The World Wide Web uses relatively simple technologies with sufficient
scalability, efficiency and utility that they have resulted in a
remarkable information space of interrelated
resourcesinterconnected space of information
and services, growing across languages, cultures, and media. In an effort to preserve these properties of
the information space as the technologies
evolve, this architecture document discusses the core design components
of the Web. They are identification of resourcesinformation and
services, representation of resource
stateinformation state and service
requests, and the protocols that support the interaction between
agents and resources in the space. We relate
core design components, constraints, and good practices to the
principles and properties they support.
This is an unofficial draft and work in progress. It has no official
standing: rather it represents a trial balloon to help the TAG decide whether to proceed with a
second edition of AWWW or
not.
This draft highlights most differences from the first edition of
AWWW, with deletions presented like this and insertions presented like this.
This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical
report can be found in the W3C
technical reports index at http://www.w3.org/TR/.
This document has been developed by W3C's Technical Architecture Group (TAG),
which, by charter
maintains a list of
architectural issues. The scope of this document is a useful subset
of those issues; it is not intended to address all of them. The TAG
intends to address the remaining (and future) issues after publication
of Volume Two as a Recommendation.
This document uses concepts and terms regarding URIs in general, and http: URIs in particular, as
defined by the IETF. In an
18 Oct 2004 announcement, the revision of RFC2396 was endorsed as
an IETF Specification, though the latest published draft as of this
writing is draft-fielding-uri-rfc2396bis-07.
The [URI]citation should reflect
publication of the relevant RFC in future revisionsspecification is the primary normative reference for URIs in
general. For http: (and https:, and the HTTP
protocol), [[!HTTP11]] is the specification currently in force, but
[[!HTTPbis]], which will update it, is nearing final approval, and we
assume it will be in force by the time this second edition is
completed.
The references and caveats wrt HTTP(bis) above will need to be
corrected as and when HTTPbis is approved HTTP11.2e, or whatever.
List of Principles, Constraints, and Good Practice Notes
The following principles, constraints, and good practice notes are
discussed in this document and listed here for convenience. There is
also a free-standing summary.
The World Wide Web (WWW, or simply
Web) is an information space in which the items
of interest, referred to as resources, are identified by global
identifiers called Uniform Resource Identifiers
(URI).
Examples such as the following travel scenario are used
throughout this document to illustrate typical behavior of Web
agents—people or software acting on this information space. A
user agent acts on behalf of a user. Software agents include
servers, proxies, spiders, browsers, and multimedia players.
Story
While planning a trip to Mexico, Nadia reads “Oaxaca weather
information: 'http://weather.example.com/oaxaca'” in a glossy
travel magazine. Nadia has enough experience with the Web to
recognize that "http://weather.example.com/oaxaca" is a URI and
that she is likely to be able to retrieve associated information
with her Web browser. When Nadia enters the URI into her browser:
The browser recognizes that what Nadia typed is a URI.
The browser performs an information retrieval action in
accordance with its configured behavior for resources identified
via the "http" URI scheme.
The authority responsible for "weather.example.com" provides
information in a response to the retrieval request.
The browser interprets the response, identified as XHTML by the
server, and performs additional retrieval actions for inline
graphics and other content as necessary.
The browser displays the retrieved information, which includes
hypertext links to other information. Nadia can follow these
hypertext links to retrieve additional information.
This scenario illustrates the three architectural bases of the Web that
are discussed in this document:
Identification. URIs are used to identify resources. In this
travel scenario, the resource is a periodically updated report on
the weather in Oaxaca, and the URI is
“http://weather.example.com/oaxaca”.
Interaction. Web agents communicate using standardized
protocols that enable interaction through the exchange of messages
which adhere to a defined syntax and semantics. By entering a URI
into a retrieval dialog or selecting a hypertext link, Nadia tells
her browser to perform a retrieval action for the resource
identified by the URI. In this example, the browser sends an HTTP
GET request (part of the HTTP protocol) to the server at
"weather.example.com", via TCP/IP port 80, and the server sends
back a message containing what it determines to be a representation
of the resource as of the time that representation was generated.
Note that this example is specific to hypertext browsing of
information—other kinds of interaction are possible, both within
browsers and through the use of other types of Web agent; our
example is intended to illustrate one common interaction, not
define the range of possible interactions or limit the ways in
which agents might use the Web.
Formats. Most protocols used for representation retrieval
and/or submission make use of a sequence of one or more messages,
which taken together contain a payload of representation data and
metadata, to transfer the representation between agents. The choice
of interaction protocol places limits on the formats of
representation data and metadata that can be transmitted. HTTP, for
example, typically transmits a single octet stream plus metadata,
and uses the "Content-Type" and "Content-Encoding" header fields to
further identify the format of the representation. In this
scenario, the representation transferred is in XHTML, as identified
by the "Content-type" HTTP header field containing the registered
Internet media type name, "application/xhtml+xml". That Internet
media type name indicates that the representation data can be
processed according to the XHTML specification.
