Fixed window counter algorithm works as follows:

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 threshold, new requests are dropped until a new time window starts.

Let us use a concrete example to see how it works. In Figure 8, the time unit is 1 second and the system allows a maximum of 3 requests per second. In each second window, if more than 3 requests are received, extra requests are dropped as shown in Figure 8.

Figure 8

Figure 8

A major problem with this algorithm is that a burst of traffic at the edges of time windows could cause more requests than allowed quota to go through. Consider the following case:

Figure 9

Figure 9

In Figure 9, 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 requests.

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 edges of a window could cause more requests than the allowed quota to go through.