Strings to things
So far you have described the page using types and properties that are
inside the page itself. But if you have to update some information that
is common to many of your pages, that could be painful to roll out...
and even if you have an automated process for updating that information
across all of your pages, there is no guarantee that anything extracting
data from your site will extract all of the updates at one time.
Fortunately, the problem of providing one copy of information on the
web was solved at the same time the web was created: via the simple
power of the link! And structured data is no different; in fact,
linked data is a term that has emerged over the past few years
marking a more pragmatic approach to building a web of structured data
than the somewhat classically academic semantic web.
The following principles of linked data were first articulated by
Tim Berners-Lee in a 2006 design note:
- Use URIs as names for things
- Use HTTP URIs so that people can look up those names.
- When someone looks up a URI, provide useful information, using the standards (RDF*, SPARQL)
- Include links to other URIs. so that they can discover more things.
Keep these principles in mind as you work through the following steps!
Continue working with the HTML file that you have been editing so far,
or for a fresh start, copy ../1_book/step3/check_d.html into a new file.
Link the authors to external pages
There are many sources of identifiers for people on the web. Some
sources that you may find familiar include:
Assuming your underlying system has the ability to store and express
identifiers, you can help the machines disambiguate and retrieve more
information about your authors by linking to their identifiers from your
catalogue page. Use the sameas property to add links from your
simple text representation of the authors of this book to external
resources.
Hint: To save you time in looking up identifiers, here are a
few for Agnes M Herzberg:
- http://id.loc.gov/authorities/names/n84200859
- http://viaf.org/viaf/64300918
Check your markup
<!DOCTYPE html>
...
<body vocab="http://schema.org/" typeof="Book" resource="#book">
...
<td class="recordAuthor" property="author copyrightHolder" typeof="Person" resource="#author1">
<a href="/Author?lookfor=%22Herzberg%2C+Agnes+M%22">
<span resource="#author1">
<link property="sameAs" href="http://id.loc.gov/authorities/names/n84200859">
<link property="sameAs" href="http://viaf.org/viaf/64300918">
<span property="name">
<span property="familyName">Herzberg</span>,
<span property="givenName">Agnes M</span>
</span>
</span>
</a>
</span>
</td>
...
Note: While it might be tempting to use the url
property, that is normally reserved for linking to a URL where the
thing that is described is available (for example, linking to a
downloadable podcast or e-book). In contrast, sameAs is
used to link to a description of the thing.
Create separate pages for the authors in your own system
Take a look at how the page has developed over time; there is now a
lot of HTML markup just to describe the author, and you can imagine
more markup if you were to express all of the see from and
see also forms that might be contained in a local authority
record. If your system uses local authority records, in fact, they
are a perfect candidate for refactoring your markup. You can move
the bulk of the markup from the bibliographic record display page
into a separate page about the author, built on your local authority
record. Then, once it is a separately displayed page, then you
can simply link to it from this page... as well as from any other
pages that want to provide information about this author.
Create a new file named agnesMHerzberg.html in your text editor,
and copy the @resource="#author1" markup into the file.
As the new file describes a single type, you can move the
declaration of the type into the <body> element of
the new page, and you can (optionally) remove the @resource
attributes from the markup that you pasted into the file. Don't forget the
@vocab declaration! Use your existing page as a template.
Use the RDFa parsers to ensure that the markup in the new file
expresses the same information as it did in the original file.
Repeat these steps to create davidFAndrews.html, using the
@resource="#author2" markup as the source of interest.
Check your agnesMHerzberg.html
<!DOCTYPE html>
<html>
<head>
<title>Agnes M Herzberg</title>
</head>
<body vocab="http://schema.org/" typeof="Person" resource="#person">
<link property="sameAs" href="http://id.loc.gov/authorities/names/n84200859">
<link property="sameAs" href="http://viaf.org/viaf/64300918">
<span property="name">
<span property="familyName">Herzberg</span>,
<span property="givenName">Agnes M</span>
</span>
</body>
</html>
Check your davidFAndrews.html
<!DOCTYPE html>
<html>
<head>
<title>David F Andrews</title>
</head>
<body vocab="http://schema.org/" typeof="Person" resource="#person">
<span property="name">
<link property="sameAs" href="http://id.loc.gov/authorities/names/n84200861">
<link property="sameAs" href="http://viaf.org/viaf/291650334">
<link property="sameAs" href="http://www.freebase.com/m/0nfj58b">
<span property="familyName">Andrews</span>,
<span property="givenName">David F</span>
</span>
</body>
</html>
Link to the author page
Now, replace the inline markup in the original page with a simple link
to your new file. You still want to state that "Author Name" is the
author of the book using the @property="author"
assertion, but now you can either add that property directly to an
<a> element that links to your new file, or use the
resource attribute to link to the external file instead of
the internal markup. This is a
signal to any RDFa parser that the linked resource contains the data
for the named property.
