<document>
  <header>
    <product>resin</product>
    <title>Comet/Server-Push Servlet</title>
    <description>
<p>Resin's server-push (Comet) servlet API enables streaming communication such
as reverse AJAX dynamic updates for browser/JavaScript applications.  The API
encapsulates of the threading and communications issues between the
request threads and the rest of the application.
</p>

</description>
  </header>

  <body>

<s1>

<p>Resin's server-push (Comet) API lets server application push new data to
the client as it becomes available.  Administration and monitoring
applications need to continually update the client when new information
becomes available to the server.
</p>

<figure src="cometcomm.png"/>

<p>The architecture in the picture uses two HTTP streams to the server
application, one for normal client-server requests, and a second
unidirectional stream to send updates from the server to the client.
In this example, we're using a browser with JavaScript, but the same
architecture applies to a more sophisticated Flash monitoring
application sending Hessian packets to update the monitoring display.</p>

</s1>

<localtoc/>

<s1 title="Example Overview">

<p>The example updates a comet.html page every two seconds with new data.
In this case, just an updated counter.</p>

<p>The components of the Comet/AJAX application look like:</p>

<ul>
<li>Protocol: JavaScript function calls with a trivial argument.</li>
<li>Client:
  <ul>
  <li>View: HTML updated by JavaScript AJAX</li>
  <li>Controller: call server with an &lt;iframe></li>
  </ul>
</li>
<li>Server:
  <ul>
  <li>Service: TimerService manages the comet connections and wakes them
with new data.</li>
  <li>Servlet: TestCometServlet generates &lt;script> protocol tags from
new data from the TimerService on each <code>resume</code>.</li>
  <li>State: CometState encapsulates both the item's state (the timer count),
and the <a href="http://caucho.com/resin-javadoc/com/caucho/servlet/comet/CometController.html">CometController</a> needed to wake the servlet and
pass updated data.</li>
  </ul>
</li>
</ul>

</s1>

<s1 title="Streaming Protocol: &lt;script> tags">

<p>The comet HTTP stream is a sequence of &lt;script> tags
containing JavaScript commands to update the browser's display.
Because the browser executes the script as part of its
progressive rendering, the user will see the updates immediately without
waiting for the entire HTTP request to complete.</p>

<p>In our example, the packet is a JavaScript
<code>comet_update(data)</code> call, which updates the text field with
new data.  Here's an example of the packet stream:</p>

<example title="Update JavaScript packets">
&lt;script type="text/javascript">
window.parent.comet_update(1);
&lt;/script>

&lt;!-- 2 second delay -->

&lt;script type="text/javascript">
window.parent.comet_update(2);
&lt;/script>

&lt;!-- 2 second delay -->

&lt;script type="text/javascript">
window.parent.comet_update(3);
&lt;/script>
</example>

<p>More sophisticated comet applications will use
a <a href="http://hessian.caucho.com/doc/metaprotocol-taxonomy.xtp#Dynamic-Typed%20Metaprotocols">dynamic-typed protocol</a> to update the client.
Browser-based applications could use JSON to update the client and
Flash-based applications might use Hessian.  In all cases, the protocol
must be kept simple and designed for the client's requirements.  Design
separate, simple protocols for Flash and JavaScript browsers, rather than
trying to create some complicated general protocol.
</p>

</s1>

<s1 title="Browser Client">

<p>The JavaScript command stream updates a parent HTML file which defines
the JavaScript commands and launches the Comet servlet request with an
&lt;iframe> tag.  Our <code>comet_update</code> function finds the
HTML tag with <code>id="content"</code> and updates its HTML content
with the new data from the server.</p>

<example title="comet.html">
&lt;html>
&lt;body>

Server Data:
&lt;span id="content">server data will be shown here&lt;/span>

&lt;script type="text/javascript">
function comet_update(value) {
  document.getElementById('content').innerHTML = value;
};
&lt;/script>

&lt;iframe src="comet"
        style="width:1px;height:1px;position:absolute;top:-1000px">&lt;/iframe>

&lt;/body>
&lt;/html>
</example>

</s1>

<s1 title="CometController">

<p>The <a href="http://caucho.com/resin-javadoc/com/caucho/servlet/comet/CometController.html">CometController</a> is Resin's thread-safe encapsulation
of control and communication from the application's service to the
Comet servlet.  Applications may safely pass the <code>CometController</code>
to different threads, wake the servlet with <code>wake()</code>, and send
data with <code>setAttribute</code>.</p>

