Simulation Simplified
16. A Web Server
Class WebServer01

import edu.rit.numeric.ExponentialPrng;
import edu.rit.sim.Event;
import edu.rit.sim.Simulation;
import edu.rit.util.Random;
import java.util.LinkedList;
public class WebServer01
   {
   private static double lambda;
   private static double mu;
   private static int R;
   private static long seed;
   private static Random prng;
   private static ExponentialPrng requestPrng;
   private static ExponentialPrng serverPrng;
   private static LinkedList<Request> requestQueue;
   private static int requestCount;
   private static Simulation sim;

   public static void main
      (String[] args)
      {
      if (args.length != 4) usage();
      lambda = Double.parseDouble (args[0]);
      mu = Double.parseDouble (args[1]);
      R = Integer.parseInt (args[2]);
      seed = Long.parseLong (args[3]);
      prng = Random.getInstance (seed);
      requestPrng = new ExponentialPrng (prng, lambda);
      serverPrng = new ExponentialPrng (prng, mu);
      requestQueue = new LinkedList<Request>();
      requestCount = 0;
      sim = new Simulation();
      generateRequest();
      sim.run();
      }

   private static class Request
      {
      private int requestNumber;
      public Request
         (int requestNumber)
         {
         this.requestNumber = requestNumber;
         }
      public String toString()
         {
         return "Request " + requestNumber;
         }
      }

   private static void generateRequest()
      {
      addToQueue (new Request (++ requestCount));
      if (requestCount < R)
         {
         sim.doAfter (requestPrng.next(), new Event()
            {
            public void perform() { generateRequest(); }
            });
         }
      }

   private static void addToQueue
      (Request request)
      {
      System.out.printf ("%.3f %s added to queue%n",
         sim.time(), request);
      requestQueue.add (request);
      if (requestQueue.size() == 1) startServing();
      }

   private static void startServing()
      {
      System.out.printf ("%.3f Started serving %s%n",
         sim.time(), requestQueue.getFirst());
      sim.doAfter (serverPrng.next(), new Event()
         {
         public void perform() { removeFromQueue(); }
         });
      }

   private static void removeFromQueue()
      {
      System.out.printf ("%.3f %s removed from queue%n",
         sim.time(), requestQueue.removeFirst());
      if (requestQueue.size() > 0) startServing();
      }

   private static void usage()
      {
      System.err.println
         ("Usage: java WebServer01 <lambda> <mu> <R> <seed>");
      System.exit (1);
      }
   }