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Re: Distributed ZooKeeper cluster design
I have to strongly disagree with ted on the mirroring idea... I think it is
likely to be really error-prone and kind of defeats the purpose of ZK in my
mind. It depends on what you're mirroring but if you're trying to keep all
the data coherent you can't sensibly do that in two clusters, so unless the
mirror is for a really small subset of the data I would stay far far away
from that.

Observers are available in 3.3.3, yes.
Unfortunately, we don't have configurable connection logic in ZK client (at
least java) right now. We have the ability to add it pretty easily, but it
hasn't been put in yet.

You're seeing slow performance for a setup that has all ZK servers in
region C for clients only in regions A and B and you can't blame the
network? That's literally the only thing you could blame, unless clients in
region C were also seeing slow performance or they have some other problem
in they way they are implemented that makes them different from clients
running in region C.


On Tue, Dec 13, 2011 at 11:09 AM, Dima Gutzeit

> Ted and Camille,
> Thanks for a very details response.
> At the moment I have an option A implemented in production and what I see
> is that ZK client in A and B have a "slow" performance (even reads) and I
> can't really blame the network since it does not look like a real
> bottleneck.
> I wonder if doing option 2 will improve the ZK client performance/speed ...
> As for my use case, its around 50/50 reads and writes.
> As for fallback, ofcourse in A and B I would want to define C as a backup,
> not sure how it can be done since as I understand if I supply several
> addresses in the connection string the client will use one, randomly.
> About Ted's suggestion to consider having several clusters and to have a
> special process to mirror, is it something available as part of ZooKeeper ?
> I also read about observers (is it available in 3.3.3 ?) and it seems to be
> a good option is my case, which brings me to the question of how to
> configure explicit fallback instead of random client selection ? If I want
> to tell ZK client in B to use the local B instance (observer) and if it
> fails then contact ANY server in the C (with a list of several).
> Thanks in advance.
> Regards,
> Dima Gutzeit.
> On Tue, Dec 13, 2011 at 5:44 PM, Camille Fournier <[EMAIL PROTECTED]
> >wrote:
> > Ted is of course right, but to speculate:
> >
> > The idea you had with 3 in C, one in A and one in B isn't bad, given
> > some caveats.
> >
> > With 3 in C, as long as they are all available, quorum should live in
> > C and you shouldn't have much slowdown from the remote servers in A
> > and B. However, if you point your A servers only to the A zookeeper,
> > you have a failover risk where your A servers will have no ZK if the
> > sever in region A goes down (same with B, of course). If you have a
> > lot of servers in the outer regions, this could be a risk. You are
> > also giving up any kind of load balancing for the A and B region ZKs,
> > which may not be important but is good to know.
> >
> > Another thing to be aware of is that the A and B region ZKs will have
> > slower write response time due to the WAN cost, and they will tend to
> > lag behind the majority cluster a bit. This shouldn't cause
> > correctness issues but could impact client performance in those
> > regions.
> >
> > Honestly, if you're doing a read-mostly workload in the A and B
> > regions, I doubt this is a bad design. It's pretty easy to test ZK
> > setups using Pat's zksmoketest utility, so you might try setting up
> > the sample cluster and running some of the smoketests on it.
> > (https://github.com/phunt/zk-smoketest/blob/master/zk-smoketest.py).
> > You could maybe also add observers in the outer regions to improve
> > client load balancing.
> >
> > C
> >
> >
> >
> > On Tue, Dec 13, 2011 at 9:05 AM, Ted Dunning <[EMAIL PROTECTED]>
> > wrote:
> > > Which option is preferred really depends on your needs.