Tombstone

A marker that a record still exists but must not be served — kept in the index rather than removed from it.

Why it matters in a memory layer

Deleting a retired memory's vector would make audit and as-of queries impossible, and the course's whole spine is supersede-never-destroy. But leaving it silently retrievable is worse: the vector is still an accurate embedding of text that is no longer believed. A tombstone separates is this a correct vector from should anyone see this, which are different questions that a content-addressed cache cannot tell apart on its own. The same shape appears in eviction, where a demoted memory stays in the log and out of default retrieval.

Connections

Taught in: Vector Stores for Data That Changes

Do not confuse with: Supersession