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Copyright © 2006, 2007, 2008 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use, and software licensing rules apply.
The Web is designed to support flexible exploration of information by human users and by automated agents. For such exploration to be productive, information published by many different sources and for a variety of purposes must be comprehensible to a wide range of Web client software, and to users of that software.
HTTP and other Web technologies can be used to deploy resource representations that are self-describing: information about the encodings used for each representation is provided explicitly within the representation. Starting with a URI, there is a standard algorithm that a user agent can apply to retrieve and interpret such representations. Furthermore, representations can be what we refer to as grounded in the Web, by ensuring that specifications required to interpret them are determined unambiguously based on the URI, and that explicit references connect the pertinent specifications to each other. Web-grounding ensures that the specifications needed to interpret information on the Web can be identified unambiguously. When such self-describing, Web-grounded resources are linked together, the Web as a whole can support reliable, ad hoc discovery of information.
This finding describes how document formats, markup conventions, attribute values, and other data formats can be designed to facilitate the deployment of self-describing, Web-grounded Web content.
This document is an editors' copy that has no official standing.
This document has been produced for the W3C Technical Architecture Group (TAG). It is an editor's draft that has not been approved by the TAG, and it includes revisions motivated by discussions held at the December 2008 Face to Face Meeting of the TAG .
Additional TAG findings, both accepted and in draft state, may also be available. The TAG may incorporate this and other findings into future versions of the [AWWW].
Please send comments on this finding to the publicly archived TAG mailing list www-tag@w3.org (archive).
1 Introduction
2 The Web's Standard Retrieval Algorithm
3 Use of Widely Deployed Standards and Formats
4 Creating New Formats and Standards
4.1 Use Existing URI Schemes, Protocols, and Media Types
4.2 URI-based Extensibility
4.2.1 Example: The Atom Syndication Format
4.2.2 Example: Microformats
4.2.3 Self-describing XML Documents
5 RDF and the Self-Describing Semantic Web
5.1 Using RDFa To Produce Self-describing HTML
5.2 Using GRDDL to Bridge From XML To RDF
6 Accountability and Grounding Information in the Web
6.1 Grounding New Specifications in the Web
7 Conclusions
8 References
Supporting ad-hoc exploration is a goal of the Web. Users must therefore be able to get useful information from documents prepared by people whom they don't know, and with whom they have not coordinated in advance. The chapters below explain in more detail how the following techniques can be used to create, deploy and access self-describing Web resource representations that can be correctly interpreted using only widely available information (note that the terms resource and representation, as used here, are defined in [AWWW]):
Each representation should include standard machine-readable indications, such as HTTP Content-type headers, XML encoding declarations, etc., of the standards and conventions used to encode it.
Documents used as Web resource representations should, when practical, be encoded using
widely deployed formats such as text/html
and image/jpeg
, and deployed using HTTP.
Machine-processable specifications for interpreting new formats should be provided on the Web, and linked from representations that use the formats. Examples of linkable specifications include OWL ontologies, RDDL documents, GRDDL transformations, etc. By following links to such specifications, user agents can dynamically obtain information needed to process new representation formats.
Web resource representations should be grounded in the Web: i.e., the specifications required for their interpretation should be discoverable by recursively following links starting with the specification for URIs [URI].
For integration with the Semantic Web, self-describing representations should convey RDF triples,
either directly in the representation, by linking to the triples (perhaps using <link>
elements in HTML or the link:
header in HTTP), or by linking to transformations using technologies such as GRDDL.
A standard HTTP-based algorithm is used to deploy, retrieve and interpret self-describing Web resource representations.
Furthermore, when self-describing representations are linked together, the Web as a whole can support reliable, ad hoc discovery of information.
Principle
Self-describing resources promote ad hoc discovery of information.
Good Practice
Web resource representations should be self-describing.
The sections below discuss in more detail the techniques needed to create self-describing content for the Web, how to extend the Web with new formats that are themselves self-describing, how to publish self-describing Semantic Web data, why it's important that interpretation of Web representations be grounded unambiguously in the core specifications of the Web, and how a standard HTTP-based algorithm enables users to retrieve and interpret self-describing resource representations.
HTTP is the most widely deployed protocol on the Web, and it is designed to facilitate the deployment of self-describing Web resource representations. Indeed, there is a standard algorithm that a user agent can employ to attempt to obtain and interpret the representation of any Web resource that is accessible using the HTTP protocol. Consider the following example, which is representative of many simple Web interactions:
Bob is reading a Web page which includes a link to
http://example.com/todaysnews
.
