gRPC error about missing an "Unimplemented" server method

Question

this my proto file:

syntax = "proto3";

option go_package = ".;pb";

service Greeter {
  rpc SayHello (HelloRequest) returns (HelloReply); // hello
}

message HelloRequest {
  string name = 1;
}

message HelloReply {
  string message = 1;
}

**

package main

import (
    "awesomeProject/grpc_test/proto"
    "context"
    "google.golang.org/grpc"
    "net"
)


type Server struct {}

func (s *Server) SayHello(ctx context.Context, request *pb.HelloRequest) (*pb.HelloReply, error) {
    return &pb.HelloReply{
        Message: "hello" + request.Name,
    }, nil
}

func main() {
    g := grpc.NewServer()
    pb.RegisterGreeterServer(g, &Server{})

    lis, err := net.Listen("tcp", "0.0.0.0:8080")
    if err != nil {
        panic("failed to listen: " + err.Error() )
    }

    err = g.Serve(lis)
    if err != nil {
        panic("failed to start grpc: " + err.Error())
    }
}

and i got an error:

cannot use &Server{} (type *Server) as type pb.GreeterServer in argument to pb.RegisterGreeterServer:*Server does not implement pb.GreeterServer (missing pb.mustEmbedUnimplementedGreeterServer method)

how to solve it?

What did you use to generate the Go code from the proto file? It looks like you are using a toolkit that requires you to embed a type into your server struct. — Burak Serdar, Oct 24 '21 at 21:59
some implementation issue. have you checkout https://buf.build/ ? it help setting up and maintaining gRPC services in Go. — twiny, Oct 24 '21 at 23:06
@BurakSerdar that happens with the official [protoc-gen-go-grpc](https://grpc.io/docs/languages/go/quickstart/) toolkit when invoked as `protoc --go_out=. --go-grpc_out=.`. It does not happen (yet) when invoked via a plugin: `protoc --go_out=plugins=grpc:.` — rustyx, Oct 25 '21 at 15:33

Brits · Answer 1 · 2021-10-25T00:08:33.077

This is covered in this README (this issue and this issue provide a lot, perhaps too much!, more info).

The solution will be to edit your struct definition (as the error suggests) to:

type Server struct {
    proto.UnimplementedGreeterServer
}

You might also want to take a look at the quickstart. Your service appears to be derived from this and you will note that main.go (/examples/helloworld/greeter_server/main.go in here) includes a similar struct definition.

Adding proto.UnimplementedGreeterServer provides a default implementation for all of the services (part of the generated code); for example:

func (UnimplementedGreeterServer) SayHello(context.Context, *HelloRequest) (*HelloReply, error) {
    return nil, status.Errorf(codes.Unimplemented, "method SayHello not implemented")
}

Including UnimplementedGreeterServer within the structure means that if you add a new service to the proto file and then recompile your application it will compile without error AND return an unimplemented error should a client attempt to call the new service. The issues referenced earlier include a lot of discussion around why this method was selected.

gRPC error about missing an "Unimplemented" server method

1 Answers1