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From Patricio Echag├╝e <patric...@gmail.com>
Subject Re: Re: Quorum, Hector, and datacenter preference
Date Thu, 24 Mar 2011 19:44:27 GMT
Doesn't CL=LOCAL_QUORUM solve your problem?

On Thu, Mar 24, 2011 at 9:33 AM, <jonathan.colby@gmail.com> wrote:

> Hi Nate -
>
> That sounds really promising and I'm looking forward to trying that out.
>
> My original question came up while thinking how to achieve quorum (with
> rf=3) with a loss of 1 of 2 data centers. My logic was that if you had 2
> replicas in the same data center where the client originally written to,
> then that client is guaranteed to be able to satisfy quorum, even if the
> other data center is unreachable.
>
> But I think there is no way to guarantee where the first write is written
> to. That would be based on the token range, which could very well be in any
> data center.
>
> Jon
>
>
>
>
> On Mar 24, 2011 3:05pm, Nate McCall <nate@datastax.com> wrote:
> > We have a load balancing policy which selects the host best on latency
> >
> > and uses a Phi convict algorithm in a method similar to DynamicSnitch.
> >
> > Using this policy, you would inherently get the closest replica
> >
> > whenever possible as that would most likely be the best performing.
> >
> >
> >
> > This policy is still in trunk and 0.7.0 tip. We should have a new
> >
> > release out containing the above in the next few days.
> >
> >
> >
> > On Thu, Mar 24, 2011 at 8:46 AM, Jonathan Colby
> >
> > jonathan.colby@gmail.com> wrote:
> >
> > > Indeed I found the big flaw in my own logic.   Even writing to the
> "local" cassandra nodes does not guarantee where the replicas will end up.
> The decision where to write the first replica is based on the token ring,
> which is spread out on all nodes regardless of datacenter.   right ?
> >
> > >
> >
> > > On Mar 24, 2011, at 2:02 PM, Jonathan Colby wrote:
> >
> > >
> >
> > >> Hi -
> >
> > >>
> >
> > >> Our cluster is spread between 2 datacenters.   We have a
> straight-forward IP assignment so that OldNetworkTopology (rackinferring
> snitch) works well.    We have cassandra clients written in Hector in each
> of those data centers.   The Hector clients all have a list of all cassandra
> nodes across both data centers.  RF=3.
> >
> > >>
> >
> > >> Is there an order as to which data center gets the first write?    In
> other words, would (or can) the Hector client do its first write to the
> cassandra nodes in its own data center?
> >
> > >>
> >
> > >> It would be ideal it Hector chose the "local" cassandra nodes.  That
> way, if one data center is unreachable, the Quorum of replicas in cassandra
> is still reached (because it was written to the working data center first).
> >
> > >>
> >
> > >> Otherwise, if the cassandra writes are really random from the Hector
> client point-of-view, a data center outage would result in a read failure
> for any data that has 2 replicas in the lost data center.
> >
> > >>
> >
> > >> Is anyone doing this?  Is there a flaw in my logic?
> >
> > >>
> >
> > >>
> >
> > >
> >
> > >
> >
>

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