Bob has had no previous contact with the owner of the referenced resource, and
his browser has not been specially configured for access to it.
The steps taken by Bob's browser when he clicks the link illustrate
a typical path through the standard retrieval algorithm of the Web
(readers unfamiliar with the HTTP protocol may find it useful to consult
either [HTTP], or one of the many HTTP introductions available on the Web).
Bob's browser...
parses the URI and, from the http:
at the beginning, determines that the http scheme has been used — this tells the browser that a representation retrieved using the HTTP protocol is authoritative
looks up the DNS name [DNS] example.com
to determine the associated IP address
opens a TCP stream to port 80 at the IP address determined above
formats an HTTP GET request for resource /todaysnews
, and sends that to the server:
GET /todaysnews HTTP/1.1 Host: example.com User-Agent: TAG Sample HttpClient v1.0 Accept: */* Accept-language: en-us
reads this response from the server:
HTTP/1.1 200 OK Date: Tue, 28 Aug 2007 01:49:33 GMT Server: Apache Content-Type: text/html; charset=utf-8 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>Today's news</title> </head> <body> <h1>Today's News: Oh boy!!</h1> [HTML FOR NEWS REPORT HERE] </body> </html>
from the status code (200) determines that the request has been successfully processed, and that a representation of the resource is available in the Content-Type
and the entity-body
inspects the returned Content-Type
and determines that it is UTF-8 encoded text/html
, a standard media type that the browser supports
passes the entity-body
to its HTML rendering engine, which uses
the markup in the HTML to determine the title of the page (Today's News), the rest of the document's structure, and so on — the browser presents the page to Bob
Neither Bob nor his browser has any advance knowledge of the nature of the
resource or the fact that its representation is provided in HTML,
yet the browser successfully retrieves the representation,
determines its format, and renders it for him.
The link could have been to an image/jpeg
picture, an application/atom+xml
feed, or to a document containing application/rdf+xml
data.
Bob's browser could in each case determine the format.
Indeed, as Bob continues to browse the Web,
his browser is able to determine the format of each representation that is retrieved, and can determine how to present it to him.
This example shows how HTTP enables the deployment of self-describing Web resources.
Consider instead a different example, in which Bob clicks on a link to ftp://example.com/todaysnews
.
Although Bob's browser can easily open an FTP connection to retrieve a file,
there is no
way for the browser to reliably determine the nature of the information received.
Even if the URI were ftp://example.com/todaysnews.html
the browser
would be guessing if it assumed that the file's contents were HTML,
since no normative specification
ensures that data from ftp URIs ending
in .html
is in any particular format.
The Web's retrieval algorithm works best when used with the core suite of protocols and formats that are most widely deployed, and that are capable of supporting retrieval of self-describing representations. These core technologies include: DNS, HTTP 1.1, HTML 4, XML, as well as widely deployed image formats such as image/jpeg and image/gif. As discussed in 5 RDF and the Self-Describing Semantic Web, RDF, OWL and GRDDL are among the additional core technologies that enable self-describing Semantic Web content. A flow diagram illustrating more details of the Web's standard retrieval algorithm is provided in A Diagram of the Web's Retrieval Algorithm.
Successful communication depends on the supplier and the consumer(s) of a document having a shared understanding of the information conveyed, and that in turn requires at least some shared assumptions about the form in which the information is represented. The simplest way to achieve this is if the media type, the document encoding, and any other conventions used for the representation are standards and are widely deployed.
Consider Susan, who buys a new digital camera.
The software supplied with her camera uploads photos to the Web
using the widely-deployed image/jpeg
media type,
and her Web server correctly labels served representations
with that Content-Type
.
Millions of user agents deployed around the world are preconfigured to
display Susan's photographs and to extract metadata
such as camera settings from them.
Search engines are likely to index them in helpful ways too.
Now consider instead Mary, who buys a different camera
with software that does not use
widely deployed Web formats.
Indeed, the camera's manufacturer has invented a
new "raw" file format that takes advantage of the camera's special features.
The provided photo management software not only uses that format
locally, it also uploads photos to Mary's Web server in that same form.
Indeed, it even uploads a .htaccess file,
configuring the server to label served representations with the
proprietary Content-Type image/x-fancyrawphotoformat
.
In this example, there are no outright violations of Web architecture,
but the decision to use an uncommon, proprietary, unregistered and apparently experimental
media type is unfortunate.
No existing Web user agents recognize the image/x-fancyrawphotoformat
media
type, search engine spiders are unlikely to extract useful information from
pictures in that format, and so on.
