Hi Mahadev,

The inter-site links are a nuisance.  We have two data-centres with 100Mb
links which I hope would be good enough for most uses, but we need a 3rd
site - and currently that only has 2Mb links to the other sites.  This might
be a problem.

The ensemble would have a lot of read traffic from applications asking which
database to connect to for each transaction - which presumably would be
mostly handled by local zookeeper servers (do we call these "nodes" as
opposed to znodes ?).  The write traffic would be mostly changes to
configuration (a rare event), and changes in the health of database servers
- also hopefully rare.  I suppose the main concern is how much ambient
zookeeper system chatter will cross the links.   Are there any measurements
of how much traffic is used by zookeeper in maintaining the ensemble ?

Another question that occurs is whether I can link sites A,B, and C in a
ring - so that if any one site drops out, the remaining 2 continue to talk.
I suppose that if the zookeeper servers are all in direct contact with each
other, this issue does not exist.


On 7 March 2010 21:43, Mahadev Konar <maha...@yahoo-inc.com> wrote:

> Hi Martin,
>  As Ted rightly mentions that ZooKeeper usually is run within a colo
> because
> of the low latency requirements of applications that it supports.
> Its definitely reasnoble to use it in a multi data center environments but
> you should realize the implications of it. The high latency/low throughput
> means that you should make minimal use of such a ZooKeeper ensemble.
> Also, there are things like the tick Time, the syncLimit and others (setup
> parameters for ZooKeeper in config) which you will need to tune a little to
> get ZooKeeper running without many hiccups in this environment.
> Thanks
> mahadev
> On 3/6/10 10:29 AM, "Ted Dunning" <ted.dunn...@gmail.com> wrote:
> > What you describe is relatively reasonable, even though Zookeeper is not
> > normally distributed across multiple data centers with all members
> getting
> > full votes.  If you account for the limited throughput that this will
> impose
> > on your applications that use ZK, then I think that this can work well.
> > Probably, you would have local ZK clusters for higher transaction rate
> > applications.
> >
> > You should also consider very carefully whether having multiple data
> centers
> > increases or decreases your overall reliability.  Unless you design very
> > carefully, this will normally substantially degrade reliability.  Making
> > sure that it increases reliability is a really big task that involves a
> lot
> > of surprising (it was to me) considerations and considerable hardware and
> > time investments.
> >
> > Good luck!
> >
> > On Sat, Mar 6, 2010 at 1:50 AM, Martin Waite <waite....@googlemail.com
> >wrote:
> >
> >> Is this a viable approach, or am I taking Zookeeper out of its
> application
> >> domain and just asking for trouble ?
> >>
> >
> >

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