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From Tom Nichols <tmnich...@gmail.com>
Subject Re: Dealing with session expired
Date Thu, 12 Feb 2009 21:43:16 GMT
On Thu, Feb 12, 2009 at 4:11 PM, Benjamin Reed <breed@yahoo-inc.com> 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
environment.

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.
-Tom


> 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.
>
> ben
> ________________________________________
> From: Tom Nichols [tmnichols@gmail.com]
> Sent: Thursday, February 12, 2009 11:53 AM
> To: zookeeper-user@hadoop.apache.org
> 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.
>
> -Tom
>
>
> On Thu, Feb 12, 2009 at 2:39 PM, Patrick Hunt <phunt@apache.org> 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
>> archi...)
>>
>> 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).
>>
>> Patrick
>>
>> Mahadev Konar wrote:
>>>
>>> Hi Tom,
>>>  The session expired event means that the the server expired the client
>>> and
>>> 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
>>> happen.
>>>
>>> mahadev
>>>
>>>
>>> On 2/12/09 11:17 AM, "Tom Nichols" <tmnichols@gmail.com> 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?
>>>>
>>>> Thanks.
>>>> -Tom
>>>>
>>>>
>>>>
>>>> On Thu, Feb 12, 2009 at 2:12 PM, Mahadev Konar <mahadev@yahoo-inc.com>
>>>> wrote:
>>>>>
>>>>> 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
>>>>> session
>>>>> makes it much simpler for users to understand the existence and
>>>>> disappearance of ephemeral nodes and watches created by a zookeeper
>>>>> client.
>>>>>
>>>>> thanks
>>>>> mahadev
>>>>>
>>>>>
>>>>> On 2/12/09 10:58 AM, "Tom Nichols" <tmnichols@gmail.com> 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.
>>>>>>
>>>>>> Thoughts?
>>>>>>
>>>>>> Thanks in advance,
>>>>>>
>>>>>> -Tom
>>>>>
>>>
>>
>

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