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From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: how to properly recreate ephemeral nodes and reset watches after a session expiry
Date Fri, 15 Nov 2013 02:01:24 GMT
In the latest release, PersistentEphemeralNode was updated and it no longer needs to use a
ConnectionStateListener. It relies on the watcher being set. ZooKeeper will call the watcher
when the connection is interrupted. The PersistentEphemeralNode just continually tries to
recreate the node until it succeeds.

-Jordan

On Nov 13, 2013, at 11:25 PM, Senecal, Shaun | Shaun | BDD <shaun.senecal@mail.rakuten.com>
wrote:

> I don't see any references to ConnectionStateListener or ConnectionState.RECONNECTED
in PersistentEphemeralNode.  How does it know when the connection state is changed to RECONNECTED?
> 
> 
> From: Bae, Jae Hyeon [metacret@gmail.com]
> Sent: Thursday, November 14, 2013 4:17 PM
> To: user@curator.apache.org
> Subject: Re: how to properly recreate ephemeral nodes and reset watches after a session
expiry
> 
> When the connection state is changed to RECONNECTED, PersistentEphemeralNode is retrying
to create the node and set the watcher.
> 
> 
> On Wed, Nov 13, 2013 at 10:47 PM, Senecal, Shaun | Shaun | BDD <shaun.senecal@mail.rakuten.com>
wrote:
> How does this work if the connection to ZK is lost for an extended period of time?  Depending
on your retry behaviour, it appears to give up retrying before the connection is re-established
and the watch isn't set.  I can't see how PersistentEphemeralNode is handling this situation,
is there some trick I am missing?
> 
> 
> Shaun
> 
> From: Jordan Zimmerman [jordan@jordanzimmerman.com]
> Sent: Wednesday, November 13, 2013 12:16 AM
> To: user@curator.apache.org
> Subject: Re: how to properly recreate ephemeral nodes and reset watches after a session
expiry
> 
> Have a look at PersistentEphemeralNode. It does this.
> 
> -Jordan
> 
> On Nov 11, 2013, at 11:20 PM, Senecal, Shaun | Shaun | BDD <shaun.senecal@mail.rakuten.com>
wrote:
> 
>> Hi,
>> 
>> I have some code that needs to be able to recreate ephemeral nodes and reset watches
after a session expiry.  The approach currently taken is to keep track of all nodes that need
to be recreated and all watches that need to be reset, then use a ConnectionStateListener
to trigger the recovery process when a RECONNECTED event is received after a LOST event. 
It has been a painful process getting this to work, because I want the logic to be able to
survive issues that occur DURING the recovery process as well.
>> 
>> While the current implementation seems to be working, I'm left feeling that there
has to be a better way to do this.  What is the "best practice" approach?
>> 
>> I came across this link (https://listserv.netflix.com/pipermail/curator-users/2012-June/000068.html)
implying that you shouldn't recreate ephemeral nodes in a ConnectionStateListener, and instead
should add a watch to each node which recreates the node when NodeDeleted or Expired is received.
 I have tested this solution and it appears to work.  The only place I see this method getting
really complicated is if I need to set watches (ie getData()) on ephemeral nodes which I want
to be reset after an expiry since I would need to worry about ensuring the ephemeral node
is recreated before attempting to reset the watch.  Is this considered the best practice?
>> 
>> 
>> 
>> Thanks,
>> 
>> Shaun


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