I've recently moved to Play framework 2.0 and there are some questions concerning me regarding how controllers actually work in play.

In [play docs](http://www.playframework.org/documentation/2.0.3/ScalaAsync) there are mentioned:

> 
> Because of the way Play 2.0 works, the action code must be as fast as
>  possible (ie. non blocking).
> 
> 
> 

However in [another part of the docs](https://github.com/playframework/Play20/wiki/AkkaCore):

```
            /actions {
                router = round-robin
                nr-of-instances = 24
            }

```

and

```
        actions-dispatcher = {
            fork-join-executor {
                parallelism-factor = 1.0
                parallelism-max = 24
            }
        }

```

It seems that there are 24 actors allocated for controllers handling. I guess every request allocates one of those actors for the lifetime of request. **Is that right?**

Also, what does `parallelism-factor` mean and how does `fork-join-executor` differ from `thread-pool`?

Also - docs should say that Async should be used for long computations. **What qualifies as a long calculation? 100ms? 300ms? 5 seconds? 10 seconds?** My guess would be anything over a second, but how to determine that?

The reason of this questioning is that testing async controller calls is way harder than regular calls. You have to spin up a fake application and do a full-fledged request instead of just calling a method and checking its return value.

Even if that wasn't the case, I doubt that wrapping everything in `Async` and `Akka.future` is the way.

I have asked for this in #playframework IRC channel but there was no answer and it seems that I'm not the only one not sure how things should be done.

Just to reiterate:

1. Is it right that every request allocates one actor from /actions pool?
2. What does `parallelism-factor` mean and why is it 1?
3. How does `fork-join-executor` differ from `thread-pool-executor`?
4. How long should a calculation be to become wrapped in `Async`?
5. Is is not possible to test async controller method without spinning up fake applications?

Thanks in advance.

**Edit: some stuff from IRC**

Some stuff from IRC.

```
<imeredith> arturaz: i cant be boethered writing up a full reply but here are key points
<imeredith> arturaz: i believe that some type of CPS goes on with async stuff which frees up request threads
<arturaz> CPS?
<imeredith> continuations
<imeredith> when the future is finished, or timedout, it then resumes the request
<imeredith> and returns data
<imeredith> arturaz: as for testing, you can do .await on the future and it will block until the data is ready
<imeredith> (i believe)
<imeredith> arturaz: as for "long" and parallelism - the longer you hold a request thread, the more parrellism you need
<imeredith> arturaz: ie servlets typically need a lot of threads because you have to hold the request thread open for a longer time then if you are using play async
<imeredith> "Is it right that every request allocates one actor from /actions pool?" - yes i belive so
<imeredith> "What does parallelism-factor mean and why is it 1?" - im guessing this is how many actors there are in the pool?
<imeredith> or not
<imeredith> "How does fork-join-executor differ from thread-pool-executor?" -no idea
<imeredith> "How long should a calculation be to become wrapped in Async?" - i think that is the same as asking "how long is a piece of string"
<imeredith> "Is is not possible to test async controller method without spinning up fake applications?" i think you should be able to get the result
<viktorklang> imeredith: A good idea is to read the documentation: http://doc.akka.io/docs/akka/2.0.3/general/configuration.html ( which says parallelism-factor is: # Parallelism (threads) ... ceil(available processors * factor))
<arturaz> viktorklang, don't get me wrong, but that's the problem - this is not documentation, it's a reminder to yourself.
<arturaz> I have absolutely no idea what that should mean
<viktorklang> arturaz: It's the number of processors available multiplied with the factor you give, and then rounded up using "ceil". I don't know how it could be more clear.
<arturaz> viktorklang, how about: This factor is used in calculation `ceil(number of processors * factor)` which describes how big is a thread pool given for your actors.
<viktorklang> arturaz: But that is not strictly true since the size is also guarded by your min and max values
<arturaz> then why is it there? :)
<viktorklang> arturaz: Parallelism (threads) ... ceil(available processors * factor) could be expanded by adding a big of conversational fluff: Parallelism ( in other words: number of threads), it is calculated using the given factor as: ceil(available processors * factor)
<viktorklang> arturaz: Because your program might not work with a parallelism less than X and you don't want to use more threads than X (i.e if you have a 48 core box and you have 4.0 as factor that'll be a crapload of threads)
<viktorklang> arturaz: I.e. scheduling overhead gives diminishing returns, especially if ctz switching is across physical slots.
<viktorklang> arturaz: Changing thread pool sizes will always require you to have at least basic understanding on Threads and thread scheduling
<viktorklang> arturaz: makes sense?
<arturaz> yes
<arturaz> and thank you
<arturaz> I'll add this to my question, but this kind of knowledge would be awesome docs ;)

```