Explorer's Guide to the Semantic Web

In these scenarios, you can see quite a few Semantic Web areas in operation at the same time. Software agents (the digital assistants) are discovering meta data and information and processing it. Logical reasoning is not only used to make inferences, it’s also explained to the human user. Assessments of trust and reliability are deduced through networks of interacting information. We see the discovery of web services. It all seems so plausible and so useful.

1.2.1 Can the Semantic Web work this way?

The task is much bigger than the building of the original World Wide Web. At that time, few people realized how many new capabilities the Web would unleash. Today, some of the basic infrastructure is already in place. There are organizations like the World Wide Web Consortium (W3C; www.w3c.org), whose purpose includes developing and advancing standards of importance to the Internet as a whole, including the Semantic Web. So the task is bigger, but the starting point is more advanced.

Can the visions be realized? Opinions vary—mine is that many of them will come to pass (some are already beginning to operate) and make a real difference in the lives of people who use the Web.

1.3 The Semantic Web’s foundation

The Web is designed around resources, standardized addressing of those resources (Uniform Resource Locators and Uniform Resource Indicators), and a small, widely understood set of commands. It’s also designed to operate over very large and complex networks in a decentralized way. Let’s look at each of these design features.

4 I’m referring in part to the so-called REST (Representation State Transfer) architecture and the controversy over whether current SOAP-based web services that don’t use this model would be better suited to the Web if they did.

1.3.1 Resources

Strictly speaking, a resource itself is not retrieved, but only a representation of the resource. For some protocols, like File Transfer Protocol (FTP), the representation is normally a copy of a file. For others, like HTTP, the representation may or may not be a copy of a file. A resource can even be represented by different forms—a PDF file, an HTML page, a voice recording, and so on.

Second, and perhaps harder to grasp, a resource can be something that doesn’t yet exist, and that may never exist. A resource can be a concept or a reference to a real or fictitious person—something that can’t be addressed and transferred over a network, but that can be talked about, thought about. For the purposes of the Semantic Web, such a resource can be referred to or identified by a URI.5

1.3.2 Standardized addressing

5 For example, RFC 1737, “Functional Requirements for Uniform Resource Names” (a subset of URIs), says, “The purpose or function of a URN is to provide a globally unique, persistent identifier used for recognition, for access to characteristics of the resource or for access to the resource itself.” (Emphasis added.)

1.3.3 Small set of commands

However, some architectures use complex messages or need to restrict the visibility of message contents, and they use an approach that’s more involved than basic HTTP. Other Internet protocols can be used, and additional messaging layers can be carried over HTTP as well (such as SOAP, whose name no longer stands for anything). There is some controversy over what methods should be used for the Web—as distinct from the Internet, which includes much more than the World Wide Web—and whether the Semantic Web architecture should restrict itself to the simpler architecture of the current Web.

1.3.4 Scalability and large networks

Second, each transaction on the Web (that is, a request and the subsequent response) contains all the information needed to handle the request. No data needs to be stored by the server from one request to another. However, many practical uses of the Web do require that some data be saved for a period of time. If you reserve a ticket and then order it on another web page, the system must store your ticket reservation and be able to connect it to your request to purchase. Since any web transaction is separate from all others, it’s harder to arrange to maintain data across a connected series of transactions. Independent interactions make possible a large, decentralized system where responses can be cached to allow faster responses and reduce network traffic.

Data that maintains some history of transactions is sometimes called state, as in “the state of the system.” Web transactions are stateless.7

If there is a business need to store information across several interactions, the server must provide special arrangements to make it happen.

6 There is some controversy over whether the web model supports security provisions better than othernetwork architectures, such as Remote Procedure Call (RPC) systems.

7 When a cookie is stored on your computer, the cookie stores some state information. Unfortunately, this state doesn’t fit the web model well, so it can sometimes cause confusion between browser, server, and user.