CODEWITHSUNDEEP
scalaclosuresakkaactor
royalflush
royalflush

Reputation: 100

Inconsistent behavior between local actor and remote actor

This is sort of a follow up to an earlier question at Scala variable binding when used with Actors

Against others' advice, I decided to make a message containing a closure and mutate the variable that closure is closed under between messages.. and explicitly wait for them. The environment is akka 1.2 on scala 2.9

Consider the following 

var minAge = 18
val isAdult = (age: Int) => age >= minAge

println((actor ? answer(19, isAdult)).get) 
minAge = 20
println((actor ? answer(19, isAdult)).get)

The message handler for answer essentially applies isAdult to the first parameter (19). When actor is local, I get the answers I expect. 

true
false

But when it is remote, I get

false
false

I am simply curious why this would be the behavior? I would have expected consistent behavior between the two..

Thanks in advance!

Upvotes: 3

Views: 176

Answers (2)

Roland Kuhn
Roland Kuhn

Reputation: 15472

What you encounter here is what I would call “greediness” of Scala closures: they never close “by-value”, presumably because of the uniform access principle. This means that the closure contains an $outer reference which it uses to obtain the value of minAge. You did not give enough context to show what the $outer looks like in your test, hence I cannot be more precise in how it is serialized, from which would follow why it prints what you show.

One word, though: don’t send closures around like that, please. It is not a recipe for happiness, as you acknowledge yourself.

Upvotes: 2

oxbow_lakes
oxbow_lakes

Reputation: 134340

Well, you have come across what may (or may not) be considered a problem for a system where the behaviour is specified by rules which are not enforced by the language. The same kind of thing happens in Java. Here:

  1. Client: Data d = rmiServer.getSomeData();
  2. Client: d.mutate()

Do you expect the mutation to happen on the server as well? A fundamental issue with any system which involves remote communication, especially when that communication is transparent to a client, is understanding where that communication is occurring and what, exactly, is going on.

  • The communication with the actor takes the form of message-passing
  • An effect can pass a boundary only by the mechanism of message-passing (that is, the effect must reside within the returned value)
  • The actor library may transparently handle the transmission of a message remotely
  • If your effect is not a message, it is not happening!

Upvotes: 3

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