Rate Limiter Module

RateLimiter.java

/*
Rate Limiter Interface 
*/
package io.aneessh18.postmaninterview;

public interface RateLimiter {
    boolean isAllowed(Request request);
}


/*
Request Model
*/

package io.aneessh18.postmaninterview;

public class Request {
    public long time;
    public String msg;
    public String userId;
}


/*
Rest API class 
*/

package io.aneessh18.postmaninterview;

import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;


@RestController
@RequestMapping("/v1/api")
public class Controller {

    private final RateLimiter rateLimiter  = new SlidingWindowRateLimiter(10, 10000);
    private final RateLimiter userIdRateLimiter = new UserIdSlidingWindowRateLimiter(10, 10000);

    @GetMapping("/echo-path/{path}")
    public ResponseEntity<String> echo(@PathVariable("path") String path,@RequestParam(value = "param", required = false) String param){
        Request request = new Request();
        request.msg = "msg";
        if (rateLimiter.isAllowed(request)){
            return ResponseEntity.ok(String.format("Request to path %s with %s param", path, param));

        }else{
            return ResponseEntity.status(429).build();
        }
    }

    @GetMapping("/echo-user/{user}")
    public ResponseEntity<String> echoUser(@PathVariable("user") String userId){
        Request request = new Request();
        request.msg = "msg";
        request.userId=userId;
        if (userIdRateLimiter.isAllowed(request)){
            return ResponseEntity.ok(String.format("Request to path %s", userId));

        }else{
            return ResponseEntity.status(429).build();
        }
    }

}

/*
Sliding Window Rate Limiter 
*/ 

package io.aneessh18.postmaninterview;

import java.util.ArrayDeque;
import java.util.Queue;


public class SlidingWindowRateLimiter implements RateLimiter {
    private final int numRequests;
    private final int window;

    private final Queue<Request> queue = new ArrayDeque<>();
    public SlidingWindowRateLimiter(int numRequests, int window) {
        this.numRequests = numRequests;
        this.window = window;
    }
    @Override
    public boolean isAllowed(Request request){
        long time = System.currentTimeMillis();
        // prune the queue
        // list -> time-window
        if(!queue.isEmpty() && queue.peek().time+window<=time){
            queue.poll();
        }
        if(queue.size()< numRequests){
            request.time=time;
            queue.add(request);
            return true;
        } else{
            return false;
        }
    }
}

/*
UserIdSlidingWindowRateLimiter
*/

package io.aneessh18.postmaninterview;

import java.util.ArrayDeque;
import java.util.HashMap;
import java.util.Queue;

public class UserIdSlidingWindowRateLimiter implements RateLimiter{
    private final int numRequests;
    private final int window;

    private final HashMap<String, Queue<Request>> userQueue;
    public UserIdSlidingWindowRateLimiter(int numRequests, int window) {
        this.numRequests = numRequests;
        this.window = window;
        this.userQueue = new HashMap<>();
    }
    @Override
    public boolean isAllowed(Request request){
        String userId = request.userId;
        userQueue.putIfAbsent(userId, new ArrayDeque<>());
        Queue<Request> queue = userQueue.get(userId);

        long time = System.currentTimeMillis();
        // prune the queue
        // list -> time-window
        if(!queue.isEmpty() && queue.peek().time+window<=time){
            queue.poll();
        }
        if(queue.size()< numRequests){
            request.time=time;
            queue.add(request);
            return true;
        } else{
            return false;
        }
    }
}