Directives

A “Directive” is a small building block used for creating arbitrarily complex route structures. Akka HTTP already pre-defines a large number of directives and you can easily construct your own:

Basics

Directives create Routes. To understand how directives work it is helpful to contrast them with the “primitive” way of creating routes.

Since Route is just a type alias for a function type Route instances can be written in any way in which function instances can be written, e.g. as a function literal:

val route: Route = { ctx => ctx.complete("yeah") }

or shorter:

val route: Route = _.complete("yeah")

With the complete directive this becomes even shorter:

val route = complete("yeah")

These three ways of writing this Route are fully equivalent, the created route will behave identically in all cases.

Let’s look at a slightly more complicated example to highlight one important point in particular. Consider these two routes:

val a: Route = {
  println("MARK")
  ctx => ctx.complete("yeah")
}

val b: Route = { ctx =>
  println("MARK")
  ctx.complete("yeah")
}

The difference between a and b is when the println statement is executed. In the case of a it is executed once, when the route is constructed, whereas in the case of b it is executed every time the route is run.

Using the complete directive the same effects are achieved like this:

val a = {
  println("MARK")
  complete("yeah")
}

val b = complete {
  println("MARK")
  "yeah"
}

This works because the argument to the complete directive is evaluated by-name, i.e. it is re-evaluated every time the produced route is run.

Let’s take things one step further:

val route: Route = { ctx =>
  if (ctx.request.method == HttpMethods.GET)
    ctx.complete("Received GET")
  else
    ctx.complete("Received something else")
}

Using the get and complete directives we can write this route like this:

val route =
  get {
    complete("Received GET")
  } ~
  complete("Received something else")

Again, the produced routes will behave identically in all cases.

Note that, if you wish, you can also mix the two styles of route creation:

val route =
  get { ctx =>
    ctx.complete("Received GET")
  } ~
  complete("Received something else")

Here, the inner route of the get directive is written as an explicit function literal.

However, as you can see from these examples, building routes with directives rather than “manually” results in code that is a lot more concise and as such more readable and maintainable. In addition it provides for better composability (as you will see in the coming sections). So, when using Akka HTTP’s Routing DSL you should almost never have to fall back to creating routes via Route function literals that directly manipulate the RequestContext.

Structure

The general anatomy of a directive is as follows:

name(arguments) { extractions =>
  ... // inner route
}

It has a name, zero or more arguments and optionally an inner route (The RouteDirectives are special in that they are always used at the leaf-level and as such cannot have inner routes). Additionally directives can “extract” a number of values and make them available to their inner routes as function arguments. When seen “from the outside” a directive with its inner route form an expression of type Route.

What Directives do

A directive can do one or more of the following:

  • Transform the incoming RequestContext before passing it on to its inner route (i.e. modify the request)
  • Filter the RequestContext according to some logic, i.e. only pass on certain requests and reject others
  • Extract values from the RequestContext and make them available to its inner route as “extractions”
  • Chain some logic into the RouteResult future transformation chain (i.e. modify the response or rejection)
  • Complete the request

This means a Directive completely wraps the functionality of its inner route and can apply arbitrarily complex transformations, both (or either) on the request and on the response side.

Composing Directives

Note

Gotcha: forgetting the ~ (tilde) character in between directives can result in perfectly valid Scala code that compiles but does not work as expected. What would be intended as a single expression would actually be multiple expressions, and only the final one would be used as the result of the parent directive. Alternatively, you might choose to use the concat combinator. concat(a, b, c) is the same as a ~ b ~ c.

As you have seen from the examples presented so far the “normal” way of composing directives is nesting. Let’s take a look at this concrete example:

val route: Route =
  path("order" / IntNumber) { id =>
    get {
      complete {
        "Received GET request for order " + id
      }
    } ~
    put {
      complete {
        "Received PUT request for order " + id
      }
    }
  }
verify(route) // hide

Here the get and put directives are chained together with the ~ operator to form a higher-level route that serves as the inner route of the path directive. To make this structure more explicit you could also write the whole thing like this:

def innerRoute(id: Int): Route =
  get {
    complete {
      "Received GET request for order " + id
    }
  } ~
  put {
    complete {
      "Received PUT request for order " + id
    }
  }

val route: Route = path("order" / IntNumber) { id => innerRoute(id) }
verify(route) // hide