Unlike Susan's, which can be viewed by almost anyone, Mary's photos are
at best useful to a few people who have the proprietary software needed
to decode them.
Good Practice
Web resource representations should be published using widely deployed standards and formats.
The techniques described above apply in the many cases where widely
deployed media types such as image/jpeg
are sufficient, but
the Web is used for a broad and continually growing
range of information.
No fixed set of formats and standards can fully meet the need to
encode all such information for machine processing.
Of course, ways can be found to convey almost any information using standard
media types.
An employment record, for example, can be transmitted as either text/plain
or text/html
.
The resulting document may be quite suitable for browsing, but it
might not facilitate automated discovery of the
employee's name, his or her date of hire, and so on.
To meet such needs, new standards must be created, e.g. for
marking up the names and dates.
Similarly, the need may arise to use new values for
individual fields such as rel
attributes on HTML link
elements (see [TAGIssue51]).
So, although the Web requires self-describing documents that can be understood using only widely deployed standards, there is also a continual need for new formats and encoding conventions. How can new formats and encodings be deployed in a manner that is self-describing? The following sections explore ways of creating new formats and encoding conventions that maximize interoperability with existing Web infrastructure, and that can be used to create self-describing documents.
Innovations can be introduced to the Web at many different architectural layers. For example:
New URI schemes can be introduced
New transfer protocols can be deployed
New media types can be introduced
New namespace-qualified markup can be defined for XML
New RDF properties and ontologies can be defined for the Semantic Web
Often, a given capability could in principle be deployed at any of several different layers. For example, new sorts of content, such as movies, could be made available using new URI schemes and/or with new protocols, but doing so would require updating hundreds of millions of user agents, servers, proxies, and so on to understand these changes to the core mechanisms of the Web. Usually it is preferable to leverage the existing core mechanisms of the Web, such as http-scheme URIs and the HTTP protocol, as these are widely deployed. Indeed, one should usually leverage as many existing layers of the Web's architecture as is practical when introducing new function.
Good Practice
When extending the Web with new formats and functions, use existing URI schemes, protocols, and media types wherever practical.
One way to do this is to use URI-based extensibility within existing media types, as described in the sections below.
Many documents, particularly those that convey machine-readable data or messages, encode information using specifications that are specialized to particular purposes. Such specifications may cover details of particular data formats such as lists of customers or inventory records, results of scientific experiments, listings for television shows, details of university course offerings, information about molecular structures or drug tests, etc. Because of the great variety and number of such formats and their specifications, it's not practical to assume that even most of them will be directly implemented by typical Web user agents. Instead, the Web provides means by which the necessary specifications can be discovered, and to a significant degree implemented, dynamically and automatically. This is done by:
ensuring that every specification, and in many cases each markup tag name or data value used, is identified with a URI
ensuring that such URIs are used in the instance either directly as data values or tag names, or else to identify the encodings used
including in Web representations URIs that identify the specifications needed to interpret those representations
In other words, it should be possible to discover from each Web representation the conventions used to encode it, and particularly in cases where those conventions are not widely deployed, to find within the representations links to specifications, ontologies and/or programs necessary for interpreting the representation. So, just as the Web may be used to dynamically discover a great wealth of resources, it can also be used to dynamically discover the specifications, ontologies, or programs needed to interpret the representations of those resources.
Good Practice
Web representations should link to the information needed to support automatic processing of those representations.
The Atom Syndication Format [ATOM] is
an XML-based format for syndicating information about blogs
and other Web resources.
Atom syndications are served with with Content-Type application/atom+xml
,
and thus can be recognized by user agents.
ATOM entries
can include <atom:link>
elements such as
the following:
<entry> <title>An interesting picture</title> <link rel="enclosure" type="image/jpeg" length="12345" href="http://example.org/interestingPic"/> <content type="xhtml" xml:lang="en" xml:base="http://example.org/"> <div xmlns="http://www.w3.org/1999/xhtml"> <p><[Update: Here's an interesting picture.]</p> </div> </content> </link> </entry>
The link elements identify external resources, in this case an
image/jpeg
photograph.
Furthermore, each link can carry a rel
attribute which
specifies the relationship between the linked resource and the ATOM entry that
links it.
In the example above, the relationship is specified as enclosure
which, according to the ATOM specification, indicates that the linked
photograph may have been too large for inline processing with the rest
of the feed.
What's of interest for this finding is the fact that values of the rel
attribute are URIs (actually [IRI]s, which are the internationalized form of URIs), or else the values can be mapped to URIs.
