Start here. This is the direct spoken answer to practice first.
Why this question matters
Design and debug rate limiting while protecting the API without blocking legitimate users and integrations. It matters in real backend work because the team needs observability, ownership, and rollback room to recover without guessing. The practical angle is abuse protection, client fairness, rollout safety, distributed enforcement, and operational behavior, tied to a concrete production decision.
Rate limiting protects the API by limiting how many requests a caller can make in a time window. I would choose the key carefully: user, tenant, client app, API key, or IP depending on the endpoint. The API should return 429 Too Many Requests with useful retry information. The goal is to reduce abuse or overload without punishing normal clients.