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From Alexander Shraer <shra...@gmail.com>
Subject Re: Node being there and not at the same time
Date Mon, 27 Aug 2012 17:40:55 GMT
Hi Bill,

agreed - if the client's session expires than this is possible.
Although I don't believe that this is what's happening here since
peers usually catch up on commits really quickly while session
expiration does take some time, so its unlikely that after expiration
the client reconnects and there is a peer that is still less
up-to-date. More likely that he's creating a new client handle or some
other issue as Camille suggests.


On Mon, Aug 27, 2012 at 10:22 AM, Bill Bridge <bill.bridge@oracle.com> wrote:
> 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
>>>>> 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
>>>>>> SyncConnected event in a Watcher of mine and in that watcher I do
>>>>>> 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,
>>>>> );
>>>>>> 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
>>>>>> is
>>>>>> that I'll get a NONODE code back. Given in this scenario is that
>>>>>> 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
>>>>>> 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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