Using a separate ExecutionContext for Slick

Question

I'm using Play 2.5 with Slick. The docs on this topic simply state that everything is managed by Slick and Play's Slick module. This example however, prints Dispatcher[akka.actor.default-dispatcher]:

class MyDbioImpl @Inject()(protected val dbConfigProvider: DatabaseConfigProvider)(implicit ec: ExecutionContext)
  with HasDatabaseConfigProvider[JdbcProfile] {
  import profile.api._
  def selectSomeStuff(): Future[MyResult] = db.run {
    println(ec)
    [...]
  }
}

Since the execution context is printed inside db.run, it seems like all of my database access will also be executed on the default execution context.

I found this answer to an older question which, at the time, solved the problem. But this solution is since deprecated, it is suggested to use dependency injection to acquire the application context. When I try to do this, I get an error saying that play.akka.actor.slick-context does not exist...

class MyDbioProvider @Inject()(actorSystem: ActorSystem,
                               protected val dbConfigProvider: DatabaseConfigProvider)
  extends Provider[MyDbioImpl] {
  override def get(): MyDbioImpl = {
    val ec = actorSystem.dispatchers.lookup("play.akka.actor.slick-context")
    new MyDbioImpl(dbConfigProvider)(ec)
  }
}

Edit: Is Slick's execution context a "normal" execution context which is defined in a config file somewhere? Where does the context switch take place? I assumed the entry point to the "database world" is at db.run.

Answer 1

According to Slick:

Every Database contains an AsyncExecutor that manages the thread pool for asynchronous execution of Database I/O Actions. Its size is the main parameter to tune for the best performance of the Database object. It should be set to the value that you would use for the size of the connection pool in a traditional, blocking application (see About Pool Sizing in the HikariCP documentation for further information). When using Database.forConfig, the thread pool is configured directly in the external configuration file together with the connection parameters. If you use any other factory method to get a Database, you can either use a default configuration or specify a custom AsyncExecutor.

Basically it says you don't need to create an isolated ExecutionContext since Slick already isolates a thread pool internally. Any call you make to Slick is non-blocking thus you should use the default ExecutionContext.