Grizzlet reloaded: Bringing Ajax Push/Comet to the Masses?

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Jean-Francois Arcand

Writing a TCP/UDP stack supporting the SIP protocol using the Grizzly Framework | Main | Comet support in GlassFish

Grizzlet reloaded: Bringing Ajax Push/Comet to the Masses?

Posted by jfarcand on February 14, 2008 at 6:02 PM PST

Last September I've introduced Grizzlet, which is a simple but powerful enough way to write Ajax Push aka Comet applications. I came with the Grizzlet concept mostly after having collaborated to multiples Comet applications build on top of the Grizzly Comet 'framework'. Although the Grizzly Comet framework can be simple to use, some of my collaborators wanted to have something more lightweight. Some of them were from the embedded devices space, where the size of the web server and the application is quite important. Even a database might want to have an embedded web server with comet inside. For those reasons, I've made the little monster small and easy to use.

Well, I was under the impression it was easy, but it wasn't perfect. The first things users didn't liked is the fact that the GrizzletContainer wasn't supporting "traditional" web packaging bundle like a .jar or a .war, but instead needed an exploded folder "a la" JRuby. Message received: I've added support and now launching your Grizzlet is more simple. You just bundle your Comet application inside a war file like you usually do when using Servlet, and do:

java -jar grizzly-grizzlet-webserver-1.8.5.4.jar -a JMakiGrizzlet.war com.sun.grizzly.grizzlet.JMakiGrizzlet

You still have to specify the Grizzlet's class name, but that's something that can easily be done when the GrizzletContainer is embedded, or used inside GlassFish v3 where more powerful deployment capability are available. The next issue users where having was with the Grizzlet Request/Response API, which were kind of incomplete (kudo to Zaid for his contributions)...not anymore :-) You can take a look at the GrizzletRequest and GrizzletResponse API. Those API are really close to the HttpServletRequest/Response object.

Next I've added a couple of new examples (download them here and here). My favorite one is the JMaki based. This demo is quite interesting (see a picture here) as it stress the server quite hard. You enter word and move the word on the screen, and your word will move in all browser. Nice little game. It was already simple to implement it using Grizzly Comet, it is more simple with Grizzlet:

     23     public void onRequest(AsyncConnection ac) throws IOException {
     24         GrizzletRequest req = ac.getRequest();
     25         GrizzletResponse res = ac.getResponse();
     26 
     27         String action = req.getParameterValues("action")[0];
     28         if (action != null) {
     29             if ("post".equals(action)) {
     30                 String message = req.getParameterValues("message")[0];
     31                 String callback = req.getParameterValues("callback")[0];
     32                 if (callback == null) {
     33                     callback = "alert";
     34                 }
     35 
     36                 // Notify other registered CometHandler.
     37                 ac.push("<script id='comet_" + counter++ + "'>" + "window.parent."
     38                         + callback + "(" + message + ");</script>");
     39                 res.write("ok");
     40                 res.flush();
     41                 return;
     42             } else if ("start".equals(action)) {
     43                 res.setContentType("text/html");
     44                 String callback = req.getParameterValues("callback")[0];
     45                 if (callback == null) {
     46                     callback = "alert";
     47                 }
     48 
     49                 String message = "{ message : 'Welcome'}";
     50                 res.write("<script id='comet_" + counter++ + "'>" + "window.parent."
     51                         + callback + "(" + message + ");");
     52                 res.write("<html><head><title>jMaki Grizzly Comet Words Sample</title></head><body bgcolor=\"#FFFFFF\">");
     53                 res.flush();
     54 
     55                 if (ac.isGet()) {
     56                     ac.suspend();
     57                 }
     58                 return;
     59             }
     60         }
     61     }
     62 
     63     public void onPush(AsyncConnection ac) throws IOException {
     64         GrizzletRequest req = ac.getRequest();
     65         GrizzletResponse res = ac.getResponse();
     66 
     67         if (ac.isResuming()) {
     68             res.write("jMaki Grizzlet Words Sample closed
");
     69             res.write("</body></html<");
     70             res.flush();
     71             res.finish();
     72         } else if (ac.hasPushEvent()) {
     73             res.write(ac.getPushEvent().toString());
     74             res.flush();
     75         }
     76     }

In less than 100 lines, we have a nice game :-).

What's next? I think the Grizzlet concept can be easily extended and ported to Web Server like Tomcat and Jetty, by changing the AsynConnection interface to support generics:

     30 /**
     31  * This class represent a possible asynchronous connection. An asynchronous
     32  * connection can always be suspended or resumed, its associated request
     33  * and response objects be used to construct a response, etc.
     34  *
     35  * @author Jeanfrancois Arcand
     36  */
     37 public interface AsyncConnection<E,F> {
     38 
     39     /**
     40      * Return true is the current connection associated with
     41      * this event has been suspended.
     42      */
     43     public boolean isSuspended();
     44 
     45 
     46     /**
     47      * Suspend the current connection. Suspended connection are parked and
     48      * eventually used when the Grizzlet Container initiates pushes.
     49      */
     50     public void suspend() throws AlreadyPausedException;
     51 
     52 
     53     /**
     54      * Resume a suspended connection. The response will be completed and the
     55      * connection become synchronous (e.g. a normal http connection).
     56      */
     57     public void resume() throws NotYetPausedException;
     58 
     59 
     60     /**
     61      * Advises the Grizzlet Container to start intiating a push operation, using
     62      * the argument message. All asynchronous connection that has
     63      * been suspended will have a chance to push the data back to their
     64      * associated clients.
     65      *
     66      * @param message The data that will be pushed.
     67      */
     68     public void push(String message) throws IOException;
     69      
     70 
     71     /**
     72      * Return the Request associated with this AsynchConnection.
     73      */
     74     public E getRequest();
     75 
     76 
     77     /**
     78      * Return the Response associated with this AsynchConnection.
     79      */
     80     public F getResponse();
     81 
     82 
     83     /**
     84      * Is this AsyncConnection being in the process of being resumed?
     85      */
     86     public boolean isResuming();
     87 
     88 
     89     /**
     90      * Is this AsyncConnection has push events ready to push back data to
     91      * its associated client.
     92      */
     93     public boolean hasPushEvent();
     94 
     95 
     96     /**
     97      * Return the message that can be pushed back.
     98      */
     99     public String getPushEvent();
    100 
    101 
    102     /**
    103      * Is the current asynchronous connection defined as an HTTP Get.
    104      */
    105     public boolean isGet();
    106 
    107 
    108     /**
    109      * Is the current asynchronous connection defined as an HTTP Get.
    110      */
    111     public boolean isPost();
    112 
    113 }

I would also think than REST can take advantage of that API...something for Paul!

Now I was looking for a project's name and came a name related to Comet: AtmoSpherE :-). The project is not yet started, but once started, it will be a good sandbox to extends the Grizzlet concept and make it work with the upcoming Servlet 3.0 Comet proposal, which is still too complex in my opinion. Being able to resume/suspend a connection is a good step in the right direction, but I think not having any support/strategies when the push operation happens is something missing. Comet applications build on top of Servlet 3.0 will have to spawn threads to avoid blocking when doing the push operations, will have to throttle the push to avoid flodding clients, etc...but that's not the goal of this blog. I will soon start my adventure in CometDaily where I will talk about those issues. For now, The Grizzlet adventures are quite challenging, thanks to the community!

technorati: grizzly jersey grizzlet jmaki comet ajax glassfish

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Jean-Francois Arcand

Grizzlet reloaded: Bringing Ajax Push/Comet to the Masses?