Scala streaming library differences (Reactive Streams/Iteratee/RxScala/Scalaz...)

Question

I'm following the Functional Reactive Programming in Scala course on Coursera and we deal with RxScala Observables (based on RxJava).

As far as I know, the Play Iteratee's library looks a bit like RxScala Observables, where Observables a bit like Enumerators and Observers are bit like Iteratees.

There's also the Scalaz Stream library, and maybe some others?

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So I'd like to know the main differences between all these libraries. In which case one could be better than another?

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PS: I wonder why Play Iteratees library has not been choosed by Martin Odersky for his course since Play is in the Typesafe stack. Does it mean Martin prefers RxScala over Play Iteratees?

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Edit: the Reactive Streams initiative has just been announced, as an attempt to standardize a common ground for achieving statically typed, high-performance, low latency, asynchronous streams of data with built-in non-blocking back pressure

Answer 1

PS: I wonder why Play Iteratees library has not been choosed by Martin Odersky for his course since Play is in the Typesafe stack. Does it mean Martin prefers RxScala over Play Iteratees?

I'll answer this. The decision of which streaming API's to push/teach is not one that has been made just by Martin, but by Typesafe as a whole. I don't know what Martin personally prefers (though I have heard him say iteratees are just too hard for newcomers), but we at Typesafe think that Iteratees require too high a learning curve to teach them to newcomers in asynchronous IO.

At the end of the day, the choice of streaming library really comes down to your use case. Play's iteratees library handles practically every streaming use case in existence, but at a cost of a very difficult to learn API (even seasoned Haskell developers often struggle with iteratees), and also some loss in performance. Other APIs handle less use cases, RX for example doesn't (currently) handle back pressure, and very few of the other APIs are suitable for simple streamed parsing. But streamed parsing is actually a pretty rare use case for end users, in most cases it suffices to simply buffer then parse. So Typesafe has chosen APIs that are easy to learn and meet the majority of the most common use cases.

Answer 2

Iteratees and Stream aren't really that similar to RxJava. The crucial difference is that they are concerned with resource safety (that is, closing files, sockets, etc. once they aren't needed anymore), which requires feedback (Iteratees can tell Enumerators they are done, but Observers don't tell anything to Observables) and makes them significantly more complex.