Backfill

Reprocessing history through a stage the records predate.

Why it matters in a memory layer

It should be optional, which requires the new field to have an honest fallback. On this course old records answer as-of queries with happened_at — when this was asserted rather than when it was true — so a store that never backfills is imprecise rather than broken, and the migration is one phase instead of two.

It should be restartable, and the cheapest way is determinism rather than checkpoints: the parser derives its value from the record's own content, so a second pass computes the same answer and changes nothing. The same property consolidation needed to be safely re-run after a crash.

And its value is not coverage. This one updates 4 of 37 records — precisely the relative-time phrases whose event time differs from the instant they were said. The other 33 were already answering correctly.

Connections

Taught in: Migrating Live Memory

Do not confuse with: Shape Change