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Quick Start

This guide gets you started with gRPC in Kotlin with a simple working example.


Prerequisites

  • Kotlin version 1.3 or higher
  • JDK version 7 or higher

Download the example

You’ll need a local copy of the example code to work through this quick start. Download the example code from our GitHub repository (the following command clones the entire repository, but you just need the examples for this quick start and other tutorials):

# Clone the repository at the latest release to get the example code:
$ git clone https://github.com/grpc/grpc-kotlin
# Navigate to the examples:
$ cd grpc-kotlin/examples

Run a gRPC application

From the examples directory:

  1. Compile the client and server

    $ ./gradlew installDist
    
  2. Run the server:

    $ ./build/install/examples/bin/hello-world-server
    
  3. From another terminal, run the client:

    $ ./build/install/examples/bin/hello-world-client
    

Congratulations! You’ve just run a client-server application with gRPC.

Update a gRPC service

Now let’s look at how to update the application with an extra method on the server for the client to call. Our gRPC service is defined using Protocol Buffers; you can find out lots more about how to define a service in a .proto file in gRPC Basics: Kotlin. For now all you need to know is that both the server and the client “stub” have a SayHello() RPC method that takes a HelloRequest parameter from the client and returns a HelloReply from the server, and that this method is defined like this:

// The greeting service definition.
service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {}
}

// The request message containing the user's name.
message HelloRequest {
  string name = 1;
}

// The response message containing the greetings
message HelloReply {
  string message = 1;
}

Let’s update this so that the Greeter service has two methods. Edit src/main/proto/hello_world.proto and update it with a new SayHelloAgain() method, with the same request and response types:

// The greeting service definition.
service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {}
  // Sends another greeting
  rpc SayHelloAgain (HelloRequest) returns (HelloReply) {}
}

// The request message containing the user's name.
message HelloRequest {
  string name = 1;
}

// The response message containing the greetings
message HelloReply {
  string message = 1;
}

Remember to save the file!

Update and run the application

When we recompile the example, normal compilation will regenerate HelloWorldGrpcKt.kt, which contains our generated gRPC client and server classes. This also regenerates classes for populating, serializing, and retrieving our request and response types.

However, we still need to implement and call the new method in the human-written parts of our example application.

Update the server

In the same directory, open src/main/kotlin/io/grpc/examples/helloworld/HelloWorldServer.kt. Implement the new method like this:

private class HelloWorldService : GreeterGrpcKt.GreeterCoroutineImplBase() {
  override suspend fun sayHello(request: HelloRequest) = HelloReply
      .newBuilder()
      .setMessage("Hello ${request.name}")
      .build()

    override suspend fun sayHelloAgain(request: HelloRequest) = HelloReply
      .newBuilder()
      .setMessage("Hello again ${request.name}")
      .build()
}

Update the client

In the same directory, open src/main/kotlin/io/grpc/examples/helloworld/HelloWorldClient.kt. Call the new method like this:

class HelloWorldClient constructor(
    private val channel: ManagedChannel
) : Closeable {
  private val stub: GreeterCoroutineStub = GreeterCoroutineStub(channel)

  suspend fun greet(name: String) = coroutineScope {
    val request = HelloRequest.newBuilder().setName(name).build()
    val response = async { stub.sayHello(request) }
    println("Received: ${response.await().message}")
    val againResponse = async { stub.sayHelloAgain(request) }
    println("Received: ${againResponse.await().message}")
  }

  override fun close() {
    channel.shutdown().awaitTermination(5, TimeUnit.SECONDS)
  }
}

Run!

Run the client and server like you did before. Execute the following commands from the examples directory:

  1. Compile the client and server:

    $ ./gradlew installDist
    
  2. Run the server:

    $ ./build/install/examples/bin/hello-world-server
    
  3. From another terminal, run the client. This time, add a name as a command-line argument:

    $ ./build/install/examples/bin/hello-world-client Alice
    

    You’ll see the following output:

    Received: Hello Alice
    Received: Hello again Alice
    

What’s next