[jira] [Commented] (SPARK-17372) Running a file stream on a directory with partitioned subdirs throw NotSerializableException/StackOverflowError

["Apache Spark (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-17372?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15469072#comment-15469072 ] 

Apache Spark commented on SPARK-17372:
--------------------------------------

User 'tdas' has created a pull request for this issue:
https://github.com/apache/spark/pull/14987

> Running a file stream on a directory with partitioned subdirs throw NotSerializableException/StackOverflowError
> ---------------------------------------------------------------------------------------------------------------
>
>                 Key: SPARK-17372
>                 URL: https://issues.apache.org/jira/browse/SPARK-17372
>             Project: Spark
>          Issue Type: Sub-task
>          Components: SQL, Streaming
>    Affects Versions: 2.0.0
>            Reporter: Tathagata Das
>            Assignee: Tathagata Das
>
> Here is the result of my investigation. When we create a filestream on a directory that has partitioned subdirs (i.e. dir/x=y/), then ListingFileCatalog.allFiles returns the files in the dir as Seq[String] which internally is a Stream[String]. This is because of this [line|https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningAwareFileCatalog.scala#L93], where a LinkedHashSet.values.toSeq returns Stream. Then when the [FileStreamSource|https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/FileStreamSource.scala#L79] filters this Stream[String] to remove the seen files, it creates a new Stream[String], which has a filter function that has a $outer reference to the FileStreamSource (in Scala 2.10). Trying to serialize this Stream[String] causes NotSerializableException. This will happened even if there is just one file in the dir.
> Its important to note that this behavior is different in Scala 2.11. There is no $outer reference to FileStreamSource, so it does not throw NotSerializableException. However, with a large sequence of files (tested with 5000 files), it throws StackOverflowError. This is because how Stream class is implemented. Its basically like a linked list, and attempting to serialize a long Stream requires *recursively* going through linked list, thus resulting in StackOverflowError.
> The right solution is to convert the seq to an array before writing to the log.



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