Custom Directives

Part of the power of akka-http directives comes from the ease with which it’s possible to define custom directives at differing levels of abstraction.

There are essentially three ways of creating custom directives:

1. By introducing new “labels” for configurations of existing directives
2. By transforming existing directives
3. By writing a directive “from scratch”

Configuration Labeling

The easiest way to create a custom directive is to simply assign a new name for a certain configuration of one or more existing directives. In fact, most of the predefined akka-http directives can be considered named configurations of more low-level directives.

The basic technique is explained in the chapter about Composing Directives, where, for example, a new directive getOrPut is defined like this:

val getOrPut = get | put

// tests:
val route = getOrPut { complete("ok") }

Get("/") ~> route ~> check {
  responseAs[String] shouldEqual "ok"
}

Put("/") ~> route ~> check {
  responseAs[String] shouldEqual "ok"
}

Another example is the MethodDirectives which are simply instances of a preconfigured method directive. The low-level directives that most often form the basis of higher-level “named configuration” directives are grouped together in the BasicDirectives trait.

Transforming Directives

The second option for creating new directives is to transform an existing one using one of the “transformation methods”, which are defined on the Directive class, the base class of all “regular” directives.

Apart from the combinator operators (| and &) and the case-class extractor (as[T]) the following transformations are also defined on all Directive instances:

• map/tmap
• flatMap/tflatMap
• require/trequire
• recover/recoverPF

map and tmap

If the Directive is a single-value Directive, the map method allows for simple transformations:

val textParam: Directive1[String] =
  parameter("text".as[String])

val lengthDirective: Directive1[Int] =
  textParam.map(text => text.length)

// tests:
Get("/?text=abcdefg") ~> lengthDirective(x => complete(x.toString)) ~> check {
  responseAs[String] === "7"
}

One example of a predefined directive relying on map is the optionalHeaderValue directive.

The tmap modifier has this signature (somewhat simplified):

def tmap[R](f: L ⇒ R): Directive[Out]

It can be used to transform the Tuple of extractions into another Tuple. The number and/or types of the extractions can be changed arbitrarily. For example if R is Tuple2[A, B] then the result will be a Directive[(A, B)]. Here is a somewhat contrived example:

val twoIntParameters: Directive[(Int, Int)] =
  parameters(("a".as[Int], "b".as[Int]))

val myDirective: Directive1[String] =
  twoIntParameters.tmap {
    case (a, b) => (a + b).toString
  }

// tests:
Get("/?a=2&b=5") ~> myDirective(x => complete(x)) ~> check {
  responseAs[String] === "7"
}

flatMap and tflatMap

With map and tmap you can transform the values a directive extracts but you cannot change the “extracting” nature of the directive. For example, if you have a directive extracting an Int you can use map to turn it into a directive that extracts that Int and doubles it, but you cannot transform it into a directive, that doubles all positive Int values and rejects all others.

In order to do the latter you need flatMap or tflatMap. The tflatMap modifier has this signature:

def tflatMap[R: Tuple](f: L ⇒ Directive[R]): Directive[R]

The given function produces a new directive depending on the Tuple of extractions of the underlying one. As in the case of map/tmap there is also a single-value variant called flatMap, which simplifies the operation for Directives only extracting one single value.

Here is the (contrived) example from above, which doubles positive Int values and rejects all others:

val intParameter: Directive1[Int] = parameter("a".as[Int])

val myDirective: Directive1[Int] =
  intParameter.flatMap {
    case a if a > 0 => provide(2 * a)
    case _          => reject
  }

// tests:
Get("/?a=21") ~> myDirective(i => complete(i.toString)) ~> check {
  responseAs[String] === "42"
}
Get("/?a=-18") ~> myDirective(i => complete(i.toString)) ~> check {
  handled === false
}

A common pattern that relies on flatMap is to first extract a value from the RequestContext with the extract directive and then flatMap with some kind of filtering logic. For example, this is the implementation of the method directive:

def method(httpMethod: HttpMethod): Directive0 =
  extractMethod.flatMap[Unit] {
    case `httpMethod` ⇒ pass
    case _            ⇒ reject(MethodRejection(httpMethod))
  } & cancelRejections(classOf[MethodRejection])

The explicit type parameter [Unit] on the flatMap is needed in this case because the result of the flatMap is directly concatenated with the cancelAllRejections directive, thereby preventing “outside-in” inference of the type parameter value.

require and trequire

The require modifier transforms a single-extraction directive into a directive without extractions, which filters the requests according the a predicate function. All requests, for which the predicate is false are rejected, all others pass unchanged.

The signature of require is this:

def require(predicate: T ⇒ Boolean, rejections: Rejection*): Directive0

One example of a predefined directive relying on require is the first overload of the host directive:

/**
 * Rejects all requests for whose host name the given predicate function returns false.
 *
 * @group host
 */
def host(predicate: String ⇒ Boolean): Directive0 = extractHost.require(predicate)

You can only call require on single-extraction directives. The trequire modifier is the more general variant, which takes a predicate of type Tuple => Boolean. It can therefore also be used on directives with several extractions.

recover and recoverPF

The recover modifier allows you “catch” rejections produced by the underlying directive and, instead of rejecting, produce an alternative directive with the same type(s) of extractions.

The signature of recover is this:

def recover[R >: L: Tuple](recovery: Seq[Rejection] ⇒ Directive[R]): Directive[R] =

In many cases the very similar recoverPF modifier might be little bit easier to use since it doesn’t require the handling of all rejections:

def recoverPF[R >: L: Tuple](
  recovery: PartialFunction[Seq[Rejection], Directive[R]]): Directive[R]

One example of a predefined directive relying recoverPF is the optionalHeaderValue directive:

def optionalHeaderValue[T](f: HttpHeader ⇒ Option[T]): Directive1[Option[T]]

Directives from Scratch

The third option for creating custom directives is to do it “from scratch”, by directly subclassing the Directive class. The Directive is defined like this (leaving away operators and modifiers):

abstract class Directive[L](implicit val ev: Tuple[L]) {

  /**
   * Calls the inner route with a tuple of extracted values of type `L`.
   *
   * `tapply` is short for "tuple-apply". Usually, you will use the regular `apply` method instead,
   * which is added by an implicit conversion (see `Directive.addDirectiveApply`).
   */
  def tapply(f: L ⇒ Route): Route
}

It only has one abstract member that you need to implement, the tapply method, which creates the Route the directives presents to the outside from its inner Route building function (taking the extractions as parameters).

Extractions are kept as a Tuple. Here are a few examples:

A Directive[Unit] extracts nothing (like the get directive). Because this type is used quite frequently akka-http defines a type alias for it:

type Directive0 = Directive[Unit]

A Directive[(String)] extracts one String value (like the hostName directive). The type alias for it is:

type Directive1[T] = Directive[Tuple1[T]]

A Directive[(Int, String)] extracts an Int value and a String value (like a parameters('a.as[Int], 'b.as[String]) directive).

Keeping extractions as Tuples has a lot of advantages, mainly great flexibility while upholding full type safety and “inferability”. However, the number of times where you’ll really have to fall back to defining a directive from scratch should be very small. In fact, if you find yourself in a position where a “from scratch” directive is your only option, we’d like to hear about it, so we can provide a higher-level “something” for other users.