Error handling

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.

Alternatively, errors can be recovered from failed effects and mapped to the error output - provided that the E type in the endpoint description is itself a subclass of exception. This can be done using the toRouteRecoverErrors method.

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” 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.

Note that the decode failure handler is used only for failures that occur during decoding of path, query, body and header parameters - while invoking Codec.decode. It does not handle any failures or exceptions that occur when invoking the logic of the endpoint.

Default failure handler

The default decode failure handler can be adapted to return responses in a different format (other than textual). To reuse the existing logic, but using a different format, create the failure handler as follows:

case class MyFailure(msg: String)
def myFailureResponse(statusCode: StatusCode, message: String): DecodeFailureHandling =
   (statusCode, MyFailure(message))
val myDecodeFailureHandler = ServerDefaults.decodeFailureHandlerUsingResponse(
  badRequestOnPathFailureIfPathShapeMatches = false

Note that when specifying that a response should be returned upon a failure, we need to provide the endpoint output which should be used to create the response, as well as a value for this output.

The default decode failure handler also has the option to return a 400 Bad Request, instead of a no-match (ultimately leading to a 404 Not Found), when the “shape” of the path matches (that is, the number of segments in the request and endpoint’s paths are the same), but when decoding some part of the path fails. By default, any kind of decode failure in the path will result in a no-match being returned.