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From Camille Fournier <cami...@apache.org>
Subject Re: missing data after restarting+expanding a ZK 3.4.0 cluster
Date Tue, 06 Dec 2011 02:22:55 GMT
Even if it's "unsupported" you've basically always found real bugs with ZK
this way, so we might as well make a JIRA tracker and try to figure out
this one.
I dunno if I'll have time to look before the weekend, so anyone else that
is interested should feel free to dig in.


On Mon, Dec 5, 2011 at 8:47 PM, Jeremy Stribling <strib@nicira.com> wrote:

> I've been trying to update to ZK 3.4.0 and have had some issues where some
> data become inaccessible after adding a node to a cluster.  My use case is
> a bit strange (as explained before on this list) in that I try to grow the
> cluster dynamically by having an external program automatically restart
> Zookeeper servers in a controlled way whenever the list of participating ZK
> servers needs to change.  I haven't made a JIRA for this yet, since I'm
> guessing the official position is that ZK doesn't support this scenario
> yet, but this used to work just fine in 3.3.3 (and before), so this
> represents a regression.
> The scenario I see is this:
> 1) Start up a 1-server ZK cluster (the server has ZK ID 0).
> 2) A client connects to the server, and makes a bunch of znodes, in
> particular a znode called "/membership".
> 3) Shut down the cluster.
> 4) Bring up a 2-server ZK cluster, including the original server 0 with
> its existing data, and a new server with ZK ID 1.
> 5) Node 0 has the highest zxid and is elected leader.
> 6) A client connecting to server 1 tries to "get /membership" and gets
> back a -101 error code (no such znode).
> 7) The same client then tries to "create /membership" and gets back a -110
> error code (znode already exists).
> 8) Clients connecting to server 0 can successfully "get /membership".
> I've attached a tarball with debug logs for both servers, annotating where
> steps #1 and #4 happen.  You can see that the election involves a proposal
> for zxid 110 from server 0, but immediately following the election server 1
> has these lines:
> 2011-12-05 17:18:48,308 9299 [QuorumPeer[myid=1]/**2901<>]
> WARN org.apache.zookeeper.server.**quorum.Learner  - Got zxid 0x100000001
> expected 0x1
> 2011-12-05 17:18:48,313 9304 [SyncThread:1] INFO
> org.apache.zookeeper.server.**persistence.FileTxnLog  - Creating new log
> file: log.100000001
> Perhaps that's not relevant, but it struck me as odd.  At the end of
> server 1's log you can see a repeated cycle of getData->create->getData as
> the client tries to make sense of the inconsistent responses.
> The other piece of information is that if I try to use the on-disk
> directories for either of the servers to start a new one-node ZK cluster,
> all the data are accessible.
> Anyone have ideas?  I haven't tried writing a program outside of my
> application to reproduce this, but I can do it very easily with some of my
> app's tests if anyone needs more information.  I happy to turn this into a
> JIRA if desired.  Thanks,
> Jeremy

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