This means that anyone, anywhere can invent a new sort of link relationship,
can assign a URI to identify that relationship, and can use that value
in the rel
attribute. For example:
<entry> <title>An interesting picture</title> <link rel="http://example.org/SomeNewATOMRelationship" type="image/jpeg" length="12345" href="http://example.org/interestingPic"/> <content type="xhtml" xml:lang="en" xml:base="http://example.org/"> <div xmlns="http://www.w3.org/1999/xhtml"> <p><[Update: Here's an interesting picture.]</p> </div> </content> </link> </entry>
Furthermore, anyone doing this can (and indeed should) provide information
about that new relationship via HTTP from the assigned URI.
For convenience, the ATOM specification also provides that short form names such as enclosure
in the first example can be registered with IANA,
and ATOM provides a deterministic mapping to a URI for each of these.
These URIs are formed by prepending the fixed base URI http://www.iana.org/assignments/relation/
to the short form.
Thus, the first example above is in fact using the relationship http://www.iana.org/assignments/relation/enclosure
.
This example shows how use of URIs for data values enables distributed assignment of new values. More importantly for this finding, the use of URIs for such values provides the opportunity for information about those values to be discovered dynamically on the Web.
[Microformats] provide a simple means of marking up data in
HTML Web pages.
The presence of a microformat is typically indicated by the appearance of
an identifying value such as vcard
in an HTML class attribute,
and particular data items are usually marked with other class values.
Indeed, somewhat confusingly, the [hCard] specification requires that hCards be identified with the root class name vcard
,
as in the example below. This hCard
provides contact information for the
North American office of the W3C:
<div class="vcard"> <a class="fn org url" href="http://www.w3.org/">World Wide Web Consortium</a> <div class="adr"> <span class="type">Work</span>: <div class="street-address">32 Vassar Street</div> <span class="locality">Cambridge</span>, <abbr class="region" title="Massachusetts">MA</abbr> <span class="postal-code">02139</span> <div class="country-name">USA</div> </div> <div class="tel"> <span class="type">Work</span> +1-617-253-2613 </div> <div class="tel"> <span class="type">Fax</span> +1-617-258-5999 </div> </div>
In general, microformats such as hCard are not self-describing, because there
is no requirement in the HTML media type specifications
that class attribute
values such as vcard
or type
be interpreted per the hCard specification.
Indeed, lacking any specific indication
that the resource owner has intended this interpretation, it is dangerous
for clients to assume hCard semantics — there is a real risk
that some HTML Web pages use values like type
,
value
or even in principle vcard
for
other purposes.
Unlike some other microformats, hCard does provide an option for deploying in a way that is self-describing. The hCard profile specifies a value for the profile attribute of the HTML 4.01 [HTML401] <HEAD>
element:
<head profile='http://www.w3.org/2006/03/hcard'>
and presence of this profile value indicates that class attributes can be reliably interpreted per the hCard specification. (Note, however, that there is ongoing discussion as to whether the profile attribute will be included as part of HTML 5, and if not, whether some other mechanism will be provided for signaling the use of extensions such as microformats.)
So, microformats are self-describing only when profiles or other
means licensed by a pertinent media type specification are used to enable them.
Unfortunately, few microformats have such profiles, and even when profiles
are available, evidence suggests that they are not universally applied.
User agents that infer the presence of microformats
without reliable indicators such as <HEAD>
element
profiles are at risk of extracting incorrect data from Web pages.
XML Namespaces [XMLNamespaces]
facilitate the creation of self-describing XML documents.
Given that a Web document is of media type
application/xml
, or in the family of
media types application/____+xml
,
recursive processing from the root element may be applied to
determine not just the overall nature of the document,
but also the meaning in context
of all sub-elements.
Doing this, however, requires understanding of the semantics of
each named element.
Although a few specific XML variants such as
application/xhtml+xml
may be directly supported by some
user agents, no user agent can build in support for the ever growing
set of XML languages used for Web representations.
This section describes how namespace documents, discoverable from the
XML tag names in the markup, can be used to make such languages
self-describing, and to enable automated processing of them.
When XML namespaces are used, each XML element is named with a qualified name, consisting of a prefix and a local name. In the following example, the root element has the qualified name <inventory:inventoryItem>
:
<inventory:inventoryItem xmlns:inventory="http://example.org/inventoryNamespace"> <inventory:itemNumber> 87354 </inventory:itemNumber> <inventory:quantityAvailable> 152 </inventory:quantityAvailable> </inventory:inventoryItem>
Qualified names map to expanded names such as {http://example.org/inventoryNamespace,inventoryItem}
, comprised of a namespace name URI (http://example.org/inventoryNamespace
) and a local name (inventoryItem
).
