idempotency

A stream is a long-lived connection to a GPU slot. Slow readers, dropped connections, half-finished answers and duplicate retries each waste that slot in a different way, and each has a specific defense.

Billing is the one pipeline where a dropped event is money and a duplicated one is a support ticket. Where tokens are counted and by whom, the usage event and its idempotency key, the aggregation that closes a month, the reconciliation that proves the ledger against the engines, and the six ways it goes wrong.

Exponential backoff spaces one client's retries and does nothing for a thousand clients that failed together, which is the case that matters. The measured difference jitter makes to the peak arrival rate, the two budgets that bound the damage, and the classification of what is safe to retry at all.

Four resources, one state machine and three semantics that separate a working API from one that corrupts state under retries. The idempotency key that makes a duplicate POST safe, why cancel returns 202 rather than 200, and the cursor that survives concurrent inserts.