fixed-window-counter

Fixed Window Counter

• Overview
• Pros
• Cons

Overview

• The algorithm divides the timeline into fix-sized time windows and assign a counter for each window. Each request increments the counter by one. Once the counter reaches the pre-defined treshold, new requests are dropped until a new time window starts.

Pros

• Memory efficient.

• Easy to understand.

• Resetting available quota at the end of a unit time window fits certain use cases.

Cons

Spike in traffic at the edghes of a window could cause more requests than the allowed quota to go through.

In this example, the system allows a maximum of 5 requests per minute, and the available quota resets at the human-friendly round minute. As seen, there are five requests between 2:00:00 and 2:01:00 and five more requests between 2:01:00 and 2:02:00. For the one-minute window between 2:00:30 and 2:01:30, 10 requests go through. That is twice as many as allowed.