[jira] [Commented] (KAFKA-3932) Consumer fails to consume in a round robin fashion

["CHIENHSING WU (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-3932?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16655279#comment-16655279 ] 

CHIENHSING WU commented on KAFKA-3932:
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I encountered the same issue as well. Upon studying the source code and the [PIP|[https://cwiki.apache.org/confluence/display/KAFKA/KIP-41%3A+KafkaConsumer+Max+Records]|https://cwiki.apache.org/confluence/display/KAFKA/KIP-41%3A+KafkaConsumer+Max+Records].], I think the issues is the statement in PIP: "As before, we'd keep track of *which partition we left off* at so that the next iteration would *begin there*." I think it should *NOT* use the last partition in the next iteration; *it should pick the next one instead.* 

The simplest solution to impose the order to pick the next one is to use the order the consumer.internals.Fetcher receives the partition messages, as determined by *completedFetches* queue in that class. To avoid parsing the partition messages repeatedly. we can *save those parsed fetches to a list and maintain the next partition to get messages there.* 

Does it sound like a good approach? If this is not the right place to discuss the design please let me know where to engage. If this is agreeable I can contribute the implementation.

> Consumer fails to consume in a round robin fashion
> --------------------------------------------------
>
>                 Key: KAFKA-3932
>                 URL: https://issues.apache.org/jira/browse/KAFKA-3932
>             Project: Kafka
>          Issue Type: Bug
>          Components: consumer
>    Affects Versions: 0.10.0.0
>            Reporter: Elias Levy
>            Priority: Major
>
> The Java consumer fails consume messages in a round robin fashion.  This can lead to an unbalance consumption.
> In our use case we have a set of consumer that can take a significant amount of time consuming messages off a topic.  For this reason, we are using the pause/poll/resume pattern to ensure the consumer session is not timeout.  The topic that is being consumed has been preloaded with message.  That means there is a significant message lag when the consumer is first started.  To limit how many messages are consumed at a time, the consumer has been configured with max.poll.records=1.
> The first initial observation is that the client receive a large batch of messages for the first partition it decides to consume from and will consume all those messages before moving on, rather than returning a message from a different partition for each call to poll.
> We solved this issue by configuring max.partition.fetch.bytes to be small enough that only a single message will be returned by the broker on each fetch, although this would not be feasible if message size were highly variable.
> The behavior of the consumer after this change is to largely consume from a small number of partitions, usually just two, iterating between them, until it exhausts them, before moving to another partition.   This behavior is problematic if the messages have some rough time semantics and need to be process roughly time ordered across all partitions.
> It would be useful if the consumer has a pluggable API that allowed custom logic to select which partition to consume from next, thus enabling the creation of a round robin partition consumer.



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