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https://issues.apache.org/jira/browse/APEXMALHAR-2063?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15314771#comment-15314771
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Thomas Weise commented on APEXMALHAR-2063:
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Got it. And when flushed with checkpoint we can rely on the checkpointed state
to track the WAL offset while otherwise we need to find it from the WAL file
name or content.
> Integrate WAL to FS WindowDataManager
> -------------------------------------
>
> Key: APEXMALHAR-2063
> URL: https://issues.apache.org/jira/browse/APEXMALHAR-2063
> Project: Apache Apex Malhar
> Issue Type: Improvement
> Reporter: Chandni Singh
> Assignee: Chandni Singh
>
> FS Window Data Manager is used to save meta-data that helps in replaying
> tuples every completed application window after failure. For this it saves
> meta-data in a file per window. Having multiple small size files on hdfs
> cause issues as highlighted here:
> http://blog.cloudera.com/blog/2009/02/the-small-files-problem/
> Instead FS Window Data Manager can utilize the WAL to write data and maintain
> a mapping of how much data was flushed to WAL each window.
> In order to use FileSystemWAL for replaying data of a finished window, there
> are few changes made to FileSystemWAL this is because of following:
> 1. WindowDataManager needs to reply data of every finished window. This
> window may not be checkpointed.
> FileSystemWAL truncates the WAL file to the checkpointed point after recovery
> so this poses a problem.
> WindowDataManager should be able to control recovery of FileSystemWAL.
> 2. FileSystemWAL writes to temporary files. The mapping of temp files to
> actual file is part of its state which is checkpointed. Since
> WindowDataManager replays data of a window not yet checkpointed, it needs to
> know the actual temporary file the data is being persisted to.
> At a high level, WindowDataManager will persist meta information on file
> system which includes following details for every window
> - start wal pointer
> - end was pointer
> - wal file path
> This is a single file which is updated every end-window along with the actual
> data in WAL file.
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