API Paradigms: REST vs GraphQL vs gRPC

// user_service.proto (Protobuf Version 3)
syntax = "proto3";
package ecommerce.v1;

service UserService {
  // Unary RPC: Single request, single response
  rpc GetUserProfile(UserRequest) returns (UserResponse);
  
  // Server Streaming RPC: Stream live trade updates
  rpc StreamOrders(UserRequest) returns (stream OrderEvent);
}

message UserRequest {
  int64 user_id = 1; // Field tag 1 (NOT the value)
}

message UserResponse {
  int64 user_id = 1;
  string username = 2;
  string email = 3;
  bool is_premium = 4;
}

# Python gRPC Server Implementation
import grpc
from concurrent import futures
import user_service_pb2, user_service_pb2_grpc

class UserServiceServicer(user_service_pb2_grpc.UserServiceServicer):
    def GetUserProfile(self, request, context):
        # Fast in-memory lookup or DB query
        return user_service_pb2.UserResponse(
            user_id=request.user_id,
            username="Alice",
            email="alice@example.com",
            is_premium=True
        )

server = grpc.server(futures.ThreadPoolExecutor(max_workers=10))
user_service_pb2_grpc.add_UserServiceServicer_to_server(UserServiceServicer(), server)
server.add_insecure_port("[::]:50051")
server.start()

// Go gRPC Server Handler
package main

import (
    "context"
    pb "myapp/proto"
)

type server struct {
    pb.UnimplementedUserServiceServer
}

func (s *server) GetUserProfile(ctx context.Context, in *pb.UserRequest) (*pb.UserResponse, error) {
    return &pb.UserResponse{
        UserId:    in.GetUserId(),
        Username:  "Alice",
        Email:     "alice@example.com",
        IsPremium: true,
    }, nil
}

// Java gRPC Service Implementation
@GrpcService
public class UserServiceImpl extends UserServiceGrpc.UserServiceImplBase {
    @Override
    public void getUserProfile(UserRequest req, StreamObserver<UserResponse> responseObserver) {
        UserResponse resp = UserResponse.newBuilder()
            .setUserId(req.getUserId())
            .setUsername("Alice")
            .setEmail("alice@example.com")
            .setIsPremium(true)
            .build();
        
        responseObserver.onNext(resp);
        responseObserver.onCompleted();
    }
}

// TypeScript @grpc/grpc-js Service
import * as grpc from '@grpc/grpc-js';

const userService: IUserServiceServer = {
  getUserProfile(call, callback) {
    const userId = call.request.userId;
    callback(null, {
      userId,
      username: 'Alice',
      email: 'alice@example.com',
      isPremium: true
    });
  }
};