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From Enis Söztutar <enis....@gmail.com>
Subject Re: On HBase Read Replicas
Date Fri, 17 Feb 2017 22:04:32 GMT
You can do gets using two different "modes":
 - Do a read with backup RPCs. In case, the algorithm that I have above
will be used. 1 RPC to primary, and 2 more RPCs after primary timeouts.
 - Do a read to a single replica. In this case, there is only 1 RPC that
will happen to that given replica.

Enis

On Fri, Feb 17, 2017 at 12:03 PM, jeff saremi <jeffsaremi@hotmail.com>
wrote:

> Enis
>
> Thanks for taking the time to reply
>
> So i thought that a read request is sent to all Replicas regardless. If we
> have the option of Sending to one, analyzing response, and then sending to
> another, this bodes well with our scenarios.
>
> Please confirm
>
> thanks
>
> ________________________________
> From: Enis Söztutar <enis.soz@gmail.com>
> Sent: Friday, February 17, 2017 11:38:42 AM
> To: hbase-user
> Subject: Re: On HBase Read Replicas
>
> You can use read-replicas to distribute the read-load if you are fine with
> stale reads. The read replicas normally have a "backup rpc" path, which
> implements a logic like this:
>  - Send the RPC to the primary replica
>  - if no response for 100ms (or configured timeout), send RPCs to the other
> replicas
>  - return the first non-exception response.
>
> However, there is also another feature for read replicas, where you can
> indicate which exact replica_id you want to read from when you are doing a
> get. If you do this:
> Get get = new Get(row);
> get.setReplicaId(2);
>
> the Get RPC will only go to the replica_id=2. Note that if you have region
> replication = 3, then you will have regions with replica ids: {0, 1, 2}
> where replica_id=0 is the primary.
>
> So you can do load-balancing with a get.setReplicaId(random() %
> num_replicas) kind of pattern.
>
> Enis
>
>
>
> On Thu, Feb 16, 2017 at 9:41 AM, Anoop John <anoop.hbase@gmail.com> wrote:
>
> > Never saw this kind of discussion.
> >
> > -Anoop-
> >
> > On Thu, Feb 16, 2017 at 10:13 PM, jeff saremi <jeffsaremi@hotmail.com>
> > wrote:
> > > Thanks Anoop.
> > >
> > > Understood.
> > >
> > > Have there been enhancement requests or discussions on load balancing
> by
> > providing additional replicas in the past? Has anyone else come up with
> > anything on this?
> > > thanks
> > >
> > > ________________________________
> > > From: Anoop John <anoop.hbase@gmail.com>
> > > Sent: Thursday, February 16, 2017 2:35:48 AM
> > > To: user@hbase.apache.org
> > > Subject: Re: On HBase Read Replicas
> > >
> > > The region replica feature came in so as to reduce the MTTR and so
> > > increase the data availability.  When the master region containing RS
> > > dies, the clients can read from the secondary regions.  But to keep
> > > one thing in mind that this data from secondary regions will be bit
> > > out of sync as the replica is eventual consistent.   Because of this
> > > said reason,  change client so as to share the load across diff RSs
> > > might be tough.
> > >
> > > -Anoop-
> > >
> > > On Sun, Feb 12, 2017 at 8:13 AM, jeff saremi <jeffsaremi@hotmail.com>
> > wrote:
> > >> Yes indeed. thank you very much Ted
> > >>
> > >> ________________________________
> > >> From: Ted Yu <yuzhihong@gmail.com>
> > >> Sent: Saturday, February 11, 2017 3:40:50 PM
> > >> To: user@hbase.apache.org
> > >> Subject: Re: On HBase Read Replicas
> > >>
> > >> Please take a look at the design doc attached to
> > >> https://issues.apache.org/jira/browse/HBASE-10070.
> > >>
> > >> Your first question would be answered by that document.
> > >>
> > >> Cheers
> > >>
> > >> On Sat, Feb 11, 2017 at 2:06 PM, jeff saremi <jeffsaremi@hotmail.com>
> > wrote:
> > >>
> > >>> The first time I heard replicas in HBase the following thought
> > immediately
> > >>> came to my mind:
> > >>> To alleviate the load in read-heavy clusters, one could assign Region
> > >>> servers to be replicas of others so that the load is distributed and
> > there
> > >>> is less pressure on the main RS.
> > >>>
> > >>> Just 2 days ago a colleague quoted a paragraph from HBase manual that
> > >>> contradicted this completely. Apparently, the replicas do not help
> > with the
> > >>> load but they actually contribute to more traffic on the network and
> > on the
> > >>> underlying file system
> > >>>
> > >>> Would someone be able to give us some insight on why anyone would
> want
> > >>> replicas?
> > >>>
> > >>> And also could one easily change this behavior in the HBase native
> Java
> > >>> client to support what I had been imagining as the concept for
> > replicas?
> > >>>
> > >>>
> > >>> thanks
> > >>>
> >
>

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