As-Of Query

A read that names its own point in time: what was true at a date, optionally as the store saw things at another date.

Why it matters in a memory layer

Two different questions hide behind "what did you know in June?". as_of(when) gives today's account of that day, corrections included. as_of(when, believed_at_time=t) gives the account the store would have given at t, mistakes and all. The second is what an audit asks for, and a system that only supports the first cannot show that it was ever wrong.

Without either, a question about the past is answered by whatever filter the read path already has. On this course's corpus that returns four facts about a job the user had not yet been offered — no layer misbehaving, every layer answering a different question correctly.

Connections

Taught in: Validity Intervals

Used in: Three Temporal Questions · Why Memory Eval Is Hard

Do not confuse with: Staleness