Content creators wishing to publish multiple versions of a given resource on the Web face a number of questions with respect to how such URIs are created, published and discovered. Questions include:
Given a resource
that can be delivered in a multiplicity of representations,
how should one publish the relevant URIs to enable automatic
discovery of these representations (AKA specific resources)?
How does one ensure that the alternative relationship amongst these various representations is available in a machine readable form, and consequently discoverable?
Here, multiple representations might include:
Representations appropriate for different delivery contexts
Alternative formats of the resource distinguished by
Different versions of the resource e.g., either by language or date
Representations in different languages
This document explores the issues that arise in this context, and attempts to define best practices that help:
Preserve the One Web while enabling content publishing to a multiplicity of delivery contexts.
Enable the creation of RESTful URIs that remain representation agnostic while delivering the correct end-user experience.
Enable automatic discovery of the available representations.
Enable web crawlers discover the relationship between a given generic resource and the specific resources that correspond to its various alternatives. This will help search engines build better Web indices and avoid the need to index all available alternatives of a given resource.
This document has been developed for discussion by the W3C Technical Architecture Group. This version, dated November 1, 2006 was approved by the TAG on its teleconference October 31, 2006; this approval hereby closes the TAG issue Generic-resources-53.
Publication of this finding does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time.
Additional TAG findings, both approved and in draft state, may also be available.
2 Use Case Scenarios
2.1 Publishing Desktop And Mobile Versions
2.1.1 Suggested Solution
2.2 Publishing In Multiple Languages
2.2.1 Suggested Solution
2.3 Publishing Continuously Updating Content
2.3.1 Suggested Solution
3 Recommended Best Practices
5 Open Issues
There has always been a need to serve user-agent specific content for a given URI — thus highlighting the distinction between Resource and Representation on the Web. The increasing importance of the mobile, multilingual Web makes this requirement even stronger. At the same time, published content (and its various representations) needs to be discoverable on the Web; as an example, crawlers and web-bots need to be able to discover the availability of alternative forms of a given resource. Documents published on the Web become discoverable via the hyperlinked structure of the Web; to enable discovery of alternative representations, the relation between these multiple representations needs to be captured by the hyperlink structure of the Web. This finding enumerates some of the issues faced by content creators on the Web today and proposes a sequence of best practices to foster the following long-term goals:
Preserve a Single Web i.e., a Web where content is universally accessible from a variety of end-user devices.
Ensure that the One Web enables the easy exchange of resources (and pointers to resources) across its different facets, i.e., mobile and desktop users should be able to share references to Web Resources (URIs) with the accessing user being able to retrieve an appropriate representation (specific resource).
Ensure that contents published to a given facet of the Web are linkable, discoverable, crawlable, searchable and browsable from any of its other facets.
Enable content providers clearly advertise the relationship between a given generic resource and the various specific resources that correspond to the available alternatives for that generic resource.
This section enumerates the candidate use case scenarios along with accompanying issues and suggested solutions. See the next section for recommended best practices that are a generalization of these solutions.
The owners of
would like to publish their content to a wide variety of end-user
devices ranging from desktop Web browsers to mobile devices such
as cell-phones and PDAs.
They also serve multiple geographies using different languages.
They know about the different markup language variants that are
currently in vogue on these devices, and are capable of
generating the representation that is most appropriate for the
accessing user-agent. In publishing their content and associated
URIs, they face the following issues.
Given generic resource
with corresponding alternatives for a desktop browser, a PDA
and a cell-phone:
Should the different alternatives have distinct URIs?
Should the generic resource have a single URI that delivers the appropriate representation?
If publishing distinct URIs for the resource and its various representations, how should the relationship between these URIs be expressed in a discoverable, machine-readable form? How should this relationship be reflected in the hyperlink structure of the Web?
We suggest the following approach for this situation:
Create representation-specific URIs
(specific resources) for each available
If no content negotiation is in place,
serve a canonical representation (generic resource) of the content at
With that same URI, use HTTP content-negotiation, along with the correct HTTP VARY headers to serve up the appropriate representation at access time. Ensure that the VARY headers capture the right parameters that were used to choose the representation that is being served — this is important for correct behavior when using cacheing proxies.
As an alternative to the previous step,
arrange for the server to generate an
redirect to automatically serve up
http://example.com/ubiquity is accessed by
This form of redirect involves an extra client/server
and may therefore be suboptimal for mobile devices.
This is a temporary redirect;
the accessing user-agent should continue to use the canonical URI
when creating bookmarks, or emailing URI.
Finally, note that to optimize link traversal out of the resulting
document, the content provider might wish to rewrite relative
links to point at the specific resource.
This will ensure that later uses of the URI results in
expected end-user results; e.g., In the following scenario:
Cell-phone user emails link
Recipiant opens message on a desktop
Clicks on the link
The user following the link from inside the email message on a desktop browser should receive the desktop version, and not the mobile version. Notice that passing around the canonical URI is critical in achieving this behavior.
Additionally, contrast this solution with using HTTP content-negotiation with VARY headers; using a redirect to the URI as a specific resource has the advantage of freezing all parameters that were used to choose that representation into the URI.
