Problem: Define AsOf / AsAt

[ylorph]

[MerrionComputing]

(Often used in accounting) These terms indicate that the state being returned is what it was at a particular point in time. In event sourcing this can be returned by running the projection up to that point.

This is one business domain way of avoiding concurrency issues because the data being returned is acknowledged to be out of date. We often see account balances "as at midnight of the previous day" and prices "as at prior close of the exchange".

Sometimes you also have bitemporal systems when you record two dates about any given event: when it actually occurred (as at) and when it became known to our system (knowledge date). This allows for projections of "what did we think this value was as at x" and "what was it actually as at x".

(Unfortunately the exact terms used can vary between industries and companies so this needs to be defined in the ubiquitous language)