Rate limiting restricts how many requests a client, identified by IP address, API key or account, may make within a period. Requests over the limit are delayed or rejected, typically with 429 Too Many Requests.
Why it matters
It protects expensive endpoints such as login, search and APIs from brute-force attempts, scraping and runaway clients, and keeps one heavy user from degrading service for everyone. It also helps absorb small application-layer floods, though it is not a full DDoS defense.
Example
An nginx limit of 10 requests per second per IP, with a short burst allowed:
limit_req_zone $binary_remote_addr zone=api:10m rate=10r/s;
server {
location /api/ {
limit_req zone=api burst=20 nodelay;
limit_req_status 429;
}
}By default nginx returns 503 for limited requests, so limit_req_status 429 is needed for the standard code. Well-behaved APIs also send Retry-After: 30 so clients know when to retry. Our own free API, for example, has a daily request quota described in the API docs.
Common mistakes
- Limiting by IP behind a proxy or CDN without restoring the real client IP, so everyone shares one bucket.
- Rate limiting your own uptime monitor and triggering false outage alerts. Allowlist it.
- Returning 503 or 200 with an error body instead of 429, which breaks client backoff logic.
- Applying one global limit instead of stricter limits on login and password-reset endpoints.
Related
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