Using Akka with Scalatra - scala

Question

My target is building a highly concurrent backend for my widgets. I'm currently exposing the backend as a web service, which receives requests to run a specific widget (using Scalatra), fetches widget's code from DB and runs it in an actor (using Akka) which then replies with the results. So imagine I'm doing something like:
get("/run:id") {
...
val actor = Actor.actorOf("...").start
val result = actor !! (("Run",id), 10000)
...
}
Now I believe this is not the best concurrent solution and I should somehow combine listening for requests and running widgets in one actor implementation. How would you design this for maximum concurrency? Thanks.

Answer 1

You can start your actors in an akka boot file or in your own ServletContextListener so that they are started without being tied to a servlet.
Then you can look for them with the akka registry.
Actor.registry.actorFor[MyActor] foreach { _ !! (("Run",id), 10000) }
Apart from that there is no real integration for akka with scalatra at this moment.
So until now the best you can do is by using blocking requests to a bunch of actors.
I'm not sure but I wouldn't necessary spawn an actor for each request but rather have a pool of widget actors which you can send those requests. If you use a supervisor hierarchy then the you can use a supervisor to resize the pool if it is too big or too small.
class MyContextListener extends ServletContextListener {
def contextInitialized(sce: ServletContextEvent) {
val factory = SupervisorFactory(
SupervisorConfig(
OneForOneStrategy(List(classOf[Exception]), 3, 1000),
Supervise(actorOf[WidgetPoolSupervisor], Permanent)
}
def contextDestroyed(sce: ServletContextEvent) {
Actor.registry.shutdownAll()
}
}
class WidgetPoolSupervisor extends Actor {
self.faultHandler = OneForOneStrategy(List(classOf[Exception]), 3, 1000)
override def preStart() {
(1 to 5) foreach { _ =>
self.spawnLink[MyWidgetProcessor]
}
Scheduler.schedule(self, 'checkPoolSize, 5, 5, TimeUnit.MINUTES)
}
protected def receive = {
case 'checkPoolSize => {
//implement logic that checks how quick the actors respond and if
//it takes to long add some actors to the pool.
//as a bonus you can keep downsizing the actor pool until it reaches 1
//or until the message starts returning too late.
}
}
}
class ScalatraApp extends ScalatraServlet {
get("/run/:id") {
// the !! construct should not appear anywhere else in your code except
// in the scalatra action. You don't want to block anywhere else, but in a
// scalatra action it's ok as the web request itself is synchronous too and needs to
// to wait for the full response to have come back anyway.
Actor.registry.actorFor[MyWidgetProcessor] foreach {
_ !! ((Run, id), 10000)
} getOrElse {
throw new HeyIExpectedAResultException()
}
}
}
Please do regard the code above as pseudo code that happens to look like scala, I just wanted to illustrate the concept.