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https://issues.apache.org/jira/browse/APEXMALHAR-2063?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15314820#comment-15314820
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Chandni Singh commented on APEXMALHAR-2063:
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Embedding the window id into the WAL was not necessary with the Meta file
solution. The meta file contained offsets from/to in WAL for each window.
Here there will be just one entry per window and the entry is the incremental
state.
Yes naming convention + finalizing part files just after rotation will help.
Currently the finalization is delayed until a window is committed. For replay
of a window the finalization should be done as soon as the part file gets
rotated. This again is because we are using it for idempotency on the input
side and not on the output side.
I think we can get around without a Meta file. However, this still requires
some changes to FSWal mainly re-factoring.
> 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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