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From Patricio Echagüe <patric...@gmail.com>
Subject Re: Re: Re: Re: Quorum, Hector, and datacenter preference
Date Fri, 25 Mar 2011 17:20:06 GMT
Glad it worked

2011/3/25 <jonathan.colby@gmail.com>

> very cool. thanks for the info. this is exactly what we need.
>
>
> On Mar 25, 2011 8:22am, Patricio Echagüe <patricioe@gmail.com> wrote:
> >
> > It's a cassandra consistency level
> > On Mar 24, 2011 11:44 PM, jonathan.colby@gmail.com> wrote:> Patricio -
> > >
> > > I haven't heard of local_quorum. Is that a cassandra setting or
> something
> >
> > > you specify in the client?
> > >
> > > On Mar 24, 2011 8:44pm, Patricio Echagüe patricioe@gmail.com> wrote:
> > >> 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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