//
   //  ========================================================================
   //  Copyright (c) 1995-2016 Mort Bay Consulting Pty. Ltd.
   //  ------------------------------------------------------------------------
   //  All rights reserved. This program and the accompanying materials
   //  are made available under the terms of the Eclipse Public License v1.0
   //  and Apache License v2.0 which accompanies this distribution.
   //
   //      The Eclipse Public License is available at
  //      http://www.eclipse.org/legal/epl-v10.html
  //
  //      The Apache License v2.0 is available at
  //      http://www.opensource.org/licenses/apache2.0.php
  //
  //  You may elect to redistribute this code under either of these licenses.
  //  ========================================================================
  //
  
  package org.eclipse.jetty.servlets;
  
  import java.io.IOException;
  import java.util.Queue;
  import java.util.concurrent.ConcurrentLinkedQueue;
  import java.util.concurrent.Semaphore;
  import java.util.concurrent.TimeUnit;
  
  import javax.servlet.AsyncContext;
  import javax.servlet.AsyncEvent;
  import javax.servlet.AsyncListener;
  import javax.servlet.Filter;
  import javax.servlet.FilterChain;
  import javax.servlet.FilterConfig;
  import javax.servlet.ServletContext;
  import javax.servlet.ServletException;
  import javax.servlet.ServletRequest;
  import javax.servlet.ServletResponse;
  import javax.servlet.http.HttpServletRequest;
  import javax.servlet.http.HttpServletResponse;
  import javax.servlet.http.HttpSession;
  
  import org.eclipse.jetty.server.handler.ContextHandler;
  import org.eclipse.jetty.util.annotation.ManagedAttribute;
  import org.eclipse.jetty.util.annotation.ManagedObject;
  import org.eclipse.jetty.util.log.Log;
  import org.eclipse.jetty.util.log.Logger;
  
  /**
   * Quality of Service Filter.
   * <p>
   * This filter limits the number of active requests to the number set by the "maxRequests" init parameter (default 10).
   * If more requests are received, they are suspended and placed on priority queues.  Priorities are determined by
   * the {@link #getPriority(ServletRequest)} method and are a value between 0 and the value given by the "maxPriority"
   * init parameter (default 10), with higher values having higher priority.
   * <p>
   * This filter is ideal to prevent wasting threads waiting for slow/limited
   * resources such as a JDBC connection pool.  It avoids the situation where all of a
   * containers thread pool may be consumed blocking on such a slow resource.
   * By limiting the number of active threads, a smaller thread pool may be used as
   * the threads are not wasted waiting.  Thus more memory may be available for use by
   * the active threads.
   * <p>
   * Furthermore, this filter uses a priority when resuming waiting requests. So that if
   * a container is under load, and there are many requests waiting for resources,
   * the {@link #getPriority(ServletRequest)} method is used, so that more important
   * requests are serviced first.     For example, this filter could be deployed with a
   * maxRequest limit slightly smaller than the containers thread pool and a high priority
   * allocated to admin users.  Thus regardless of load, admin users would always be
   * able to access the web application.
   * <p>
   * The maxRequest limit is policed by a {@link Semaphore} and the filter will wait a short while attempting to acquire
   * the semaphore. This wait is controlled by the "waitMs" init parameter and allows the expense of a suspend to be
   * avoided if the semaphore is shortly available.  If the semaphore cannot be obtained, the request will be suspended
   * for the default suspend period of the container or the valued set as the "suspendMs" init parameter.
   * <p>
   * If the "managedAttr" init parameter is set to true, then this servlet is set as a {@link ServletContext} attribute with the
   * filter name as the attribute name.  This allows context external mechanism (eg JMX via {@link ContextHandler#MANAGED_ATTRIBUTES}) to
   * manage the configuration of the filter.
   */
  @ManagedObject("Quality of Service Filter")
  public class QoSFilter implements Filter
  {
      private static final Logger LOG = Log.getLogger(QoSFilter.class);
  
      static final int __DEFAULT_MAX_PRIORITY = 10;
      static final int __DEFAULT_PASSES = 10;
      static final int __DEFAULT_WAIT_MS = 50;
      static final long __DEFAULT_TIMEOUT_MS = -1;
  
      static final String MANAGED_ATTR_INIT_PARAM = "managedAttr";
      static final String MAX_REQUESTS_INIT_PARAM = "maxRequests";
      static final String MAX_PRIORITY_INIT_PARAM = "maxPriority";
      static final String MAX_WAIT_INIT_PARAM = "waitMs";
      static final String SUSPEND_INIT_PARAM = "suspendMs";
  
