Java Fixed window Rate limit Algorithm

Fixed Window Algorithm:

The Fixed Window Algorithm is the simplest rate limiting algorithm. It limits the rate by counting the number of requests made within a fixed time window. If the number of requests exceeds the limit, then any subsequent requests are blocked until the start of the next window. For example, if the limit is set to 10 requests per second, then the algorithm counts the number of requests made in the last second and blocks any additional requests until the start of the next second.

Java Code for Fixed Window Algorithm:

import java.time.Instant;
import java.util.ArrayDeque;
import java.util.Queue;

public class FixedWindowRateLimiter {
    private final int limit; // Maximum number of requests allowed within the time window
    private final int windowSeconds; // Duration of the time window in seconds
    private final Queue<Instant> requestQueue; // Queue to store request timestamps

    public FixedWindowRateLimiter(int limit, int windowSeconds) {
        this.limit = limit;
        this.windowSeconds = windowSeconds;
        this.requestQueue = new ArrayDeque<>();

    public synchronized boolean allowRequest() {
        clearExpiredRequests(); // Clear expired requests from the queue

        if (requestQueue.size() < limit) {
            requestQueue.offer(; // Add current request timestamp to the queue
            return true; // Request is allowed
        } else {
            return false; // Request is not allowed

    private void clearExpiredRequests() {
        Instant now =;
        Instant windowStart = now.minusSeconds(windowSeconds);

        while (!requestQueue.isEmpty() && requestQueue.peek().isBefore(windowStart)) {
            requestQueue.poll(); // Remove expired requests from the queue

In the above code, we have a FixedWindowRateLimiter class that implements the Fixed Window Algorithm for rate limiting. It takes the maximum number of requests allowed within a time window (limit) and the duration of the time window in seconds (windowSeconds) as parameters in the constructor.

The allowRequest() method is used to check if a request is allowed or not. It synchronizes the method to ensure thread safety.

The clearExpiredRequests() method is called before checking the request queue for the number of requests. It clears any expired requests from the queue based on the current time and the window duration. The expired requests are removed from the queue to keep track of only the requests within the defined time window.

You can create an instance of FixedWindowRateLimiter and use the allowRequest() method to control the rate at which requests are allowed within the fixed window.

It’s important to note that the Fixed Window Algorithm resets the request count at the start of each time window. This means that the rate limit is enforced strictly within each window, but it doesn’t take into account the distribution of requests within the window.