Kezhu Wang created ZOOKEEPER-4698:
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Summary: Persistent watch events lost after reconnection
Key: ZOOKEEPER-4698
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-4698
Project: ZooKeeper
Issue Type: Bug
Components: server
Affects Versions: 3.8.1, 3.7.1
Reporter: Kezhu Wang
I found this in reply to [apache#1950
(comment)|https://github.com/apache/zookeeper/pull/1950#issuecomment-1553742525].
But it turns out a known issue [apache#1106
(comment)|https://github.com/apache/zookeeper/pull/1106#issuecomment-543860329].
I think it is worth to note separately in jira for potential future discussions
and fix. I have pushed a [test
case|https://github.com/kezhuw/zookeeper/commit/31d89e9829380559066fc2b83e3d38462380c5d4]
for this. It fails as expected.
{noformat}
[ERROR] Failures:
[ERROR] WatchEventWhenAutoResetTest.testPersistentRecursiveWatch:237 do not
receive a NodeDataChanged ==> expected: not <null>
[ERROR] WatchEventWhenAutoResetTest.testPersistentWatch:211 do not receive a
NodeDataChanged ==> expected: not <null>
{noformat}
It is hard to fix this with sole {{DataTree}}. Two independent comments
[pointed|https://github.com/apache/zookeeper/pull/1106#issuecomment-1366449561]
[out|https://github.com/kezhuw/zookeeper/commit/31d89e9829380559066fc2b83e3d38462380c5d4#diff-cfd09b7021c88da6631872e8a4a271f830162f7c5a63a140839ba029048493fdR227-R230]
this. I guess we have to walk through txn log to deliver a correct fix.
{quote}
Watches will not be received while disconnected from a server. When a client
reconnects, any previously registered watches will be reregistered and
triggered if needed. In general this all occurs transparently. There is one
case where a watch may be missed: a watch for the existence of a znode not yet
created will be missed if the znode is created and deleted while disconnected.
{quote}
This is [what our programer's guide
says|https://zookeeper.apache.org/doc/r3.8.1/zookeeperProgrammers.html#ch_zkWatches].
It is well-known, at least for me, that we can lose some transiently
intermediate events in reconnection. But in case of persistent watch, we can
lose more. This forces clients to rebuild their knowledge on reconnection.
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