What must always be true in your app, and how do you keep it that way?
- Module
- 01/05
- To read
- ~6 min
A payment integration assumed every amount was in US dollars. Months later, the payment API started accepting euros too. What do you think happened on the first European order?
In the module you make your guess first, then read what happened and why.
Pre-conditions: What must be true before execution
Post-conditions: What must be true after execution
Invariants: What must be true before and after every operation
Schema contracts at every integration boundary
The rules for one connection in your app
A spec for your own app, as a text file you download. The module also gives you the prompt for Claude Code, Codex or Antigravity, a checklist to check what it built, and recommended skills.
- API gateways
- Invariants in the database
- Observability