Common server options

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.

Server options

Each interpreter accepts an implicit options value, which contains configuration values for:

  • how to create a file (when receiving a response that is mapped to a file, or when reading a file-mapped multipart part)
  • how to handle decode failures

Handling decode failures

Quite often user input will be malformed and decoding will fail. Should the request be completed with a 400 Bad Request response, or should the request be forwarded to another endpoint? By default, tapir follows OpenAPI conventions, that an endpoint is uniquely identified by the method and served path. That’s why:

  • an “endpoint doesn’t match” result is returned if the request method or path doesn’t match. The http library should attempt to serve this request with the next endpoint.
  • otherwise, we assume that this is the correct endpoint to serve the request, but the parameters are somehow malformed. A 400 Bad Request response is returned if a query parameter, header or body is missing / decoding fails, or if the decoding a path capture fails with an error (but not a “missing” decode result).

This can be customised by providing an implicit instance of tapir.server.DecodeFailureHandler, which basing on the request, failing input and failure description can decide, whether to return a “no match”, an endpoint-specific error value, or a specific response.

Only the first failure is passed to the DecodeFailureHandler. Inputs are decoded in the following order: method, path, query, header, body.

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 the results can both contain errors (represented as Eithers), and are wrapped in a container. 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 a helper extension method, andThenRight. If the first function returns an error, that error is propagated to the final result; otherwise, the result is passed as input to the second function.

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 tapir.server.akkahttp._
val r: Route = myEndpoint.toRoute((authFn _).andThenRight(logicFn _))

Writing down the types, here are the generic signatures when using andThenRight:

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

Exception handling

There’s no exception handling built into tapir. However, tapir contains a more general error handling mechanism, as the endpoints can contain dedicated error outputs.

If the logic function, which is passed to the server interpreter, fails (i.e. throws an exception, which results in a failed Future or IO/Task), this is propagated to the library (akka-http or http4s).

However, any exceptions can be recovered from and mapped to an error value. For example:

type ErrorInfo = String

def logic(s: String): Future[Int] = ...

def handleErrors[T](f: Future[T]): Future[Either[ErrorInfo, T]] =
  f.transform {
    case Success(v) => Success(Right(v))
    case Failure(e) =>
      logger.error("Exception when running endpoint logic", e)

  .toRoute((logic _).andThen(handleErrors))

In the above example, errors are represented as Strings (aliased to ErrorInfo for readability). When the logic completes successfully an Int is returned. Any exceptions that are raised are logged, and represented as a value of type ErrorInfo.

Following the convention, the left side of the Either[ErrorInfo, T] represents an error, and the right side success.