What are rate limiting and throttling?

Rate limiting is the practice of restricting the number of requests a client can make to an API in a given time window (e.g., 100 requests per minute). It's commonly used to prevent abuse, ensure fair usage, and protect backend systems.

Throttling, while often used interchangeably with rate limiting, typically refers to delaying or slowing down requests once a client hits a certain threshold, instead of outright rejecting them.

How do they work?

Rate limiting

  • Uses counters or tokens to track how many requests a client has made.
  • If the client exceeds the allowed limit, the server responds with a 429 Too Many Requests status.
  • Limits can be set per IP, user, token, or API key.

Throttling

  • Instead of rejecting, it queues or delays requests.
  • Useful for smoothing traffic bursts and avoiding resource spikes.
  • Can be applied dynamically based on server load or user behavior.

Basic example (Express.js)

const rateLimit = require("express-rate-limit");

const limiter = rateLimit({
  windowMs: 1 * 60 * 1000, // 1 minute
  max: 100, // limit each IP to 100 requests per windowMs
  message: "Too many requests, please try again later.",
});

app.use("/api/", limiter);

Why use rate limiting and throttling?

BenefitExplanation
Protect backendPrevents overload or abuse from malicious users/bots
Ensure fair usageStops one user from monopolizing server resources
Improve reliabilityHelps maintain consistent performance under high load
Enhance securityMitigates brute-force and denial-of-service attacks

Important notes

  • Always return appropriate headers like:
  • X-RateLimit-Limit
  • X-RateLimit-Remaining
  • Retry-After
  • Rate limiting applies to both public and private APIs.
  • Throttling may still let requests through—it's better for user experience in softer restrictions.
  • Use distributed rate limiting (e.g., Redis-based) if your API is running on multiple servers.

Best practices

  • Set sensible default limits (e.g., per IP or token).
  • Provide clear error messages and headers for clients to handle limits gracefully.
  • Use exponential backoff or retries in your client apps.
  • Combine with authentication to set tiered limits (e.g., higher limits for premium users).
  • Use tools like:
  • express-rate-limit (Node.js)
  • NGINX rate limiting modules
  • Cloudflare / API Gateway built-in protections

Conclusion

Rate limiting and throttling are essential techniques for securing and scaling APIs. By controlling traffic flow and preventing abuse, you can maintain service availability and performance even under heavy usage. Implementing these strategies is a key part of building reliable, production-ready APIs.