What is the difference between REST and gRPC?

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**REST** — an architectural style using HTTP/1.1 or HTTP/2 with JSON for data exchange. **gRPC** — a high-performance RPC framework using HTTP/2 and Protocol Buffers (binary serialization). **System Design Context:** REST dominates public APIs due to simplicity and tooling. gRPC excels in internal service-to-service communication where performance matters. Many modern architectures use both: gRPC internally, REST externally. **REST — Detailed Characteristics:** - **Protocol**: HTTP/1.1 or HTTP/2. - **Serialization**: JSON (text-based, human-readable). - **Contract**: OpenAPI/Swagger specification. - **Streaming**: Request-response only (or SSE/WebSocket for real-time). - **Browser support**: Native. - **Tooling**: Mature — Postman, curl, every language has HTTP clients. ```java // REST controller @RestController @RequestMapping("/api") public class UserController { private final UserService userService; @GetMapping("/users/{id}") public ResponseEntity getUser(@PathVariable Long id) { return userService.findById(id) .map(ResponseEntity::ok) .orElse(ResponseEntity.notFound().build()); } @GetMapping("/users") public ResponseEntity> listUsers( @RequestParam(defaultValue = "0") int page, @RequestParam(defaultValue = "20") int size) { return ResponseEntity.ok(userService.findAll(page, size)); } } ``` **gRPC — Detailed Characteristics:** - **Protocol**: HTTP/2 only. - **Serialization**: Protocol Buffers (binary, compact). - **Contract**: .proto files (code-generated). - **Streaming**: Unary, Server-streaming, Client-streaming, Bidirectional. - **Browser support**: Requires gRPC-Web proxy. - **Performance**: 2-10x faster than REST for internal calls. ```protobuf // user_service.proto syntax = "proto3"; package com.example; service UserService { rpc GetUser (GetUserRequest) returns (User); rpc ListUsers (ListUsersRequest) returns (stream User); rpc CreateUser (CreateUserRequest) returns (User); } message User { int64 id = 1; string name = 2; string email = 3; } message GetUserRequest { int64 id = 1; } message ListUsersRequest { int32 page_size = 1; string page_token = 2; } ``` ```java // gRPC server implementation @GrpcService public class UserServiceImpl extends UserServiceGrpc.UserServiceImplBase { private final UserRepository userRepository; @Override public void getUser(GetUserRequest request, StreamObserver responseObserver) { User user = userRepository.findById(request.getId()) .map(this::toProto) .orElseThrow(() -> Status.NOT_FOUND .withDescription("User not found") .asRuntimeException()); responseObserver.onNext(user); responseObserver.onCompleted(); } @Override public void listUsers(ListUsersRequest request, StreamObserver responseObserver) { userRepository.findAll() .map(this::toProto) .forEach(responseObserver::onNext); responseObserver.onCompleted(); } } ``` **Performance Comparison:** - REST (JSON): ~500 bytes for a simple user object, ~2ms serialization. - gRPC (Protobuf): ~100 bytes for the same object, ~0.2ms serialization. - gRPC supports multiplexed streams over a single HTTP/2 connection. **Common Pitfalls:** - **gRPC + browser**: needs gRPC-Web or Envoy proxy. - **Proto backward compatibility**: must use field numbers, never reuse deleted field numbers. - **REST over-engineering**: gRPC adds complexity; use REST for simple CRUD APIs. **When to Use:** - **REST** — public APIs, simple CRUD, browser-facing, broad tooling needed. - **gRPC** — internal microservices, high-throughput, streaming, polyglot environments.

Java: What is the difference between REST and gRPC?