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From Enis Söztutar <enis....@gmail.com>
Subject Re: On HBase Read Replicas
Date Fri, 17 Feb 2017 19:38:42 GMT
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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