Junior
Hold each API key to its request limit, and tell clients when to slow down.
Build the rate limiter for a public API. Customers' apps call the API with an API key. Last month one customer's runaway script sent thousands of requests a second and slowed the API down for everyone else. Each key should be held to its own limit, and a client that goes over it should be told to slow down and when it can try again, rather than getting errors or timeouts.
Each plan comes with its own limit, and support sometimes raises or lowers the limit for a single customer.