Snapshot Isolation

A job records which ids it read and is allowed an opinion about exactly those; everything else survives its write-back untouched.

Why it matters in a memory layer

The recorded id set is the whole mechanism, because consolidation legitimately removes memories. A merged duplicate is absent from the output on purpose, and a memory that arrived after the read is absent because the job never saw it — the two are indistinguishable in the output alone. A write-back that infers intent from absence treats both as deletions, which is exactly how replace destroys data.

With the id set, the merge is not merely non-destructive but correct: racing a job against every fourth turn yields a store identical to the serialised run. It is also replayable, because scoped write-back plus an already-idempotent consolidation means a crashed job can simply be run again.

A lock would also work and would block the conversation for a full pass over the store — the cost that moving the work off the turn was meant to avoid.

Connections

Taught in: Background Job Mechanics

Used in: Promotion as a Release

Do not confuse with: Lost Update