Introduction

The Akka HTTP modules implement a full server- and client-side HTTP stack on top of akka-actor and akka-stream. It’s not a web-framework but rather a more general toolkit for providing and consuming HTTP-based services. While interaction with a browser is of course also in scope it is not the primary focus of Akka HTTP.

Akka HTTP follows a rather open design and many times offers several different API levels for “doing the same thing”. You get to pick the API level of abstraction that is most suitable for your application. This means that, if you have trouble achieving something using a high-level API, there’s a good chance that you can get it done with a low-level API, which offers more flexibility but might require you to write more application code.

Philosophy

Akka HTTP has been driven with a clear focus on providing tools for building integration layers rather than application cores. As such it regards itself as a suite of libraries rather than a framework.

A framework, as we’d like to think of the term, gives you a “frame”, in which you build your application. It comes with a lot of decisions already pre-made and provides a foundation including support structures that lets you get started and deliver results quickly. In a way a framework is like a skeleton onto which you put the “flesh” of your application in order to have it come alive. As such frameworks work best if you choose them before you start application development and try to stick to the frameworks “way of doing things” as you go along.

For example, if you are building a browser-facing web application it makes sense to choose a web framework and build your application on top of it because the “core” of the application is the interaction of a browser with your code on the web-server. The framework makers have chosen one “proven” way of designing such applications and let you “fill in the blanks” of a more or less flexible “application-template”. Being able to rely on best-practice architecture like this can be a great asset for getting things done quickly.

However, if your application is not primarily a web application because its core is not browser-interaction but some specialized maybe complex business service and you are merely trying to connect it to the world via a REST/HTTP interface a web-framework might not be what you need. In this case the application architecture should be dictated by what makes sense for the core not the interface layer. Also, you probably won’t benefit from the possibly existing browser-specific framework components like view templating, asset management, JavaScript- and CSS generation/manipulation/minification, localization support, AJAX support, etc.

Akka HTTP was designed specifically as “not-a-framework”, not because we don’t like frameworks, but for use cases where a framework is not the right choice. Akka HTTP is made for building integration layers based on HTTP and as such tries to “stay on the sidelines”. Therefore you normally don’t build your application “on top of” Akka HTTP, but you build your application on top of whatever makes sense and use Akka HTTP merely for the HTTP integration needs.

On the other hand, if you prefer to build your applications with the guidance of a framework, you should give Play Framework or Lagom a try, which both use Akka internally.

Using Akka HTTP

Akka HTTP is provided in a separate jar file, to use it make sure to include the following dependency:

"com.typesafe.akka" %% "akka-http" % "10.0.3" 

Mind that akka-http comes in two modules: akka-http and akka-http-core. Because akka-http depends on akka-http-core you don’t need to bring the latter explicitly. Still you may need to this in case you rely solely on low-level API; make sure the scala version should be a recent release of version 2.11.

Routing DSL for HTTP servers

The high-level, routing API of Akka HTTP provides a DSL to describe HTTP “routes” and how they should be handled. Each route is composed of one or more level of Directive s that narrows down to handling one specific type of request.

For example one route might start with matching the path of the request, only matching if it is “/hello”, then narrowing it down to only handle HTTP get requests and then complete those with a string literal, which will be sent back as a HTTP OK with the string as response body.

Transforming request and response bodies between over-the-wire formats and objects to be used in your application is done separately from the route declarations, in marshallers, which are pulled in implicitly using the “magnet” pattern. This means that you can complete a request with any kind of object a as long as there is an implicit marshaller available in scope.

Default marshallers are provided for simple objects like String or ByteString, and you can define your own for example for JSON. An additional module provides JSON serialization using the spray-json library (see JSON Support for details).

The Route created using the Route DSL is then “bound” to a port to start serving HTTP requests:

import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model._
import akka.http.scaladsl.server.Directives._
import akka.stream.ActorMaterializer
import scala.io.StdIn

object WebServer {
  def main(args: Array[String]) {

    implicit val system = ActorSystem("my-system")
    implicit val materializer = ActorMaterializer()
    // needed for the future flatMap/onComplete in the end
    implicit val executionContext = system.dispatcher

    val route =
      path("hello") {
        get {
          complete(HttpEntity(ContentTypes.`text/html(UTF-8)`, "<h1>Say hello to akka-http</h1>"))
        }
      }

    val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)

    println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
    StdIn.readLine() // let it run until user presses return
    bindingFuture
      .flatMap(_.unbind()) // trigger unbinding from the port
      .onComplete(_ => system.terminate()) // and shutdown when done
  }
}

A common use case is to reply to a request using a model object having the marshaller transform it into JSON. In this case shown by two separate routes. The first route queries an asynchronous database and marshalls the Future[Option[Item]] result into a JSON response. The second unmarshalls an Order from the incoming request saves it to the database and replies with an OK when done.

