Fixed Window Rate Limiter

Pros:

Simplicity: Easy to implement and understand.

Performance: Efficient under low to moderate load due to minimal computational overhead.

Cons:

Burst Traffic Problem: If a burst of requests occurs near the end of one window and the beginning of the next, it can double the allowed limit. For example, if the limit is 10 requests per second, 10 requests at the end of one window and 10 at the start of the next could allow 20 requests in a short time.

import java.time.Instant;
import java.util.concurrent.atomic.AtomicInteger;

public class FixedWindowRateLimiter {
    private final int maxRequests; // Maximum requests allowed per window
    private final long windowDurationMillis; // Duration of each window in milliseconds
    private AtomicInteger requestCount; // Count of requests in the current window
    private long windowStartTime; // Start time of the current window

    public FixedWindowRateLimiter(int maxRequests, long windowDurationMillis) {
        this.maxRequests = maxRequests;
        this.windowDurationMillis = windowDurationMillis;
        this.requestCount = new AtomicInteger(0);
        this.windowStartTime = Instant.now().toEpochMilli();
    }

    public synchronized boolean isAllowed() {
        long currentTime = Instant.now().toEpochMilli();
        // Check if the current time is outside the current window
        if (currentTime - windowStartTime >= windowDurationMillis) {
            // Reset the window
            windowStartTime = currentTime;
            requestCount.set(0);
        }
        return requestCount.incrementAndGet() <= maxRequests;
    }

    public static void main(String[] args) throws InterruptedException {
        // Example: Allow up to 5 requests per 10 seconds window
        FixedWindowRateLimiter rateLimiter = new FixedWindowRateLimiter(5, 10000);
        // Simulate 22 incoming requests
        for (int i = 0; i < 22; i++) {
            Instant now = Instant.now();
            if (rateLimiter.isAllowed()) {
                System.out.println("Request " + (i + 1) + ": Allowed at "+ now);
            } else {
                System.out.println("Request " + (i + 1) + ": Throttled at "+ now);
            }
            Thread.sleep(1000); // Sleep for 1 second between requests
        }
    }
}

Execution Output :

Request 1: Allowed at 2024-11-09T02:18:34.391914600Z
Request 2: Allowed at 2024-11-09T02:18:35.437279700Z
Request 3: Allowed at 2024-11-09T02:18:36.448884Z
Request 4: Allowed at 2024-11-09T02:18:37.449886600Z
Request 5: Allowed at 2024-11-09T02:18:38.452924Z
Request 6: Throttled at 2024-11-09T02:18:39.456711400Z
Request 7: Throttled at 2024-11-09T02:18:40.464231600Z
Request 8: Throttled at 2024-11-09T02:18:41.478928700Z
Request 9: Throttled at 2024-11-09T02:18:42.494495800Z
Request 10: Throttled at 2024-11-09T02:18:43.508222600Z
Request 11: Allowed at 2024-11-09T02:18:44.523985700Z
Request 12: Allowed at 2024-11-09T02:18:45.526747900Z
Request 13: Allowed at 2024-11-09T02:18:46.541044400Z
Request 14: Allowed at 2024-11-09T02:18:47.544245700Z
Request 15: Allowed at 2024-11-09T02:18:48.545852600Z
Request 16: Throttled at 2024-11-09T02:18:49.561031500Z
Request 17: Throttled at 2024-11-09T02:18:50.563952100Z
Request 18: Throttled at 2024-11-09T02:18:51.566120200Z
Request 19: Throttled at 2024-11-09T02:18:52.567776600Z
Request 20: Throttled at 2024-11-09T02:18:53.571288700Z
Request 21: Allowed at 2024-11-09T02:18:54.576001300Z
Request 22: Allowed at 2024-11-09T02:18:55.588448300Z

Simulating random delay between requests : Thread.sleep((long)(Math.random() * 3500));

Sleep randomly between 0 to 3.5 second between requests

Request 1: Allowed at 2024-11-09T01:48:54.155444200Z
Request 2: Allowed at 2024-11-09T01:48:56.627097500Z
Request 3: Allowed at 2024-11-09T01:48:59.404685200Z
Request 4: Allowed at 2024-11-09T01:48:59.560314200Z
Request 5: Allowed at 2024-11-09T01:49:02.487695700Z
Request 6: Throttled at 2024-11-09T01:49:03.102031400Z
Request 7: Throttled at 2024-11-09T01:49:04.114203700Z
Request 8: Allowed at 2024-11-09T01:49:05.489638Z
Request 9: Allowed at 2024-11-09T01:49:07.868572500Z
Request 10: Allowed at 2024-11-09T01:49:08.069096400Z
Request 11: Allowed at 2024-11-09T01:49:09.860865Z
Request 12: Allowed at 2024-11-09T01:49:13.226450800Z
Request 13: Allowed at 2024-11-09T01:49:16.537556200Z
Request 14: Allowed at 2024-11-09T01:49:19.958231400Z
Request 15: Allowed at 2024-11-09T01:49:22.875345400Z
Request 16: Allowed at 2024-11-09T01:49:24.022313200Z
Request 17: Allowed at 2024-11-09T01:49:26.143643200Z
Request 18: Allowed at 2024-11-09T01:49:29.561869100Z
Request 19: Allowed at 2024-11-09T01:49:30.477400500Z
Request 20: Allowed at 2024-11-09T01:49:31.424657900Z
Request 21: Allowed at 2024-11-09T01:49:33.937672600Z
Request 22: Allowed at 2024-11-09T01:49:36.796995200Z

References :

https://www.rdiachenko.com/posts/arch/rate-limiting/fixed-window-algorithm

https://medium.com/@devenchan/implementing-rate-limiting-in-java-from-scratch-fixed-window-and-sliding-window-implementation-a6e8d6407d17