However, this difference in structure can make it more difficult for people to read the RDF/XML document and actually see the relationships between the data, one of the more common complaints about RDF/XML. With non-RDF XML, you can, at a glance, see that the history element is directly related to this specific resource element and so on. In addition, even this small example demonstrates that RDF adds a layer of complexity on the XML that can be off-putting when working with it manually. Within an automated process, though, the RDF/XML structure is actually an advantage.
When processing XML, an element isn't actually complete until you reach its end tag. If an application is parsing an XML document into elements in memory before transferring them into another persisted form of data, this means that the elements that contain other elements must be retained in memory until their internal data members are processed. This can result in some fairly significant strain on memory use, particularly with larger XML documents.
RDF/XML, on the other hand, would allow you to process the first element quickly because its "contained" data is actually stored in another element somewhere else in the document. As long as the relationship between the two elements can be established through the URI, we'll always be able to reconstruct the original data regardless of how it's been transformed.
Another advantage to the RDF/XML approach is when querying the data. Again, in XML, if you're looking for a specific piece of data, you basically have to provide the entire structure of all the elements preceding the piece of data in order to ensure you have the proper value. As you'll see in RDF/XML, all you have to do is remember the triple nature of the specification, and look for a triple with a pattern matching a specific resource URI, such as a property URI, and you'll find the specific value. Returning to the RDF/XML shown earlier, you can find the reason for the specific movement just by looking for the following pattern:
<http://www.yasd.com/dynaearth/monsters3.htm> pstcn:reason ?
Rdf方便进行数据合并交换
Another reason you would use RDF/XML over non-RDF XML is the ability to join data from two disparate vocabularies easily, without having to negotiate structural differences between the two. Since the XML from both data sets is based on the same model (RDF) and since both make use of namespaces (which prevent element name collision—the same element name appearing in both vocabularies), combining data from both vocabularies can occur immediately, and with no preliminary work. This is essential for the Semantic Web, the basis for the work on RDF and RDF/XML. However, this is also essential in any business that may need to combine data from two different companies, such as a supplier of raw goods and a manufacturer that uses these raw goods.
Rdf不同于其他技术的特点,以及用rdf进行数据建模的优点
As excellent as these two reasons (less strain on memory and joining vocabularies) are for utilizing RDF as a model for data and RDF/XML as a format, for certain instances of data stored on the Web, RDF is clearly not a replacement. As an example, RDF is not a replacement for XHTML for defining web pages that are displayed in a browser. RDF is also not a replacement for CSS, which is used to control how that data is displayed. Both CSS and XHTML are optimized for their particular uses, organizing and displaying data in a web browser. RDF's purpose differs—it's used to capture specific statements about a resource, statements that help form a more complete picture of the resource. RDF isn't concerned about either page organization or display.
何时选择RDF
Ultimately, the decision about using RDF/XML in place of XML is based on whether there's a good reason to do so—a business rather than a technical need to use the model and related XML structure. If the data isn't processed automatically, if it isn't persisted and combined with data from other vocabularies, and if you don't need RDF's optimized querying capability, then you should use non-RDF XML. However, if you do need these things, consider the use of RDF/XML.
XML 和RDF 和OWL的关系
The following quote from the OWL Use Cases and Requirements document, one of many the Ontology Working Group is creating, defines the relationship between XML, RDF/XML, and OWL:
The Semantic Web will build on XML's ability to define customized tagging schemes and RDF's flexible approach to representing data. The next element required for the Semantic Web is a Web ontology language which can formally describe the semantics of classes and properties used in web documents. In order for machines to perform useful reasoning tasks on these documents, the language must go beyond the basic semantics of RDF Schema.
RSS使用RDF的值得怀疑的地方
Given the transitory nature of RSS feeds as I just described them, it is difficult to justify the use of RDF for RSS. If RDF's purpose is to record assertions about resources that can be discovered and possibly merged with other assertions to form a more complete picture of the resource, then that implies some form of permanence to this data, that the data hangs around long enough to be discovered. If the data has a life span of only a minute, hour, or day, its use within a larger overall "semantic web" tends to be dubious, at best.
究竟何时可以使用rdf
I'll cover the issue of RSS in more detail in Chapter 13, but for now the point to focus on is that when to use RDF isn't always obvious. The key to knowing when to make extra effort necessary to overlay an RDF model on the data isn't necessarily based on the original purpose for the data or even the transitory nature of the data—but on the data itself. If the data is of interest, descriptive, and not easily discovered by any other means, little RDF alarms should be ringing in our minds.
