cornerman / sloth Edit

Type safe RPC in scala

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Type safe RPC in scala

Sloth is essentially a pair of macros (server and client) which takes an API definition in the form of a scala trait and then generates code for routing in the server as well as generating an API implementation in the client.

This library is inspired by autowire. Some differences:

  • No macro application on the call-site in the client (.call()), just one macro for creating an instance of an API trait
  • Return types of Api traits are not restricted to Future. You can use any higher-kinded generic return types (cats.MonadError in client, cats.Functor in server)

Get started

Get latest release:

libraryDependencies += "com.github.cornerman" %%% "sloth" % "0.4.0"

Or get development snapshots via jitpack:

resolvers += "jitpack" at ""
libraryDependencies += "com.github.cornerman.sloth" %%% "sloth" % "master-SNAPSHOT"

Example usage

Define a trait as your Api:

trait Api {
    def fun(a: Int): Future[Int]


Implement your Api:

object ApiImpl extends Api {
    def fun(a: Int): Future[Int] = Future.successful(a + 1)

Define a router where we can use, e.g., boopickle for serializing the arguments and result of a method:

import sloth._
import boopickle.Default._
import chameleon.ext.boopickle._
import java.nio.ByteBuffer
import cats.implicits._

val router = Router[ByteBuffer, Future].route[Api](ApiImpl)

Use it to route requests to your Api implementation:

val result = router(Request[ByteBuffer]("Api" :: "fun" :: Nil, bytes))
// Now result contains the serialized Int result returned by the method


Generate an implementation for Api on the client side:

import sloth._
import boopickle.Default._
import chameleon.ext.boopickle._
import java.nio.ByteBuffer
import cats.implicits._

object Transport extends RequestTransport[PickleType, Future] {
    // implement the transport layer. this example just calls the router directly.
    // in reality, the request would be sent over a connection.
    override def apply(request: Request[PickleType]): Future[PickleType] =
        router(request).toEither match {
            case Right(result) => result
            case Left(err) => Future.failed(new Exception(err.toString))

val client = Client[PickleType, Future, ClientException](Transport)
val api: Api = client.wire[Api]

Make requests to the server like normal method calls: { num =>
  println(s"Got response: $num")

Additional features

Generic return type

Sometimes it can be useful to have a different return type on the server and client, you can do so by making your API generic:

trait Api[F[_]] {
    def fun(a: Int): F[Int]

In your server, you can use any cats.Functor as F, for example:

type ServerResult[T] = User => T

object ApiImpl extends Api[ServerResult] {
    def fun(a: Int): User => Int = { user =>
        println(s"User: $user")
        a + 1

val router = Router[ByteBuffer, ServerResult]

In your client, you can use any cats.MonadError that can capture a ClientFailure (see ClientFailureConvert for using your own failure type):

type ClientResult[T] = Either[ClientFailure, T]

val client = Client[PickleType, ClientResult, ClientFailure](Transport)
val api: Api = client.wire[Api[ClientResult]]

Multiple routes

It is possible to have multiple APIs routed through the same router:

val router = Router[ByteBuffer, Future]

Router result

The router in the server returns a RouterResult[PickleType, Result[_]] which either returns a result or fails with a ServerFailure. Furthermore, it gives access to the deserialized request:

router(request) match {
    case RouterResult.Success(arguments, result) => println(s"Success (arguments: $arguments): $result")
    case RouterResult.Failure(arguments, error) => println(s"Error (arguments: $arguments): $error")

Or you can just convert the result to an Either[ServerFailure, Result[PickleType]]:

router(request).toEither match {
    case Right(result) => println(s"Success: $result")
    case Left(error) => println(s"Error: $error")

Client logging

For logging, you can define a LogHandler, which can log each request including the deserialized request and response. Define it when creating the Client:

object MyLogHandler extends LogHandler[ClientResult[_]] {
  def logRequest[T](path: List[String], argumentObject: Any, result: ClientResult[T]): ClientResult[T] = ???

val client = Client[PickleType, ClientResult, ClientFailure](Transport, MyLogHandler)

Method overloading

When overloading methods with different parameter lists, sloth does not have a unique path (because it is derived from the trait name and the method name). Here you will need to provide your own path name:

trait Api {
    def fun(i: Int): F[Int]
    def fun(i: Int, s: String): F[Int]


For serialization, we make use of the typeclasses provided by chameleon. You can use existing libraries like circe, upickle, scodec or boopickle out of the box or define a serializer yourself (see the project readme). So you need a Serializer and Deserializer for each type you are using in the method signature of your API methods.

How does it work

Sloth derives all information about an API from a scala trait. For example:

// @PathName("apiName")
trait Api {
    // @PathName("funName")
    def fun(a: Int, b: String)(c: Double): F[Int]

For each declared method in this trait (in this case fun):

  • Calculate method path: List("Api", "fun") (PathName annotations on the trait or method are taken into account).
  • Serialize the method parameters as tuples.


When calling router.route[Api](impl), a macro generates a function that maps a method path and a pickled case class to a pickled result. This basically boils down to:

HashMap("Api" -> HashMap("fun" -> { payload =>
    // deserialize payload
    // call Api implementation impl with arguments
    // return serialized response


When calling client.wire[Api], a macro generates an instance of Api by implementing each method using the provided transport:

new Api {
    def fun(a: Int, b: String)(c: Double): F[Int] = {
        // serialize arguments
        // call RequestTransport transport with method path and arguments
        // return deserialized response

Experimental: Checksum for Apis

In order to check the compatability of the client and server Api trait, you can calculate a checksum of your Api:

import sloth.ChecksumCalculator._

trait Api {
    def fun(s: String): Int

val checksum:Int = checksumOf[Api]

The checksum of an Api trait is calculated from its Name and its methods (including names and types of parameters and result type).


  • Type parameters on methods in the API trait are not supported.
  • All public methods in an API trait need to return the same higher kinded result type.
  • Your chosen serialization library needs to support serializing tuples, which are generated by the macro for the parameter lists of each method in the API trait. This is normally the case.