Day 34 - FastAPI Clean Architecture

Date: 2025-10-23 (Thursday)
Status: “Done”

Lecture Notes

Clean Architecture Overview

  • Separate config, models, routes, and core logic to simplify scaling.
  • Keep main.py lightweight—only boot the app, load config, and mount routers.
  • Use Pydantic models to standardize request/response shapes so the contract matches OpenAPI.
backend/
├── main.py
├── core/
│   └── config.py
├── models/
│   └── book.py
├── routes/
│   └── books.py
└── services/
    └── books.py

Configuration & Dependencies

  • core/config.py reads environment variables and centralizes CORS, API prefixes, and debug flags.
  • Use FastAPI’s dependency injection to pass the service layer into routers.
  • Enables swapping data sources (in-memory → PostgreSQL) without changing function contracts.

CORS & API Stability

  • Restrict CORS to required origins (http://localhost:3000 during development).
  • Allow only GET for the first slice to shrink the attack surface.
  • Keep /openapi.json accessible so contract-testing tools can pull the spec.

Start Simple, Refactor Later

  • Begin with an in-memory repository for fast demos, then add a real database.
  • Document TODOs clearly to avoid losing them between sprints.
  • Keep logging minimal and focus on critical faults (timeouts, data mismatches).

Hands-On Labs

  • Refactor main.py so it only handles app bootstrapping and router registration.
  • Build the get_book_detail(id) service layer using spec-driven fake data.
  • Configure CORSMiddleware to match the frontend mock and production URLs.