[GitHub] [zookeeper] hanm opened a new pull request #1069: ZOOKEEPER-3056: Fails to load database with missing snapshot file but with valid transaction log file.

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hanm opened a new pull request #1069: ZOOKEEPER-3056: Fails to load database with missing snapshot file but with valid transaction log file.
URL: https://github.com/apache/zookeeper/pull/1069
 
 
   ZOOKEEPER-2325 introduced a check on snapshot and transaction log files during recovery, which will treat empty or missing snapshot files with the presence of transaction log files illegal state, and abort start up process. 
   
   For old versions of ZooKeeper, it's possible to have valid transaction log files but no snapshot. For example, if snap count set too low, or server crashes before the first snapshot was taken. For new versions of ZooKeeper with ZOOKEEPER-2325, this is not a problem as ZooKeeper will make sure the presence of at least one snapshot file (e.g. on a fresh start with empty local data, ZooKeeper will create one snapshot file as part of the start up.).
   
   So we need provide a smooth upgrade path for users with old version of ZooKeeper. This patch introduces a system property flag, once set, will skip the empty snapshot validation during start up. The flag will reset itself once start up is finished, to allow the feature introduced in ZOOKEEPER-2325 continuing work without restart the ZooKeeper process. The default value of this flag is false, as we don't want to disable the features introduced in ZOOKEEPER-2325. We also want the flag to be explicitly set by admin, as ZooKeeper itself is not able to tell when to trust missing snapshot or not.
   

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