XML Reference Guide

Mar 14, 2003

␡

⎙ Print

< Back Page 230 of 278 Next >

There is a perception in XML-land that RDF, or the Resource Description Framework, is somehow "complex" or "difficult to understand". This perception has been so strong that it has actually led to resistance to technologies that make use of it, such as the Semantic Web or RSS 1.0. It doesn't have to be that way.

Let's start at the beginning. RDF doesn't actually describe anything. There, I've said it. Just as XML isn't actually a language, but rather a system of rules for creating a new language -- documents must be well-formed, elements must be nested properly, and so on -- RDF is quite literally a framework for describing resources.

So what's a resource? A resource can be just about anything, but at this point it's usually something accessible via a URL, such as a web page or a graphic. (RDF isn't limited to these types of resources, but to keep it simple we'll limit this discussion to them for now.

Then what are we describing? We're describing properties of that resource. For example, we might make the following assertions:

The Vanguard Science Fiction Report has a creator of Nicholas Chase
XML Primer Plus has an ISBN of 0672324229
The InformIT Reference Guide has an image of 
                http://www.informit.com/images-psts/topic_icons/xml_e.gif

Now, those might sound a little awkward from a grammatical point of view, but that's how we phrase an assertion:

<resource> has a <property> of <literal value>

(You might also look at it as

Subject predicate object

)

You can take that triple and express it as a Resource Graph:

Resource Graph: VSFR has creator of Nicholas Chase

One requirement is that the resource that's being described -- in this case, The Vanguard Science Fiction Report -- has to be specified via a Uniform Resource Identifier (URI). The most common type of URI is a URL, or Uniform Resource Locator, so we can change the graph to show that, as in:

Resource Graph: VSFR (URL) has creator of Nicholas Chase

From here, that's pretty clear to a human reading it. Or is it? The term "creator" has lots of different meanings. We could mean the person who wrote the content, or the person who authored the web page, or even God, for that matter. So it would help if we could refer to a specific definition, such as "An entity primarily responsible for making the content of the resource."

Now, as it happens, there is such a defintion that we can reference. The Dublin Core Metadata Initiative (from Dublin Ohio, not Ireland) has created a set of definitions, such as creator, subject, type, and contributor. We could specify that we mean the Dublin Core creator property by including it's full identifier:

Resource Graph: VSFR (URL) has creator (URI) of Nicholas Chase

At this point the assertion is fairly unambiguous. We know what resource we're describing, and we're clear on exactly the property we're setting. Some might argue that the actual value, "Nicholas Chase," is still ambiguous, and you could, by rights, specify a URI that represents me, but the Dublin Core definition of "creator" recommends that you use a human readable name here. See, we're standardizing things already, and we haven't even put this into XML yet!

Speaking of which, let's do that. What we want to do is create an XML structure that defines the resource that we're describing and provides an unabiguous way of describing it. We'll start with an overall structure:

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <rdf:Description rdf:about="http://www.vanguardreport.com/">

  </rdf:Description>
</rdf:RDF>

Here we've simply specified that we're describing a resource, and the resource the descriptions are about is http://www.vanguardreport.com. We'll add the actual descriptions in a moment, but before we move on, let's take a good, hard look at what we've got here.

Specifically, note the use of namespaces. If you've only got a passing familiarity with namespaces, you may not have made the connection that the rdf: in rdf:RDF is actually just the alias. It's not the actual namespace. The actual namespace is http://www.w3.org/1999/02/22-rdf-syntax-ns#. This means that the element

rdf:RDF

is actually

http://www.w3.org/1999/02/22-rdf-syntax-ns#RDF

What this means for us is that we can use namespace information to easily specify not only a property, but the full URI for a property, as in:

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:dc="http://purl.org/dc/elements/1.1/">
  <rdf:Description rdf:about="http://www.vanguardreport.com/">
    <dc:title>The Vanguard Science Fiction Report</dc:title> 
    <dc:creator>Nicholas Chase</dc:creator>
  </rdf:Description>
</rdf:RDF>

By creating the Dublin Core namespace, with an alias of dc:, we've quickly specified the http://purl.org/dc/elements/1.1/creator and http://purl.org/dc/elements/1.1/title properties.