What you can’t see from this snippet is that directives are not implemented as simple methods but rather as stand-alone objects of type Directive. This gives you more flexibility when composing directives. For example you can also use the | operator on directives. Here is yet another way to write the example:

val route =
  path("order" / IntNumber) { id =>
    (get | put) { ctx =>
      ctx.complete(s"Received ${ctx.request.method.name} request for order $id")
    }
  }
verify(route) // hide

Or better (without dropping down to writing an explicit Route function manually):

val route =
  path("order" / IntNumber) { id =>
    (get | put) {
      extractMethod { m =>
        complete(s"Received ${m.name} request for order $id")
      }
    }
  }
verify(route) // hide

If you have a larger route structure where the (get | put) snippet appears several times you could also factor it out like this:

val getOrPut = get | put
val route =
  path("order" / IntNumber) { id =>
    getOrPut {
      extractMethod { m =>
        complete(s"Received ${m.name} request for order $id")
      }
    }
  }
verify(route) // hide

Note that, because getOrPut doesn’t take any parameters, it can be a val here.

As an alternative to nesting you can also use the & operator:

val getOrPut = get | put
val route =
  (path("order" / IntNumber) & getOrPut & extractMethod) { (id, m) =>
    complete(s"Received ${m.name} request for order $id")
  }
verify(route) // hide

Here you can see that, when directives producing extractions are combined with &, the resulting “super-directive” simply extracts the concatenation of its sub-extractions.

And once again, you can factor things out if you want, thereby pushing the “factoring out” of directive configurations to its extreme:

val orderGetOrPutWithMethod =
  path("order" / IntNumber) & (get | put) & extractMethod
val route =
  orderGetOrPutWithMethod { (id, m) =>
    complete(s"Received ${m.name} request for order $id")
  }
verify(route) // hide

This type of combining directives with the | and & operators as well as “saving” more complex directive configurations as a val works across the board, with all directives taking inner routes.

Note that going this far with “compressing” several directives into a single one probably doesn’t result in the most readable and therefore maintainable routing code. It might even be that the very first of this series of examples is in fact the most readable one.

Still, the purpose of the exercise presented here is to show you how flexible directives can be and how you can use their power to define your web service behavior at the level of abstraction that is right for your application.

Type Safety of Directives

When you combine directives with the | and & operators the routing DSL makes sure that all extractions work as expected and logical constraints are enforced at compile-time.

For example you cannot | a directive producing an extraction with one that doesn’t:

val route = path("order" / IntNumber) | get // doesn't compile

Also the number of extractions and their types have to match up:

val route = path("order" / IntNumber) | path("order" / DoubleNumber)   // doesn't compile
val route = path("order" / IntNumber) | parameter('order.as[Int])      // ok

When you combine directives producing extractions with the & operator all extractions will be properly gathered up:

val order = path("order" / IntNumber) & parameters('oem, 'expired ?)
val route =
  order { (orderId, oem, expired) =>
    ...
  }

Directives offer a great way of constructing your web service logic from small building blocks in a plug and play fashion while maintaining DRYness and full type-safety. If the large range of Predefined Directives does not fully satisfy your needs you can also easily create Custom Directives.

Automatic Tuple extraction (flattening)

Convenient Scala DSL syntax described in Basics, and Composing Directives are made possible by Tuple extraction internally. Let’s see how this works with examples.

val futureOfInt: Future[Int] = Future.successful(1)
val route =
  path("success") {
    onSuccess(futureOfInt) { //: Directive[Tuple1[Int]]
      i => complete("Future was completed.")
    }
  }

Looking at the above code, onSuccess(futureOfInt) returns a Directive1[Int] = Directive[Tuple1[Int]].

val futureOfTuple2: Future[Tuple2[Int,Int]] = Future.successful( (1,2) )
val route =
  path("success") {
    onSuccess(futureOfTuple2) { //: Directive[Tuple2[Int,Int]]
      (i, j) => complete("Future was completed.")
    }
  }

Similarly, onSuccess(futureOfTuple2) returns a Directive1[Tuple2[Int,Int]] = Directive[Tuple1[Tuple2[Int,Int]]], but this will be automatically converted to Directive[Tuple2[Int,Int]] to avoid nested Tuples.

val futureOfUnit: Future[Unit] = Future.successful( () )
val route =
  path("success") {
    onSuccess(futureOfUnit) { //: Directive0
        complete("Future was completed.")
    }
  }

If the future returns Future[Unit], it is a bit special case as it results in Directive0. Looking at the above code, onSuccess(futureOfUnit) returns a Directive1[Unit] = Directive[Tuple1[Unit]]. However, the DSL interprets Unit as Tuple0, and automatically converts the result to Directive[Unit] = Directive0,