A deep technical guide to designing a robust, provider-agnostic billing pipeline for usage-based SaaS. Covers subscription state machines, idempotent event processing, automated dunning logic, credit notes, and metered invoice generation with full SQL schemas, Mermaid diagrams, and Go snippets.

A complete system design of Relay, a multi-tenant AI API gateway. Covers multi-tenant database architecture (silo vs pool vs bridge), API key authentication, provider routing with failover, dual-layer rate limiting, token-based billing, response caching, row-level security, tenant provisioning, and observability. Full Postgres schemas, mermaid diagrams, and Go snippets included.