This is a demonstration on using gRPC as a mechanism to communicate between services. There are 4 method types in this example: simple, server streaming, client streaming, bidirectional streaming.
It also considers several aspects that we might encouter in a real solution, such as

• Authentication with TSL
  Server:

      var credentials = TlsServerCredentials.newBuilder()
              .keyManager(certChainFilePath, privateKeyFilePath)
              .trustManager(trustCertFilePath) // to use mutual SSL
              .build();
  
      var server = Grpc.newServerBuilderForPort(4000, credentials)

  Client:

      var chanelCredentials = TlsChannelCredentials.newBuilder()
              .trustManager(trustedCertFilePath)
              .keyManager(certChainFilePath, privateKeyFilePath) // to use mutual SSL
              .build();
  
      var channel = Grpc.newChannelBuilderForAddress("localhost", 4000, chanelCredentials).build();

• Error handling
  Server:

      @Override
      public void getAllCategories(Blank request, StreamObserver<Category> responseObserver) {
          // Result in UNKNOWN error without description & cause
          responseObserver.onError(new IllegalArgumentException("exception 2"));
          responseObserver.onCompleted();
      }

      @Override
      public void onCompleted() {
          // Cause hasn't been passed to client
          responseObserver.onError(Status.NOT_FOUND
                  .withDescription("Couldn't find any categories")
                  .withCause(new RuntimeException("exception 3"))
                  .asRuntimeException());
      }

  Client:

      @Override
      public void onError(Throwable t) {
          var status = Status.fromThrowable(t);
          System.out.println(status);
      }

• Deadline or timeout
  Blocking stub:

      var blockingStub = CategoryServiceGrpc.newBlockingStub(channel)
              .withDeadlineAfter(500, TimeUnit.MILLISECONDS);

  Asynchronous stub:

      var stub = CategoryServiceGrpc.newStub(channel)
              .withDeadlineAfter(500, TimeUnit.MILLISECONDS);

Compare to REST:

• gRPC use binary message format (Protocol Buffers) so it's faster, especially when the message is large.
• REST only supports request/response but gRPC also supports streaming communication.
• gRPC helps to avoid the difficulty specifying the right VERBs in REST
• gRPC use HTTP/2, REST use HTTP/1.1