Event-Driven Billing System

Invoices and payments modelled as events, designed so a retry never charges anyone twice.

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The problem

Billing is the one place where "at least once" is not good enough. A retried payment event must never become a second charge.

An event log of invoice and payment events, with idempotent consumers and a read model rebuilt from the log.

The first read model assumed events arrive in order. Partition rebalances proved otherwise within a day of load testing.

More builds

I am happy to talk about the boring parts — design, failure modes, and the places I would not trust the first version.