So far so good, we haven’t seen too much code or implemented any business logic. In the previous sections we have seen some protocol definitions with Scala annotations and the generation of these definitions from IDL files. Finally, in this section, we are going to see how to complete our quickstart example. We’ll take a look at both sides, the server and the client.


Predictably, generating the server code is just creating an implementation for your service algebra.

First, here’s our Greeter RPC protocol definition:

Next, our dummy Greeter server implementation:

import cats.effect.Async
import cats.syntax.applicative._
import monix.execution.Scheduler
import monix.reactive.Observable
import service._

import scala.concurrent.ExecutionContext

class ServiceHandler[F[_]: Async](implicit S: Scheduler) extends Greeter[F] {

  private[this] val dummyObservableResponse: Observable[HelloResponse] =
    Observable.fromIterable(1 to 5 map (i => HelloResponse(s"Reply $i")))

  override def sayHello(request: HelloRequest): F[HelloResponse] =
    HelloResponse(reply = "Good bye!").pure[F]

  override def lotsOfReplies(request: HelloRequest): Observable[HelloResponse] =

  override def lotsOfGreetings(request: Observable[HelloRequest]): F[HelloResponse] =
      .foldLeftL((0, HelloResponse(""))) {
        case ((i, response), currentRequest) =>
          val currentReply: String = response.reply
            i + 1,
              reply = s"$currentReply\nRequest ${currentRequest.greeting} -> Response: Reply $i"))

  override def bidiHello(request: Observable[HelloRequest]): Observable[HelloResponse] =
      .flatMap { request: HelloRequest =>
        println(s"Saving $request...")
      .onErrorHandle { e =>
        throw e

As you can see, we need a monix.execution.Scheduler implicit. This is needed in order to resolve the cats.effect instances for Monix.

That’s it! We have exposed a potential implementation on the server side.

Server Runtime

As you can see, the generic handler above requires F as the type parameter, which corresponds with our target Monad when interpreting our program. In this section, we will satisfy all the runtime requirements, in order to make our server runnable.

Creating a runtime

Since mu relies on ConcurrentEffect from the cats-effect library, we’ll need a runtime for executing our effects.

We’ll be using IO from cats-effect, but you can use any type that has a ConcurrentEffect instance.

For executing IO we need a ContextShift[IO] used for running IO instances and a Timer[IO] that is used for scheduling, let’s go ahead and create them.

Note: You’d need an implicit monix.execution.Scheduler in the case you’re using Monix observables.

trait CommonRuntime {

  val EC: scala.concurrent.ExecutionContext =

  implicit val S: monix.execution.Scheduler =

  implicit val timer: cats.effect.Timer[cats.effect.IO]     = cats.effect.IO.timer(EC)
  implicit val cs: cats.effect.ContextShift[cats.effect.IO] = cats.effect.IO.contextShift(EC)


As a side note, CommonRuntime will also be used later on for the client example program.

Transport Implicits

For the server bootstrapping, remember to add the mu-rpc-server dependency to your build.

Now, we need to implicitly provide two things:

  • A runtime interpreter of our ServiceHandler tied to a specific type. In our case, we’ll use cats.effects.IO.
  • A ServerW implicit evidence, compounded by:
    • RPC port where the server will bootstrap.
    • The set of configurations we want to add to our gRPC server, like our Greeter service definition. All these configurations are aggregated in a List[GrpcConfig]. Later on, an internal builder will build the final server based on this list. The full list of exposed settings is available in this file.

In summary, the result would be as follows:

import cats.effect.IO
import monix.execution.Scheduler
import service._

object gserver {

  trait Implicits extends CommonRuntime {

    implicit val greeterServiceHandler: ServiceHandler[IO] = new ServiceHandler[IO]


  object implicits extends Implicits


Server Bootstrap

What else is needed? We just need to define a main method:

import cats.effect.IO

object RPCServer {

  import gserver.implicits._

  def main(args: Array[String]): Unit = {

    val runServer = for {
      grpcConfig <- Greeter.bindService[IO]
      server     <- GrpcServer.default[IO](8080, List(AddService(grpcConfig)))
      runServer  <- GrpcServer.server[IO](server)
    } yield runServer



The Server will bootstrap on port 8080.

