rate limiting

Fixed Window Counter

Divide time into fixed-size windows with a counter per window

Window Counter0 / 5
Window Progress95%
Allowed0
Rejected0
5 req / 4s window
W13[0/5]W14[0/5]W15[0/5]

⚠ boundary problem

Try sending 5 requests at the end of one window and 5 more at the start of the next. You'll get 10 requests through in a very short burst — double the limit! This is the fundamental flaw of fixed windows.

// how it works

Time is divided into fixed windows (e.g. 1 minute). Each window has a counter.

  • New request arrives → find current window
  • If counter < limit → allow, increment counter
  • If counter ≥ limit → reject
  • Window expires → counter resets to 0

// trade-offs

  • O(1) time and space — extremely simple
  • Low memory: only 1 counter per key
  • Boundary problem: 2× burst at window edges
  • Not suitable when smooth rate enforcement is critical