The namespace name URI serves at least two roles: the most obvious and the most widely understood is to distinguish expanded names in one namespace from those in another; the other role, and the one that is most important for purposes of this finding, is that it provides Web identification for the namespace itself.
The namespace is a Web resource, and like any other resource, it can and should provide representations using HTTP.
A user agent processing an XML document can retrieve descriptions of the namespaces used in that document, and
can use that retrieved information to determine how to correctly process the XML markup.
The TAG Finding "Associating Resources with Namespaces"
[NamespaceDocuments],
recommends the use of [RDDL] as a preferred means of
documenting namespaces.
RDDL is itself extensible, but it is commonly used to suggest XML Schemas (in any of several languages), XSLT Stylesheets, etc. that are usable with markup from the namespace being described.
Example: assume that user Bob is browsing the Web, and that he follows a link to a resource that returns the XML above as its representation.
Specifically, Bob's browser uses 2 The Web's Standard Retrieval Algorithm to retrieve the representation,
to determine its character encoding, and to discover that its Content-type
is application/inventory+xml
.
Of course, it's very unlikely that Bob's browser has built in knowledge of the inventory XML language, but the Content-type makes clear [XMLMediaType] that
the representation can be interpreted as XML with Namespaces.
The root element tag is from namespace http://example.org/inventoryNamespace
, which uses the http scheme,
so Bob's browser does an HTTP GET from that URI.
What comes back is a
RDDL document containing the following <rddl:resource>
element:
<rddl:resource xlink:role="http://www.w3.org/1999/XSL/Transform" xlink:arcrole="http://www.w3.org/1999/xhtml" xlink:href="http://example.org/InventoryToBrowsableHTML.xslt" xlink:title="Transform Inventory XML to HTML for Browsing"> </rddl:resource>
This designates a stylesheet (http://example.org/InventoryToBrowsableHTML.xslt
) that can be
applied to format the inventory XML as HTML — the
browser automatically retrieves and applies
the stylesheet, producing HTML that is
rendered on the screen.
Without any manual intervention from Bob, his browser automatically displays the inventory record in a format that is convenient to read and print.
Bob's browser may also be enabled for XML validation,
in which case it can look in the RDDL for a link to a
schema to validate the inventory markup.
Bob's browser has, by retrieving and processing the RDDL document, extended itself for processing of the inventory markup language. Unless the RDDL provides a link to one or more executable programs that process inventory records, it's unlikely that Bob's browser can automatically discover everything that one might reasonably want to know about processing inventory markup. Still, even the limited automatic function described above is very useful, and RDDL is an extensible framework that can be easily adapted to provide new kinds of information about namespaces. Note that because RDDL documents are themselves XML, GRDDL can be applied to derive RDF statements from them (see 5.2 Using GRDDL to Bridge From XML To RDF). In this way, self-describing XML documents can be integrated with the self-describing Semantic web. [NamespaceDocuments] describes this technique in more detail.
RDF [RDF] provides an interoperable means of publishing and linking self-describing Web data resources, and for integrating representations rendered using other technologies such as XML. The result is a single, global self-describing Semantic Web that integrates not only resources that are themselves built or represented using RDF, but also the other Web resources to which that RDF links, as well as those that can be mapped to RDF using technologies such as [GRDDL] . Readers unfamiliar with RDF should consult the RDF primer [RDFPrimer] or the N3 Primer [N3Primer] as a prerequisite to understanding the discussion below.
Each RDF statement is a triple consisting of a subject, a predicate (typically the identifier for a property, or for a relationship between two Web resources), and an object (the value of the property or the referent of the relationship).
The subject, the predicate, and often the object as well,
are themselves identified by URIs,
enabling the dynamic discovery introduced in 4.2 URI-based Extensibility above — if
a user agent has no built in knowledge of some particular RDF subject,
relationship, or object,
it can often use the URI to retrieve the
information necessary for processing.
Indeed, RDF's Schema [RDFSchema] and OWL Ontology technologies [OWL]
together offer
a standard, machine-processable means of describing relationships between RDF statements, e.g. that two seemingly
differing predicates are the "owl:sameAs
" each other.
As described in 2 The Web's Standard Retrieval Algorithm, the principal purpose of the Web's core retrieval algorithm is to obtain self-describing representations of Web resources. For the self-describing Semantic Web, the algorithm is extended to achieve a more particular goal: to directly obtain RDF triples that represent or indirectly obtain RDF triples that describe the referenced resource.
