Hi, Sabarish Sasidharan, The key point is to make your KV-store update idempotent. So, if the offset associated with the aggregated value are written in the same row in RocksDB (i.e. atomicity is achieved here), I think that your approach would work. As Robert mentioned, offsets are always committed last in Samza. Hence, any failure recovery is guaranteed to replay some of the old messages. If the flushed state store has the aggregated value together w/ the offset, you can use the offset to de-dup the replayed old messages that are already applied to the aggregated results.
@Robert, yes, the order you listed would be maintained. Thanks! -Yi On Fri, Apr 15, 2016 at 12:16 PM, Robert Crim <rjc...@gmail.com> wrote: > Looking at: > > https://github.com/apache/samza/blob/f02386464d31b5a496bb0578838f51a0331bfffa/samza-core/src/main/scala/org/apache/samza/container/TaskInstance.scala#L171 > > > The commit function, in order, does: > 1. Flushes metrics > 2. Flushes stores > 3. Produces messages from the collectors > 4. Write offsets > > So I would reason that it would be OK to store an offset you've seen in the > store and use that to skip the messages if you've already mutated your data > -- but be aware any of 2 (if multiple stores) ,3, or 4 may not have > happened so you might want to do those again. You'd need to be careful if > your changes span multiple stores or keys since multiple writes to > changelogs are not atomic. > > Question to maintainers: is it safe for Samza users to relay on this order? > > On Fri, Apr 15, 2016 at 11:31 AM, Sabarish Sasidharan < > sabarish....@gmail.com> wrote: > > > Hi Guozhang > > > > Thanks. Assuming the checkpoint would typically be behind the offset > > persisted in my store (+ changelog), when the messages are replayed > > starting from the checkpoint, I can very well skip those by comparing > > against the offset in my store right? So I am not understanding why > > duplicates would affect my state. > > > > Regards > > Sab > > > > On Fri, Apr 15, 2016 at 10:07 PM, Guozhang Wang <wangg...@gmail.com> > > wrote: > > > > > Hi Sab, > > > > > > For stateful processing where you have persistent state stores, you > need > > to > > > maintain the checkpoint which includes the committed offsets as well as > > the > > > store flushed in sync, but right not these two operations are not done > > > atomically, and hence if you fail in between, you could still get > > > duplicates where you consume from the committed offsets while some of > > them > > > have already updated the stores. > > > > > > Guozhang > > > > > > > > > On Thu, Apr 14, 2016 at 11:56 PM, Sasidharan, Sabarish < > > > sabarish.sasidha...@harman.com> wrote: > > > > > > > Hi > > > > > > > > To achieve exactly once processing for my aggregates, wouldn’t it be > > > > enough if I maintain the latest offset processed for the aggregate > and > > > > check against that offset when messages are replayed on recovery? Am > I > > > > missing something here? > > > > > > > > Thanks > > > > > > > > Regards > > > > Sab > > > > > > > > > > > > > > > -- > > > -- Guozhang > > > > > >
