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From Bill Bridge <bill.bri...@oracle.com>
Subject Re: Node being there and not at the same time
Date Mon, 27 Aug 2012 17:22:17 GMT
Alex,
You certainly know the code much better than I, so I may be mistaken 
here. It looks to me like waitForEpochAck() is about changes in the set 
of peers, and is not related to client connect/disconnects. I do not see 
how this would be called if a client disconnected due to some problem of 
his own, such as too slow to heartbeat, then reconnected to a different 
peer or observer.

You suggest that a reconnecting client should ensure the new server has 
seen all transactions that the client has seen. This sounds like the 
right thing to do. This would certainly eliminate the race condition I 
postulated. This sounds like the kind of thing someone would have 
already thought of. If this is not already done then it would be a good 
change to make. I do not know where the code to do that would be. It 
could be part of the server reconnect code or it could be a sync() in 
the client library.

If Mattias's code creates a new session when reconnecting, rather than 
reconnecting to the same session, then he could have the problem 
described even if reconnect ensures the client is not ahead of the 
server. He could fix this either by reconnecting to the same session, or 
simply doing a sync() when necessary.

Thanks,
Bill

On 8/24/2012 6:11 PM, Alexander Shraer wrote:
> Bill,  if I understand correctly this shouldn't be possible - the
> client will not be able to connect to a server that is
> less up-to-date than that same client. So if the create completed at
> the client before it disconnects the new server will have to know
> about it too otherwise the connection will fail. See
> Leader.waitForEpochAck:
>
> if (ss.isMoreRecentThan(leaderStateSummary)) {
>                      throw new IOException("Follower is ahead of the
> leader, leader summary: "
>                                                      +
> leaderStateSummary.getCurrentEpoch()
>                                                      + " (current epoch), "
>                                                      +
> leaderStateSummary.getLastZxid()
>                                                      + " (last zxid)");
>                  }
>
> of course its possible that another client connected to a different
> server doesn't see the create.
>
> Alex
>
>
> On Fri, Aug 24, 2012 at 5:15 PM, Bill Bridge <bill.bridge@oracle.com> wrote:
>> Mattias,
>>
>> Is it possible that after you get NODEEXISTS from creation and before you do
>> the second getData(), you reconnect to another ZooKeeper instance? If so,
>> maybe the new connection is to a follower that has not yet seen the
>> creation. If this is what is happening, then a sync() after the second
>> NONODE with a third getData() should work. By only doing the sync() when you
>> hit the unusual race condition it will have no performance impact.
>>
>> Bill
>>
>>
>> On 8/23/2012 8:21 AM, Mattias Persson wrote:
>>> Hi David,
>>>
>>> There is nowhere in the code where that node gets deleted. If we refrain
>>> from that suspicion, could there be something else?
>>>
>>> 2012/8/23 David Nickerson <davidnickerson4mailinglists@gmail.com>
>>>
>>>> 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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