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From Mahadev Konar <maha...@yahoo-inc.com>
Subject Re: Semantics of ConnectionLoss exception
Date Thu, 26 Mar 2009 20:27:01 GMT
The problem is that we cannot differentiate between the servers being down
and a network problem from the client to servers.

If the servers are down and we expire the session for a client on the client
side -- the servers would come up and would still have the session as valid
(though it will be expired in a few session timeout seconds) but we pre
maturely would have expired the session at the client.

You can look at it this way -- the client does not know whats going on with
the zookeeper service, so instead of giving back a false answer to the
application it waits for a true answer from the servers. Also, we are
assuming that the client would not be able to proceed further neway if it
cannot contact the servers.

Hope this answers your question.


On 3/26/09 12:09 PM, "Nitay" <nitayj@gmail.com> wrote:

> Why is it done that way? How am I supposed to reliably detect that my
> ephemeral nodes are gone? Why not deliver the Session Expired event on the
> client side after the right time has passed without communication to any
> server?
> On Thu, Mar 26, 2009 at 10:58 AM, Mahadev Konar <mahadev@yahoo-inc.com>wrote:
>>> Isn't it the case that the client won't get session expired until it's
>>> able to connect to a server, right? So what might happen is that the
>>> client loses connection to the server, the server eventually expires the
>>> client and deletes ephemerals (notifying all watchers) but the client
>>> won't see the "session expiration" until it is able to reconnect to one
>>> of the servers. ie the client doesn't know it's been expired until it's
>>> able to reconnect to the cluster, at which point it's notified that it's
>>> been expired.
>> You are right pat!
>> mahadev
>> http://hadoop.apache.org/zookeeper/docs/r3.0.1/zookeeperProgrammers.html
>>>> Has this information scattered around, but we should put it in the FAQ
>>>> specifically.
>>> 3.0.1 is a bit old, try this for the latest docs:
>> http://hadoop.apache.org/zookeeper/docs/current/zookeeperProgrammers.html
>>>> - Is the ZooKeeper handle I'm using dead after this event?
>>>> Again no. your handle is valid until you get an session expiry event or
>> you
>>>> do a zoo_close on your handle.
>>>> Thanks
>>>> mahadev
>>>> On 3/25/09 5:42 PM, "Nitay" <nitayj@gmail.com> wrote:
>>>>> I'm a little unclear about the ConnectionLoss exception as it's
>> described in
>>>>> the FAQ and would like some clarification.
>>>>> From the state diagram, http://wiki.apache.org/hadoop/ZooKeeper/FAQ#1,
>> there
>>>>> are three events that cause a ConnectionLoss:
>>>>> 1) In Connecting state, call close().
>>>>> 2) In Connected state, call close().
>>>>> 3) In Connected state, get disconnected.
>>>>> It's the third one I'm unclear about.
>>>>> - Does this event happening mean my ephemeral nodes will go away?
>>>>> - Is the ZooKeeper handle I'm using dead after this event? Meaning
>> that,
>>>>> similar to the SessionExpired case, I need to construct a new
>> connection
>>>>> handle to ZooKeeper and take care of the restarting myself. It seems
>> from
>>>>> the diagram that this should not be the case. Rather, seeing as the
>>>>> disconnected event sends the user back to the Connecting state, my
>> handle
>>>>> should be fine and the library will keep trying to reconnect to
>> ZooKeeper
>>>>> internally? I understand my current operation may have failed, what I'm
>>>>> asking about is future operations.
>>>>> Thanks,
>>>>> -n

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