Consider Amy, who uses an RDF-enabled user agent to retrieve an RDF/XML document containing the following element:
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:employeeData="http://example.org/EmployeeInformation#"> <employeeData:employee rdf:about="http://example.org/Employees#BobSmith"> <employeeData:name>Bob Smith</employeeData:name> <employeeData:email rdf:resource="mailto:BobSmith@example.org"/> </employeeData:employee> </rdf:RDF>
The user agent is general purpose, and although it has
rules for certain commonly used ontologies, it
has no built in code to handle the employeeData
properties
in the above example.
To dynamically acquire the necessary function, the agent does an HTTP
GET for http://example.org/EmployeeInformation
.
The GET returns an OWL ontology, from which the agent discovers
that http://example.org/EmployeeInformation#email
is rdfs:subPropertyOf
the
http://www.w3.org/2001/vcard-rdf/3.0#email
property,
one that the agent recognizes as designating a person's e-mail address.
The agent offers Amy the option to send e-mail to Bob Smith.
Amy's browser has, like Bob's in the example above, automatically extended
itself for processing the employee data.
Note that in this case, the user agent has built in
capabilities corresponding to the URI,
HTTP, XML, RDF, RDFS and FOAF specifications.
Additional capability was enabled dynamically based on
the linked ontology information acquired at runtime.
Good Practice
Representations provided directly in RDF, or those for which automated means can be used to discover corresponding RDF, contribute to the self-describing Semantic Web.
Because its model is uniform, because all of its self-description is provided in the same model as the data itself, and because all RDF information is linked into the Web as a whole, RDF provides uniquely powerful facilities for dynamic integration of a self-describing Web. Therefore, it's particularly important that information not originally supplied in an RDF-specific format be convertible into RDF. The sections below discuss two means of doing this: the first shows how RDFa can integrate HTML documents into the Semantic Web, and the second illustrates the use of GRDDL to extract RDF from XML documents.
[RDFa] is a W3C Recommendation for embedding Semantic Web statements into XHTML Web pages (see also [RDFaSyntaxandProcessing]). The following example illustrates how RDFa can integrate HTML into the self-describing Semantic Web:
Mary is exploring the Web using a browser that has been enhanced with capabilities for interpreting RDFa. Her browser knows to look through each XHTML Web page that she browses, picking out information from the RDFa, and helping her to use it. For example, the page might contain the following HTML, which represents a FOAF-based contact listing. (This example is adapted from one in [RDFa]):
<?xml version="1.0" encoding="UTF-8"?> <html xmlns="http://www.w3.org/1999/xhtml" version="XHTML+RDFa 1.0" xml:lang="en"> <head> <title>FOAF/RDFa Demo</title> </head> <body> <div typeof="foaf:Person" xmlns:foaf="http://xmlns.com/foaf/0.1/" about="http://example.org/staff/Alice"> <p property="foaf:name"> Alice Birpemswick </p> <p> Email: <a rel="foaf:mbox" href="mailto:alice@example.com">alice@example.com</a> </p> <p> Phone: <a rel="foaf:phone" href="tel:+1-617-555-7332">+1 617.555.7332</a> </p> </div> </body> </html>
Even though this document is of media type
application/xhtml+xml
[XHTMLMediaType],
which is not a member of the RDF family
of media types, an RDFa-enabled user agent can extract RDF from this document.
This document conveys as RDF a set of semantic Web statements about the Web resource
http://example.org/staff/Alice
. The predicates are all named with the
same base URI http://xmlns.com/foaf/0.1/
, for which the
shorthand prefix foaf
is established in the HTML.
Using this syntax, the RDFa carries triples for relationships such as the
full name of the contact
(http://xmlns.com/foaf/0.1/#name
), which is Alice Birpemswick
,
the e-mail address (http://xmlns.com/foaf/0.1/#mbox
) which is
mailto:alice@example.com
,
and so on.
An RDFa-enabled user agent can extract these triples and use them
to help Mary work with the data they contain,
or to integrate with
other Semantic Web information.
Indeed RDF is designed for such use because,
as discussed above in 5 RDF and the Self-Describing Semantic Web,
Semantic Web triples are inherently self-describing.
If a user agent needs more information about the processing of
the email triple it can, like Amy's user agent, do an HTTP GET
for URI http://xmlns.com/foaf/0.1/
,
and use the results to
get more information.
That information may allow the agent
to automatically discover that,
in the example,
mailto:alice@example.org
can indeed
be used to send mail to the person
named Alice Birpemswick
.
