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From Abraham Fine <af...@apache.org>
Subject Re: Zookeeper session expiration
Date Mon, 04 Dec 2017 20:09:39 GMT
Hello Anthony and Shawn-

To the best of my knowledge ZooKeeper does not use the "wall clock" time
anywhere. So that should not be the problem. 

Please consider enabling debug logging, which should allow you to track
the "pings". 

Thanks,
Abe

On Mon, Dec 4, 2017, at 11:51, Anthony Shaya wrote:
> Thanks Shawn, should I message the developer mailing list for a more
> definitive answer?
> 
> Thanks again for the reply.
> 
> -----Original Message-----
> From: Shawn Heisey [mailto:apache@elyograg.org] 
> Sent: Monday, December 4, 2017 2:49 PM
> To: user@zookeeper.apache.org
> Subject: Re: Zookeeper session expiration
> 
> On 12/4/2017 8:22 AM, Anthony Shaya wrote:
> > My question is related to how session expiration works, I noticed on many of the
client machines the times across these machines were all off (by anywhere from 1 minute to
20 minutes - which was resolved after discovery - haven't verified this completely yet). Can
this directly affect session expiration within the zookeeper cluster?
> >
> >    *   I read the following in https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwiki.apache.org%2Fhadoop%2FZooKeeper%2FFAQ&data=02%7C01%7C%7C6d6643860a4e4a8194c808d53b5023ec%7Cc61157e903cb47589165ee7845cb0ca3%7C0%7C0%7C636480137750841475&sdata=RwGGH19FLeYFmXMrg5GBkSLJ65ANj1EXkTvwyk6OLd4%3D&reserved=0
, "Expirations happens when the cluster does not hear from the client within the specified
session timeout period (i.e. no heartbeat).". So in some case it seems like if the times were
wrong across the machines its possible one of the clients could of effectively sent a heart
beat in the past (not sure about this tbh) and then the cluster expires the session?
> 
> I make these comments without any knowledge of what ZK code actually
> does.  I am a member of this list because I'm a representative of the
> Apache Solr project, which uses the ZK client in order to maintain a
> cluster.
> 
> IMHO, any software which makes actual decisions based on the timestamps
> in messages from another system is badly designed.  I would hope that the
> ZK designers know this, and always make any decisions related to time
> using the clock in the local system only.
> 
> If ZK's designers did the right thing, then a session timeout would
> indicate that quite literally no heartbeats were received in X seconds,
> as measured by the local clock, and the local clock ONLY ... NOT from
> timestamp information received from another system.
> 
> Although such a lack of communication could be caused by any number of
> things, including network hardware failure, one of the most common
> reasons I have seen for problems like this is extreme java garbage
> collection pauses in the client software.
> 
> Situations where the heap is a little bit too small can cause a java
> program to basically be doing garbage collection constantly, so it
> doesn't have much time to do anything else, like send heartbeats to ZK
> servers.
> 
> Situations where the heap is HUGE and garbage collection is not well
> tuned can lead to pauses of a minute or longer while Java does a massive
> full GC.
> 
> >    *   I don't have the zookeeper node log for the above time to see what was going
on in zookeeper when the cluster determined the session expired.
> >
> >    *   Is there any additional logging I can turn on to troubleshoot zk session
expiration issues?
> 
> Hopefully your ZK clients also have logging.  Failing that, you could
> turn on GC logging for the software with the ZK client (assuming it's a
> Java client) and find a program or website that can examine the log and
> give you statistics or a graph of GC pauses.
> 
> If there is a problem in software using the client and whatever logging
> is available doesn't help you figure out what's wrong, you're generally
> going to need to talk to whoever wrote that software for help
> troubleshooting it.
> 
> Thanks,
> Shawn
> 
> 
> 
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