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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