[jira] [Resolved] (KAFKA-13008) Stream will stop processing data for a long time while waiting for the partition lag

["Guozhang Wang (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KAFKA-13008?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Guozhang Wang resolved KAFKA-13008.
-----------------------------------
      Assignee: Guozhang Wang
    Resolution: Fixed

> Stream will stop processing data for a long time while waiting for the partition lag
> ------------------------------------------------------------------------------------
>
>                 Key: KAFKA-13008
>                 URL: https://issues.apache.org/jira/browse/KAFKA-13008
>             Project: Kafka
>          Issue Type: Bug
>          Components: streams
>    Affects Versions: 3.0.0
>            Reporter: Luke Chen
>            Assignee: Guozhang Wang
>            Priority: Blocker
>             Fix For: 3.0.0
>
>         Attachments: image-2021-07-07-11-19-55-630.png
>
>
> In KIP-695, we improved the task idling mechanism by checking partition lag. It's a good improvement for timestamp sync. But I found it will cause the stream stop processing the data for a long time while waiting for the partition metadata.
>  
> I've been investigating this case for a while, and figuring out the issue will happen in below situation (or similar situation):
>  # start 2 streams (each with 1 thread) to consume from a topicA (with 3 partitions: A-0, A-1, A-2)
>  # After 2 streams started, the partitions assignment are: (I skipped some other processing related partitions for simplicity)
>  stream1-thread1: A-0, A-1 
>  stream2-thread1: A-2
>  # start processing some data, assume now, the position and high watermark is:
>  A-0: offset: 2, highWM: 2
>  A-1: offset: 2, highWM: 2
>  A-2: offset: 2, highWM: 2
>  # Now, stream3 joined, so trigger rebalance with this assignment:
>  stream1-thread1: A-0 
>  stream2-thread1: A-2
>  stream3-thread1: A-1
>  # Suddenly, stream3 left, so now, rebalance again, with the step 2 assignment:
>  stream1-thread1: A-0, *A-1* 
>  stream2-thread1: A-2
>  (note: after initialization, the  position of A-1 will be: position: null, highWM: null)
>  # Now, note that, the partition A-1 used to get assigned to stream1-thread1, and now, it's back. And also, assume the partition A-1 has slow input (ex: 1 record per 30 mins), and partition A-0 has fast input (ex: 10K records / sec). So, now, the stream1-thread1 won't process any data until we got input from partition A-1 (even if partition A-0 is buffered a lot, and we have `{{max.task.idle.ms}}` set to 0).
>  
> The reason why the stream1-thread1 won't process any data is because we can't get the lag of partition A-1. And why we can't get the lag? It's because
>  # In KIP-695, we use consumer's cache to get the partition lag, to avoid remote call
>  # The lag for a partition will be cleared if the assignment in this round doesn't have this partition. check [here|https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/consumer/internals/SubscriptionState.java#L272]. So, in the above example, the metadata cache for partition A-1 will be cleared in step 4, and re-initialized (to null) in step 5
>  # In KIP-227, we introduced a fetch session to have incremental fetch request/response. That is, if the session existed, the client(consumer) will get the update only when the fetched partition have update (ex: new data). So, in the above case, the partition A-1 has slow input (ex: 1 record per 30 mins), it won't have update until next 30 mins, or wait for the fetch session become inactive for (default) 2 mins to be evicted. Either case, the metadata won't be updated for a while.
>  
> In KIP-695, if we don't get the partition lag, we can't determine the partition data status to do timestamp sync, so we'll keep waiting and not processing any data. That's why this issue will happen.
>  
> *Proposed solution:*
>  # If we don't get the current lag for a partition, or the current lag > 0, we start to wait for max.task.idle.ms, and reset the deadline when we get the partition lag, like what we did in previous KIP-353
>  # Introduce a waiting time config when no partition lag, or partition lag keeps > 0 (need KIP)
> [~vvcephei] [~guozhang] , any suggestions?
>  
> cc [~ableegoldman]  [~mjsax] , this is the root cause that in [https://github.com/apache/kafka/pull/10736,] we discussed and thought there's a data lose situation. FYI.



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