<p>In the example, the <code>TimerService</code> passes the updated
count to the servlet by calling
<code>setAttribute("caucho.count", count)</code>, and wakes the servlet
by calling <code>wake()</code>.  When the servlet resumes, it will
retrieve the count using <code>request.getAttribute("caucho.count")</code>.
Note, applications must only use the thread-safe
<code>CometController</code> in other threads.  As with other servlets, the
<code>ServletRequest</code>, <code>ServletResponse</code>, writers and
output stream can only be used by the servlet thread itself, never by
any other threads.</p>

<example title="com.caucho.servlet.comet.CometController">
package com.caucho.servlet.comet;

public interface CometController
{
  public void wake();

  public Object getAttribute(String name);
  public void setAttribute(String name, Object value);
  public void removeAttribute(String name);

  public void close();
}
</example>

</s1>

<s1 title="Comet Servlet">

<p>The comet servlet has three major responsibilities:</p>

<ol>
<li>Process the initial request (<code>service</code>).</li>
<li>Register the <code>CometController</code> with the service (<code>service</code>).</li>
<li>Send streaming data as it becomes available (<code>resume</code>).</li>
</ol>

<p>
Like other servlets, only the comet servlet
may use the <code>ServletRequest</code>, <code>ServletResponse</code> or any
output writer or stream.  No other thread may use these servlet objects,
and the application must never store these objects in fields or objects
accessible by other threads.  Even in a comet servlet, the servlet objects
are not thread-safe.  Other services and threads must use
the <code>CometController</code> to communicate with the servlet.</p>

<p><b>Process the initial request:</b> our servlet just calls
<code>setContentType("text/html")</code>, since it's a trivial example.
A real application would do necessary database lookups and possibly
send more complicated data to the client.</p>

<p><b>Register the <code>CometController</code>:</b>
our servlet registers the controller with the timer service by calling
<code>addCometState</code>. In general, the application state object will
contain the <code>CometController</code> as part of the registration
process.</p>

<p><b>Send streaming data:</b>.  The <code>TimerService</code> will set new
data in the <code>"comet.count"</code> attribute and <code>wake()</code>
the controller.  When the servlet executes the <code>resume()</code> method,
it will retrieve the data, and send the next packet to the client.</p>

<example title="example/TestCometServlet.java">
package example;

import java.io.*;

import javax.servlet.http.*;
import javax.servlet.*;

import com.caucho.servlet.comet.GenericCometServlet;
import com.caucho.servlet.comet.CometController;

public class TestComet extends GenericCometServlet {
  @Override
  public boolean service(ServletRequest request,
                         ServletResponse response,
                         CometController controller)
    throws IOException, ServletException
  {
    HttpServletRequest req = (HttpServletRequest) request;
    HttpServletResponse res = (HttpServletResponse) response;

    res.setContentType("text/html");

    TestState state = new TestState(controller);

    _service.addCometState(state);

    return true;
  }
  
  @Override
  public boolean resume(ServletRequest request,
                        ServletResponse response,
                        CometController controller)
    throws IOException, ServletException
  {
    HttpServletRequest req = (HttpServletRequest) request;
    HttpServletResponse res = (HttpServletResponse) response;

    PrintWriter out = res.getWriter();

    String count = req.getAttribute("comet.count");

    out.print("&lt;script type='text/javascript'>");
    out.print("comet_update(" + count + ");");
    out.print("&lt;/script>");

    return true;
  }
}
</example>

<p>The connection can close for a number of reasons.  Either the
<code>service()</code> or <code>resume()</code> methods may return false,
telling Resin to close the connection.  The service might call
<code>CometController.close()</code> which will also close the connection.
Finally, the client may close the connection itself.</p>

<p>The sequence of calls for the example looks like the following:</p>

<ol>
<li><code>servlet.service()</code> is called for the initial request</li>
<li><code>_service.addCometState()</code> registers with the <code>TimerService</code></li>
<li>after the <code>service()</code> completes, Resin suspends the servlet.</li>
<li>The <code>TimerService</code> detects an event, in this case the timer event.</li>
<li>The <code>TimerService</code> calls <code>controller.setAttribute()</code> to send new data.</li>
<li>The <code>TimerService</code> calls <code>controller.wake()</code> to wake the servlet.</li>
<li><code>servlet.resume()</code> processes the data and sends the next packet.</li>
<li>After the <code>resume()</code> completes, Resin suspends the servlet again and we repeat as after step #3.</li>
<li>After the 10th data, the <code>TimerService</code> calls <code>controller.close()</code>, closing the servlet connection.</li>
</ol>

<s2 title="Comet Servlet State Machine">

<p>The sequence of comet servlet calls looks like the following state
machine.  After the initial request, the servlet spends most of its
time suspended, waiting for the <code>TimerService</code> to call
<code>wake()</code>.</p>

<figure src="cometstate.png"/>

</s2>
</s1>

  </body>
</document>
