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From "Benjamin Reed (JIRA)" <j...@apache.org>
Subject [jira] Created: (ZOOKEEPER-582) ZooKeeper can revert to old data when a snapshot is created outside of normal processing
Date Tue, 17 Nov 2009 20:58:39 GMT
ZooKeeper can revert to old data when a snapshot is created outside of normal processing
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                 Key: ZOOKEEPER-582
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-582
             Project: Zookeeper
          Issue Type: Bug
          Components: server
    Affects Versions: 3.2.1
            Reporter: Benjamin Reed
             Fix For: 3.2.2


when zookeeper starts up it will restore the most recent state (latest zxid) it finds in the
data directory. unfortunately, in the quorum version of zookeeper updates are logged using
an epoch based on the latest log file in a directory. if there is a snapshot with a higher
epoch than the log files, the zookeeper server will start logging using an epoch one higher
than the highest log file.

so if a data directory has a snapshot with an epoch of 27 and there are no log files, zookeeper
will start logging changes using epoch 1. if the cluster restarts the state will be restored
from the snapshot with the epoch of 27, which in effect, restores old data.

normal operation of zookeeper will never result in this situation.

this does not effect standalone zookeeper.

a fix should make sure to use an epoch one higher than the current state, whether it comes
from the snapshot or log, and should include a sanity check to make sure that a follower never
connects to a leader that has a lower epoch than its own.

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