      private final String _suspended = "QoSFilter@" + Integer.toHexString(hashCode()) + ".SUSPENDED";
      private final String _resumed = "QoSFilter@" + Integer.toHexString(hashCode()) + ".RESUMED";
      private long _waitMs;
      private long _suspendMs;
      private int _maxRequests;
     private Semaphore _passes;
     private Queue<AsyncContext>[] _queues;
     private AsyncListener[] _listeners;
 
     public void init(FilterConfig filterConfig)
     {
         int max_priority = __DEFAULT_MAX_PRIORITY;
         if (filterConfig.getInitParameter(MAX_PRIORITY_INIT_PARAM) != null)
             max_priority = Integer.parseInt(filterConfig.getInitParameter(MAX_PRIORITY_INIT_PARAM));
         _queues = new Queue[max_priority + 1];
         _listeners = new AsyncListener[_queues.length];
         for (int p = 0; p < _queues.length; ++p)
         {
             _queues[p] = new ConcurrentLinkedQueue<>();
             _listeners[p] = new QoSAsyncListener(p);
         }
 
         int maxRequests = __DEFAULT_PASSES;
         if (filterConfig.getInitParameter(MAX_REQUESTS_INIT_PARAM) != null)
             maxRequests = Integer.parseInt(filterConfig.getInitParameter(MAX_REQUESTS_INIT_PARAM));
         _passes = new Semaphore(maxRequests, true);
         _maxRequests = maxRequests;
 
         long wait = __DEFAULT_WAIT_MS;
         if (filterConfig.getInitParameter(MAX_WAIT_INIT_PARAM) != null)
             wait = Integer.parseInt(filterConfig.getInitParameter(MAX_WAIT_INIT_PARAM));
         _waitMs = wait;
 
         long suspend = __DEFAULT_TIMEOUT_MS;
         if (filterConfig.getInitParameter(SUSPEND_INIT_PARAM) != null)
             suspend = Integer.parseInt(filterConfig.getInitParameter(SUSPEND_INIT_PARAM));
         _suspendMs = suspend;
 
         ServletContext context = filterConfig.getServletContext();
         if (context != null && Boolean.parseBoolean(filterConfig.getInitParameter(MANAGED_ATTR_INIT_PARAM)))
             context.setAttribute(filterConfig.getFilterName(), this);
     }
 
     public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException
     {
         boolean accepted = false;
         try
         {
             Boolean suspended = (Boolean)request.getAttribute(_suspended);
             if (suspended == null)
             {
                 accepted = _passes.tryAcquire(getWaitMs(), TimeUnit.MILLISECONDS);
                 if (accepted)
                 {
                     request.setAttribute(_suspended, Boolean.FALSE);
                     if (LOG.isDebugEnabled())
                         LOG.debug("Accepted {}", request);
                 }
                 else
                 {
                     request.setAttribute(_suspended, Boolean.TRUE);
                     int priority = getPriority(request);
                     AsyncContext asyncContext = request.startAsync();
                     long suspendMs = getSuspendMs();
                     if (suspendMs > 0)
                         asyncContext.setTimeout(suspendMs);
                     asyncContext.addListener(_listeners[priority]);
                     _queues[priority].add(asyncContext);
                     if (LOG.isDebugEnabled())
                         LOG.debug("Suspended {}", request);
                     return;
                 }
             }
             else
             {
                 if (suspended)
                 {
                     request.setAttribute(_suspended, Boolean.FALSE);
                     Boolean resumed = (Boolean)request.getAttribute(_resumed);
                     if (resumed == Boolean.TRUE)
                     {
                         _passes.acquire();
                         accepted = true;
                         if (LOG.isDebugEnabled())
                             LOG.debug("Resumed {}", request);
                     }
                     else
                     {
                         // Timeout! try 1 more time.
                         accepted = _passes.tryAcquire(getWaitMs(), TimeUnit.MILLISECONDS);
                         if (LOG.isDebugEnabled())
                             LOG.debug("Timeout {}", request);
                     }
                 }
                 else
                 {
                     // Pass through resume of previously accepted request.
                     _passes.acquire();
                     accepted = true;
                     if (LOG.isDebugEnabled())
                         LOG.debug("Passthrough {}", request);
                 }
             }
 
