Separating Business Logic

When to use

• Designing a new feature from scratch
• Splitting a god service into layers
• Moving business rules out of controllers or infrastructure

Core rules

1. Domain layer: Pure business rules, no framework, no I/O, no console.log
2. Application layer: Use-cases orchestrate domain + ports, no HTTP/DB direct
3. Infrastructure layer: Adapters for HTTP, DB, 3rd-party, implements ports
4. Controllers/handlers: only parse input, call one use-case, map Result to response
5. Domain entities contain invariants (rules that must always hold) and behavior

Reference shape (TypeScript)