typelevel / fs2-grpc   2.5.4

MIT License GitHub

gRPC implementation for FS2/cats-effect

Scala versions: 2.12 2.13 3.x
sbt plugins: 1.0

fs2-grpc - gRPC implementation for FS2/cats-effect

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SBT configuration


addSbtPlugin("org.typelevel" % "sbt-fs2-grpc" % "<latest-version>")



Depending if you wish to use grpc-netty or grpc-okhttp, add one of the following dependencies:

libraryDependencies += "io.grpc" % "grpc-netty-shaded" % scalapb.compiler.Version.grpcJavaVersion


libraryDependencies += "io.grpc" % "grpc-okhttp" % scalapb.compiler.Version.grpcJavaVersion

Protocol buffer files

The protobuf files should be stored in the directory <project_root>/src/main/protobuf.

Multiple projects

If the generated code is used by multiple projects, you may build the client/server code in a common project which other projects depend on. For example:

lazy val protobuf =

lazy val client =

lazy val server =

Creating a client

A ManagedChannel is the type used by grpc-java to manage a connection to a particular server. This library provides syntax for ManagedChannelBuilder which creates a Resource which can manage the shutdown of the channel, by calling .resource[F] where F has an instance of the Sync typeclass. This implementation will do a drain of the channel, and attempt to shut down the channel, forcefully closing after 30 seconds. An example of the syntax using grpc-netty is:

import io.grpc.netty.shaded.io.grpc.netty.NettyChannelBuilder
import fs2.grpc.syntax.all._

val managedChannelResource: Resource[IO, ManagedChannel] =
    .forAddress("", 9999)

The syntax also offers the method resourceWithShutdown which takes a function ManagedChannel => F[Unit] which is used to manage the shutdown. This may be used where requirements before shutdown do not match the default behaviour.

The generated code provides a method stubResource[F], for any F which has a Async instance, and it takes a parameter of type Channel. It returns a Resource with an implementation of the service (in a trait), which can be used to make calls.

Moreover, the generated code provides method overloads that take ClientOptions used for configuring calls.

def runProgram(stub: MyFs2Grpc[IO]): IO[Unit] = ???

val run: IO[Unit] = managedChannelResource
  .flatMap(ch => MyFs2Grpc.stubResource[IO](ch))

Creating a server

The generated code provides a method bindServiceResource[F], for any F which has a Async instance, and it takes an implementation of the service (in a trait), which is used to serve responses to RPC calls. It returns a Resource[F, ServerServiceDefinition] which is given to the server builder when setting up the service. Furthermore, the generated code provides method overloads that take ServerOptions used for configuring service calls.

A Server is the type used by grpc-java to manage the server connections and lifecycle. This library provides syntax for ServerBuilder, which mirrors the pattern for the client. An example is:

import io.grpc.netty.shaded.io.grpc.netty.NettyServerBuilder
import fs2.grpc.syntax.all._

val helloService: Resource[IO, ServerServiceDefinition] = 
  MyFs2Grpc.bindServiceResource[IO](new MyImpl())

def run(service: ServerServiceDefinition) = NettyServerBuilder
  .evalMap(server => IO(server.start()))


Code generation options

To alter code generation, you can set some flags with scalapbCodeGeneratorOptions, e.g.:

scalapbCodeGeneratorOptions += CodeGeneratorOption.FlatPackage

The full set of options available are:

  • CodeGeneratorOption.FlatPackage - If true, the compiler does not append the proto base file name
  • CodeGeneratorOption.JavaConversions - Enable Java conversions for protobuf
  • CodeGeneratorOption.Grpc (included by default) - generate grpc bindings based on Observables
  • CodeGeneratorOption.Fs2Grpc (included by default) - generate grpc bindings for FS2/cats
  • CodeGeneratorOption.SingleLineToProtoString - toProtoString generates single line
  • CodeGeneratorOption.AsciiFormatToString - toString uses toProtoString functionality

Pass additional protoc options

PB.protocOptions in Compile := Seq("-xyz")

Tool Sponsorship

Development of fs2-grpc is generously supported in part by YourKit through the use of their excellent Java profiler.