📜 WAYPOINT LESSON
Layered Architecture
⭐⭐ intermediate⏳ 13 min read📍 Lesson 106 of 180
Controller → service → repository, the arrows-point-down rule, and what each layer tests like.
Layered architecture
Three layers cover most services:
Controller/UI → parses input, calls service, maps result
↓ depends on
Service → business rules, orchestration, transactions
↓ depends on
Repository → persistence (Module 11)
The dependency rule: arrows point down only. A service never calls a controller; a repository never calls a service. Cross-layer calls flow through narrow interfaces.
Why layers earn their keep:
- business rules live in ONE place (the service), not scattered in UI and SQL
- each layer tests differently: fast unit tests for services, contract tests for repositories
- swapping delivery (CLI → REST) doesn't touch the service
The failure mode this prevents is the god class — 1,200 lines that parse, validate, compute, persist, and print, none of it testable in isolation.