Fortunately, once all the runtime requirements are in place (import gserver.implicits._), we only have to write the previous piece of code, which primarily, should be the same in all cases (except if your target Monad is different from cats.effects.IO).

Service testing

Thanks to withServerChannel from the package mu.rpc.testing.servers, you will be able to run in-memory instances of the server, which is very convenient for testing purposes. Below, a very simple property-based test for proving Greeter.sayHello:

import cats.effect.Resource
import io.grpc.{ManagedChannel, ServerServiceDefinition}
import org.scalatestplus.scalacheck.Checkers
import org.scalatest._
import org.scalacheck.Gen
import org.scalacheck.Prop.forAll
import service._

class ServiceSpec extends FunSuite with Matchers with Checkers with OneInstancePerTest {

  import gserver.implicits._
  def withClient[Client, A](
      serviceDef: IO[ServerServiceDefinition],
      resourceBuilder: IO[ManagedChannel] => Resource[IO, Client])(
      f: Client => A): A =
      .flatMap(sc => resourceBuilder(IO(
      .use(client => IO(f(client)))

  def sayHelloTest(requestGen: Gen[HelloRequest], expected: HelloResponse): Assertion =
      Greeter.clientFromChannel[IO](_)) { client =>
        check {
          forAll(requestGen) { request =>
            client.sayHello(request).unsafeRunSync() == expected

  val requestGen: Gen[HelloRequest] = Gen.alphaStr map HelloRequest

  test("Get a valid response when a proper request is passed") {
    sayHelloTest(requestGen, HelloResponse(reply = "Good bye!"))


Greeter.bindService is an auto-derived method which creates, behind the scenes, the binding service for gRPC. It requires F[_] as the type parameter, the target/concurrent monad, in our example: cats.effects.IO.

Running the test:

scala> run(new ServiceSpec)
- Get a valid response when a proper request is passed


mu derives a client automatically based on the protocol. This is especially useful because you can distribute it depending on the protocol/service definitions. If you change something in your protocol definition, you will get a new client for free without having to write anything.

You will need to add either mu-rpc-netty or mu-rpc-okhttp to your build.

Client Runtime

Similarly in this section, just like we saw for the server, we are defining all the client runtime configurations needed for communication with the server.

Creating a runtime

In our example, we are going to use the same runtime described for the server. Remember we’re relying on the ConcurrentEffect, so you can use any type that has a ConcurrentEffect instance.

Transport Implicits

First, we need to configure how the client will reach the server in terms of the transport layer. There are two supported methods:

  • By Address (host/port): enables us to create a channel with the target’s address and port number.
  • By Target: we can create a channel with a target string, which can be either a valid NameResolver-compliant URI or an authority string.

Additionally, we can add more optional configurations that can be used when the connection occurs. All the options are available here.

Given the transport settings and a list of optional configurations, we can create the ManagedChannel.html object, using the ManagedChannelInterpreter builder.

As we will see shortly in our example, we are going relay on the default behaviour passing directly the ChannelFor to the client builder.

So, taking into account all of that, how would our code look?

import cats.effect.{IO, Resource}
import service._

object gclient {

  trait Implicits extends CommonRuntime {

    val channelFor: ChannelFor =
      ConfigForAddress[IO]("", "rpc.port").unsafeRunSync

    implicit val serviceClient: Resource[IO, Greeter[IO]] =

  object implicits extends Implicits


  • In the code above, host and port would be read from an application configuration file.
  • To be able to use the ConfigForAddress helper, you need to add the mu-config dependency to your build.

Client Program

Once we have our runtime configuration defined as above, everything gets easier on the client side. This is an example of a client application, following our dummy quickstart:

import cats.effect.IO
import service._
import gclient.implicits._

object RPCDemoApp {

  def main(args: Array[String]): Unit = {

    val hello = serviceClient.use(_.sayHello(HelloRequest("foo"))).flatMap { result =>
      IO(println(s"Result = $result"))