The field guide · rule 01 of 16
Keep each fact in one place
Decide where each important fact lives, and let everything else read it from there.
What it means
Before building anything, decide what the system stores, who owns each fact, and where the one true copy lives. If the same fact is kept in two places, sooner or later the two copies disagree, and nobody knows which one is right.
What goes wrong without it
A customer's balance shows one figure in the app and another in the payout table. Support cannot answer, and the books cannot be trusted.
How I build it
- The spending limit in my AI agent system reads the same usage record that the meter writes, so there is no second spending figure to drift.
- The Space.ke assistant takes its facts from the same data the site's pages use, so the chat cannot contradict the pages.
- Prices in my chain-intelligence service live in one table the admin can edit, with safe defaults in the code if that row is missing.
Ask your team
- For each important fact, can you name the one record that counts?
- If two records disagree, does anything warn you?
- Could a new developer find where a price or a balance is stored within an hour?
How a review checks this
Every review starts with a map of where each important fact lives, and marks any fact kept in two places.
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