I think you are right, The rack awareness is used to spread the partitions on creation, assignment -etc so get as many racks as your replication count.
/svante 2017-08-20 13:33 GMT+02:00 Carl Samuelson <c...@brightbobbin.com>: > Hi > > I asked this question on SO here: > https://stackoverflow.com/questions/45778455/kafka-rack- > id-and-min-in-sync-replicas. > Basically, I am trying to understand how rack-id helps in DR situations. > > Kafka has introduced rack-id to provide redundancy capabilities if a whole > rack fails. There is a min in-sync replica setting to specify the minimum > number of replicas that need to be in-sync before a producer receives an > ack (-1 / all config). There is an unclean leader election setting to > specify whether a leader can be elected when it is not in-sync. > > So, given the following scenario: > > - Two racks. Rack 1, 2. > - Replication count is 4. > - Min in-sync replicas = 2 > - Producer ack=-1 (all). > - Unclean leader election = false > > Is it possible that there is a moment where all 4 replicas are available, > but the two in-sync replicas both come from rack 1, so the producer > receives an ack and at that point rack 1 crashes (before any replicas from > rack 2 are in-sync)? This means that rack 2 will only contain unclean > replicas and no producers would be able to add messages to the partition > essentially grinding to a halt. The replicas would be unclean in any case, > so no new leader could be elected in any case. > > Is my analysis correct, or is there something under the hood to ensure that > the replicas forming min in-sync replicas have to be from different racks? > Since replicas on the same rack would have lower latency it seems that the > above scenario is reasonably likely. > > <the SO post has an image showing the scenario> > > Thanks, > > Carl >
