comet/server-push servlet

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.

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.

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.

Example Overview

Streaming Protocol: <script> tags

Browser Client

AsyncContext

Comet Servlet

Comet Servlet State Machine

Example Overview

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

The components of the Comet/AJAX application look like:

Protocol: JavaScript function calls with a trivial argument.
Client:
- View: HTML updated by JavaScript AJAX
- Controller: call server with an <iframe>
Server:
- Service: TimerService manages the comet connections and wakes them with new data.
- Servlet: TestCometServlet generates <script> protocol tags from new data from the TimerService on each resume.
- State: CometState encapsulates both the item's state (the timer count), and the AsyncContext needed to wake the servlet and pass updated data.

Streaming Protocol: <script> tags

The comet HTTP stream is a sequence of <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.

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

Update JavaScript packets

<script type="text/javascript">
window.parent.comet_update(1);
</script>

<!-- 2 second delay -->

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

<!-- 2 second delay -->

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

More sophisticated comet applications will use a dynamic-typed protocol 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.

Browser Client

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

comet.html

<html>
<body>

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

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

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

</body>
</html>

AsyncContext

The AsyncContext is the Servlet 3.0 encapsulation of control and communication from the application's service to the Comet servlet. Applications may safely pass the AsyncContext to different threads, and wake the servlet with dispatch(), and return the request and response with getRequest(). The servlet request methods are not thread safe.

In the example, the TimerService passes the updated count to the servlet by calling setAttribute("caucho.count", count) on the request, and wakes the servlet by calling dispatch(). When the servlet resumes, it will retrieve the count using request.getAttribute("caucho.count"). Note, these calls are not thread safe. As with other servlets, the ServletRequest, ServletResponse, writers and output stream can only be used by the servlet thread itself, never by any other threads.

javax.servlet.AsyncContext

package javax.servlet;

public interface CometController
{
  public void dispatch();
  public void complete();

  public ServletRequest getRequest();
  public ServletResponse getResponse();
}

Comet Servlet

The comet servlet has three major responsibilities:

Process the initial request (service).
Register the AsyncContext with the service (service).
Send streaming data as it becomes available (resume).

Like other servlets, the ServletRequest, ServletResponse are not thread-safe. Normally, only the servlet thread will access these methods. If the application needs to use the output stream or pass attributes, it must synchronize with the servlet. Even in a comet servlet, the servlet objects are not thread-safe.

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

Register the AsyncContext: our servlet registers the controller with the timer service by calling addCometState. In general, the application state object will contain the AsyncContext as part of the registration process.

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

example/TestCometServlet.java

package example;

import java.io.*;

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

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

    if (req.isAsyncStarted()) {
      resume(req, res);
      return;
    }

    res.setContentType("text/html");

    TestState state = new TestState(req.startAsync());

    _service.addCometState(state);
  }
  
  private void resume(HttpServletRequest req,
                      HttpServletResponse req)
    throws IOException, ServletException
  {

    PrintWriter out = res.getWriter();

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

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

The connection can close for a number of reasons. The service might call AsyncContext.complete() which will close the connection. Finally, the client may close the connection itself.

The sequence of calls for the example looks like the following:

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

Comet Servlet State Machine

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 TimerService to call wake().