An Introduction To

Rishabh VermaSoftware ConsultantKnoldus Software LLP

Agenda

Why do we need a Http module ?

Overview Of Akka-Http

Low level API

High level API

Rest Api with Akka-Http(demo)

Need of Http module

Akka is about building distributed and highly concurrent applications.

For distribution across threads : (Actors)

For distribution across machines : (Akka clusters)

For distribution across Internet (i.e interaction with external world) : (Akka-Http)

Overview of Akka-Http

Toolkit for providing and consuming Http based services.

Goal To interact with external world.

Implemented on top of Akka Streams, which is implemented on top of Akka Actors.

Used for building REST APIs .

Why Akka-http over Spray?

Weaknesses of Spray:Spray is no longer being supported

Handling chunked messages is quite difficult with spray.

Missing features like Websockets support etc.

Akka-http

Akka-http is Spray 2.0

Simplified model structure and fully stream based

Stream topology

Akka-Http Structure

Akka-http

Provides high level client and server side APIs.

Marshalling of custom types Http Entities.

Unmarshalling custom types Http Entities.

Provides High level server side API.

Routing DSL.

Can handle chunked messages.

Can receive message headers before the entity.

Marshalling

"Marshalling" is the process of converting a higher-level (object) structure into some kind of lower-level representation.

Some pre-defined marshallers :

Array[Byte]

String

ByteString

example:val responseFuture = Marshal(420 -> errorMsg).to[HttpResponse]

UnMarshalling

To convert lower level objects into higher level object.

Some pre-defined unmarshallers :

Byte, Boolean

Short, Int, Long

Client Side API

Server Side API

Low level API

The low-level Akka HTTP server APIs allows for handling connections or individual requests by accepting HttpRequests and answering them by producing HttpResponses.

This is provided by the akka-http-core module.

Lower level API based simply on mapping request to response.

Low level API (cont..)

val requestHandler: HttpRequest => HttpResponse = { case HttpRequest(GET, Uri.Path("/"), _, _, _) => HttpResponse(entity = HttpEntity( ContentTypes.`text/html(UTF-8)`, "Hello world!")) case HttpRequest(GET, Uri.Path("/ping"), _, _, _) => HttpResponse(entity = "PONG!") case HttpRequest(GET, Uri.Path("/crash"), _, _, _) => sys.error("BOOM!")

High Level API

Low level API becomes uneasy to handle when we need to create large number to routes.

For this we should use higher level API.

High level API is provided by Akka-http.

High Level API (cont..)

Directives

Routing DSL

Rejection and Exception Handling

Path matcher

High Level API (cont..)

Directives :A "Directive" is a small building block used for creatingroutes.

There are some predefined directives( get, post, complete etc.)

We can also define our custom directives.Exmaple: with get and complete directive val exampleRoute: Route = get { complete("Received GET") } ~

High Level API (cont..)

Routing Dsl :The "Route" is the central concept of the routing DSL since all structures you can build with it are instances of a Route. Ex: type Route = RequestContext => Future[RouteResult].

Path Matcher:

The path matcher decides to which block the incoming request will be mapped.

High Level API (cont..)

Rejection :~ operator was introduced, which connects two routes in a way that allows a second route to get a go at a request if the first route "rejected" it.Example:path("order") { get { complete("Received GET") } ~ post { complete("Received POST") }

Http Server

object Server extends App {

implicit val actorSystem = ActorSystem("akka-system") implicit val flowMaterializer = ActorMaterializer()

val interface = "localhost" val route: Route = MainService.route

val binding = Http().bindAndHandle(route, "localhost", 8080) println(s"Server is now online at http:///8080\nPress RETURN to stop...") StdIn.readLine()

binding.flatMap(_.unbind()).onComplete(_ => actorSystem.shutdown() println("Server is down...")}

References

http://doc.akka.io/docs/akka/2.4.8/scala/http/

https://www.youtube.com/watch?v=y_slPbktLr0

https://danielasfregola.com/2016/02/07/how-to-build-a-rest-api-with-akka-http/