I think the KIP looks good. I also think we need the metadata topic in-order to provide sane guarantees on what data will be processed.
As Matthias has outlined in the KIP we need to know when to stop consuming from intermediate topics, i.e, topics that are part of the same application but are used for re-partitioning or through etc. Without the metadata topic the consumption from the intermediate topics would always be one run behind. In the case of a join requiring partitioning this would result in no output for the first run and then in subsequent runs you'd get the output from the previous run - i'd find this a bit odd. Also I think having a fixed HWM IMO is a good idea. If you are running your streams app in some shared environment, then you don't want to get into a situation where the app fails (for random reasons), restarts with a new HMW, fails, restarts... and then continues to consume resources for ever as the HMW is constantly moving forward. So i think the approach in the KIP helps batch-mode streams apps to be good-citizens when running in shared environments. Thanks, Damian On Wed, 30 Nov 2016 at 10:40 Eno Thereska <eno.there...@gmail.com> wrote: > Hi Matthias, > > I like the first part of the KIP. However, the second part with the > failure modes and metadata topic is quite complex and I'm worried it > doesn't solve the problems you mention under failure. For example, the > application can fail before writing to the metadata topic. In that case, it > is not clear what the second app instance should do (for the handling of > intermediate topics case). So in general, we have the problem of failures > during writes to the metadata topic itself. > > Also, for the intermediate topics example, I feel like we are trying to > provide some sort of synchronisation between app instances with this > approach. By default today such synchronisation does not exist. One > instances writes to the intermediate topic, and the other reads from it, > but only eventually. That is a nice way to decouple instances in my opinion. > > The user can always run the batch processing multiple times and eventually > all instances will produce some output. The user's app can check whether > the output size is satisfactory and then not run any further loops. So I > feel they can already get a lot with the simpler first part of the KIP. > > Thanks > Eno > > > > On 30 Nov 2016, at 05:45, Matthias J. Sax <matth...@confluent.io> wrote: > > > > Thanks for your input. > > > > To clarify: the main reason to add the metadata topic is to cope with > > subtopologies that are connected via intermediate topic (either > > user-defined via through() or internally created for data > repartitioning). > > > > Without this handling, the behavior would be odd and user experience > > would be bad. > > > > Thus, using the metadata topic for have a "fixed HW" is just a small > > add-on -- and more or less for free, because the metadata topic is > > already there. > > > > > > -Matthias > > > > > > On 11/29/16 7:53 PM, Neha Narkhede wrote: > >> Thanks for initiating this. I think this is a good first step towards > >> unifying batch and stream processing in Kafka. > >> > >> I understood this capability to be simple yet very useful; it allows a > >> Streams program to process a log, in batch, in arbitrary windows > defined by > >> the difference between the HW and the current offset. Basically, it > >> provides a simple means for a Streams program to "stop" after > processing a > >> batch, stop (just like a batch program would) and continue where it left > >> off when restarted. In other words, it allows batch processing behavior > for > >> a Streams app without code changes. > >> > >> This feature is useful but I do not think there is a necessity to add a > >> metadata topic. After all, the user doesn't really care as much about > >> exactly where the batch ends. This feature allows an app to "process as > >> much as there is data to process" and the way it determines how much > data > >> there is to process is by reading the HW on startup. If there is new > data > >> written to the log right after it starts up, it will process it when > >> restarted the next time. If it starts, reads HW but fails, it will > restart > >> and process a little more before it stops again. The fact that the HW > >> changes in some scenarios isn't an issue since a batch program that > behaves > >> this way doesn't really care exactly what that HW is. > >> > >> There might be cases which require adding more topics but I would shy > away > >> from adding complexity wherever possible as it complicates operations > and > >> reduces simplicity. > >> > >> Other than this issue, I'm +1 on adding this feature. I think it is > pretty > >> powerful. > >> > >> > >> On Mon, Nov 28, 2016 at 10:48 AM Matthias J. Sax <matth...@confluent.io > > > >> wrote: > >> > >>> Hi all, > >>> > >>> I want to start a discussion about KIP-95: > >>> > >>> > >>> > https://cwiki.apache.org/confluence/display/KAFKA/KIP-95%3A+Incremental+Batch+Processing+for+Kafka+Streams > >>> > >>> Looking forward to your feedback. > >>> > >>> > >>> -Matthias > >>> > >>> > >>> -- > >> Thanks, > >> Neha > >> > > > >
