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From Svante Karlsson <svante.karls...@csi.se>
Subject Re: Reliability against rack failure
Date Sun, 05 Aug 2018 18:43:42 GMT
You need 3 racks for your zookeepers anyway. It needs 2 out of three. How
have you solved that?

Den sön 5 aug. 2018 20:31Sanjay Awatramani <Sanjay.Awatramani@guavus.com>
skrev:

> Thanks for the quick response Svante.
> I forgot to mention that the deployment I am looking at has 2 racks. We
> came up with this solution, but for this specific deployment adding a rack
> is out of question.
> Is there a way to resolve this with 2 racks ?
>
> Regards,
> Sanjay
>
> On 05/08/18, 11:57 PM, "Svante Karlsson" <svante.karlsson@csi.se> wrote:
>
> >3 racks,  Replication Factor = 3, min.insync.replicas=2, ack=all
> >
> >2018-08-05 20:21 GMT+02:00 Sanjay Awatramani
> ><Sanjay.Awatramani@guavus.com>:
> >
> >> Hi,
> >>
> >> I have done some experiments and gone through kafka documentation, which
> >> makes me conclude that there is a small chance of data loss or
> >>availability
> >> in a rack scenario. Can someone please validate my understanding ?
> >>
> >> The minimum configuration for a single rack system against single
> >>machine
> >> failure is Replication Factor = 3, min.insync.replicas=2, ack=all. This
> >> will ensure that leader + at least one replica receives the data
> >>written by
> >> a producer and there will be no data loss as well as the system
> >>continues
> >> to be available for further writes by the producer when a broker goes
> >>down.
> >>
> >> With rack awareness enabled, Kafka will distribute replicas of a
> >>partition
> >> across racks, giving reliability in case of rack failure. However rack
> >> awareness is only concerned with distribution of replicas, not
> >>prioritising
> >> the order of replication when followers catch up with the leader.
> >>
> >> Moving to a rack aware setup which has 2 racks, the above configuration
> >> would create a problem because one of the racks might get 2 replicas
> >>and if
> >> that rack goes down, data will be lost.
> >>
> >> Extending the minimum configuration for a 2 rack setup, Replication
> >>Factor
> >> = 4, min.insync.replicas=2, ack=all. This will ensure that when a rack
> >>goes
> >> down, one of the replicas will be available as it would be on a
> >>different
> >> rack than the leader. This was my understanding and I cannot find any
> >> documentation to back this. I studied the mechanism by which producer
> >> writes to leader - all IN SYNC REPLICAS (ISR) pull the newest data, and
> >>if
> >> the leader confirms that at least min.insync.replicas have got the
> >>newest
> >> data, it sends an ack back to the producer. In a rack aware system, I
> >>think
> >> Kafka will send an ack even if the 2 replicas which are in sync are on
> >>the
> >> same rack. And at this instant if that rack goes down, data is lost.
> >>
> >> If we make min.insync.replicas=3, we can guarantee that one of the
> >> replicas will be on a different rack and data will not be lost. However
> >>if
> >> any rack goes down, producer¹s writes will start failing as it won¹t
> >>have
> >> the requisite replicas available.
> >>
> >> Is my understanding correct ? Is there a way to configure Kafka in a
> >>rack
> >> scenario to make it tolerant to data loss as well as make it available
> >>for
> >> further writes even when a single node or an entire rack goes down ?
> >>
> >> Regards,
> >> Sanjay
> >>
> >>
>
>

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