import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.stream.ActorMaterializer
import akka.Done
import akka.http.scaladsl.server.Route
import akka.http.scaladsl.server.Directives._
import akka.http.scaladsl.model.StatusCodes
import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport._
import spray.json.DefaultJsonProtocol._

import scala.io.StdIn

import scala.concurrent.Future

object WebServer {

  // domain model
  final case class Item(name: String, id: Long)
  final case class Order(items: List[Item])

  // formats for unmarshalling and marshalling
  implicit val itemFormat = jsonFormat2(Item)
  implicit val orderFormat = jsonFormat1(Order)

  // (fake) async database query api
  def fetchItem(itemId: Long): Future[Option[Item]] = ???
  def saveOrder(order: Order): Future[Done] = ???

  def main(args: Array[String]) {

    // needed to run the route
    implicit val system = ActorSystem()
    implicit val materializer = ActorMaterializer()
    // needed for the future map/flatmap in the end
    implicit val executionContext = system.dispatcher

    val route: Route =
      get {
        pathPrefix("item" / LongNumber) { id =>
          // there might be no item for a given id
          val maybeItem: Future[Option[Item]] = fetchItem(id)

          onSuccess(maybeItem) {
            case Some(item) => complete(item)
            case None       => complete(StatusCodes.NotFound)
          }
        }
      } ~
        post {
          path("create-order") {
            entity(as[Order]) { order =>
              val saved: Future[Done] = saveOrder(order)
              onComplete(saved) { done =>
                complete("order created")
              }
            }
          }
        }

    val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)
    println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
    StdIn.readLine() // let it run until user presses return
    bindingFuture
      .flatMap(_.unbind()) // trigger unbinding from the port
      .onComplete(_ ⇒ system.terminate()) // and shutdown when done

  }
}

The logic for the marshalling and unmarshalling JSON in this example is provided by the “spray-json” library (details on how to use that here: JSON Support).

One of the strengths of Akka HTTP is that streaming data is at its heart meaning that both request and response bodies can be streamed through the server achieving constant memory usage even for very large requests or responses. Streaming responses will be backpressured by the remote client so that the server will not push data faster than the client can handle, streaming requests means that the server decides how fast the remote client can push the data of the request body.

Example that streams random numbers as long as the client accepts them:

import akka.actor.ActorSystem
import akka.stream.scaladsl._
import akka.util.ByteString
import akka.http.scaladsl.Http
import akka.http.scaladsl.model.{HttpEntity, ContentTypes}
import akka.http.scaladsl.server.Directives._
import akka.stream.ActorMaterializer
import scala.util.Random
import scala.io.StdIn

object WebServer {

  def main(args: Array[String]) {

    implicit val system = ActorSystem()
    implicit val materializer = ActorMaterializer()
    // needed for the future flatMap/onComplete in the end
    implicit val executionContext = system.dispatcher

    // streams are re-usable so we can define it here
    // and use it for every request
    val numbers = Source.fromIterator(() =>
      Iterator.continually(Random.nextInt()))

    val route =
      path("random") {
        get {
          complete(
            HttpEntity(
              ContentTypes.`text/plain(UTF-8)`,
              // transform each number to a chunk of bytes
              numbers.map(n => ByteString(s"$n\n"))
            )
          )
        }
      }

    val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)
    println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
    StdIn.readLine() // let it run until user presses return
    bindingFuture
      .flatMap(_.unbind()) // trigger unbinding from the port
      .onComplete(_ => system.terminate()) // and shutdown when done
  }
}

Connecting to this service with a slow HTTP client would backpressure so that the next random number is produced on demand with constant memory usage on the server. This can be seen using curl and limiting the rate curl --limit-rate 50b 127.0.0.1:8080/random

Akka HTTP routes easily interacts with actors. In this example one route allows for placing bids in a fire-and-forget style while the second route contains a request-response interaction with an actor. The resulting response is rendered as json and returned when the response arrives from the actor.

import akka.actor.{Actor, ActorSystem, Props}
import akka.http.scaladsl.Http
import akka.http.scaladsl.model.StatusCodes
import akka.http.scaladsl.server.Directives._
import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport._
import akka.pattern.ask
import akka.stream.ActorMaterializer
import akka.util.Timeout
import spray.json.DefaultJsonProtocol._
import scala.concurrent.duration._
import scala.io.StdIn

object WebServer {

  case class Bid(userId: String, bid: Int)
  case object GetBids
  case class Bids(bids: List[Bid])

  class Auction extends Actor {
    def receive = {
      case Bid(userId, bid) => println(s"Bid complete: $userId, $bid")
      case _ => println("Invalid message")
    }
  }

