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From "Fournier, Camille F." <Camille.Fourn...@gs.com>
Subject RE: devops/admin/client question: What do you do when you rollback?
Date Thu, 04 Aug 2011 19:05:55 GMT
Right now the server just detects that the zxid is wrong, and calls close on the client. The
client logs:
15:01:47,593 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1159] - Unable
to read additional data from server sessionid 0x131962b00540000, likely server has closed
socket, closing socket connection and attempting reconnect
(branch 3.3.3)

I will poke around and see if I can figure out a nicer way to indicate this condition. The
expired state is perfectly fine for me in my use case.

C


-----Original Message-----
From: Patrick Hunt [mailto:phunt@apache.org] 
Sent: Thursday, August 04, 2011 1:51 PM
To: user@zookeeper.apache.org
Subject: Re: devops/admin/client question: What do you do when you rollback?

On Thu, Aug 4, 2011 at 10:29 AM, Fournier, Camille F.
<Camille.Fournier@gs.com> wrote:
> We had an issue here the other day where the ZK servers were running poorly, and in an
effort to get them healthy again we ended up rolling back the cluster state. While this was,
in retrospect, not the right solution to the problem we were facing, it brought up another
problem. Namely, that many of our clients couldn't reconnect with their sessions because their
zxid was too high (expected), but that the error they got when trying to do that reconnection
was just a vanilla disconnected error. The result was that most of our clients had to be bounced.

Hi Camille, there's a long standing jira on this:
https://issues.apache.org/jira/browse/ZOOKEEPER-523

> Aside from trying hard to avoid ever rolling back the cluster state, does anyone have
a way they deal with this situation if it occurs? Should we consider enhancing the error message
to the client so we could track the fact that we were ahead of the quorum zxid and react sensibly?
Alternately, since we were sending a sessionId along with the zxid, perhaps it would be nice
to check to see if the sessionId exists before checking the zxid, which would send an expired
state signal which my client code could handle cleanly.

It seems reasonable that if the client connects to all servers in the
ensemble (that it knows about) and sees that it's ahead of each one,
it should consider the session expired (we could add a new state, but
seems like just treating as expired with a good log message would be
better from b/w compat standpoint).

I can't recall, does the client have sufficient information to make
this determination, or is the server just disconnecting?

Patrick

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