Nadia's browser is configured and programmed to interpret the
receipt of an "application/xhtml+xml" typed representation as an
instruction to render the content of that representation according
to the XHTML rendering model, including any subsidiary interactions
(such as requests for external style sheets or in-line images)
called for by the representation. In the scenario, the XHTML
representation data received from the initial request instructs
Nadia's browser to also retrieve and render in-line the weather
maps, each identified by a URI and thus causing an additional
retrieval action, resulting in additional representations that are
processed by the browser according to their own data formats (e.g.,
"application/svg+xml" indicates the SVG data format), and this
process continues until all of the data formats have been rendered.
The result of all of this processing, once the browser has reached
an application steady-state that completes Nadia's initial
requested action, is commonly referred to as a "Web page".
The following illustration shows the relationship between identifier,
resource, and representation.
About this Document
This document describes the properties we desire of the Web and the
design choices that have been made to achieve them. It promotes the
reuse of existing standards when suitable, and gives guidance on how
to innovate in a manner consistent with Web architecture.
The terms MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY are used in the
principles, constraints, and good practice notes in accordance with
RFC 2119 [[!RFC2119]].
This document does not include conformance provisions for these
reasons:
Conforming software is expected to be so diverse that it would
not be useful to be able to refer to the class of conforming software
agents.
Some of the good practice notes concern people; specifications
generally define conformance for software, not people.
We do not believe that the addition of a conformance section is
likely to increase the utility of the document.
Audience of this Document
This document is intended to inform discussions about issues of Web
architecture. The intended audience for this document includes:
Participants in W3C Activities
Other groups and individuals designing technologies to be
integrated into the Web
Implementers of W3C specifications
Web content authors and publishers
Note: This document does not distinguish in any
formal way the terms "language" and "format." Context determines
which term is used. The phrase "specification designer" encompasses
language, format, and protocol designers.
Scope of this Document
This document presents the general architecture of the Web. Other
groups inside and outside W3C also address specialized aspects of
Web architecture, including accessibility, quality assurance,
internationalization, device independence, and Web Services. The
section on Architectural Specifications includes references
to these related specifications.
This document strives for a balance between brevity and precision
while including illustrative examples. TAG findings are
informational documents that complement the current document by
providing more detail about selected topics. This document includes
some excerpts from the findings. Since the findings evolve
independently, this document includes references to approved TAG
findings. For other TAG issues covered by this document but without
an approved finding, references are to entries in the TAG issues list.
Many of the examples in this document that involve human activity
suppose the familiar Web interaction model (illustrated at the
beginning of the Introduction) where a person follows a link via a
user agent, the user agent retrieves and presents data, the user
follows another link, etc. This document does not discuss in any
detail other interaction models such as voice browsing (see, for
example, [[!VOICEXML20]]). The choice of interaction model may have
an impact on expected agent behavior. For instance, when a
graphical user agent running on a laptop computer or hand-held
device encounters an error, the user agent can report errors
directly to the user through visual and audio cues, and present the
user with options for resolving the errors. On the other hand, when
someone is browsing the Web through voice input and audio-only
output, stopping the dialog to wait for user input may reduce
usability since it is so easy to "lose one's place" when browsing
with only audio-output. This document does not discuss how the
principles, constraints, and good practices identified here apply
in all interaction contexts.
Principles, Constraints, and Good Practice Notes
The important points of this document are categorized as follows:
Principle
An architectural principle is a fundamental rule that applies to
a large number of situations and variables. Architectural
principles include "separation of concerns", "generic interface",
"self-descriptive syntax," "visible semantics," "network effect"
(Metcalfe's Law), and Amdahl's Law: "The speed of a system is
limited by its slowest component."
Constraint
In the design of the Web, some choices, like the names of the
p and li elements in HTML, the choice
of the colon (:) character in URIs, or grouping bits into
eight-bit units (octets), are somewhat arbitrary; if
paragraph had been chosen instead of p
or asterisk (*) instead of colon, the large-scale result would,
most likely, have been the same. This document focuses on more
fundamental design choices: design choices that lead to
constraints, i.e., restrictions in behavior or interaction within
the system. Constraints may be imposed for technical, policy, or
other reasons to achieve desirable properties in the system, such
as accessibility, global scope, relative ease of evolution,
efficiency, and dynamic extensibility.
Good practice
Good practice—by software developers, content authors, site
managers, users, and specification designers—increases the value
of the Web.
Identification
In order to communicate internally, a community agrees (to a reasonable
extent) on a set of terms and their meanings. One goal of the Web,
since its inception, has been to build a global community in which any
party can share information with any other party. To achieve this goal,
the Web makes use of a single global identification system: the URI.
