Keeping one source of truth REQUIRED BACKGROUND: the skill. Overview Every fact about the system lives in exactly one place, and every other part of the system reads it from there. This outranks convenience. The doctrine 1. Before adding data, find who already owns it. Extend that owner; do not start a rival. 2. Derive rather than store. If the platform or an existing source can answer it at read time, read it there; do not copy the answer into a second source where it can go stale. 3. Absorb duplicates you find on the way. When you touch code that hardcodes what a file already knows (or the…