gRPC MP Implementation

[spericas]

Description

Integration of H3 gRPC MP Implementation into H4. gRPC in H3 MP started its own Netty-based server on a new port, the new implementation in H4 is fully intergrated with the web server, which is now capable of detecting the type of a connection and route its data to the correct handler. As a result of this new integration, there is new gRPC server configuration (there is no gRPC server anymore) and a few other integration points have changed.

Module Changes

• New helidon-microprofile-grpc-core module
• New helidon-microprofile-grpc-server module
• Additions and updates to helidon-grpc-core and helidon-webserver-grpc modules

gRPC MP Support

Just like in H3, there is now support for gRPC annotated endpoints:

    @Grpc.GrpcService
    @ApplicationScoped
    public static class EchoService {

        @Grpc.Unary("Echo")
        public void echo(Echo.EchoRequest request, StreamObserver<Echo.EchoResponse> observer) {
            try {
                String message = request.getMessage();
                Echo.EchoResponse response = Echo.EchoResponse.newBuilder().setMessage(message).build();
                complete(observer, response);
            } catch (IllegalStateException e) {
                observer.onError(e);
            }
        }
    }

In addition to annotated endpoints, the gRPC CDI extension will scan and automatically register any beans that implement GrpcService (from SE) and BindableService (io.grpc), just like it did in H3.

Server interceptors from the io.grpc API are support only on annotated types either by direct reference using the @GrpcInterceptors or via CDI name binding.

    @Grpc.GrpcService
    @ApplicationScoped
    @EchoInterceptorBinding       // name binding
    @Grpc.GrpcInterceptors(EchoInterceptor1.class)   // direct reference
    public static class EchoService { ... }

    @Grpc.GrpcInterceptor
    @ApplicationScoped
    public static class EchoInterceptor1 implements ServerInterceptor {

        @Override
        public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> call,
                                                                     Metadata headers,
                                                                     ServerCallHandler<ReqT, RespT> next) {
            ...
        }
    }

    @Grpc.GrpcInterceptor
    @EchoInterceptorBinding
    @ApplicationScoped
    public static class EchoInterceptor2 implements ServerInterceptor {

        @Override
        public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> call,
                                                                     Metadata headers,
                                                                     ServerCallHandler<ReqT, RespT> next) {
            ...
        }
    }

Additional Features in PR

• The GrpcMpContext SPI has also been ported to H4. It enables users to write CDI extensions that are given access to the CDI BeanManager and the server's GrpcRouting builder.
• A Context interceptor is automatically registered for all annotated endpoints
• A Tracing interceptor is registered for all annotated endpoints if grpc.tracing.enabled is set to true (defaults tofalse)

[tomas-langer]

• I think the annotations should be in helidon-microprofile-grpc-api module
• please make sure all public constructors are replaced with factory methods (unless they are allowed by our guidelines)
• please make sure we never accept null as a parameter value

There is a lot of reflection used here. It may be better to use annotation processing to generate all the needed stuff to be reflection free (maybe even use ServiceRegistry for this?).