Use linking mechanisms provided by the
representation being served to create
links to the other available
representations. As an example, when using HTML, one
elements to advertize the availability of alternate
representations. In this context, note that there are
two distinct types of such links:
Links for human consumption that are to be presented to the user
And links for machine consumption, that are used by the user agent to provide additional functionality.
As an example, links to available alternatives meant
for human consumption might use the HTML
a element since these are rendered by
user-agents. In contrast, links meant for use by bots
might use the HTML
link element —
as an example, this reflects present practice when
publishing pointers to
In either case, notice that following these steps creates a mini-graph comprising of the canonical URI and URIs for its various representations.
The owners of
publish their content in a multiplicity of languages.
They wish to publish any given announcement at a
while retaining the ability to serve up a version in a language
that is most appropriate for the user.
Further, they wish to create URIs for
each available language to facilitate hyperlinking and
discovery. At the same time, they do not wish to hard-wire the
language in which a given announcement is accessed when such URIs
are passed around by end-users.
For a design pattern that has worked well over the years, see the W3C practice of publishing press releases in multiple languages. Here are its salient characteristics:
Press releases announced with a canonical URI.
Accessing this canonical URI with the appropriate
Language header results in an automatic redirect
that delivers the document in the desired language.
Each language version contains links to URI's that in turn serve a representation in one of the other available languages.
Since these translations are typically for
human consumption, these links
are encoded as HTML
a elements so that they get displayed in
The owners of
publish up-to-date content. Once published, they would like users
to be able to reliably bookmark the published content.
At the same time, they would like end-users to be able to always
access a canonical URL when looking for the most
recently published content.
The issue identified here has been faced by and solved successfully during the last few years by the blogging community.
Accessing a blog's canonical URI retrieves recent posts.
Posted items have a bookmark or permalink pointer that can be used to reliably access postings from the past.
Pointers to alternative content are encoded
link elements. This enables agents
such as blog-readers, content-aggregators and Web
crawlers to discover the availability of alternative
versions. Note that this design pattern is widely
deployed on the Web in the context of RSS/ATOM feeds
to advertize permalinks and other pointers to make
them discoverable. In the case of RSS/Atom feeds,
this has enabled Web sites to embed such links within
head element of HTML pages, and have
them revealed to the user by Web browsers
that are capable of consuming such feeds.
As can be seen from the use-cases and suggested solutions enumerated in the previous section, pointers to Web Resources (URIs) can either:
Be canonical URIs, i.e., have no context hard-wired. Such canonical URIs identify a generic resource.
Encapsulate partial context, e.g., language. Such URIs identify a specific resource that is one possible alternative of a generic resource.
Encapsulate multiple context bits, e.g., language and device profile.
Capture all context, i.e., the creator of the URI guarantees that all state is completely captured by the URI.
Our primary take-aways from the these observations are:
URIs are cheap, we suggest creating as many distinctive URIs as is meaningful.
The hyperlink structure of the Web is crucial for content discovery; when creating a multiplicity of URIs for a given canonical resource, ensure that the relationship amongst these multiple URIs is captured by the hyperlink structure of the content. This will ensure that Web user-agents (both human-facing as well as web crawlers) are able to discover the various available alternatives and even more importantly, discover the inter-relationship amongst these specific resources, and their mutual relationship to the generic resource.
Encourage users and user-agents to work with canonical URIs; leave it to the underlying infrastructure to generate appropriate redirects in order to serve users the appropriate representation (specific resource). For each such available representation that is generated as a function of user context, ensure that there is a URI that can reproduce that representation (specific resource) in the absence of user context; or equivalently: for every representation, ensure that there is a URI that hard-wires all user context e.g., language, device preference etc., required to generate that specific resource.
URIs are cheap.Create them as needed, publish them to the Web, and ensure that they are appropriately linked in to the rest of the Web. Thus, each representation of interest should get it's own URI (become a specific resource) and there should be one additional URI representing the generic resource.
Enable discoverability of alternative representations by leveraging the hyperlink structure of the Web. Thus, given one of the alternatives for a resource, ensure that one can reach the corresponding generic resource by traversing a contained hyperlink. When creating a generic resource with multiple alternatives, encode hyperlinks to the available alternatives with the generic resource. This will enable crawlers and other web agents discover the availability of these alternatives, and to establish the correct semantic linkage amongst the various alternatives.
Hyperlinks can be designed either for human consumption
a element), purely for machine consumption
link element), or both.
To maintain a single Web,
ensure that the hyperlink structure of the Web is leveraged to
create a graph structure whose transitive closure includes all
available representations of a given generic resource.
This finding has highlighted the need to capture the
relationship between a generic resource and its specific
alternatives. We have illustrated such linking using the present
practice of using
link elements with an appropriate
It would be useful for groups defining various hypertext formats
to arrive at a common set of values for the
that appropriately capture the various types of relationships
that are envisioned amongst a generic resources and its specific
alternatives --- for some initial ideas, see W3C Architecture: Generic
which sketches an ontology;
also, see TAG
Issue 51 (Standardized Field Values).
This figure shows a Generic Resource along with its multiple representations. In addition to its generic representation, the resource is available in print and mobile versions in both English and Japanese. URIs are assigned to each of these possible representations, and the illustration shows that these individual representations (specific resources) have links to/from the Generic Resource. Additional dotted arcs indicate that the content provider may create additional links that connect specific resources.