The browser can then offer Mary the option to send e-mail to Alice,
or to add Alice to
her address book.
Good Practice
RDFa should be used to make information conveyed in HTML self-describing.
RDFa documents such as the one shown above are grounded in the Web of data when
served using the Content-type application/xhtml+xml
.
The specification for that media type [XHTMLMediaType]
designates it as a member of the
family of application/____+xml
media types,
and the specification for those types [XMLMediaType]
in turn provides for markup to be interpreted based on
the namespaces used in the document. Finally, the namespace document for XHTML
[XHTMLNamespace] allows use of XHTML with RDFa.
So, taken together, these specifications provide normatively for use of
RDFa in documents served with Content-type application/xhtml+xml
.
Note that in this case, the user agent has built in
capabilities corresponding to the
URI, HTTP, XML, XHTML and RDFa and FOAF specifications.
The server used
these technologies to publish, and there was successful communication.
RDFa provides a standard means of encoding RDF information in XHTML documents, but many other XML variants lack that capability. Furthermore, RDFa requires explicit encoding of each triple in the XHTML instance, and that may in some cases be impractical. [GRDDL] provides a standard means of extracting triples from a broad range of XML document formats. Each GRDDL-enabled XML document links to a transformation that, when applied to the document, produces RDF triples. Typically, the same GRDDL transformation can be used on entire families of similar XML documents.
For example, assume that Albert uses a GRDDL-enabled user agent to retrieve an XML document containing the following fragment:
<employees xmlns="http://example.org/employeeNS"> <employee name="Bob Smith"> <email>BobSmith@example.org</email> </employee> </employees>
Note that, unlike the earlier examples, this is neither in HTML nor in RDF;
we can assume that http://example.org/employeeNS
is a namespace
created by some particular business for use in its own business documents.
Albert's agent has no built in knowledge of this namespace, and so can
not do much with it.
Now assume that Albert instead retrieves a different document.
Most of the markup and data in it is identical to the first,
but this document is
GRDDL enabled:
<employees xmlns="http://example.org/employeeNS" xmlns:grddl="http://www.w3.org/2003/g/data-view#" grddl:transformation= "http://example.org/GRDDL_For_employeeNS.xsl> <employee name="Bob Smith"> <email>BobSmith@example.org</email> </employee> </employees>
Albert's user agent is GRDDL aware, so
it transforms the <employees>
information
to RDF using the supplied GRDDL_For_employeeNS.xsl
transformation, producing RDF triples
that the agent recognizes as designating Bob Smith's email address.
As in the example above, Albert's agent offers to send mail
to Bob Smith.
(The agent might also dynamically discover how to interpret the triples using the techniques described above in 5 RDF and the Self-Describing Semantic Web.)
Good Practice
GRDDL should be used to integrate XML documents into the self-describing Semantic Web.
Note that in this case, the user agent was programmed to understand URI, HTTP, XML, XHTML, GRDDL and the vCard ontology. The information needed for translation from the XML vocabulary into RDF was loaded dynamically.
The Web is an important medium for publishing information, and so it is often important to get agreement about what has in fact been published, and by whom. Consider the following example:
Senator Smith alleges that he has been libeled by an article published on the Web,
and so the senator files suit against the purported publishers.
He produces in court the log of a response to an HTTP GET
for URI http://publisher.example.com/oursenator.html
:
HTTP/1.1 200 OK Date: Tue, 22 Oct 2008 02:43:22 GMT Server: Apache Content-Type: text/html; charset=utf-8 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>The problem with Senator Smith</title> </head> <body> <h1>Senator Smith is a Liar and a Thief!</h1> <p>Our senator, Mr. Smith, steals money from children, and he lies on his income tax returns! </p> </body> </html>
When confronted by the judge, the owners of the Web site attempt to claim that they have not in fact published information about the senator. "Yes", they say, "our server did accept a TCP/IP connection at port 80 and it responded with some bits, but those bits don't mean what you think they mean. They might look to you like HTML, but that's not what we intended. We haven't said anything derogatory about the senator."
In fact, the specifications for the Web can be very useful in proving that a document was indeed published containing the statement: "Senator Smith is a Liar and a Thief!". The pertinent Web specifications indicate that the bits returned are to be interpreted as the UTF-8 encoding of Unicode characters, that those characters are to be interpreted as HTML, that the text within the HTML is to be read in English, and thus that the entity body is indeed to be read as containing the sentence quoted above.
