Hey Nick, A few high-level thoughts:
1. We definitely don't want to piggyback on the SUSPENDED task state, as this is currently more like an intermediate state that a task passes through as it's being closed/migrated elsewhere, it doesn't really mean that a task is "suspended" and there's no logic to suspend processing on it. What you want is probably closer in spirit to the concept of a paused "named topology", where we basically freeze processing on a specific task (or set of tasks). 2. More importantly however, the SUSPENDED state was only ever needed to support efficient eager rebalancing, and we plan to remove the eager rebalancing protocol from Streams entirely in the near future. And unfortunately, the named topologies feature was never fully implemented and will probably be ripped out sometime soon as well. 3. In short, to go this route, you'd probably need to implement a PAUSED state from scratch. This wouldn't be impossible, but we are planning to basically revamp the entire thread model and decouple the consumer (potentially including the deserialization step) from the processing threads. Much as I love the idea of this feature, it might not make a lot of sense to spend time implementing right now when much of that work could be scrapped in the mid-term future. We don't have a timeline for this, however, so I don't think this should discourage you if the feature seems worth it, just want to give you a sense of the upcoming roadmap. 4. As for the feature itself, my only concern is that this feels like a very advanced feature but it would be easy for new users to accidentally abuse it and get their application in trouble. Specifically I'm worried about how this could be harmful to applications for which some degree of synchronization is required, such as a join. Correct join semantics rely heavily on receiving records from both sides of the join and carefully selecting the next one to process based on timestamp. Imagine if a DeserializationException occurs upstream of a repartition feeding into one side of a join (but not the other) and the user opts to PAUSE this task. If the join continues as usual it could lead to missed or incorrect results when processing is enforced with no records present on one side of the join but usual traffic flowing through the other. Maybe we could somehow signal to also PAUSE all downstream/dependent tasks? Should be able to add this information to the subscription metadata and send to all clients via a rebalance. There might be better options I'm not seeing. Or we could just decide to trust the users not to shoot themselves in the foot -- as long as we write a clear warning in the javadocs this might be fine Thanks for all the great KIPs! On Thu, Oct 12, 2023 at 9:51 AM Nick Telford <nick.telf...@gmail.com> wrote: > Hi everyone, > > This is a Streams KIP to add a new DeserializationHandlerResponse, > "SUSPEND", that suspends the failing Task but continues to process other > Tasks normally. > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-990%3A+Capability+to+SUSPEND+Tasks+on+DeserializationException > > I'm not yet completely convinced that this is practical, as I suspect it > might be abusing the SUSPENDED Task.State for something it was not designed > for. The intent is to pause an active Task *without* re-assigning it to > another instance, which causes cascading failures when the FAIL > DeserializationHandlerResponse is used. > > Let me know what you think! > > Regards, > Nick >
