Public API. Limits and quotas

An API for developers with Free and Pro tiers: a core with headroom within budget, and when a Free key starts downloading the whole database, deciding whom to limit and whom to pay for.

Diagram of a public API: a gateway lets the Pro lane flow freely and meters the Free lane, throttling one key that floods it, so the core keeps headroom.

System Design — Load balancers, caches, queues and regions: build a system that holds the load