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https://issues.apache.org/jira/browse/SPARK-22552?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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sachin malhotra resolved SPARK-22552.
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Resolution: Not A Problem
> Cannot Union multiple kafka streams
> -----------------------------------
>
> Key: SPARK-22552
> URL: https://issues.apache.org/jira/browse/SPARK-22552
> Project: Spark
> Issue Type: Bug
> Components: Structured Streaming
> Affects Versions: 2.2.0
> Reporter: sachin malhotra
> Priority: Minor
>
> When unioning multiple kafka streams I learned that the resulting dataframe
> only contains the data that exists in the dataframe that initiated the union
> i.e. if df1.union(df2) (or a chaining of unions) the result will only contain
> the rows that exist in df1.
> Now to be more specific this occurs when data comes in during the same
> micro-batch for all three streams. If you wait for each single row to be
> processed for each stream the union does return the right results.
> For example, if you have 3 kafka streams and you:
> send message 1 to stream 1, WAIT for batch to finish, send message 2 to
> stream 2, wait for batch to finish, send message 3 to stream 3, wait for
> batch to finish. Union will return the right data.
> But if you,
> send message 1,2,3, WAIT for batch to finish, you only receive data in the
> first stream when unioning all three dataframes
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