Fixed Window Counter

1. One-Liner

The Fixed Window Counter divides time into fixed-size windows (e.g., 1 minute) and limits how many requests are allowed per window. When the window changes, the counter resets.

2. The Problem It Solves

Rate limiting protects APIs from abuse. Fixed Window is one of the simplest approaches: count requests in the current time bucket. When the bucket (window) expires, start fresh.

3. The Core Idea

Imagine a whiteboard that gets wiped every minute:

You can write up to 5 tally marks per minute.

When the clock hits the next minute, someone erases the board.

You start counting from zero again.

That's Fixed Window: fixed time chunks, count per chunk, reset on boundary.

4. How It Works

Step	What Happens
1. Get time	Get current timestamp
2. Calc window	window_id = now / window_size
3. Check window	Did we cross into a new window?
4. Reset	If new window, set count = 0
5. Check limit	Is count >= limit?
6a. YES → Deny	Window full, reject request
6b. NO → Allow	Increment count, allow request

5. Key Properties

Property	Value
Edge burst?	Yes — 2x burst possible at window boundary
Memory	O(1) — window_id + count
Simplicity	Very simple to implement
Accuracy	Lower than sliding window

Implementation Example (PYTHON)

import time

class FixedWindowCounter:
    def __init__(self, window_size, limit):
        self.window_size = window_size
        self.limit = limit
        self.current_window = 0
        self.count = 0

    def try_allow(self):
        now = time.monotonic()
        window_id = int(now // self.window_size)
        if window_id != self.current_window:
            self.current_window = window_id
            self.count = 0
        if self.count >= self.limit:
            return False   # DENY
        self.count += 1
        return True    # ALLOW