API Rate Limiting Explained: Top 4 Algorithms for System Scalability and Protection

Introduction

API Rate Limiting is a defensive mechanism that controls the number of requests a user or client can make to an API over a specific period. This prevents API abuse, protects against DoS/DDoS attacks, and ensures fair resource allocation. The effectiveness of your rate limit depends on the underlying algorithm you choose.

The Top 4 Rate Limiting Algorithms

1. Fixed Window Counter ⏱️

• **How it Works:** Divides time into fixed, non-overlapping intervals (e.g., 60 seconds). A counter increments with each request. Once the limit is hit, all subsequent requests until the end of the window are rejected.
• **Pro:** Simple and easy to implement.
• **Con (The "Burst" Problem):** Allows a double-capacity burst of requests near the window edges (e.g., limit of 100/min. A client can send 100 requests at 0:59 and 100 more at 1:00).

2. Sliding Log 📜

• **How it Works:** For each client, the Gateway keeps a timestamped log of all successful requests. For a new request, it counts all timestamps within the last *X* seconds (the window). If the count exceeds the limit, the request is denied.
• **Pro:** Extremely accurate and avoids the Fixed Window burst problem.
• **Con:** High memory consumption, as every request's timestamp must be stored and queried. Not suitable for massive scale.

3. Sliding Window Counter (Hybrid) 📊

• **How it Works:** A hybrid approach using two fixed windows (the current and the previous). It smoothly estimates the request count using a weighted average. For example, if 70% of the previous window has passed, it credits the current window with 30% of the previous count.
• **Pro:** Good compromise between accuracy and memory efficiency. Smoother rate limiting than the Fixed Window.
• **Con:** Still an approximation, which can slightly over- or under-limit the traffic.

4. Token Bucket (Best for Burst Tolerance) 🪙

• **How it Works:** A virtual bucket holds a fixed number of "tokens." Requests consume one token, and tokens are added to the bucket at a fixed refill rate. If the bucket is empty, the request is denied.
• **Pro:** Allows for controlled bursts (up to the size of the bucket) while limiting the long-term average rate to the refill rate. Excellent for managing short, spikey loads.
• **Con:** Requires tuning two parameters: bucket size and refill rate.

Conclusion

For most high-traffic API Gateways, the **Token Bucket** and **Sliding Window Counter** offer the best balance of performance, accuracy, and resource efficiency. Choose the algorithm that best aligns with your application's tolerance for bursts and your acceptable level of memory usage.