Details
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Improvement
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Status: Closed
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Major
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Resolution: Fixed
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Description
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.
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