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From David Alves <davidral...@gmail.com>
Subject Re: A few questions on zookeeper
Date Tue, 07 Dec 2010 21:30:41 GMT
Hi Ted

	Thank you again for your insight.
	Regarding the rolling reconfigure solution...  Is there any documentation on this procedure?
	Regarding the two zk instances on a single machine... wouldn't that make that machine a SPOF?
the objective here was to maintain a sustainable two machine config for a single server (2
nodes active-passive) highly available deployment, and take advantage of the fact that zk
is already being used for larger (<3 active nodes) deployments.


On Dec 7, 2010, at 8:38 PM, Ted Dunning wrote:

> You can make "quorum disk alike" happen by running two ZK instances on one
> of your two machines.
> You can also do this dynamically by doing the standard rolling reconfigure
> trick.
> On Tue, Dec 7, 2010 at 12:32 PM, David Alves <davidralves@gmail.com> wrote:
>> Hi Ted and Jared
>>       Thank you for your replies. Sorry in advance if my questions are too
>> absurd.
>>       I know that, currently, with two servers the cluster would have a
>> higher failure probability (since the ensemble started with three and it
>> needs a two nodes majority, if any one fails there is no longer a majority
>> and the cluster hangs right?).
>>       The purpose of the quorum disk would be to function has a tie
>> breaker in even node deployments (namely two), by representing an external,
>> storage only, resource where cluster state would be maintained. I know there
>> are some paxos-based deployments that use this technique, although I'm not
>> sufficiently into the internals of zookeeper to assert whether it would
>> work.
>>       A question about an alternative... would it be possible to change
>> the majority function (dynamically) in two node deployments?
>>       Regarding zookeeper-107, that is very interesting as we were
>> considering using zookeeper in an "elastic" setting. Is there progress on
>> this issue? can I help?
>> Regards
>> David
>> On Dec 6, 2010, at 5:02 PM, Ted Dunning wrote:
>> David,
>> For your second point, take a look at
>> https://issues.apache.org/jira/browse/ZOOKEEPER-107.  Unfortunately,
>> Zookeeper does not support this feature yet.
>> ~Jared
>>> You can definitely do this, but your reliability will be different from
>> what
>>> might be expected.  The probability of the cluster hanging due to a
>> failure
>>> is higher than for a single machine.  The probability of data loss will
>> be
>>> lower than for a single machine.  Similarly, any maintenance that
>> requires
>>> one of the two machines to be down will cause your cluster to hang.
>>> Can you clarify what you mean by 2 server + 1 quorum disk?
>>> Btw... one way around the downtime during maintenance is to reconfigure
>> the
>>> cluster on the fly to use a single server
>>> during the maintenance window.  You will still have a short window of
>> freeze
>>> because you need to take down the
>>> server that is exiting the cluster first before bouncing the other server
>>> with the new config.  If your maintenance period is less than a few
>> minutes,
>>> this isn't usually worthwhile.
>>> On Mon, Dec 6, 2010 at 7:43 AM, David Alves <davidralves@gmail.com>
>> wrote:
>>>>      1- Feasibility of a two node cluster:
>>>>              Q- I know zookeeper runs over (a approx. version of) paxos
>>>> and tolerates f failures in 2f+1 nodes but would it be possible to use
>> in a
>>>> 2 server + 1 quorum disk deployment (much like windows clusters do)? The
>>>> objective would be to use ZK  both for distributed (processing nodes >2)
>> and
>>>> single processing node (2 nodes active-passive) highly available
>>>> deployments.

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