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https://issues.apache.org/jira/browse/CURATOR-311?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15207595#comment-15207595
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Jian Fang edited comment on CURATOR-311 at 3/23/16 5:05 PM:
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I don't have time to create unit tests to reproduce this because it won't be
easy. But I did observe this behavior very often in my clusters.
For example, we have three EC2 instances, i.e., m1, m2, and m3. Each instance
runs a zookeeper peer. For some reason, m3 is terminated and a new EC2 instance
m4 is provisioned to replace m3. We called zookeeper reconfig() API to update
the membership. But unfortunately, from time to time, we observed that one
process that used the curator client read the stall data before the replacement
(we updated the data after the replacement). Thus, that process stuck with old
data and failed the whole system. Manually restarted the process or used the
mechanism I described above to force SharedValue to call readValue() when
connection state changed did resolve this issue.
I looked at the code. SharedValue only used the watcher to update the value
in-memory. That is why I suspected that the watcher may be lost or the session
reconnection logic did not handled the watcher properly.
Anyway, I wonder why SharedValue only used the watcher for value updates. There
are always race conditions in a distributed system to lose events or lose the
watcher since the watcher is set for each zookeeper API call. For example, if a
Zookeeper session expires and a new session could not be established in time,
then the events would be lost. Furthermore, if the session handling logic could
not re-register the watcher properly, the value update would never happen
anymore. From my own experiences, anything could happen in production.
Shouldn't a backup mechanism be used to check the data under certain conditions
or make sure that the watcher is re-registered properly after a new session is
established?
was (Author: john.jian.fang):
I don't have time to create unit tests to reproduce this because it won't be
easy. But I did observe this behavior very often in my clusters.
For example, we have three EC2 instances, i.e., m1, m2, and m3. For some
reason, m3 is terminated and a new EC2 instance m4 is provisioned to replace
m3. We called zookeeper reconfig() API to update the membership. But
unfortunately, from time to time, we observed that one old instance always read
the stall data before the replacement (we updated the data after the
replacement). Thus, that instance stuck. Manually restarted the process or
used the mechanism I described above to force SharedValue to call readValue()
when connection state changed did resolve this issue.
I looked at the code. SharedValue only used the watcher to update the value
in-memory. That is why I suspected that the watcher may be lost or the session
reconnection logic did not handled the watcher properly.
Anyway, I wonder why SharedValue only used the watcher for value updates. There
are always race conditions in a distributed system to lose events or lose the
watcher since the watcher is set for each zookeeper API call. Shouldn't a
backup mechanism be used to prevent that from happening?
> SharedValue could hold stall data when quourm membership changes
> ----------------------------------------------------------------
>
> Key: CURATOR-311
> URL: https://issues.apache.org/jira/browse/CURATOR-311
> Project: Apache Curator
> Issue Type: Bug
> Components: Recipes
> Affects Versions: 3.1.0
> Environment: Linux
> Reporter: Jian Fang
>
> We run a Zookeeper 3.5.1-alpha quorum on EC2 instances and the quorum members
> could be changed, for example, one peer could be replaced by a new EC2
> instance due to EC2 instance termination. We use Apache Curator 3.1.0 as the
> zookeeper client. During our testing, we found the SharedValue data structure
> could hold stall data during and after one peer is replaced and thus led to
> the system failure.
> We look into the SharedValue code. Seems it always returns the value from an
> in-memory reference variable and the value is only updated by a watcher. If
> for any reason, the watch is lost, then the value would never get a chance to
> be updated again.
>
> Right now, we added a connection state listener to force SharedValue to call
> readValue(), i.e., read the data from zookeeper directly, if the connection
> state has been changed to RECONNECTED to work around this issue.
> It would be great if this issue could be fixed in Curator directly.
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