org.mortbay.servlet
Class ThrottlingFilter

java.lang.Object
  org.mortbay.servlet.ThrottlingFilter

All Implemented Interfaces:

Filter

public class ThrottlingFilter
extends Object
implements Filter

This filter protects a web application from having to handle an unmanageable load.

For servers where there is 1 application with standardized resource restrictions, then this affect can be easily controlled by limiting the size of the server thread pool, however where there are multiple applications, or a single application has different resource requirements for different URLs, then this filter can assist in managing the number of requests being services at any point in time.

The filter has 3 configurable values, which may be set as init parameters:

1. maximum determines the maxmimum number of requests that may be on the filter chain at any point in time. (See below for a more detailed explanation)
2. block determines how long (in milliseconds) a request will be queued before it is rejected. Set this to -1 to block indefinately.
3. queue determines how many requests can be queued simultaneously - any additional requests will be rejected. Set this to 0 to turn off queueing.

Request Counting: The filter counts how many requests are currently being services by the rest of the filter chain (including any servlets that may be configured to handle the request). Request counting is per instance of the filter. There is no syncronization between virtual machines, and the request count is not shared between multiple instances of the filter. Thus a web.xml file such as

  <filter><filter-name>throttle1</filter-name>
          <filter-class>org.adjective.spiral.filter.ThrottlingFilter</filter-class>
  <filter><filter-name>throttle2</filter-name>
          <filter-class>org.adjective.spiral.filter.ThrottlingFilter</filter-class>

creates 2 separate filters with individual request counts.

Queueing: When the number of active requests exceed the maximum requests will be queued. This queue regulates the flow of connections. Once the number of requests on the queue reached the queue threshold, then any new requests will be rejected. Requests are queued for a maximum of block milliseconds - is no capacity is made available in this time then the request will be rejected. The oldest pending request is removed from the queue and processed as soon as the number of pending requests falls below the maximum value (i.e. when a request is completed)

Rejection: Requests are rejected when the number of requests in progress has reached maximum and either the queue is full; or a request has been queued for more than block milliseconds. The rejection is performed by calling the method rejectRequest(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse). By default this method sends the HTTP status code 503, but this may be over-ridden in derived classes.

This filter works best with the SelectChannelConnector, as RetryRequest based Continuations can be used to free the thread and other resources associated with the queued requests.

Author:

- Tim Vernum

Constructor Summary

ThrottlingFilter()

Method Summary

void	destroy()
void	doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain)
void	doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
void	init(FilterConfig filterConfig)
protected void	rejectRequest(HttpServletRequest request, HttpServletResponse response)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ThrottlingFilter

public ThrottlingFilter()

Method Detail

init

public void init(FilterConfig filterConfig)
          throws ServletException

Specified by:

init in interface Filter

Throws:

ServletException

doFilter

public void doFilter(ServletRequest request,
                     ServletResponse response,
                     FilterChain chain)
              throws IOException,
                     ServletException

Specified by:

doFilter in interface Filter

Throws:

IOException

ServletException

doFilter

public void doFilter(HttpServletRequest request,
                     HttpServletResponse response,
                     FilterChain chain)
              throws IOException,
                     ServletException

Throws:

IOException

ServletException

rejectRequest

protected void rejectRequest(HttpServletRequest request,
                             HttpServletResponse response)
                      throws IOException

Throws:

IOException

destroy

public void destroy()

Specified by:

destroy in interface Filter