Comments inline. On Thu, Aug 11, 2016 at 8:06 PM, Gyula Fóra <gyula.f...@gmail.com> wrote: > Option 1: > I think the main problem here is sending all the state everywhere will not > scale at all. I think this will even fail for some internal Flink operators > (window timers I think are kept like this, maybe Im wrong here). The > general problem here what we don't have with the key-value states is that > the system can't do the repartitioning automatically. I think we should try > to make abstractions that would allow the system to do this.
The state size can definitely become a problem. For Kafka sources for example I don' think it would be problematic, but the timers it might be, yes. It definitely depends on the use case. In theory, we could also redistribute the list elements automatically, for example in a round robing fashion. The question is whether this will be enough in general. > > Option 2: > To be honest I don't completely get this approach, what do the indices mean > in the get set methods? What happens if the same index is used from > multiple operators? > This may also suffers in scalability like option 1 (but as I said I dont > get this completely :() Yes, I don't like it either. It's actually similar to Option 1 (from runtime perspective). I think the main question with Option 2 is whether we expose the API as an interface or a state class. If we go for this kind of interface we could parameterize the restore behaviour via the descriptor (e.g. flag to merge/union etc.). That should be more extensible than providing interfaces. > I think another approach could be (might be similar what option 2 is trying > to achieve) to provide a Set<T> (or Map) like abstraction to keep the non > partitioned states. Users could add/remove things from it at their on will, > but the system would be free to redistribute the Sets between the > operators. In practice this would mean for instance that the Kafka sources > would store (partition, offset) tuples in the set but and every time in the > open method they would check what is assigned to them (the system is free > to decide). This of course would only work well if we can assume that > distributing the states by equal numbers is desirable. I think the same point applies to redistributing the list automatically (what I meant with whether it is "general enough"). I think what you describe here could be the list w/o unioning it. > > Broadcast states: > This might be a good time to think about broadcast states. Non-partitioned > states that are the same at all subtasks, I think this comes up in a lot of > use-cases (I know at least one myself haha) and it is pretty straight > forward from a runtime perspective, the bigger question is the API. Can you explain this a little more? ======== Another open question (not addressed in the FLIP yet) is how we treat operators that have both keyed and non-keyed state. The current API kind of moves this question to the user.
