Usage-Based API Platform

An end-to-end study of how modern API products are built, metered, and billed. A FastAPI service issues and verifies API keys, Redis holds the hot-path rate-limit and usage counters, and a durable ledger backs the invoice so the fast path can fail without the money being wrong. A Next.js console surfaces per-key reporting, idempotency keys guard every write that touches money, and Stripe handles subscriptions. The goal was to make every operational concern — rate limits, retries, reconciliation — visible rather than hidden.

More detailed notes — architecture, decisions, tradeoffs, and outcomes — get published here as the project matures, alongside source code and live deployments.