Exactly-Once Semantics

1. The Incident

The store meters partner API usage and bills for it: a streaming job counts each partner's calls per day and feeds invoices. It's the most money-shaped pipeline in the company, and it has been quietly wrong in both directions.

Month one: overbilling. A routine crash-and-rewind (the stream lesson's ordinary recovery) reprocessed forty minutes of events: and the running counts absorbed them twice. Partner A was billed for 1,041 calls; their own logs show 987. They audited