[jira] [Resolved] (SPARK-28650) Fix the guarantee of ForeachWriter

["Shixiong Zhu (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SPARK-28650?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Shixiong Zhu resolved SPARK-28650.
----------------------------------
    Fix Version/s: 2.4.5
         Assignee: Jungtaek Lim
       Resolution: Fixed

> Fix the guarantee of ForeachWriter
> ----------------------------------
>
>                 Key: SPARK-28650
>                 URL: https://issues.apache.org/jira/browse/SPARK-28650
>             Project: Spark
>          Issue Type: Documentation
>          Components: Structured Streaming
>    Affects Versions: 2.4.3
>            Reporter: Shixiong Zhu
>            Assignee: Jungtaek Lim
>            Priority: Major
>             Fix For: 2.4.5
>
>
> Right now ForeachWriter has the following guarantee:
> {code}
> If the streaming query is being executed in the micro-batch mode, then every partition
> represented by a unique tuple (partitionId, epochId) is guaranteed to have the same data.
> Hence, (partitionId, epochId) can be used to deduplicate and/or transactionally commit data
> and achieve exactly-once guarantees.
> {code}
>  
> But we can break this easily actually when restarting a query but a batch is re-run (e.g., upgrade Spark)
>  * Source returns a different DataFrame that has a different partition number (e.g., we start to not create empty partitions in Kafka Source V2).
>  * A new added optimization rule may change the number of partitions in the new run.
>  * Change the file split size in the new run.
> Since we cannot guarantee that the same (partitionId, epochId) has the same data. We should update the document for "ForeachWriter".



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