How, though, can we know whether these specifications apply at all? Perhaps the publishers could claim that some other specifications applied? Unfortunately for them, the Web is clear on that point as well.
Starting with a URI such as http://localnewspaper.example.com/oursenator.html
and
the specification for URIs [URI], all the applicable
specifications for interpreting an http-scheme Web representation
can be unambiguously discovered by, recursively, following references to other specifications.
Using this specific example to illustrate:
The [URI] specification indicates that the registration of URI schemes is provided for in [RFC2717]. This in turn indicates that IANA maintains a registry of URI schemes, which is at [IANASchemeRegistry]. That page in turn shows that URIs employing the http scheme are governed by RFC 2616 [HTTP].
RFC 2616 describes the interpretation of the Content-type header, indicating
that the value of this header is an Internet Media Type, and provides a reference to [RFC1590] for looking up the interpretation of particular media types. RFC 1590 in turn indicates that a registry of such media types
is available from IANA, and from that [IANAMediaTypeRegistry] one can discover that the
documentation of media type text/html
is found at [HTMLMediaType]. This
finally provides the normative interpretation of the charset parameter, verifying that the
information is indeed to be interpreted as UTF-8, and that the markup in the HTTP entity body
is HTML.
Similar techniques can be used to determine that the lang
attribute in the HTML
does indeed call for text in the document to be read in English.
...and so on.
Of course, determining whether Senator Smith has been libeled is beyond the scope of Web Architecture. The architecture does, however, provide the means by which one can determine unambiguously the specifications that apply to interpretation of the data retrieved. In general, it is possible to determine which specifications apply to interactions with resources identified by any particular URI; as explained above, these specifications can be located by recursively following references from the specifications for URIs themselves. Whatever the other legal arguments about the case, the interpretation of the document is well defined.
Documents published using HTTP (or other schemes and protocols that are appropriately registered) are thus self-describing not just in the general sense of being interpretable using widely available information, but in the particular sense of having an interpretation that follows from the URI used for access and from the core specifications of the Web. We therefore refer to such documents as being grounded in the Web. From the URI referenced and the specification for URIs [URI], the other specifications for interacting with the resource and for interpreting its representations are determined unambiguously. Furthermore, this characteristic implies an important degree of accountability for those who serve information on the Web. As illustrated by the example above, Web architecture settles many important questions relating to what information has been published, and by whom.
As explained in 4 Creating New Formats and Standards, new formats and protocols are occasionally needed for use with the Web. For information published using those new technologies to be grounded in the Web, it's essential that the pertinent specifications be reachable by references, preferably in the form of Web links, from the specifications for existing Web technologies. Means by which this can be achieved include:
The new specification can be registered in a suitable registry. Examples of registries that are available today include the IANA registries for URI schemes [IANASchemeRegistry] or Internet media types [IANAMediaTypeRegistry]. Suitable registries are those that are themselves normatively linked or referenced from applicable specifications for the Web (e.g. the IANA scheme registry is discussed in [RFC2717], which in turn is referenced from [URI]). Registries like this can be extremely valuable, but there is also a logistical and social cost to maintaining them, and a cost to developers who must arrange for the registration of new entries. In return for these costs, the community benefits from avoidance of collisions in the allocation of identifiers, and in many cases from careful review of new registrations.
Existing specifications can be edited and republished to link explicitly to specifications for the new technology.
In certain cases, the new specification may be discoverable automatically. For example, when a new namespace is used in XML, it may be sufficient to publish a namespace document (see [XMLNamespaceDocuments]), as [XMLMediaType] is itself properly linked, and it already provides for extension of XML using Namespaces.
Good Practice
Specifications for interactions with Web resources, and for interpretation of resource representations, should be linked (directly or indirectly) from the specification for URIs [URI].
Ad hoc exploration of the Web is possible only if resource representations are self-describing. Using the techniques described above and starting with an http- or https-scheme URI, a user agent can proceed step by step to retrieve a representation, reliably discover the conventions that have been used to encode it, and if necessary, dynamically find instructions for processing it. Furthermore, it is possible starting with a URI and the specification for URIs [URI], to follow successive references to specifications that apply to interactions with the identified resource. These specifications, recursively, define the interpretation of information published in the Web.
Those who invent new document formats, new markup tags, or new conventions for encoding particular data values should use the techniques described above to make those formats self-describing, and to ensure that the pertinent specifications can be discovered by following references from existing ones. When these techniques are used, and when self-describing representations are linked together, the Web as a whole can support reliable, ad hoc discovery of information, and can grow to include ever more powerful languages and systems.