Platinum Edition Using HTML 4, XML, and Java 1.2
(Publisher: Macmillan Computer Publishing)
Author(s): Eric Ladd
ISBN: 078971759x
Publication Date: 11/01/98

CHAPTER 17
CDF and Active Desktop Components

by Eric Ladd

In this chapter

Microsoft’s Approach to Webcasting 416

Microsoft’s Channel Definition Format (CDF) 420

Setting Up a Software Distribution Channel 427

Setting Up an Active Desktop Component 429

Controlling Log Recording 431

Making a Channel Password-Protected 432

CDF-Compliant Software 432

Creating Channels with Microsoft FrontPage 432

Microsoft’s Approach to Webcasting

When Microsoft released Internet Explorer 4.0, it put out an integrated suite of Internet-related software tools that enables you to create and browse Web pages, read electronic mail and UseNet newsgroups, participate in virtual meetings with colleagues at a distance, and collaboratively work on Microsoft Office documents. The browser component of the Internet Explorer suite also included support for Webcasting channels—a medium by which content providers could “broadcast” their work right to a user’s desktop.

The word “desktop” in the last sentence isn’t used metaphorically, either. Another facet of Internet Explorer 4 is the Active Desktop, a Windows configuration that blurs the boundaries between the traditional desktop and the Internet. Under the Active Desktop, you have a single “Explorer” that enables you to examine files on your system as well as view documents on the Web. Users with the Active Desktop turned on are also able to view Active Desktop Components that content providers build into their Webcast channels.

• See Chapter 11, “Introduction to XML,” p. 305.

This chapter introduces the ideas of Webcasting, channels, and Active Desktop Components. After gaining an understanding of Microsoft’s Webcasting philosophy, you’ll explore the Channel Definition Format (CDF)—a channel description language derived from the eXtensible Markup Language (XML)—and learn how to use it to convert your Web site into a Webcast channel. Finally, you’ll see how to build Active Desktop Components into your channel so that users can have dynamic content sitting right on their desktop (no browser required!).

At some point during your experience with the Web, you’ve probably heard about the concept of “push technology,” in which content is selectively pushed down to a user’s computer at that user’s request. Push technology has also found a role in corporate intranets because it gives system administrators an easy way to distribute software updates to all users.

But what exactly is push technology? In many cases, what some people call “push” is actually a misnomer. That’s why Microsoft has proposed a three-tiered model of what it calls Webcasting. Push technology is part of this model, as are some simpler, less “intelligent” ways of managing content delivery.

The three components of the Microsoft Webcasting model are

•  Subscriptions—The most basic form of Webcasting, in which a user subscribes to a specific Web page and is notified by Internet Explorer whenever the page changes.

•  Channels—An intermediate form of Webcasting, in which content providers can create a “channel” from their existing content, enabling them to manage what users see and how frequently updates are made.

•  Push technology—True push technology, content delivery that’s handled completely on the server side, is the highest form of Webcasting.

According to Microsoft, the first two tiers of the model do not represent true push technology. Rather, they are more accurately described as an “intelligent pull” of content. “Pull” suggests that the movement of content is initiated by the browser rather than by a server, and the intelligence comes in through regularly scheduled site crawls, also initiated by your browser, which looks for information that has changed.

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NOTE:  For a full treatment of Microsoft’s take on Webcasting, consult its Webcasting white paper at http://www.microsoft.com/ie/press/techinfo-f.htm?/ie/press/whitepaper/pushwp.htm.

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The next few sections take a closer look at each of these tiers.

Subscriptions

When visitors use Internet Explorer to subscribe to a page, they’re really giving the browser instructions to look for changes to the page on a regular basis. If any changes are found, Internet Explorer can notify the users in the following ways:

•  By placing a “gleam” (red asterisk) on an updated Favorite

•  By sending an email message

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NOTE:  Internet Explorer assumes that if you like a page enough to subscribe to it, it should also have a place in your Favorites folder. That’s why a change in a page you’ve subscribed to shows up as a gleam in the Favorites listing.

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Either way, users find out about the changed content and make a decision about whether to go to the page and check out what’s been updated. And if they don’t want to look at the page right then, Internet Explorer can download it and store a copy locally for later offline browsing. This can save you big bucks in connect time charges if your Internet service provider charges you based on how long you’ve been connected.

Subscriptions, although considered to be the most basic form of Webcasting, have several inherent advantages:

•  They’re free and easy to use.

•  Users have complete control over how often Internet Explorer checks a page for changes.

•  Downloading of updated pages enables portability of content to a laptop, which means you can take the pages with you.

•  Site administrators don’t have to make any changes to their sites so that users can subscribe to them.

•  Subscriptions are maintained by Internet Explorer rather than by a separate add-on program.

One major drawback to subscriptions is that Internet Explorer has to do a site crawl to determine whether a page has changed. A site crawl might end up generating too much information, leaving users to sift through everything it found to figure out what’s relevant. Additionally, some sites do not permit site-crawling programs (frequently called spiders) to access them at all, so even if you do subscribe to a page, Internet Explorer might not be able to check whether any updates were made.