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Common Continuation Patterns
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## Common Continuation Patterns

### Suspend Resume Pattern

The suspend/resume style is used when a servlet and/or filter is used to generate the response after an asynchronous wait that is terminated by an asynchronous handler. Typically a request attribute is used to pass results and to indicate if the request has already been suspended.

    void doGet(HttpServletRequest request, HttpServletResponse response)
    {
         // if we need to get asynchronous results
         Object results = request.getAttribute("results");
         if (results==null)
         {
           final Continuation continuation = ContinuationSupport.getContinuation(request);

           // if this is not a timeout
           if (continuation.isExpired())
           {
             sendMyTimeoutResponse(response);
             return;
           }

           // suspend the request
           continuation.suspend(); // always suspend before registration

           // register with async service.  The code here will depend on the
           // the service used (see Jetty HttpClient for example)
           myAsyncHandler.register(new MyHandler()
           {
              public void onMyEvent(Object result)
              {
                continuation.setAttribute("results",results);
                continuation.resume();
              }
           });
           return; // or continuation.undispatch();
         }

         // Send the results
         sendMyResultResponse(response,results);
    }

This style is very good when the response needs the facilities of the servlet container (e.g., it uses a web framework) or if one event may resume many requests so the container’s thread pool can be used to handle each of them.

### Suspend Continue Pattern

The suspend/complete style is used when an asynchronous handler is used to generate the response:

    void doGet(HttpServletRequest request, HttpServletResponse response)
    {
         final Continuation continuation = ContinuationSupport.getContinuation(request);

         // if this is not a timeout
         if (continuation.isExpired())
         {
           sendMyTimeoutResponse(request,response);
           return;
         }

         // suspend the request
         continuation.suspend(); // response may be wrapped.

         // register with async service.  The code here will depend on the
         // the service used (see Jetty HttpClient for example)
         myAsyncHandler.register(new MyHandler()
         {
           public void onMyEvent(Object result)
           {
             sendMyResultResponse(continuation.getServletResponse(),results);
             continuation.complete();
           }
         });
    }

This style is very good when the response does not need the facilities of the servlet container (e.g., it does not use a web framework) and if an event will resume only one continuation. If many responses are to be sent (e.g., a chat room), then writing one response may block and cause a DOS on the other responses.

### Examples

- The ChatServlet example shows how the suspend/resume style can be used to directly code a chat room (See similar example using Async Servlets). The same principles are applied to frameworks like cometd which provide an richer environment for such applications, based on Continuations.

- The QoSFilter uses suspend/resume style to limit the number of requests simultaneously within the filter. This can be used to protect a JDBC connection pool or other limited resource from too many simultaneous requests.

  If too many requests are received, the extra requests wait for a short time on a semaphore, before being suspended. As requests within the filter return, they use a priority queue to resume the suspended requests. This allows your authenticated or priority users to get a better share of your server’s resources when the machine is under load.

- The DosFilter is similar to the QoSFilter, but protects a web application from a denial of service attack, as much as is possible from within a web application.

  If too many requests are detected coming from one source, then those requests are suspended and a warning generated. This works on the assumption that the attacker may be written in simple blocking style, so by suspending you are hopefully consuming their resources. True protection from DOS can only be achieved by network devices (or eugenics :)).

- The ProxyServlet uses the suspend/complete style and the Jetty asynchronous HTTP client to implement a scalable Proxy server (or transparent proxy).

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