Cost and billing
Usage event
What is usage event?
A usage event is an immutable record of consumption — what was used, when, by whom, and under which price revision. It is the atom of a billing system and the thing an invoice must be reconstructible from.
Recording the pricing revision on the event is what makes historical invoices reproducible after prices change. Without it, recomputing an old invoice silently applies today’s rates.
Events should be immutable and append-only. Correcting a mistake means adding a compensating event, not editing history, so that the record of what happened survives the correction.
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