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the relationship between these two entities will be something like creator. Acommon relationship is is-a, as in the author is a person. This relationship crops up a lot in software where the entities are classes or type, and particular items are individual members of that class or have that type (in English you might say my home page is a Web page, for example). The entities themselves can be said to have attributes, a set of qualities of different types. For example, a person entity will probably have an attribute name, so you could say (in English) the author s name is Charles Dickens. The entity-relationship way of looking at systems led to the development of specific set of techniques for modeling, and this approach is (not surprisingly) known as Entity-Relationship Modeling (E-R). Most CASE (Computer-Aided Software Engineering) tools allow you to describe systems in terms of entities and relationships one way or another because it s a fundamental way of developing a data model. In the context of modeling feeds, it s not necessary to go the entire formal E-R route to take advantage of the basic ideas. For a particular kind of data, if you can select the entities, the attributes of those entities and the relationships between them, you have the essence of a data model. Entities in Feeds The entities in feeds roughly correspond to the elements in the XML format; there is the feed or channel itself, and the individual items. The feed has a set of attributes (title, description, and so on) as does each item. However, there is a slight snag in that some of the attributes could equally well be described as entities in their own right. The part of an RSS 2.0 feed might contain a whole document, the text in the
element will be the URL of a document on the Web. Although in modeling you are looking at things at a conceptual level, what you decide should be entities and what should be attributes of those entities will be determined very much by what you want to do with them in practice. In the context of this book that means storing and manipulating feed data. The approaches covered in this chapter are XML-based, relational database-based, Object-Oriented and RDF-based, and as you will see the E-R models would look a little different in each case. Relationships in Feeds Starting at the top, the primary relationship between a feed and its items is one of containment. This can be seen as a variation of the document-oriented view that, at a given point in time, the document retrievable from a feed URL contains items [X, Y, Z]. Amore useful view is that the conceptual feed (identified with the URI) contains every item ever published and every item that will ever be published from that address. This is a better fit for the item-oriented approach, although obviously some awareness of particular items will be needed there s not a lot useful information associated with blog posts that haven t yet been written. The relationship between an item and its associated elements is a little trickier to describe. On the one hand, the