HTTP The existence of a (Cheap web hosting) physical network infrastructure
HTTP The existence of a physical network infrastructure was, of course, essential to being able to move information feeds around once they were created. In addition a transport protocol, or pair of transport protocols (in this context TCP/IP), needed to be in place. All information on the Web is sent in TCP/IP packets. On a physical system that s not totally reliable, the use of TCP/IP produces a remarkable approximation to reliability. Over those protocols HTTP, the Hypertext Transport Protocol, is in place. When information feeds are retrieved by a browser or by an aggregator, retrieval is done using HTTP. For example, the URL for the information feed for my blog is located at http://www.tfosorcim.org/blog/index.rdf. HTTP is used in retrieving the RSS 1.0 document, which constitutes the information feed document for the blog. If the physical and software infrastructure had not been in place, it would not have been possible to implement generally available information feeds, at least not in the way that they were implemented. HTML HTML, the HyperText Markup Language, was also a key part in the use and acceptance of information feeds. HTML, and more recently XHTML, has been the display format chosen for the Web pages represented as items in an information feed. Converting RSS (and later Atom) information feeds to HTML was a relatively straightforward task. Scripting languages, such as Perl, were familiar to many programmers. And software modules were created which allowed the creation of HTML (or XHTML) from RSS and, later, Atom. Some software modules use XML parsing (discussed in more detail in the next section), whereas others treated an information feed as text. When faced with a malformed feed that didn t meet the well-formedness constraints of XML (discussed in Chapter 6), the use of regular expressions can support appropriate handling of the character sequences in the information feed and an HTML page can be created that the aggregator or a Web browser can display to the user. XML If an RSS or Atom feed has been created correctly, then the information feed document should be wellformed XML. In that case the feed document can be parsed using widely available XML parsers and each component part of the feed document can be displayed according to choices specified by the author of the aggregator or by the user. XML software tools are available for many platforms. For example, XSLT transformations can be used to transform all or part of the feed document in ways to make it suitable for display. Polling the Web An aggregator polls a specified URL, using HTTP, to find an information feed. Once the feed document has been retrieved, it is parsed and, depending on the approach taken by the aggregator writer, all or parts of the retrieved information is displayed to the users. Figure 2-1 shows the appearance of the information feed from www.tfosorcim.org/blog/index.rdf in Mozilla Thunderbird 0.9. 14 Chapter 2
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