In response to Joel's point, I think I do understand that messages can be lost, if in fact we have dropped down to only 1 member in the ISR at the time the message is written, and then that 1 node goes down.
What I'm not clear on, is the conditions under which a node can drop out of the ISR. You said: "- ISR = 0, leader = 0; (so 1 is out of the ISR - say if broker 0 is slow (but up))" Did you mean to say "if broker *1* is slow (but up)"? I assume by "slow", you mean when a follower hasn't made a fetch request within "replica.lag.time.max.ms"? The default for this is 10000 ms, so it would have to be in pretty bad shape to be "up" but "slow", no? It also does seem odd that a node can be too slow to remain in an ISR, but then be made available to compete in a ISR leader election..... Jason On Fri, Nov 1, 2013 at 5:00 PM, Neha Narkhede <neha.narkh...@gmail.com>wrote: > For supporting more durability at the expense of availability, we have a > JIRA that we will fix on trunk. This will allow you to configure the > default as well as per topic durability vs availability behavior - > https://issues.apache.org/jira/browse/KAFKA-1028 > > Thanks, > Neha > > > On Fri, Nov 1, 2013 at 1:43 PM, Joel Koshy <jjkosh...@gmail.com> wrote: > > > >>>> > > >>>> Unclean shutdown could result in data loss - since you are moving > > >>>> leadership to a replica that has fallen out of ISR. i.e., it's log > end > > >>>> offset is behind the last committed message to this partition. > > >>>> > > >>>> > > >>> But if data is written with 'request.required.acks=-1', no data > should > > be > > >>> lost, no? Or will partitions be truncated wholesale after an unclean > > >>> shutdown? > > > > Sorry about the delayed reply to this, but it is an important point - > > data can be lost even in this case. -1 means ack after all replicas in > > the current ISR have received the message. So for example: > > - assigned replicas for some partition = 0,1 > > - ISR = 0, leader = 0; (so 1 is out of the ISR - say if broker 0 is > > slow (but up)) > > - messages with acks=-1 will get committed (i.e., the producer will > > receive the ack) > > - shutdown broker 0. So this will result in an unclean leader election > > since there are no other replicas in ISR; and if the broker 1 is up it > > will become the new leader > > - however it only has messages until the earlier watermark > > - when broker 0 comes back up it will become a follower to 1. Its > > initial fetch will be for its last checkpointed high watermark (which > > is larger than the current leader's hw), but the leader will piggy > > back the current hw in the fetch response to the follower which will > > then truncate its log to the leader's current hw. At least this is my > > recollection of how it should be - but I'm staring at the code and > > think there may be an issue with it. Will update this thread if this > > is incorrect. > > > > Thanks, > > > > Joel > > >
