tag, and Netscape's Dynamic HTML (except for the World Wide Web Consortium Cascading Style Sheets standard) cannot be considered standards at present. Meanwhile, as potential standards go, the CDF (Channel Definition Format) has a lot going for it.
The CDF was jointly developed by Microsoft and PointCast and is written using XML (eXtensible Markup Language), which is likely to replace HTML as the general-purpose Web markup language. XML is a true nonproprietary technology, currently being rapidly pushed through the standards process. Since CDF is an XML DTD (Document Type Definition), it will be easy for vendors to adopt future implementations of it.
Though neither XML nor CDF is a ratified standard yet, both are likely to achieve that status quickly. CDF has also proven to be attractive to existing push vendors such as PointCast and BackWeb, who have pledged to support CDF in their products and develop clients that integrate with IE 4.0.
Cross-Platform Channels
Channel is
a formidable sounding word, but don't let it scare you. Push channels are fairly easy to implement-both Microsoft and Netscape claim they can be created without any changes to a Web site. While that's true in the simplest cases, nearly everyone who implements a channel will likely want to make some changes to enhance the end user's experience with those sites.
For IE 4.0's Subscriptions feature and Netscape's Netcaster channels, you can take any existing Web site and make it a push channel simply by entering the site's URL into the appropriate preferences dialog box in IE 4.0 and Netcaster. You'll also want to enter other key information, such as how often to poll the Web site to see if updated information is available.
Both Netcaster and Subscriptions use the HTTP GET method with the "If-Modified-Since" header element to determine whether a Web page has been modified. This form of the HTTP GET method requests that the server send the Web page only if the page has been modified since a certain date.
Typically, that date is the last date you accessed the page and is stored by Netcaster and Subscriptions.
Unfortunately, some Web servers still don't transmit Web-page modification dates accurately. When a Web page is updated, its new modification date should be reflected to any requests, but some Web servers will still send the modification date from the old version of the page in the server cache.
Manually entering all the preference information for a Netcaster channel is tedious. In fact, this shouldn't be done by end users anyway, since they are unlikely to know how often a site will be updated or how many levels deep to "crawl" looking for updated pages. Fortunately, Netscape created a special method in JavaScript 1.2, called addChannel, that automatically adds preference information about a site to your Netcaster preferences.
The figure at left shows a typical script that you attach to a button on your Web site. Users who visit your Web site using Communicator can click on that button to
add your site to their Netcaster channels automatically. To make use of the script in this figure, you'll want to make it a function call and then have it executed when the user clicks an appropriate button or link.
To make Web sites work with IE 4.0's CDF push client, you must create a CDF file and make that available to subscribers with a clickable link. Clicking on the link in the IE 4.0 browser causes the CDF file to download to your machine, where the push client reads it and formats the information in IE 4.0's push interface. The CDF file typically contains a list of the subject headings available on the push channel, although some sites may wish to list all the articles available on the site within the CDF file.
The CDF specification is flexible, perhaps too flexible, for channel creation. It's likely that some CDF files will work only for particular push clients, such as PointCast, since it's possible for a CDF file to contain extensions that other clients won't understand.
Perhaps the
easiest way to create a CDF file now-or just see what the format looks like-is to download the PointCast 2.0 beta from PointCast's Web site (www. pointcast.com). PointCast 2.0 contains a built-in CDF editor that can be used to generate CDF files for any Web site.
Building Web sites that work efficiently with both CDF and Netcaster requires some structuring. Most sites will want to have a root page containing links to all the subject areas covered by the channel. One level below that page will be pages containing all of the actual content of the channel.
You'll then need to instruct Netcaster, and configure your CDF file, to search two levels deep for content updates. You have to be careful not to have advertisements or other stray links on the root page, or else Netcaster or the CDF push client will crawl to those linked pages and download that content.
Conclusion
Microsoft and Netscape each offer an easy push solution that anyone can freely implement-a Web page. To make those pages
compelling, however, requires some work and, unfortunately, two sets of competing technologies.
Meanwhile, it remains to be seen whether end users will decide to purchase either Netscape's Communicator or Microsoft's Internet Explorer 4.0 simply because of the push client features each supports. In either case, it's certain that content providers will want to support both.
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fter a year filled with various proprietary push solutions battling for meager market share, the smoke has cleared. Microsoft and Netscape Communications have unveiled their push solutions and, practically overnight, the push market has realigned behind the two browser leaders. l That's particularly good news for companies trying to decide how to push corporate and product information on their intranet or outside on the public Internet. Businesses that may have wanted to create content channels for their corporate intranets but didn't want to risk time and resources on proprietary solutions that would have locked them into a si
ngle vendor now can look to the push solutions offered by Microsoft and Netscape.
