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From Bill Bridge <bill.bri...@oracle.com>
Subject Re: Node being there and not at the same time
Date Fri, 31 Aug 2012 05:50:02 GMT
Nothing to be sorry about, I was wrong to suggest a client could see an 
old state by reconnecting. When you said that it should not be allowed I 
realized that had to be the case. I saw that email too and realized it 
had something to do with this subject.

It would seem nicer to simply do a sync() when this happens rather than 
refusing the connection. We could destroy the connection if the client 
is still in the future after a sync(). There is something seriously 
wrong if the client is still in the future after a sync(). If this 
happened with the current code the client would just keep trying until 
the connection finally worked and we would not find out that something 
is wrong. I suppose the client's last zxid could have been corrupted in 
his memory causing this problem. It would be good to have this 
disconnect and fail the client rather than spin.

Without the connection you cannot do the sync() yourself. It is 
conceivable that it will be a few seconds before there is another server 
that is current enough to connect with. Maybe the other servers are in 
different data centers and would not be efficient to connect to them.

On 8/30/2012 10:21 PM, Alexander Shraer wrote:
> Bill,
> I'm sorry - you were right and I totally quoted the wrong place in the 
> code. The code that ensures that a client doesn't "go back in time" by 
> connecting to a server that is less up to date than that client is 
> most probably this one from ZooKeeperServer.java. I realized it after 
> looking on the question of Simon today in the mailing list...
> if (connReq.getLastZxidSeen() > zkDb.dataTree.lastProcessedZxid)
>             String msg = "Refusing session request for client "
>                 + cnxn.getRemoteSocketAddress()
>                 + " as it has seen zxid 0x"
>                 + Long.toHexString(connReq.getLastZxidSeen())
>                 + " our last zxid is 0x"
>                 + 
> Long.toHexString(getZKDatabase().getDataTreeLastProcessedZxid())
>                 + " client must try another server";
> On Mon, Aug 27, 2012 at 10:22 AM, Bill Bridge <bill.bridge@oracle.com 
> <mailto: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 <mailto: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
>                 <mailto: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
>                     <mailto: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
>                         <mailto:mattias@neotechnology.com>]
>                         Hacker, Neo Technology
>                         www.neotechnology.com
>                         <http://www.neotechnology.com>

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