Channel Definition Format was an open standard that defines a «smart pull» technology for webcasting information to users desktops.
What is CDF (Channel Definition Format)
Channel Definition Format was an open standard created by Microsoft for Microsoft Internet Explorer version 4 (and proposed as a standard to the World Wide Web Consortium) that defines a «smart pull» technology for webcasting information to users desktops. Based on the Extensible Markup Language (XML), Channel Definition Format (CDF) lets administrators create Active Channels for delivery of content through the users Web browser, and Active Desktop elements and channel screen savers for delivery directly to the users desktops.
Channel content can be personalized, and delivery can be scheduled according to users needs and preferences. Using CDF also reduces server load and allows delivery of just the needed content, instead of requiring users to download large quantities of unnecessary content.
How it works
Let’s consider the delivery of Web content to the user’s browser using Active Channels. A Web site can be made into an Active Channel through the addition of a CDF file. The CDF file is a simple text file that is formatted using XML. It forms a kind of table of contents of the logical subset of the Web site that comprises the Active Channel. A link is then created to the CDF file on the Web site. The user clicks the link to subscribe to the Active Channel and download the CDF file. The Active Channel then appears on the channel bar on the user’s desktop. The content for the channel is downloaded to a cache on the user’s system. Channel updates are accomplished by scheduled Web crawls, using either the publisher’s predefined schedule or a user’s customized one. Users can also receive updates to channels by e-mail.
Some of the advantages of using CDF for the distribution of Web information to users include
- Simplicity: Turning an existing Web site into a channel merely involves creating a CDF file with a text editor and creating a hyperlink to this file.
- Structure: CDF describes how to logically group information in a hierarchical structure, independent of the content format.
- Personalization: Standard Hypertext Transfer Protocol (HTTP) cookies can be used to deliver personalized information to users.
- Administrator control: The administrator can control how much of the site can be downloaded by users.
- User control: The user can use CDF to specify which portions of a site to download to his or her browser, instead of pulling a lot of content off the site and hoping that it contains the needed information.
CDF is not true webcasting in the sense of Internet Protocol (IP) multicasting because it is a “pull” technology. True webcasting is supported by Microsoft NetShow for delivery of content using IP multicasting.
CDF Files are text files used for creating Active Channels, Active Desktop items, and channel screen savers for managed webcasting of content to user’s desktops. CDF files are based on the Channel Definition Format (CDF) standard. CDF files provide a mechanism for allowing users to select the content they want to download from a Web site, and they let administrators schedule content for delivery to user’s desktops.
How CDF Files work
CDF files are used to convert existing Web sites into Active Channels without the need to change the existing site in any way. You simply create a CDF file using a text editor such as Microsoft Notepad and include it in your site. This will allow the content of the site to be webcast to user’s browsers. The CDF file must be saved with the extension .cdf, and a link on your site should point to this file so that users can subscribe to the channel.
A typical CDF file defines a channel hierarchy for the different Web sites making up the Active Channel. This channel hierarchy contains a table of contents for webcasting the content and consists of a top-level channel, subchannels, and actual content items (Web pages). The simplest format for a CDF file is a list of Uniform Resource Locators (URLs) that point to specific Web pages in the site. More advanced CDF files can contain information such as:
- A map of the hierarchical structure of the URLs in the Web site
- Logical groupings of different content items within a site that can differ from the observable link structure of the site itself
- The title of each referenced Web page and a brief abstract of its contents
- Information controlling the scheduling of content updates
The syntax of advanced CDF file items is based on the Extensible Markup Language (XML), an open specification that provides extensibility to standard Hypertext Markup Language (HTML) files. More than one CDF file can be created for a site, allowing users to subscribe to information in different fashions. For example, a news site can have separate CDF files for news, sports, and weather subscriptions.
Channels in Active Channel enable personalized delivery of Web content using Web applications designed for Internet Information Server (IIS) for Windows NT (Internet Information Services for Windows 2000). Active Server Pages (ASP) can be used for dynamically generating personalized CDF files for users. Cookies can also be used for dynamically generating customized CDF files for users. These CDF files can be customized on the basis of preferences that a user specifies on an HTML form prior to subscribing to the channel.