             if (accepted)
             {
                 chain.doFilter(request, response);
             }
             else
             {
                 if (LOG.isDebugEnabled())
                     LOG.debug("Rejected {}", request);
                 ((HttpServletResponse)response).sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE);
             }
         }
         catch (InterruptedException e)
         {
             ((HttpServletResponse)response).sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE);
         }
         finally
         {
             if (accepted)
             {
                 for (int p = _queues.length - 1; p >= 0; --p)
                 {
                     AsyncContext asyncContext = _queues[p].poll();
                     if (asyncContext != null)
                     {
                         ServletRequest candidate = asyncContext.getRequest();
                         Boolean suspended = (Boolean)candidate.getAttribute(_suspended);
                         if (suspended == Boolean.TRUE)
                         {
                             candidate.setAttribute(_resumed, Boolean.TRUE);
                             asyncContext.dispatch();
                             break;
                         }
                     }
                 }
                 _passes.release();
             }
         }
     }
 
     /**
      * Computes the request priority.
      * <p>
      * The default implementation assigns the following priorities:
      * <ul>
      * <li> 2 - for an authenticated request
      * <li> 1 - for a request with valid / non new session
      * <li> 0 - for all other requests.
      * </ul>
      * This method may be overridden to provide application specific priorities.
      *
      * @param request the incoming request
      * @return the computed request priority
      */
     protected int getPriority(ServletRequest request)
     {
         HttpServletRequest baseRequest = (HttpServletRequest)request;
         if (baseRequest.getUserPrincipal() != null)
         {
             return 2;
         }
         else
         {
             HttpSession session = baseRequest.getSession(false);
             if (session != null && !session.isNew())
                 return 1;
             else
                 return 0;
         }
     }
 
     public void destroy()
     {
     }
 
     /**
      * Get the (short) amount of time (in milliseconds) that the filter would wait
      * for the semaphore to become available before suspending a request.
      *
      * @return wait time (in milliseconds)
      */
     @ManagedAttribute("(short) amount of time filter will wait before suspending request (in ms)")
     public long getWaitMs()
     {
         return _waitMs;
     }
 
     /**
      * Set the (short) amount of time (in milliseconds) that the filter would wait
      * for the semaphore to become available before suspending a request.
      *
      * @param value wait time (in milliseconds)
      */
     public void setWaitMs(long value)
     {
         _waitMs = value;
     }
 
     /**
      * Get the amount of time (in milliseconds) that the filter would suspend
      * a request for while waiting for the semaphore to become available.
      *
      * @return suspend time (in milliseconds)
      */
     @ManagedAttribute("amount of time filter will suspend a request for while waiting for the semaphore to become available (in ms)")
     public long getSuspendMs()
     {
         return _suspendMs;
     }
 
     /**
      * Set the amount of time (in milliseconds) that the filter would suspend
      * a request for while waiting for the semaphore to become available.
      *
      * @param value suspend time (in milliseconds)
      */
     public void setSuspendMs(long value)
     {
         _suspendMs = value;
     }
 
     /**
      * Get the maximum number of requests allowed to be processed
      * at the same time.
      *
      * @return maximum number of requests
      */
     @ManagedAttribute("maximum number of requests to allow processing of at the same time")
     public int getMaxRequests()
     {
         return _maxRequests;
     }
 
     /**
      * Set the maximum number of requests allowed to be processed
      * at the same time.
      *
      * @param value the number of requests
      */
     public void setMaxRequests(int value)
     {
         _passes = new Semaphore((value - getMaxRequests() + _passes.availablePermits()), true);
         _maxRequests = value;
     }
 
     private class QoSAsyncListener implements AsyncListener
     {
         private final int priority;
 
         public QoSAsyncListener(int priority)
         {
             this.priority = priority;
         }
 
         @Override
         public void onStartAsync(AsyncEvent event) throws IOException
         {
         }
 
         @Override
         public void onComplete(AsyncEvent event) throws IOException
         {
         }
 
         @Override
         public void onTimeout(AsyncEvent event) throws IOException
         {
             // Remove before it's redispatched, so it won't be
             // redispatched again at the end of the filtering.
             AsyncContext asyncContext = event.getAsyncContext();
             _queues[priority].remove(asyncContext);
             asyncContext.dispatch();
         }
 
         @Override
         public void onError(AsyncEvent event) throws IOException
         {
         }
     }
 }