------
>
> Key: KAFKA-4547
> URL: https://issues.apache.org/jira/browse/KAFKA-4547
> Project: Kafka
> Issue Type: Bug
> Components: clients
> Affects Versions: 0.10.1.0
> Environment: Windows Kafka 0.10.1.0
> Reporter: Pranav Nakhe
> Assignee: Vahid Hashemian
> Labels: clients
> Fix For: 0.10.2.0
>
> Attachments: issuerep.zip
>
>
> Consider the following code -
> KafkaConsumer<String, String> consumer = new KafkaConsumer<String, String>(props);
> List<TopicPartition> listOfPartitions = new ArrayList();
> for (int i = 0; i < consumer.partitionsFor("IssueTopic").size(); i++) {
> listOfPartitions.add(new TopicPartition("IssueTopic", i));
> }
> consumer.assign(listOfPartitions);
> consumer.pause(listOfPartitions);
> consumer.seekToEnd(listOfPartitions);
> // consumer.resume(listOfPartitions); -- commented out
> for(int i = 0; i < listOfPartitions.size(); i++) {
> System.out.println(consumer.position(listOfPartitions.get(i)));
> }
>
> I have created a topic IssueTopic with 3 partitions with a single replica on my single node kafka installation (0.10.1.0)
> The behavior noticed for Kafka client 0.10.1.0 as against Kafka client 0.10.0.1
> A) Initially when there are no messages on IssueTopic running the above program returns
> 0.10.1.0
> 0
> 0
> 0
> 0.10.0.1
> 0
> 0
> 0
> B) Next I send 6 messages and see that the messages have been evenly distributed across the three partitions. Running the above program now returns
> 0.10.1.0
> 0
> 0
> 2
> 0.10.0.1
> 2
> 2
> 2
> Clearly there is a difference in behavior for the 2 clients.
> Now after seekToEnd call if I make a call to resume (uncomment the resume call in code above) then the behavior is
> 0.10.1.0
> 2
> 2
> 2
> 0.10.0.1
> 2
> 2
> 2
> This is an issue I came across when using the spark kafka integration for 0.10. When I use kafka 0.10.1.0 I started seeing this issue. I had raised a pull request to resolve that issue [SPARK-18779] but when looking at the kafka client implementation/documentation now it seems the issue is with kafka and not with spark. There does not seem to be any documentation which specifies/implies that we need to call resume after seekToEnd for position to return the correct value. Also there is a clear difference in the behavior in the two kafka client implementations.
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