Under the Hood

Fable, Deconstructed

A shipped production app's real architecture — the TDDs, the trade-offs, and what changed once it met actual users.

Twelve chapters, one shipped group-expenses app. Every curriculum topic cross-links here so the theory always lands on a real decision, a real trade-off, and — often — a real production incident.

  1. 01Requirements & constraints
  2. 02High-level architecture
  3. 03Data model
  4. 04Authentication
  5. 05Real-time chat
  6. 06Expense engine
  7. 07Settlement flow
  8. 08Media pipeline
  9. 09Push notifications
  10. 10Infra evolution
  11. 11Security, privacy & observability
  12. 12Scaling story & retrospective

Requirements & constraints: what Fable had to be

Before any architecture, the forces that shaped every later decision. Fable is a group-expenses app, which sounds simple until you write down what 'simple' actually demands: money that's exactly right forever, a mobile client on flaky networks that you can't hot-patch, one developer's time and a near-zero budget, and real users who split dinners on trains. This opening chapter states the functional and non-functional requirements and the hard constraints — and shows how each one is the reason a later chapter chose what it chose.