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From David Nickerson <davidnickerson4mailingli...@gmail.com>
Subject Re: Node being there and not at the same time
Date Thu, 23 Aug 2012 14:53:06 GMT
It's a little difficult to guess what your application is doing, but it
sounds like there's "someone else" who can create and delete the nodes
you're trying to work with. So when you create the node and check its data,
someone else might have deleted it before you got the chance to check the
data. The same is true when you check that it exists and then check the
data. You could ensure that the node won't be deleted by using ACLs or
giving the node a sequential ephemeral child.

On Thu, Aug 23, 2012 at 6:30 AM, Mattias Persson
<mattias@neotechnology.com>wrote:

> Hi,
>
> I've got a problem that I've seen at only a few occasions and which
> confuses me a bit. Basically I construct a ZooKeeper client (I'm running
> version 3.3.2) where there's a ZK quorum of size 3 running. I get a
> SyncConnected event in a Watcher of mine and in that watcher I do a
> get-or-create(-if-absent) behaviour where I first do a:
>
>   zooKeeper.getData( myPath, false, null );
>
> if that produces a NONODE code I'll try to create it with:
>
>   zooKeeper.create( myPath, smallByteArray, OPEN_ACL_UNSAFE, PERSISTENT );
>
> If that fails with NODEEXISTS code I'll just get it, assuming someone else
> made it before me. What I see from this getData call that I do after
> getting this NODEEXISTS code, which is the same as the first one btw, is
> that I'll get a NONODE code back. Given in this scenario is that I'm 100%
> certain that this node exists in the quorum at myPath in the first place
> even.
>
> Questions:
> 1) How can this happen?
> 2) Do I use ZooKeeper here in an improper way?
> 3) Will a later version fix any potential issue I might have hit?
> 4) What's the guarantees around the state of my ZooKeeper instance after a
> receive a SyncConnected event, is it fully synced with the master at that
> point, or will a call to sync() be necessary first?
>
> Best,
> Mattias
>
> --
> Mattias Persson, [mattias@neotechnology.com]
> Hacker, Neo Technology
> www.neotechnology.com
>

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