java client watches inconsistently triggered on reconnect
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Key: ZOOKEEPER-1412
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1412
Project: ZooKeeper
Issue Type: Bug
Components: java client
Affects Versions: 3.4.3, 3.4.2, 3.4.1, 3.4.0, 3.3.4, 3.3.3
Reporter: Botond Hejj
Fix For: 3.3.5, 3.4.4
I've observed an inconsistent behavior in java client watches. The
inconsistency relates to the behavior after the client reconnects to the
zookeeper ensemble.
After the client reconnects to the ensemble only those watches should trigger
which should have been triggered also if the connections was not lost. This
means if I watch for changes in node /foo and there is no change there than my
watch should not be triggered on reconnecting to the ensemble.
This is not always the case in the java client.
I've debugged the issues and I could locate the case when the watch is always
triggered on reconnect. This is consistently happening if I connect to a
follower in the ensemble and I don't do any operation which goes through the
leader.
Looking at the code I see that the client stores the lastzxid and sends that
with its request. This is 0 on startup and will be updated everytime from the
server replies. This lastzxid is also sent to the server after reconnect
together with watches. The server decides which watch to trigger based on this
lastzxid probably because that should mean the last known state of the client.
If this lastzxid is 0 than all the watches are triggered.
I've checked why is this lastzxid 0. I thought it shouldn't be since there was
already a request to the server to set the watch and in the reply the server
could have sent back the zxid but it turns out that it sends just 0. Looking at
the server code I see that for requests which doesn't go through the leader the
follower server just sends back the same zxid that the client sent.
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