Server logic

The logic that should be run when an endpoint is invoked can be passed when interpreting the endpoint as a server, and converting to an server-implementation-specific route. But there are other options available as well, which might make it easier to work with collections of endpoints.

Defining an endpoint together with the server logic

It’s possible to combine an endpoint description with the server logic in a single object, ServerEndpoint[I, E, O, S, F]. Such an endpoint contains not only an endpoint of type Endpoint[I, E, O, S], but also a logic function I => F[Either[E, O]], for some effect F.

For example, the book example can be more concisely written as follows:

import sttp.tapir._
import sttp.tapir.server.akkahttp._
import scala.concurrent.Future
import akka.http.scaladsl.server.Route

val countCharactersServerEndpoint: ServerEndpoint[String, Unit, Int, Nothing, Future] =[Int]).serverLogic { s =>
    Future.successful(Right[Unit, Int](s.length))

val countCharactersRoute: Route = countCharactersServerEndpoint.toRoute

A ServerEndpoint can then be converted to a route using .toRoute/.toRoutes methods (without any additional parameters), or to documentation.

Moreover, a list of server endpoints can be converted to routes or documentation as well:

val endpoint1 ="hello").out(stringBody)
  .serverLogic { _ => Future.successful("world") }

val endpoint2 ="ping").out(stringBody)
  .serverLogic { _ => Future.successful("pong") }

val route: Route = List(endpoint1, endpoint2).toRoute

Note that when dealing with endpoints which have multiple input parameters, the server logic function is a function of a single argument, which is a tuple; hence you’ll need to pattern-match using case to extract the parameters:

val echoEndpoint = endpoint
  .serverLogic { case (count, body) =>
     Future.successful(body * count)

Extracting common route logic

Quite often, especially for authentication, some part of the route logic is shared among multiple endpoints. However, these functions don’t compose in a straightforward way, as authentication usually operates on a single input, which is only a part of the whole logic’s input. Suppose you have the following methods:

type AuthToken = String

def authFn(token: AuthToken): Future[Either[ErrorInfo, User]]
def logicFn(user: User, data: String, limit: Int): Future[Either[ErrorInfo, Result]]

which you’d like to apply to an endpoint with type:

val myEndpoint: Endpoint[(AuthToken, String, Int), ErrorInfo, Result, Nothing] = ...

To avoid composing these functions by hand, tapir defines helper extension methods, andThenFirst and andTheFirstE. The first one should be used when errors are represented as failed wrapper types (e.g. failed futures), the second is errors are represented as Eithers.

This extension method is defined in the same traits as the route interpreters, both for Future (in the akka-http interpreter) and for an arbitrary monad (in the http4s interpreter), so importing the package is sufficient to use it:

import sttp.tapir.server.akkahttp._
val r: Route = myEndpoint.toRoute((authFn _).andThenFirstE((logicFn _).tupled))

Writing down the types, here are the generic signatures when using andThenFirst and andThenFirstE:

f1: T => Future[U]
f2: (U, A1, A2, ...) => Future[O]
(f1 _).andThenFirst(f2): (T, A1, A2, ...) => Future[O]

f1: T => Future[Either[E, U]]
f2: (U, A1, A2, ...) => Future[Either[E, O]]
(f1 _).andThenFirstE(f2): (T, A1, A2, ...) => Future[Either[E, O]]

Status codes

By default, successful responses are returned with the 200 OK status code, and errors with 400 Bad Request. However, this can be customised by specifying how an output maps to the status code.