Tom, you might try changing the log4j default log level to DEBUG for the rootlogger and appender if you have not already done so (servers and clients both). You'll get more information to aid debugging if it does occur again.

Also, are you seeing timeouts on the client, or just session expiration on the server?

The stat command, detailed here, may also be of use to you:

Knowing more about your env, OS & java version in particular, would also help us help you narrow things down. :-)


Tom Nichols wrote:
On Thu, Feb 12, 2009 at 4:11 PM, Benjamin Reed <> wrote:
idleness is not a problem. the client library sends heartbeats to keep the 
session alive. the client library will also handle reconnects automatically if 
a server dies.

That's odd then that I'm seeing this problem.  I have a local, 3-node
zookeeper quorum, and I have 3 instances of the client also running on
the same box.  The session expiry doesn't seem to be in response to
any severe load on the machine or anything like that.  I'll keep an
eye on it and see if I can't reproduce the behavior in a distributed

I've realized a relatively easy way to deal with this problem -- I can
let my thread throw a fatal unchecked exception and then use a
ThreadGroup implementation that catches the exception.  This in turn
spawns a new client thread and adds it back to the same threadGroup.

Thanks again guys.

since session expiration really is a rare catastrophic event. (or at least it 
should be.) it is probably easiest to deal with it by starting with a fresh 
instance if your session expires.

From: Tom Nichols []
Sent: Thursday, February 12, 2009 11:53 AM
Subject: Re: Dealing with session expired

I'm using a timeout of 5000ms.  Now let me ask this:  Suppose all of
my clients are waiting on some external event -- not ZooKeeper -- so
they are all idle and are not touching ZK nodes, nor are they calling
exists, getChildren, etc etc.  Can that idleness cause session expiry?

I'm running a local quorum of 3 nodes.  That is, I have an Ant script
that kicks off 3 <java> tasks in parallel to run ConsumerPeerMain,
each with its own config file.

Regarding handling of the failure, I suspect I will just have to
reinitialize by creating a new instance of my client(s) that
themselves will have a new ZK instance.  I'm using Spring to wire
everything together, which is why it's particularly difficult to
simply re-create a new ZK instance and pass it to the classes using it
(those classes have no knowledge of each other).  But I _can_ just
pull a freshly-created (prototype) instance from the Spring
application context, which is where a new ZK client will be wired in.

The only ramification there is I have to throw the KeeperException as
a fatal exception rather than letting that client try to re-elect.  Or
maybe add in some logic to say "if I can't re-elect, _then_ throw an
exception and consider it fatal."

Thanks guys.


On Thu, Feb 12, 2009 at 2:39 PM, Patrick Hunt <> wrote:
Regardless of frequency Tom's code still has to handle this situation.

I would suggest that the "two classes" Tom is referring to in his mail, the
ones that use ZK client object, should either be able to "reinitialize" with
a new zk session, or they themselves should be discarded and new instances
created using the new session (not sure what makes more sense for his

Regardless of whether we reuse the session object or create a new one I
believe the code using the session needs to "reinitialize" in some way --
there's been a dramatic break from the cluster.

As I mentioned, you can decrease the likelihood of expiration by increasing
the timeout - but the downside is that you are less sensitive to clients
dying (because their ephemeral nodes don't get deleted till close/expire and
if you are doing something like leader election among your clients it will
take longer for the followers to be notified).


Mahadev Konar wrote:
Hi Tom,
 The session expired event means that the the server expired the client
that means the watches and ephemrals will go away for that node.

How are you running your zookeeper quorum? Session expiry event should be
really rare event . If you have a quorum of servers it should rarely


On 2/12/09 11:17 AM, "Tom Nichols" <> wrote:

So if a session expires, my ephemeral nodes and watches have already
disappeared?  I suppose creating a new ZK instance with the old
session ID would not do me any good in that case.  Correct?


On Thu, Feb 12, 2009 at 2:12 PM, Mahadev Konar <>
Hi Tom,
 We prefer to discard the zookeeper instance if a session expires.
Maintaining a one to one relationship between a client handle and a
makes it much simpler for users to understand the existence and
disappearance of ephemeral nodes and watches created by a zookeeper


On 2/12/09 10:58 AM, "Tom Nichols" <> wrote:

I've come across the situation where a ZK instance will have an
expired connection and therefore all operations fail.  Now AFAIK the
only way to recover is to create  a new ZK instance with the old
session ID, correct?

Now, my problem is, the ZK instance may be shared -- not between
threads -- but maybe two classes in the same thread synchronize on
different nodes by using different watchers.  So it makes sense that
one ZK client instance can handle this.  Except that even if I detect
the session expiration by catching the KeeperException, if I want to
"resume" the session, I have to create a new ZK instance and pass it
to any classes who were previously sharing the same instance.  Does
this make sense so far?

Anyway, bottom line is, it would be nice if a ZK instance could itself
recover a session rather than discarding that instance and creating a
new one.


Thanks in advance,


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