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From Jean-Daniel Cryans <jdcry...@apache.org>
Subject Re: Confused about KeeperState.Disconnected and KeeperState.Expired
Date Wed, 24 Jun 2009 18:55:28 GMT
I understand, maybe the common client is the best place.

In our situation, if a HBase region server is in the state of being
disconnected for too long, the regions it's holding cannot be reached so
this is a major problem. Also, if the HMaster node gets the event that an
ephemeral is gone, it will begin processing its region server's WAL and if
the region server is still able to talk to HDFS, we have a lease handling
problem. So, in that state of disconnection, the region server should just
kill itself and completely restart.


On Wed, Jun 24, 2009 at 2:00 PM, Benjamin Reed <breed@yahoo-inc.com> wrote:

> perhaps it would fit into the common client that stefan is proposing. we
> don't have such a timer currently in the client code that we just need to
> expose, so it will be something we need to add. one thing to be careful of
> is trying to be too tricky. you don't want to trigger right after the
> session timeout because things can be in flight and a session renewal
> response might actually be on the way or the service bounced due to a leader
> failure, which is why i was recommending something like twice the session
> timeout.
> to be honest i think most of our applications just sit there trying to
> reconnect forever. after all if you do close the session and try to move on,
> the ZooKeeper service is still down, so trying with a new ZooKeeper handle
> isn't going to help anything.
> ben
> Jean-Daniel Cryans wrote:
>> Ben,
>> Thank you, I now see the rationale in not telling the client it's session
>> is
>> over because you can't be sure it actually is. But would it make sense to
>> add a new state in KeeperState representing that corner case? Something
>> like
>> AfterSessionTimeout. I'm pretty sure other would find that useful for the
>> same reason as us.
>> If anyone +1 on that, I'll open a jira and give it a try.
>> J-D
>> On Tue, Jun 23, 2009 at 6:04 PM, Benjamin Reed <breed@yahoo-inc.com>
>> wrote:
>>> ZooKeeper only tells you about states that it is sure about, so you will
>>> not get the Expired event until you reconnect to ZooKeeper. if you never
>>> connect again to ZooKeeper, you will not get the Expired event. if you
>>> want
>>> to timeout using some sanity value, 2 times the session timeout for
>>> example,
>>> you can implement that yourself by setting a timer when you get the
>>> disconnected event and then close the session explicitly when the timer
>>> goes
>>> off.
>>> there is a caveat in doing this: if your whole cluster goes down for 20
>>> mins and then comes back up, your session timeout will get reset and the
>>> session will still be alive even though you have closed it. it will then
>>> have to timeout before it actually goes away. closing the session when
>>> the
>>> client is disconnected just stops the client from trying to reconnect.
>>> does this make sense?
>>> ben
>>> Jean-Daniel Cryans wrote:
>>>> Hey all,
>>>> Working on integrating HBase with ZK, we came around an issue that we
>>>> are unable to resolve. I was trying to see how was our handling of
>>>> network partitions and session expirations and what I did is just
>>>> starting a single ZK instance with a very simple HBase setup, then I
>>>> killed the ZK server. The only thing I got from Zookeeper was a
>>>> KeeperState.Disconnected then... nothing (for like 20+ minutes).
>>>> Normally if I had a quorum I would still get that message but then I
>>>> would get another one telling me it's connected to another ZK quorum
>>>> server. So how do I know if I'm really partitioned from the ZK quroum?
>>>> Shouldn't we get a session expired at some point? From what I
>>>> understand you can only get a KeeperState.Expired when you connect
>>>> back to the quorum after x time, but what if you can "never" connect
>>>> back to it?
>>>> BTW this is r785019.
>>>> Thx a lot!
>>>> J-D

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