gRPC vs REST

// user.proto — service definition
syntax = "proto3";
package user;

service UserService {
  rpc GetUser(GetUserRequest) returns (UserResponse);                    // Unary
  rpc ListUsers(ListUsersRequest) returns (stream UserResponse);        // Server streaming
  rpc UpdateUsers(stream UpdateUserRequest) returns (UpdateSummary);   // Client streaming
  rpc Chat(stream ChatMessage) returns (stream ChatMessage);           // Bidirectional
}

message GetUserRequest { string user_id = 1; }
message UserResponse {
  string id      = 1;
  string name    = 2;
  string email   = 3;
  int64  created = 4;
}

// Compile: protoc --java_out=. --grpc-java_out=. user.proto
// Generates: UserServiceGrpc.java (stubs), UserProto.java (POJOs)

// Java gRPC server (Spring Boot with grpc-spring-boot-starter):
@GrpcService
public class UserGrpcService extends UserServiceGrpc.UserServiceImplBase {
    @Override
    public void getUser(GetUserRequest req, StreamObserver<UserResponse> observer) {
        UserResponse response = UserResponse.newBuilder()
            .setId(req.getUserId())
            .setName("Alice")
            .setEmail("alice@example.com")
            .build();
        observer.onNext(response);
        observer.onCompleted();
    }
}

// gRPC vs REST comparison:
// ┌─────────────────────┬──────────────────────┬──────────────────────┐
// │ Property            │ REST + JSON           │ gRPC + Protobuf      │
// ├─────────────────────┼──────────────────────┼──────────────────────┤
// │ Protocol            │ HTTP/1.1 or HTTP/2   │ HTTP/2 required      │
// │ Payload format      │ JSON (text)           │ Protobuf (binary)    │
// │ Payload size        │ Larger                │ 5–10× smaller        │
// │ Performance         │ Baseline              │ ~5–10× faster        │
// │ Streaming           │ No (SSE workaround)   │ Yes (4 modes)        │
// │ Type safety         │ No schema (OpenAPI)   │ Strict .proto schema │
// │ Browser support     │ Native                │ gRPC-Web proxy needed│
// │ Debugging           │ Easy (curl, Postman)  │ Hard (binary)        │
// │ Code generation     │ Optional (OpenAPI)    │ Required (protoc)    │
// │ Best for            │ Public APIs, browser  │ Internal services    │
// └─────────────────────┴──────────────────────┴──────────────────────┘

// Java gRPC client (blocking stub):
ManagedChannel channel = ManagedChannelBuilder
    .forAddress("user-service", 9090)
    .usePlaintext()                         // use .useTransportSecurity() in prod
    .build();

UserServiceGrpc.UserServiceBlockingStub stub = UserServiceGrpc.newBlockingStub(channel);
UserResponse user = stub.getUser(
    GetUserRequest.newBuilder().setUserId("u-123").build()
);
System.out.println(user.getName());         // type-safe — no JSON parsing