作者:傲气战歌网 来源:www.27zg.com 发表时间:2014-03-14 02:02 
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Extending RSSby Danny Ayers
July 23, 2003
The boom of weblogs has boosted interest in techniques for syndicating news-like material. In response a family of applications, known as aggregators or newsreaders, have been developed. Aggregators or newsreaders consume and display metadata feeds derived from the content. Currently there are two major formats for these data feeds: RSS 1.0 and RSS 2.0. Mark Pilgrim covers these two flavors of RSS in his XML.com article "What is RSS?"
The names are misleading -- the specifications differ not only in version number but also in philosophy and implementation. If you want to syndicate simple news items there is little difference between the formats in terms of capability or implementation requirement. However, if you want to extend into distributing more sophisticated or diverse forms of material, then the differences become more apparent.
The decision over which RSS version to favor really boils down to a single trade-off: syntactic complexity versus descriptive power. RSS 2.0 is extremely easy for humans to read and generate manually. RSS 1.0 isn't quite so easy, as it uses RDF. It is, however, interoperable with other RDF languages and is eminently readable and processible by machines.
This article shows how the RDF foundation of RSS 1.0 helps when you want to extend RSS 1.0 for uses outside of strict news item syndication, and how existing RDF vocabularies can be incorporated into RSS 1.0. It concludes by providing a way to reuse these developments in RSS 2.0 feeds while keeping the formal definitions made with RDF.
RSS 1.0 Terms Have a Formal DefinitionRSS 1.0 documents conforms to the RDF/XML Syntax Specification. This means that they are expressed in the language described in RDF Concepts and Abstract Syntax, which has the precise formal semantics defined in RDF Semantics. Unless you're a logician or have masochistic tendencies, you probably won't want to follow the path all the way to the formal base. For most developers the RDF Primer contains plenty to get started. The take-home message is that, unlike with plain XML, which is just syntax, there is well-known meaning that programs can derive from an RDF/XML document.
There is another part of the RDF specification that we need to consider when talking about RSS 1.0: RDF Schema. In the jargon, the RDF Schema specification defines an ontology language. An ontology gives names to concepts and relationships between those concepts. An ontology is really just a tightly controlled vocabulary; to some extent in this context the words "ontology", "vocabulary", and "schema" are interchangeable (in the RSS world, module is often used to refer to essentially the same thing).
RSS 1.0 may be a format defined in human language in the main specification document, but it is also an ontology that is specified in formal language in the RSS 1.0 RDF schema. Consider the RSS 1.0 snippet below.
<rdf:RDFThe example uses the <item> and <title> terms, and they can be found in the schema defined like this :
...The main things being said here are that item in RSS 1.0 is an RDF class and that title is a property. RDF classes more or less correspond to concepts, and properties are used to describe the relationships between those concepts. So returning to our example, it can be demonstrated that there's more being said in the example than is immediately obvious:
...This says first of all that the resource identified as #9 is an instance of the class item. The RDF/XML syntax provides a specific interpretation of the nesting of the XML, which allows us to determine that the resource has a property title, and the value of the property is the literal string "The Joy of Blogs". This still doesn't seem to offer much advantage over plain XML. But what we have isn't just given in terms of human-readable documentation, it's defined with unambiguous definitions throughout, traceable back to the logical formalism of RDF. These semantics allow us to not only make statements about the item but to reason programmatically with those statements.
