An Introduction to Akka http

An Introduction ToAn Introduction To
Rishabh Verma
Software Consultant
Knoldus 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
● 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

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)`,
"<html><body>Hello world!</body></html>"))
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

Directives :
● A "Directive" is a small building block used for creating
routes.
● 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")
} ~

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.

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:///8080nPress 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/

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