URIs are a cornerstone of Web architecture, providing identification
that is common across the Web. The global scope of URIs promotes
large-scale "network effects": the value of an identifier increases the
more it is used consistently (for example, the more it is used in
hypertext links).
This principle dates back at least as far as Douglas Engelbart's
seminal work on open hypertext systems; see section Every Object
Addressable in [[!Eng90]].
Benefits of URIs
The choice of syntax for global identifiers is somewhat arbitrary; it
is their global scope that is important. The Uniform Resource
Identifier, [[!URI]], has been successfully deployed since the
creation of the Web. There are substantial benefits to participating
in the existing network of URIs, including linking, bookmarking,
caching, and indexing by search engines, and there are substantial
costs to creating a new identification system that has the same
properties as URIs.
To benefit from and increase the value of the World Wide Web,
agents should provide URIs as identifiers for resources.
A resource should have an associated URI if another party might
reasonably want to create a hypertext link to it, make or refute
assertions about it, retrieve or cache a representation of it,
include all or part of it by reference into another representation,
annotate it, or perform other operations on it. Software developers
should expect that sharing URIs across applications will be useful,
even if that utility is not initially evident. The TAG finding
"URIs,
Addressability, and the use of HTTP GET and POST"
discusses additional benefits and considerations of URI
addressability.
Note: Some URI schemes (such as the "ftp" URI scheme
specification) use the term "designate" where this document uses
"identify."
URI/Resource Relationships
By design a URI identifies one resource. We do not limit the scope of
what might be a resource. The term "resource" is used in a
general sense for whatever might be identified by a URI. It is
conventional on the hypertext Web to describe Web pages, images,
product catalogs, etc. as “resources”. The distinguishing
characteristic of these resources is that all of their essential
characteristics can be conveyed in a message. We identify this set as
“information resources.”
This document is an example of an information resource. It consists
of words and punctuation symbols and graphics and other artifacts
that can be encoded, with varying degrees of fidelity, into a
sequence of bits. There is nothing about the essential information
content of this document that cannot in principle be transfered in a
message. In the case of this document, the message payload is the
representation of this document.
However, our use of the term resource is intentionally more broad.
Other things, such as cars and dogs (and, if you've printed this
document on physical sheets of paper, the artifact that you are
holding in your hand), are resources too. They are not information
resources, however, because their essence is not information.
Although it is possible to describe a great many things about a car
or a dog in a sequence of bits, the sum of those things will
invariably be an approximation of the essential character of the
resource.
We define the term “information resource” because we observe that it
is useful in discussions of Web technology and may be useful in
constructing specifications for facilities built for use on the Web.
Since the scope of a URI is global, the resource identified by a URI
does not depend on the context in which the URI appears (see also the
section about indirect identification).
[[!URI]] is an agreement about how the Internet community allocates
names and associates them with the resources they identify. URIs are
divided into schemes that define, via their scheme
specification, the mechanism by which scheme-specific identifiers are
associated with resources. For example, the "http" URI scheme
([[!HTTP11]]) uses DNS and TCP-based HTTP servers for the purpose of
identifier allocation and resolution. As a result, identifiers such
as "http://example.com/somepath#someFrag" often take on meaning
through the community experience of performing an HTTP GET request on
the identifier and, if given a successful response, interpreting the
response as a representation of the identified resource. (See also
Fragment Identifiers.) Of course, a retrieval action like GET
is not the only way to obtain information about a resource. One might
also publish a document that purports to define the meaning of a
particular URI. These other sources of information may suggest
meanings for such identifiers, but it's a local policy decision
whether those suggestions should be heeded.
Just as one might wish to refer to a person by different names (by
full name, first name only, sports nickname, romantic nickname, and
so forth), Web architecture allows the association of more than one
URI with a resource. URIs that identify the same resource are called
URI aliases. The section on URI aliases discusses
some of the potential costs of creating multiple URIs for the same
resource.
Several sections of this document address questions about the
relationship between URIs and resources, including:
How much can I tell about a resource by inspection of a URI that
identifies it? See the sections on URI schemes and URI
opacity.
Who determines what resource a URI identifies? See the section on
URI allocation.
By design, a URI identifies one resource. Using the same URI to
directly identify different resources produces a URI
collision. Collision often imposes a cost in communication
due to the effort required to resolve ambiguities.
Suppose, for example, that one organization makes use of a URI to
refer to the movie The Sting, and another organization
uses the same URI to refer to a discussion forum about The
Sting. To a third party, aware of both organizations, this
collision creates confusion about what the URI identifies,
undermining the value of the URI. If one wanted to talk about the
creation date of the resource identified by the URI, for instance,
it would not be clear whether this meant "when the movie was
created" or "when the discussion forum about the movie was
created."
