Rate Limiter

Whenever you expose a web service / api endpoint, you need to implement a rate limiter to prevent abuse of the service (DOS attacks).

Implement a RateLimiter Class with an isAllow method. Every request comes in with a unique clientID, deny a request if that client has made more than 100 requests in the past second.

public class RateLimiter {
    private final Map<String, LinkedList<Long>> clientHitMap = new HashMap<>();
    private final int REQUEST_LIMIT = 3;
    private final long TIME_LIMIT = 1000L;

    public boolean isAllow(String client_id) {
        LinkedList<Long> list = clientHitMap.get(client_id);
        long curTime = System.currentTimeMillis();
        if (list == null) {
            clientHitMap.put(client_id, new LinkedList<Long>());
        }
        while (!list.isEmpty() && curTime - list.peek() >= TIME_LIMIT) {
            list.poll();
        }
        if (list.size() < REQUEST_LIMIT) {
            list.offer(curTime);
            return true;
        }
        return false;
    }
}