  // these are from spray-json
  implicit val bidFormat = jsonFormat2(Bid)
  implicit val bidsFormat = jsonFormat1(Bids)

  def main(args: Array[String]) {
    implicit val system = ActorSystem()
    implicit val materializer = ActorMaterializer()
    // needed for the future flatMap/onComplete in the end
    implicit val executionContext = system.dispatcher

    val auction = system.actorOf(Props[Auction], "auction")

    val route =
      path("auction") {
        put {
          parameter("bid".as[Int], "user") { (bid, user) =>
            // place a bid, fire-and-forget
            auction ! Bid(user, bid)
            complete((StatusCodes.Accepted, "bid placed"))
          }
        }
        get {
          implicit val timeout: Timeout = 5.seconds

          // query the actor for the current auction state
          val bids: Future[Bids] = (auction ? GetBids).mapTo[Bids]
          complete(bids)
        }
      }

    val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)
    println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
    StdIn.readLine() // let it run until user presses return
    bindingFuture
      .flatMap(_.unbind()) // trigger unbinding from the port
      .onComplete(_ => system.terminate()) // and shutdown when done

  }
}

Again the logic for the marshalling and unmarshalling JSON in this example is provided by the “spray-json” library (details on how to use that here: JSON Support)

Read more about the details of the high level APIs in the section High-level Server-Side API.

Low-level HTTP server APIs

The low-level Akka HTTP server APIs allows for handling connections or individual requests by accepting HttpRequest s and answering them by producing HttpResponse s. This is provided by the akka-http-core module. APIs for handling such request-responses as function calls and as a Flow[HttpRequest, HttpResponse, _] are available.

import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model.HttpMethods._
import akka.http.scaladsl.model._
import akka.stream.ActorMaterializer
import scala.io.StdIn

object WebServer {

  def main(args: Array[String]) {
    implicit val system = ActorSystem()
    implicit val materializer = ActorMaterializer()
    // needed for the future map/flatmap in the end
    implicit val executionContext = system.dispatcher

    val requestHandler: HttpRequest => HttpResponse = {
      case HttpRequest(GET, Uri.Path("/"), _, _, _) =>
        HttpResponse(entity = HttpEntity(
          ContentTypes.`text/html(UTF-8)`,
          "<html><body>Hello world!</body></html>"))

      case HttpRequest(GET, Uri.Path("/ping"), _, _, _) =>
        HttpResponse(entity = "PONG!")

      case HttpRequest(GET, Uri.Path("/crash"), _, _, _) =>
        sys.error("BOOM!")

      case r: HttpRequest =>
        r.discardEntityBytes() // important to drain incoming HTTP Entity stream
        HttpResponse(404, entity = "Unknown resource!")
    }

    val bindingFuture = Http().bindAndHandleSync(requestHandler, "localhost", 8080)
    println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
    StdIn.readLine() // let it run until user presses return
    bindingFuture
      .flatMap(_.unbind()) // trigger unbinding from the port
      .onComplete(_ => system.terminate()) // and shutdown when done

  }
}

Read more details about the low level APIs in the section Low-Level Server-Side API.

HTTP client API

The client APIs provide methods for calling a HTTP server using the same HttpRequest and HttpResponse abstractions that Akka HTTP server uses but adds the concept of connection pools to allow multiple requests to the same server to be handled more performantly by re-using TCP connections to the server.

Example simple request:

import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model._
import akka.stream.ActorMaterializer

import scala.concurrent.Future

implicit val system = ActorSystem()
implicit val materializer = ActorMaterializer()

val responseFuture: Future[HttpResponse] =
  Http().singleRequest(HttpRequest(uri = "http://akka.io"))

Read more about the details of the client APIs in the section Consuming HTTP-based Services (Client-Side).

The modules that make up Akka HTTP

Akka HTTP is structured into several modules:

akka-http
Higher-level functionality, like (un)marshalling, (de)compression as well as a powerful DSL for defining HTTP-based APIs on the server-side, this is the recommended way to write HTTP servers with Akka HTTP. Details can be found in the section High-level Server-Side API
akka-http-core
A complete, mostly low-level, server- and client-side implementation of HTTP (incl. WebSockets) Details can be found in sections Low-Level Server-Side API and Consuming HTTP-based Services (Client-Side)
akka-http-testkit
A test harness and set of utilities for verifying server-side service implementations
akka-http-spray-json
Predefined glue-code for (de)serializing custom types from/to JSON with spray-json Details can be found here: JSON Support
akka-http-xml
Predefined glue-code for (de)serializing custom types from/to XML with scala-xml Details can be found here: XML Support
The source code for this page can be found here.