Spray API blocked by bad Execution Context in Scala?

Question

I use the Spray API to listen for requests from a server. A computation in one specific scala class ends up blocking Spray from responding across the whole application. This is a slight simplification of the problem, but I can provide more info. if needed.

class SomeClass(implicit execc: ExecutionContext){
    implicit val x = ...
    val foo = Await.result(...someFunc(x))
}

I added this import and it resolved my issue:

import scala.concurrent.ExecutionContext.Implicits.global

Can anyone explain how or why this worked?

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Edit:

OuterClass instantiates SomeClass, but itself is never instantiated with the ExecutionContext parameter. It appears that it may be using the global execution context by default, and that is why it is blocking then?

class OuterClass(executor: ExecutionContext){
    val s = new someClass
}

val x = (new OuterClass).someFunction

Answer 1

Spray route handler is a single actor that receives HTTP requests from Spray IO/Spray-can/library and passes them to the route handling function - essentially a partial function that has no concurrency on it's own. Thus if your route blocks, Spray will also block and requests will queue in the route handler actor queue.

There are 3 ways to properly handle blocking request processing in the route: spawn an actor per request, return Future of response or take request completion function and use it somewhere else unblocking the route (search for more detailed explanation if interested).

I can't be sure which execution context was used in your case, but it must have been very limited in terms of allocated threads and/or shared by your Spray route handler and long running task. This would lead to them both running on the same thread.

If you didn't have any execution context imported explicitly it must have been found through implicit resolution from regular scopes. You must have had one since you have it as a constructor parameter. Try to check your implicits in the scope to see which one it was. I'm curious myself whether it was one provided by Spray or something else.