The most trivial self-hosted REST-like web service in Scala? - scala

Question

I have a function of the form:
(inp:CaseClass) => SomeOtherCaseClass
In other words, it takes a function with a single argument whose type is a Case Class. It returns a single value whose argument is a different kind of case class.
Just about every public function in my application will be of this form.
I'd like to expose this as a web-service so that a client can use this function by HTTP-Posting some JSON. The client will receive the response as a JSON encoded document.
An ideal solution will be:
Simple (e.g. few lines of code, using mostly non-obscure language features)
Should automatically marshal and unmarshal JSON - I don't want to have to write manual converters.
As easy to use as Flask (a popular Python micro-framework for web-stuff).
Things I don't yet care about:
High performance
Authentication / Encryption
I do have a working implementation based on Scalatra. It's OK-ish, but not particularly pretty because it includes quite a bit of boilerplate code just to make it start a server. I'm wondering if I can go for something even more minimal.
This solution was based on some 3-year old code samples I found at work. I'm sure that there must be something more appropriate which has been developed in the last few years?

Answer 1

Have a look at akka-http. It's relatively new (rewrite of the now obsolete spray library) and has a large support community. It's easy to get started for your simple use case but if you ever need more advanced features in the future, they're probably supported. JSON de-/serialization is done using spray-json adapter. You may also use other libraries, like circe with minimal amount of boilerplate. Here's a simple web server implementation accepting POST requests (copied from here):
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
}
}
Does this suit your needs?