Spray.io: When (not) to use non-blocking route handling?

问题

If we are thinking of production grade REST API, should we use non-blocking as much as possible, e.g.

def insertDbAsync(rows: RowList): Future[Unit] = ...
...
val route =
path("database" / "insertRowList") {
  post {
    entity(as[RowList]) { rows =>
      log.info(s"${rows.length} rows received")
      val async = insertDbAsync(rows)
      onComplete(async) {
        case Success(response) =>
          complete("success")
        case Failure(t) =>
          complete("error")
      }
    }
  }
}

I'm thinking that the answer will most likely be a 'yes', but what are some guidelines in deciding what should and should not be a blocking code, and why?

回答1:

Spray uses Akka as underlying platform, so recommendations are same as for actors (Blocking Needs Careful Management). Blocking code may require too much threads, which may:

kill actor's lightweightness: millions of actors may operate on one thread by default. Let's say one non-blocked actor requires 0.001 threads for example. One blocked actor (which blocking time is, let's say, 100 times more than usual) will take 1 thread avg (not always same thread). First, The more threads you have - the more memory you loose - every blocked thread holds full callstack allocated before blocking, including references from stack (so GC can't erase them). Second, if you have more than number_of_processors threads - you will loose the performance. Third, if you use some dynamical pool - adding new thread may take some significant amount of time.
cause thread's starvation - you may have pool filled with threads, which doing nothing - so new tasks can't be processed before blocking operation complete (0 % CPU load, but 100500 messages waiting to be processed). It may even cause deadlocks. However, Akka uses Fork-Join-Pool by default so if your blocking code is managed (surrounded with scala.concurrent.blocking - Await.result have such surrounding inside ) - it will prevent starvation by cost of creating new thread instead of blocked one, but it won't compensate other problems.
traditionally cause deadlocks, so it's bad for design

If code is blocking from outside, you can surround it with future:

 import scala.concurrent._
 val f = Future {
     someNonBlockingCode()
     blocking { //mark this thread as "blocked" so fork-join-pool may create another one to compensate
        someBlocking()
     }  
 }

Inside separate actor:

 f pipeTo sender //will send the result to `sender` actor

Inside spray routing:

 onComplete(f) { .. }

It's better to execute such futures inside separate pool/dispatcher (fork-join-pool based).

P.S. As an alternative to futures (they may not be much convinient from design perspectvive) you may consider Akka I/O, Continuations/Coroutines, Actor's pools (also inside separate dispatcher), Disruptor etc.

回答2:

If you're using spray everything must be non-blocking as a matter of correctness - otherwise you'll block the (very small number of) dispatch threads and your server will stop responding.

来源：https://stackoverflow.com/questions/29384347/spray-io-when-not-to-use-non-blocking-route-handling

标签

scala

akka

spray