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Identity It s necessary to identify things before you can talk about them, no matter how abstract the things are. For humans, identification comes down to naming or labeling the things of interest. Similarly, from a computer s point of view, naming and labeling are the key. The Internet is a big place, and there s a potential risk of two different things being given the same name. There are ways of avoiding this, however, by providing Uniform Resource Identifiers (URIs). Uniform and Unique To avoid ambiguity it s desirable to give anything else you want to talk about on the Web a URI. It isn t essential, and some things aren t very amenable to this kind of identification people, for example. But when the resources are on the Web anyhow, it can make life easier. There are really two pieces to identification on the Web, URIs, and URI References (URIRefs). A URI is generally the absolute part, of the form http://example.org/thing. A URIRef may be a URI, a relative reference, or have an additional fragment identifier like this: http://example.org/ thing#part. These are rather evasive concepts that seem to behave differently at different times, whether they re used as identifiers (such as in RDF) or as locators (over HTTP). The whole issue of URIs and URIRefs has proved something of a nightmare for the W3C s Technical Architecture Group, though fortunately in practice rarely causes problems. Aside from the arcane details of URIs themselves, there s an identity issue specific to syndication formats. To locate and obtain a feed in the first place it s necessary to know its URI, but when it comes to identifying individual items, there are complications. It can be difficult to tell the relationship between an item URI, a link URI, and any inline content there may be. This issue has its roots in the use of RSS as pure metadata, without any content. The original RSS 0.9 included a link element like this:
http://www.example.org/stuff … The item being described is the resource identified by that URI. Similarly, the RDF/XML-based RSS 1.0 give items an identifier like this: As the attribute name suggests, the properties of the item (title, description, and so on) will all be about the resource identified by that URI. But RSS 1.0 also adds a link for The item s URL. This has generally been taken to mean the URI of an HTML version of the item, and will usually be the same URI:
http://www.example.org/stuff … The non-RDF thread of RSS also included a link element, which appeared in RSS 0.91 looking the same as in RSS 0.9: 254 Chapter 20
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