How Rate Limiting Shields APIs From Abuse, Overload, and Traffic Spikes

Rate limiting is a mechanism that controls how many requests a client can make to an API within a defined time window, typically returning an HTTP 429 error when the limit is exceeded. Without such controls, a sudden surge of requests — whether from a viral launch, a buggy client, or a deliberate attack — can overload servers, spike costs, and cause outages for legitimate users. Different endpoints can carry different limits, with security-sensitive routes like login or password reset set far stricter than general browsing endpoints. Several algorithms are commonly used to enforce these limits, including Fixed Window, Sliding Window, Token Bucket, and Leaky Bucket, each with its own trade-offs in complexity and accuracy. Rate limiting is also distinct from throttling, where the former restricts how much traffic is allowed while the latter controls the speed at which requests are processed.