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From Christian Ziech <christian.zi...@nokia.com>
Subject Re: Zookeeper on short lived VMs and ZOOKEEPER-107
Date Thu, 15 Mar 2012 09:50:46 GMT
I don't think that we could be running into a split brain problem in our 
use case.
Let me try to describe the scenario we are worried about (assuming an 
ensemble of 5 nodes A,B,C,D,E):
- The ensemble is up and running and in sync
- Node A with the host name "zookeeperA.whatever-domain.priv" goes down 
because the VM has gone away
- That removal of the VM is detected and a new VM is spawned with the 
same host name "zookeeperA.whatever-domain.priv" - let's call that node A'
- Node A' zookeeper wants to join the cluster - right now this gets 
rejected by the others since A' has a different IP address than A (and 
the old one is "cached" in the InetSocketAddress of the QuorumPeer instance)

We could ensure that at any given time there is only at most one node 
with host name "zookeeperA.whatever-domain.priv" known by the ensemble 
and that once one node is replaced, it would not come back. Also we 
could make sure that our ensemble is big enough to compensate for a 
replacement of more than x nodes at a time (setting it to x*2 + 1 nodes).

So if I did not misestimate our problem it should be (due to the 
restrictions) simpler than the problem to be solved by zookeeper-107. My 
intention is basically by solving this smaller discrete problem to not 
need to wait for that zookeeper-107 makes it into a release (the 
assumption is that a smaller fix has a possibly a chance to make it into 
the 3.4.x branch even).

Am 15.03.2012 07:46, schrieb ext Alexander Shraer:
> Hi Christian,
>
> ZK-107 would indeed allow you to add/remove servers and change their 
> addresses.
>
> > We could ensure that we always have a more or less fixed quorum of 
> zookeeper servers with a fixed set of host names.
>
> You should probably also ensure that a majority of the old ensemble 
> intersects with a majority of the new one.
> Otherwise you have to run a reconfiguration protocol similarly to 
> ZK-107. For example, if you have 3 servers A B and C, and now you're 
> adding D and E that replace B and C, how would this work ?  it is 
> probable that D and E don't have the latest state (as you mention) and 
> A is down or doesn't have the latest state too (a minority might not 
> have the latest state). Also, how do you prevent split brain in this 
> case ? meaning B and C thinking that they are still operational ? 
> perhaps I'm missing something but I suspect that the change you 
> propose won't be enough...
>
> Best Regards,
> Alex
>
>
> On Wed, Mar 14, 2012 at 10:01 AM, Christian Ziech 
> <christian.ziech@nokia.com <mailto:christian.ziech@nokia.com>> wrote:
>
>     Just a small addition: In my opinion the patch could really boil
>     down to add a
>
>       quorumServer.electionAddr = new
>       InetSocketAddress(electionAddr.getHostName(),
>     electionAddr.getPort());
>
>     in the catch(IOException e) clause of the connectOne() method of
>     the QuorumCnxManager. In addition on should perhaps make the
>     electionAddr field in the QuorumPeer.QuorumServer class volatile
>     to prevent races.
>
>     I haven't checked this change yet fully for implications but doing
>     a quick test on some machines at least showed it would solve our
>     use case. What do the more expert users / maintainers think - is
>     it even worthwhile to go that route?
>
>     Am 14.03.2012 17:04, schrieb ext Christian Ziech:
>
>         LEt me describe our upcoming use case in a few words: We are
>         planning to use zookeeper in a cloud were typically nodes come
>         and go unpredictably. We could ensure that we always have a
>         more or less fixed quorum of zookeeper servers with a fixed
>         set of host names. However the IPs associated with the host
>         names would change every time a new server comes up. I browsed
>         the code a little and it seems right now that the only problem
>         is that the zookeeper server is remembering the resolved
>         InetSocketAddress in its QuorumPeer hash map.
>
>         I saw that possibly ZOOKEEPER-107 would also solve that
>         problem but possibly in a more generic way than actually
>         needed (perhaps here I underestimate the impact of joining as
>         a server with an empty data directory to replace a server that
>         previously had one).
>
>         Given that - from looking at ZOOKEEPER-107 - it seems that it
>         will still take some time for the proposed fix to make it into
>         a release, would it make sense to invest time into a smaller
>         work fix just for this "replacing a dropped server without
>         rolling restarts" use case? Would there be a chance that a fix
>         for this makes it into the 3.4.x branch?
>
>         Are there perhaps other ways to get this use case supported
>         without the need for doing rolling restarts whenever we need
>         to replace one of the zookeeper servers?
>

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