Hello,
I think that the new fusing support in akka-streams may have more
consequences than I initially suspected. It quite drastically changes how
concurrency is handled in akka-streams.
So far the model was quite clear: every stage gets materialised into an
actor. If there's a split/broadcast, then all branches will be executed in
parallel.
Now, with auto-fusing that changes. By default, if there's a split, the
branches won't be executed in parallel. I think that's quite a big
semantical change. A similar effect is when there are e.g. two
computationally expensive, consecutive .map stages (they will now be
processed sequentially, not concurrently), however with splits I think it
was a very natural expectation that things will be processed in parallel.
Hence I have four questions:
1. if I have a split, where should the async boundary go? My first attempt
was adding the async boundary attribute to the split graph stage itself,
but that didn't work. By experimentation, I have to add it to the stages
that are connected to the split's outputs.
2. how to insert async boundaries in linear pipelines? E.g.:
Source(List(1, 2, 3))
.map(_ + 1)
.addAttributes(Attributes.asyncBoundary)
.map(_ * 2)
doesn't compile
3. are you sure it's a good idea to make all processing stages fuseable by
default? E.g. conflate seems to be naturally concurrent, as it deals with
slower/faster components, which doesn't make much sense if everything is in
one thread :)
4. which built-in stages are fuseable by default? The docs are quite vague
here saying only "linear" ones. Which ones are these?
If akka-streams now has *explicit* concurrency control, instead of the
*implicit* one so far, maybe it would make sense to be explicit in the api
(different name?) which stages will run concurrently by default?
Thanks,
Adam
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