As covered in Comet Programming: Using Ajax to Simulate
Server Push, Comet is a Web application model that enables Web servers to
send data to the client without having to explicitly request it. Hence, the
Comet model provides an alternate mechanism to classical polling to update page
content and/or data. In that article, we learned how to use XMLHttpRequest long
polling to refresh page components and keep cached data in synch with the server.
Another Comet strategy, sometimes referred to as the "Forever Frame" technique,
is to use a hidden IFrame. As with XMLHttpRequest long polling, the "Forever
Frame" technique also relies on browser-native technologies, rather than on
proprietary plugins or other special technologies.
IFrames Technique Overview
IFrame stands for Inline Frame. It allows a Web page to embed one HTML document inside another HTML element. As you can imagine, it's a simple way to create a "mashup", whereby the page combines data from several sources into a single integrated document:
Normally, data delivered in HTTP responses is sent in one piece. The length
of the data is indicated by the
Content-Length header field. With chunked encoding,
the data is broken up into a series of blocks of data and transmitted in one
or more "chunks" so that a server may start sending data before it knows the
final size of the content that it's sending. Note that the size of the blocks
may or may not be the same:
Two benefits of the IFrame method are that it's fairly easy to implement and
it works in every common browser. On the cons side, there is no way to handle errors reliably
nor is it possible to track the state of the request calling process. Therefore, if you want
to track the progress of the request, you'd best stick with the XMLHttpRequest
The CD Sales Page Revisited
You'll remember this example from the Comet Programming:
Using Ajax to Simulate Server Push article. We'll modify it in order to
highlight the differences between the Ajax and IFrame techniques.
updateCount() function refreshes the CD count within the "CD Count"
<SPAN> element. The innerHTML property is used because some browsers do
not support innerText.
Integrated PHP Code
Another departure from the Ajax Comet page is that all the required functionality is contained within the same file. PHP, like ASP and JSP is ideal for inserting within the HTML markup. By integrating code in one file, the results of actions that take place on the server are then available to the rest of the page or can be included in place using the echo() function. Whereas the PHP and HTML were housed in separate files last time in order to clearly distinguish server-side components from client-side, the CometHiddenIFrame.php file contains all the necessary code. This was not done merely for the sake of convenience; but contributes to setting the execution order of the code. For example, at the top of the page, there is a short PHP script to send the headers to the browser. It is necessary to do this before sending any HTML to the browser, as by that time, it is too late. In fact, trying to send headers after the page content has already begun to download will result in a "Headers Already Sent" PHP error.
set_time_limit() function is called with a value of
0 to turn off the maximum
script execution time (which is 30 seconds by default). The headers that we are sending prevent browser
caching - always important when sending dynamic content. Finally the
function is called to make sure that the header information is sent to the client:
The rest of the PHP code is contained in a script at the end of the page. It
must be placed there so that the page has a chance to load, as the
loop will prevent the rest of the page from displaying. The random order generation
has also been moved from the
getCdCount() function so that we can call
without a delay when the
Purchase button is clicked. In fact, the
function is now deprecated, and can be removed: