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

SBT configuration

project/plugins.sbt:

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

build.sbt:

enablePlugins(Fs2Grpc)

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 =
  project
    .in(file("protobuf"))
    .enablePlugins(Fs2Grpc)

lazy val client =
  project
    .in(file("client"))
    .dependsOn(protobuf)

lazy val server =
  project
    .in(file("server"))
    .dependsOn(protobuf)

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 is:

import fs2.grpc.syntax.all._

val managedChannelResource: Resource[IO, ManagedChannel] =
  ManagedChannelBuilder
    .forAddress("127.0.0.1", 9999)
    .resource[IO]

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.

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

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

If a ManagedChannelProvider isn't found on your classpath you may receive an error similar to

io.grpc.ManagedChannelProvider$ProviderNotFoundException: No functional channel service provider found. Try adding a dependency on the grpc-okhttp or grpc-netty artifact

This can be fixed by adding a dependency to the netty provider e.g.

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

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.

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 fs2.grpc.syntax.all._

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

def run(service: ServerServiceDefinition) = ServerBuilder
  .forPort(9999)
  .addService(service)
  .resource[IO]
  .evalMap(server => IO(server.start()))
  .useForever

helloService.use(run)

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.