Parallel Elements There are clear parallels between the (Web hosts)
Parallel Elements There are clear parallels between the entities supported by the different syndication formats after all, they share the same historical roots and aim to fulfill more or less the same purpose. The following table lines up these formats against each other, with the Model column using the word that s most commonly used when talking about these entities. Model RSS 1.0 RSS 2.0 Atom Feed rss:channel channel atom:feed Item rss:item item atom:entry Title dc:title title atom:title Date dc:date pubDate atom:issued Content dc:description, description, atom:content content:encoded xhtml:body Author dc:creator author atom:author There are various restrictions on the values that can appear in the elements and attributes in the RSS and Atom specifications. But as noted earlier, these aren t much for use as constraints, and for modeling purposes most can be reduced to simple strings. What causes a lot of work in practice is that the data appearing in the feeds is expressed in different ways, for example RSS 1.0 dates follow the W3CDTF (ISO 8601) format, whereas RSS 2.0 uses the RFC 822 format. But again this is a detail of the data, and doesn t really affect the overall models. Groups of Feeds Any application that uses data from multiple feeds will need to keep track of the feed addresses. In its simplest form, a subscription list could just be a set of feed URIs held in a text file. However, most applications will want to present the user with a little more information than this, such as the name of the feed, the author, and perhaps her e-mail address, and so on. Afairly common requirement is the ability to exchange lists of feeds or render them in a readable form. Afew alternate approaches and formats have appeared for this purpose, and support for them in syndication tools varies. You will see examples of these later in the chapter. Extending Structures RSS and Atom systems are generally designed for syndication, aggregation, and viewing of news-like material. As a result the formats primarily cover the essentials for this job: title, description, and so on. However, the technologies can allow much richer material to be syndicated and aggregated. In terms of the formats, this means including terms from other XML or RDF vocabularies in the feed. In terms of the model, this can mean that completely arbitrary structures can appear. In practice there aren t yet a great deal of extensions currently in use, it s something of a chicken and egg situation with the publication and subscription tools. 256 Chapter 20
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