async action in scala

Turns out Scala has a pretty badass native library for handling asynchronous data handling.

A quick use case:

• we have an HTTP server, and want to log all the requests being made to it (say we’re writing each request to a DB)
• however performance is important and we don’t want to take a hit for the log I/O
• so we need a way to dispatch a log message creation request without having to wait for it to finish.

Scala Future can help us out. From the docs:

A Future is a placeholder object for a value that may not yet exist.

example.scala

import scala.concurrent.{Future}
import scala.concurrent.ExecutionContext.Implicits.global // note: more on implicits soon!

def asyncLogAction(msg: String): Future[Unit] = {
  Future {
    Thread.sleep(1000)
    println(msg)
  }
}

def handleRequest(): Unit = {
  println("request received")
  println("doing some stuff here to handle the request")
  asyncLogAction("request logged")
  println("done, returning json response body")
}

handleRequest()
Thread.sleep(5000)

scala example.scala

# request received
# doing some stuff here to handle the request
# done, returning json response body
# request logged

So this is a heavy handed sample, but it shows that the hot path handleRequest executed without waiting for the asyncLogAction to complete.

Now let’s edit the code slightly to make it “blocking” by calling Await on our method that is returning the future:

import scala.concurrent.{Await, Future}
import scala.concurrent.ExecutionContext.Implicits.global // note: more on implicits soon!
import scala.concurrent.duration._
import scala.language.postfixOps

def asyncLogAction(msg: String): Future[Unit] = {
  Future {
    Thread.sleep(1000)
    println(msg)
  }
}

def handleRequest(): Unit = {
  println("request received")
  println("doing some stuff here to handle the request")
  Await.result(asyncLogAction("request logged"), 2 seconds)
  println("done, returning json response body")
}

handleRequest()
Thread.sleep(5000)

Now when we run the code again, it executes in “order” as we are forcing the execution to wait on the future to be resolved.

# request received
# doing some stuff here to handle the request
# request logged
# done, returning json response body