grpc/Step_3.md

Step-3: Implement a server.

This step extends the generated server skeleton code to write a simple server that provides the hello service. This introduces two new classes:

• a service implementation GreetingsImpl.java.

• a server that hosts the service implementation and allows access over the network: GreetingsServer.java.

Service implementation

GreetingsImpl.java implements the behaviour we require of our GreetingService. There are a number of important features of gRPC being used here:

  public void hello(Helloworld.HelloRequest req,
      StreamObserver<Helloworld.HelloReply> responseObserver) {
    Helloworld.HelloReply reply = Helloworld.HelloReply.newBuilder().setMessage(
        "Hello " + req.getName()).build();
    responseObserver.onValue(reply);
    responseObserver.onCompleted();
  }

• it provides a class GreetingsImpl that implements a generated interface GreetingsGrpc.Greetings
• GreetingsGrpc.Greetings declares the method hello that was declared in the proto IDL
• hello's signature is typesafe: hello(Helloworld.HelloRequest req, StreamObserver<Helloworld.HelloReply> responseObserver)
• hello takes two parameters: Helloworld.HelloRequest: the request StreamObserver<Helloworld.HelloReply>: a response observer, an interface to be called with the response value
• to complete the call
  • the return value is constructed
  • the responseObserver.onValue() is called with the response
  • responseObserver.onCompleted() is called to indicate that no more work will done on the RPC.

Server implementation

GreetingsServer.java shows the other main feature required to provde the gRPC service; how to allow a service implementation to be accessed from the network.

  private void start() throws Exception {
    server = NettyServerBuilder.forPort(port)
             .addService(GreetingsGrpc.bindService(new GreetingsImpl()))
             .build();
    server.startAsync();
    server.awaitRunning(5, TimeUnit.SECONDS);
  }

• it provides a class GreetingsServer that holds a ServerImpl that will run the server
• in the start method, GreetingServer binds the GreetingsService implementation to a port and begins running it
• there is also a stop method that takes care of shutting down the service and cleaning up when the program exits

Build it

This is the same as before: our client and server are part of the same maven package so the same command builds both.

$ mvn package

Try them out

We've added simple shell scripts to simplifying running the examples. Now that they are built, you can run the server with:

$ ./run_greetings_server.sh

and in another terminal window confirm that it receives a message.

$ ./run_greetings_client.sh