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 & constraintsComing soon
  2. 02High-level architectureComing soon
  3. 03Data model
  4. 04AuthenticationComing soon
  5. 05Real-time chatComing soon
  6. 06Expense engineComing soon
  7. 07Settlement flowComing soon
  8. 08Media pipelineComing soon
  9. 09Push notificationsComing soon
  10. 10Infra evolution
  11. 11Security, privacy & observabilityComing soon
  12. 12Scaling story & retrospectiveComing soon

Data model: money you can trust and ids you can sort

How Fable's schema is built around two decisions most apps get wrong — never storing money as a float, and never using an id you can't generate on the client. This chapter walks the real Prisma schema: paisa-as-bigint with a currency-branded Minor type, ULIDs over auto-increment and UUIDv4, splits that always sum to the total, and the idempotency-key unique constraint that makes a retried payment write safe. Then the drift: 33 tables planned, 37 shipped, and a session-lookup column added after cold-start refresh got slow.