How do you implement a gRPC server in Go, including error handling and interceptors?

Implementing a gRPC server follows a code-generation-first workflow: define the proto, generate Go stubs, implement the interface, and start the server. Interceptors (gRPC's equivalent of HTTP middleware) add cross-cutting concerns like logging and auth.

// Step 1: implement the generated server interface
type userServiceServer struct {
    pb.UnimplementedUserServiceServer // embed for forward compatibility
    repo UserRepository
    log  *slog.Logger
}

func (s *userServiceServer) GetUser(
    ctx context.Context,
    req *pb.GetUserRequest,
) (*pb.User, error) {
    if req.Id <= 0 {
        return nil, status.Errorf(codes.InvalidArgument,
            "id must be positive, got %d", req.Id)
    }
    user, err := s.repo.FindByID(ctx, int(req.Id))
    if err != nil {
        if errors.Is(err, ErrNotFound) {
            return nil, status.Errorf(codes.NotFound,
                "user %d not found", req.Id)
        }
        s.log.Error("repo error", "err", err)
        return nil, status.Errorf(codes.Internal, "internal error")
    }
    return &pb.User{Id: int64(user.ID), Name: user.Name, Email: user.Email}, nil
}

// Interceptor (middleware equivalent)
func loggingInterceptor(log *slog.Logger) grpc.UnaryServerInterceptor {
    return func(
        ctx context.Context,
        req any,
        info *grpc.UnaryServerInfo,
        handler grpc.UnaryHandler,
    ) (any, error) {
        start := time.Now()
        resp, err := handler(ctx, req)
        log.Info("RPC",
            "method", info.FullMethod,
            "duration", time.Since(start),
            "error", err,
        )
        return resp, err
    }
}

// Step 2: start the server
func main() {
    lis, _ := net.Listen("tcp", ":9090")
    srv := grpc.NewServer(
        grpc.ChainUnaryInterceptor(
            loggingInterceptor(log),
            recoveryInterceptor(),
            authInterceptor(secret),
        ),
    )
    pb.RegisterUserServiceServer(srv, &userServiceServer{repo: repo})
    reflection.Register(srv) // enables grpcurl inspection
    srv.Serve(lis)
}

gRPC status codes: always return structured status errors using status.Errorf(codes.X, ...). Clients receive the code and message and can handle them programmatically. The mapping to HTTP status codes is standardised (NotFound→404, InvalidArgument→400, Internal→500).

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