A practical walkthrough comparing REST and gRPC for internal microservice communication in Go. Covers the implicit contract problem with REST (runtime breakages from JSON field changes), then builds a gRPC service step-by-step: defining a .proto contract, generating Go code with protoc, implementing a server (auth-service) and client (billing-service), and extending to server-side streaming for real-time event delivery. Concludes with honest tradeoffs: gRPC is ideal for internal service-to-service calls, while REST remains appropriate for public APIs and browser clients.