I do not understand why Scala's
Future.onComplete returns a unit, so the invocations cannot be chained. According to the scala docs, "note that this design is intentional, to avoid suggesting that chained invocations may imply an ordering on the execution of the registered callbacks (callbacks registered on the same future are unordered)."
To compose the future, use combinators like
filter, which all return a Future.
Check out http://docs.scala-lang.org/overviews/core/futures.html#functional-composition-and-for-comprehensions for more details
I think cancellation is quite nuanced. See this discussion for more details: How to cancel Future in Scala?