Comet and Reverse Ajax: The Next-Generation Ajax 2.0 by By (author) Phil McCarthy By (author) Dave Crane PDF

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Extra info for Comet and Reverse Ajax: The Next-Generation Ajax 2.0 (Firstpress)

Example text

While this approach works, it places a heavy load on available resources of both the server itself and the network. Every connected client is continually transmitting data—usually to be told by the server that nothing has changed—and the server must handle each of these connections. You can lighten the load by increasing the interval between polls, but that decreases the system's responsiveness to updates, which is often one of the fundamental requirements of a collaborative application. Piggybacking provides a partial salve to these problems but will only really help in situations where the user frequently sends updates to the server anyway.

ResponseText, you get back the total response to date, not just the last message sent, so you need to do a bit of string manipulation to extract the last line. The onComplete () callback to the Aj ax. Request simply cancels the timer and tidies up the user interface, allowing the user to click the button again. As this section illustrates, you can handle Comet using the standard XHR object that underpins most Ajax frameworks. Support for Comet is far from ideal, particularly in the way that we had to identify the latest piece of content to arrive.

We're going to use DWR to demonstrate how simply you can develop a Comet application when you don't have to worry about all of the complexity under the hood. Direct Web demoting Before we begin, though, let's take a quick look into the history of DWR. DWR first emerged in early 2005 as a web remoting tool built on Ajax. In Java, the standard way of exposing behavior to the web is to write servlets, basically the Java equivalent of CGI scripts. Servlets parse HTTP request parameters, perform operations on server-side objects as required, and 54 firstPress Comet and Reverse Ajax return an HTTP response representing the results of these operations, usually in the form of an HTML document.

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