In some cases, the value of the property isn't a literal value, but a separate resource. For example, we can create a RSS 1.0 feed that includes RDF information:

(Note to the observant: Yes, the URL's have been shortened for display purposes.)

<?xml version="1.0" encoding="utf-8"?> 
<rdf:RDF 
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" 
  xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
  xmlns="http://purl.org/rss/1.0/"> 

  <channel rdf:about="http://www.informit.com/isapi/topics/guide_rss.xml">
    <title>InformIT :: XML Reference Guide</title>
    <link>http://www.informit.com/</link>
    <description>The latest weblog and content updates for the InformIT XML 
                 Reference Guide</description>
    <sy:updatePeriod>hourly</sy:updatePeriod>
    <sy:updateFrequency>6</sy:updateFrequency>
    <sy:updateBase>2003-01-01T12:00+00:00</sy:updateBase>

    <image rdf:resource="http://www.informit.com/images-psts/topic_icons/xml_e.gif" />

    <items>
      <rdf:Seq>
        <rdf:li rdf:resource=
           "http://www.informit.com/weblog/showComments.asp?weblog_id={367F6DB57E04}" />
        <rdf:li rdf:resource=
           "http://www.informit.com/weblog/showComments.asp?weblog_id={878C86686856}" />
      </rdf:Seq>
    </items>

  </channel>

  <image rdf:about="http://www.informit.com/images-psts/topic_icons/xml_e.gif">
    <title>InformIT XML Reference Guide</title>
    <url>http://http://www.informit.com/images-psts/topic_icons/xml_e.gif</url>
    <link>http://www.informit.com/</link>
  </image>

  <item rdf:about=
      "http://www.informit.com/weblog/showComments.asp?weblog_id={367F6DB57E04}">
    <title>Blog :: Netscape is dead; Long live Mozilla</title> 
    <link>http://www.informit.com/weblog/showComments.asp?weblog_id={367F6DB57E04}</link>
    <description>When Netscape released its browser as open-source, it was the Mozilla project 
      that took up the ...</description>
  </item> 

  <item rdf:about=
      "http://www.informit.com/weblog/showComments.asp?weblog_id={878C86686856}">
    <title>Blog :: RSS 2.0 transferred to neutral party</title>
    <link>http://www.informit.com/weblog/showComments.asp?weblog_id={878C86686856}</link>
    <description>Part of the RSS controversy has centered around the perception 
       that RSS 2.0 was somehow ...</description>
  </item>

</rdf:RDF>

We've got a few things going on in this file, so let's take them one at a time. First of all, just because there's no prefix for the RSS namespace (http://purl.org/rss/1.0/) doesn't mean that it doesn't define a set of properties. For example, the image element describes the http://purl.org/rss/1.0/image property of the http://www.informit.com/isapi/topics/guide_rss.xml resource. Second, it just so happens that the value of the image property isn't the string http://www.informit.com/images-psts/topic_icons/xml_e.gif, but rather the resource at that location.

(If the value were the string, it would have been written as:

<image>http://www.informit.com/images-psts/topic_icons/xml_e.gif</image>)

Finally, we're getting a brief glimpse of some of the structure that RDF can provide with the items element. The value of the items property is actually a sequence of resources. The Resource Description Framework defines a number of structures to assist in situations like this.

Overall, we couldn't even begin to cover all of what RDF has to offer in this short piece, but it all boils down to this: RDF creates a way to specify metadata, or data about data, by specifying a resource, the property, and the value of the property. Other organizations can then take this framework and define properties that everyone can agree on so that both people and automated systems can make sense of your information.

< Back Page 230 of 278 Next >

🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Privacy Notice

Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children

This site is not directed to children under the age of 13.

Marketing

Pearson may send or direct marketing communications to users, provided that

Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
Such marketing is consistent with applicable law and Pearson's legal obligations.
Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

As required by law.
With the consent of the individual (or their parent, if the individual is a minor)
In response to a subpoena, court order or legal process, to the extent permitted or required by law
To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
To investigate or address actual or suspected fraud or other illegal activities
To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020

Email Address