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From Lei Gao <l...@linkedin.com>
Subject Re: Question on maintaining leader/membership status in zookeeper
Date Fri, 30 Apr 2010 20:26:23 GMT
Hi Henry,

I am not talking about the leader election within zookeeper cluster. I guess
I didn't make the discussion context clear. In my case, I run a cluster that
uses zookeeper for doing the leader election. Yes, nodes in my cluster are
the clients of zookeeper.  Those nodes depend on zookeeper to elect a new
leader and figure out what the current leader is. So if the zookeeper (think
of it as a stand-alone entity) becomes unavailabe in the way I've described
earlier, how can I handle such situation so my cluster can still function
while a majority of nodes still connect to each other (but not to the



On 4/30/10 1:10 PM, "Henry Robinson" <henry@cloudera.com> wrote:

> Hi Lei -
> The 'user cluster' (by which I think you mean the set of clients of
> ZooKeeper?) plays no part in leader election. If a majority of ZooKeeper
> server nodes can talk to each other, a new leader can be elected. Clients of
> the minority server partition will be disconnected - if they too cannot
> reach the majority partition then they will not be able to reconnect.
> Hope this helps,
> Henry
> On 30 April 2010 12:45, Lei Gao <lgao@linkedin.com> wrote:
>> Hi Ted,
>> I 100% agree with what you said. But my question is more about what if my
>> zookeeper service cluster is partitioned from a majority of nodes in my USER
>> CLUSTER.  In this case, the majority nodes in one network partition can¹t
>> select a new leader because zookeeper is out of reach.
>> Another example will be that if there is an asymmetric network failure
>> where a majority of nodes from the USER CLUSTER can¹t reach the leader while
>> the zookeeper still can. How does zookeeper handle such situation?
>> Thanks,
>> Lei
>> On 4/30/10 12:24 PM, "Ted Dunning" <ted.dunning@gmail.com> wrote:
>> There are a variety of situations that can trigger a new leader election
>> and a few that can cause the cluster to be unable to elect a new leader.
>>  Isolation of just the leader is one of the situations that will cause a new
>> leader election.  Isolation of nodes into groups smaller than the quorum
>> will result in the cluster freezing.
>> On Fri, Apr 30, 2010 at 11:56 AM, Lei Gao <lgao@linkedin.com> wrote:
>> Hi,
>> I have a general question on how zookeeper can maintain its view of the
>> user cluster (that zookeeper manages) that is consistent with the nodes in
>> the user cluster. In other words, when zookeeper considers the current
>> leader is unavailable, does it really guarantee that a majority of nodes in
>> the user cluster can¹t reach the current leader? The same question applies
>> to the membership service as well. Because the zookeeper can be partitioned
>> from a majority of the nodes in the user cluster. How does the zookeeper
>> handle situations like this?
>> Thanks,
>> Lei

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