ZooKeeper can revert to old data when a snapshot is created outside of normal 

                 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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