Note: "when the element contains the href (or
src) attribute, @property is automatically
associated with the value of this attribute rather than the textual
content of the <a> element" (Adida, Ben;
Birbeck, Mark; Herman, Ivan; Sporny, Manu. RDFa 1.1 Primer - Second
edition). Using a @property attribute on the
same element as a @resource attribute works in a similar
fashion; the target of the @resource attribute is used as
the value of the @property attribute.
Check your markup
<!DOCTYPE html>
...
<body vocab="http://schema.org/" typeof="Book" resource="#book">
...
<tr valign="top">
<th>Weitere Autoren:</th>
<td class="recordSecAuthor" property="author copyrightHolder" resource="davidFAndrews.html#person">
<a href="/Author?lookfor=%22Andrews%2C+David+F%22">Andrews, David F</a>
</span>
</td>
</tr>
...
Checkpoint: Your original HTML page should now look
like ../1_book/step4/check_e.html and your new
author HTML pages should look like ../1_book/step4/agnesMHerzberg.html and ../1_book/step4/davidFAndrews.html.
Augment the author page
Now that you have created an entirely separate author page, you can
add much more information about the author; for example, you can
include an email address, links to their personal web sites and social
media accounts, a list of their publications and previous talks... far
more information than you would have wanted to publish inline in the
book itself.
Following the principles of linked data can lead not only to more
efficient maintenance of information and (potentially) more useful
results in search engines and other aggregators of data, but also to
a better information design and experience for your users.
Use the Person
properties to flesh out the "about this author" page with properties
such as address, birthDate, email, follows, and telephone. Be
adventurous, and remember to try to use nested types and ranges
appropriately!
Linking to authored works with @rev
If you tried creating a list of the works that David F Andrews has
written, you might have been frustrated by that, while the Person type has a performerIn, there is no
equivalent property for "author of". There is the author property for saying that
a book was written by a given person, but it only works in one direction:
the domain is CreativeWork and
the range is Organization or
Person.
Fortunately, rather than having to create two properties to cover both
possible directions for relationships between two entities, RDFa allows you
to use the @rev attribute to declare that the relationship
direction for this particular property is the reverse of what it would be
if you used @property.
Go ahead and use the @rev attribute to enhance your description of David F. Andrews to show that he
authored the following books:
- Data:
A Collection of Problems from Many Fields for the Student and Research
Worker
- Robust
Estimates of Location: Survey and Advances
- Symbolic
Computation for Statistical Inference
Check your markup
<!DOCTYPE html>
...
<body vocab="http://schema.org/" typeof="Person" resource="#person">
<span property="name">
<link property="sameAs" href="http://id.loc.gov/authorities/names/n84200861">
<link property="sameAs" href="http://viaf.org/viaf/291650334">
<link property="sameAs" href="http://www.freebase.com/m/0nfj58b">
<span property="familyName">Andrews</span>,
<span property="givenName">David F</span>
</span>
<h2>Publications:</h2>
<ul>
<li><a rev="author" typeof="Book"
href="http://www.amazon.ca/Data-Collection-Problems-Student-Research/dp/0387961259"><span
property="name">Data: A Collection of Problems from Many Fields for the
Student and Research Worker</span></a>
</li>
<li><a rev="author" typeof="Book"
href="http://www.amazon.ca/Robust-Estimates-Location-Survey-Advances/dp/0691081166"><span
property="name">Robust Estimates of Location: Survey and Advances</span></a>
</li>
<li><a rev="author" typeof="Book"
href="http://www.amazon.ca/Symbolic-Computation-Statistical-Inference-Andrews/dp/0198507054"><span
property="name">Symbolic Computation for Statistical Inference</span></a>
</li>
</ul>
</body>
</html>
Note: As with any advanced technique, a given schema.org consumer
might not understand the meaning of the @rev RDFa attribute.
Lessons learned
In this exercise, you learned:
- The basic principles of linked data
- How to combine
@property and @href
attributes to link to data on another page
- Some of the rich metadata you can express about people
on a page using the Person type!
- How to use
@rev to express the inverse relationship
of a property between two entities.
