Store Invariant

Something that must always be true of the store, checked continuously rather than at deploy time.

Why it matters in a memory layer

An index has few invariants worth asserting because it is derived — if it disagrees with the corpus, rebuild it, and drift is an operational nuisance. A memory store is the truth, so a violated invariant is not a stale derivation; it is data that is now wrong and will stay wrong.

Label them by kind, not severity. Structural means only a bug produces this — a cross-tenant memory visible, a belief retired before it was recorded, a dangling reference. Policy means unusual data produces this — a slot with more live beliefs than expected. Reported together unlabelled, a real bug and a heavy user look identical.

And test the checker. Running this course's seven against an earlier profile produces exactly one failure with a known cause — the belief retired nine months before it was recorded — which is the cheapest way to find out the invariants work at all.

Connections

Taught in: Invariants and Drift Detection

Used in: Hardening Pass

Do not confuse with: Leave-One-Out