XPackage is a way to describe how several "things" are grouped together and how they relate to each other. XPackage allows you to express statements such as the following:
My XML document contains two JPEG images named "image1.jpg" and "image2.jpg", it uses the Latin and Greek Unicode scripts, and should be styled according to the information in the CSS stylesheets "mystyle.css".
I'm giving you a collection of five images. I've noted which images are JPEG, which are PNG, and which are SVG.
On this CD-ROM is a local copy of my entire personal website. Some of the HTML pages reference images reside elsewhere on the Internet, and I've provided those images as well so no Internet connection is needed to access the information. Rather than modifying the original HTML pages, I've noted the original address of the images so that they may be displayed when the HTML pages request them by their original addresses.
Here are a series of images to be shown as a slide show. I've arranged them in the correct order in which they should be shown.
I've created a new XML namespace, and I've described which XML DTD is usually used to validate XML documents in that namespace, which stylesheets are usually used to present those documents, as well as my author and copyright information.
There is no standard syntax and vocabulary for expressing how resources are grouped together and how they relate. The W3C attempted to start an XML Packaging Working Group at one point [W3C XML Packaging WG Charter], but it never gained momentum and has since stopped work. The Open eBook Forum's OEB Publication Structure 1.0.1 created a simple packaging structure [OEBPS1], as did the IMS Content Packaging format [IMS Content Packaging], but neither claims to apply to packaging use cases in general. RDDL [RDDL] meets a small subset of packaging use cases, but is not extensible to meet all packaging use cases. Finally, the MPEG-21 Digital Identifier Declaration format [DIDL] attempts to provide a general packaging framework, but is not based upon any existing frameworks such as [RDF] or [XLink].
XPackage is really just an application of RDF that defines several ontologies, or sets of resources and properties, for use with RDF. In a general package description instance (XPackage used in generic RDF), the XPackage ontologies are not unlike other RDF ontologies such as the Dublin Core [DC] and Annotea.
rdf:about attribute?The rdf:about attribute assigns a unique identifier to a resource. (This attribute is so-named because it indicates which resource you are talking about.) A resource must be assigned a unique identifier using rdf:about before it can be referenced by another resource.
The use of the rdf:about attribute is similar to the common use of the id attribute in generic XML documents, except that rdf:about must always contain a complete Uniform Resource Identifier (URI).
rdf:about attribute contain a URI?Each of the resources described by XPackage and RDF is unique, and may at some point be described in other locations, not just within a package description document. Identifiers must therefore be globally unique, and simple identifiers such as "myBook" or "home-page" could easily clash with other resources described elsewhere. In RDF+XML, such simple identifiers are considered relative URIs, and are resolved to the local XML base designation or the URI of the XML document.
A URI allows identifiers to be partitioned into separate domains governed by each creator, reducing the likelihood of using duplicate identifiers. (See [RDF Primer] for more information.) It is easy for GlobalMentor, Inc., for example, to ensure that the URL identifier named "http://globalmentor.com/identfiers/myBook" does not clash with any other identifiers that begin with "http://globalmentor.com/identfiers/". Similarly, a URN identifier of "urn:isbn:123456789" can be ensured unique by the body that governs the allocations of ISBN number.
rdf:about attribute, does it "point" to anything?The rdf:about attribute can have a URL (one type of URI) as its value, but this URL need not refer to any particular document on the web. This URI serves merely as a unique identifier for the resource being described.
rdf:resource attribute?When one resource refers to another, it uses the other resource's identifier in an rdf:resource attribute. This value must match the value of the rdf:about attribute of another resource. The rdf:resource attribute is similar to the common use of the idref attribute in generic XML documents.
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