Social and technical solutions have been devised to help avoid URI
collision. However, the success or failure of these different
approaches depends on the extent to which there is consensus in the
Internet community on abiding by the defining specifications.
The section on URI allocation examines
approaches for establishing the authoritative source of information
about what resource a URI identifies.
URIs are sometimes used for indirect identification. This
does not necessarily lead to collisions.
URI allocation
URI allocation is the process of associating a URI with a resource.
Allocation can be performed both by resource owners and by other
parties. It is important to avoid URI collision.
URI ownership
URI ownership is a relation between a URI and a social
entity, such as a person, organization, or specification. URI
ownership gives the relevant social entity certain rights,
including:
to pass on ownership of some or all owned URIs to another
owner—delegation; and
to associate a resource with an owned URI—URI allocation.
By social convention, URI ownership is delegated from the IANA
URI scheme registry [[!IANASchemes]], itself a social entity, to
IANA-registered URI scheme specifications. Some URI scheme
specifications further delegate ownership to subordinate
registries or to other nominated owners, who may further delegate
ownership. In the case of a specification, ownership ultimately
lies with the community that maintains the specification.
The approach taken for the "http" URI scheme, for example,
follows the pattern whereby the Internet community delegates
authority, via the IANA URI scheme registry and the DNS, over a
set of URIs with a common prefix to one particular owner. One
consequence of this approach is the Web's heavy reliance on the
central DNS registry. A different approach is taken by the URN
Syntax scheme [[!RFC2141] which delegates ownership of portions
of URN space to URN Namespace specifications which themselves are
registered in an IANA-maintained registry of URN Namespace
Identifiers.
URI owners are responsible for avoiding the assignment of
equivalent URIs to multiple resources. Thus, if a URI scheme
specification does provide for the delegation of individual or
organized sets of URIs, it should take pains to ensure that
ownership ultimately resides in the hands of a single social
entity. Allowing multiple owners increases the likelihood of URI
collisions.
URI owners may organize or deploy infrastruture to ensure that
representations of associated resources are available and, where
appropriate, interaction with the resource is possible through
the exchange of representations. There are social expectations
for responsible representation management
by URI owners. Additional social implications of URI ownership
are not discussed here.
See TAG issue siteData-36,
which concerns the expropriation of naming authority.
Other allocation schemes
Some schemes use techniques other than delegated ownership to
avoid collision. For example, the specification for the data URL
(sic) scheme [[!RFC2397]] specifies that the resource identified
by a data scheme URI has only one possible representation. The
representation data makes up the URI that identifies that
resource. Thus, the specification itself determines how data URIs
are allocated; no delegation is possible.
Other schemes (such as "news:comp.text.xml") rely on a social
process.
Indirect Identification
To say that the URI "mailto:nadia@example.com" identifies both an
Internet mailbox and Nadia, the person, introduces a URI collision.
However, we can use the URI to indirectly identify Nadia.
Identifiers are commonly used in this way.
Listening to a news broadcast, one might hear a report on Britain
that begins, "Today, 10 Downing Street announced a series of new
economic measures." Generally, "10 Downing Street" identifies the
official residence of Britain's Prime Minister. In this context,
the news reporter is using it (as English rhetoric allows) to
indirectly identify the British government. Similarly, URIs
identify resources, but they can also be used in many constructs to
indirectly identify other resources. Globally adopted assignment
policies make some URIs appealing as general-purpose identifiers.
Local policy establishes what they indirectly identify.
Suppose that nadia@example.com is Nadia's email
address. The organizers of a conference Nadia attends might use
"mailto:nadia@example.com" to refer indirectly to her (e.g., by
using the URI as a database key in their database of conference
participants). This does not introduce a URI collision.
URI Comparisons
URIs that are identical, character-by-character, refer to the same
resource. Since Web Architecture allows the association of multiple
URIs with a given resource, two URIs that are not
character-by-character identical may still refer to the same
resource. Different URIs do not necessarily refer to different
resources but there is generally a higher computational cost to
determine that different URIs refer to the same resource.
To reduce the risk of a false negative (i.e., an incorrect conclusion
that two URIs do not refer to the same resource) or a false positive
(i.e., an incorrect conclusion that two URIs do refer to the same
resource), some specifications describe equivalence tests in addition
to character-by-character comparison. Agents that reach conclusions
based on comparisons that are not licensed by the relevant
specifications take responsibility for any problems that result; see
the section on error handling for more
information about responsible behavior when reaching unlicensed
conclusions. Section 6 of [[!URI]] provides more information about
comparing URIs and reducing the risk of false negatives and
positives.
Although there are benefits (such as naming flexibility) to URI
aliases, there are also costs. URI aliases are harmful when they
divide the Web of related resources. A corollary of Metcalfe's
Principle (the "network effect") is that the value of a given
resource can be measured by the number and value of other resources
in its network neighborhood, that is, the resources that link to
it.
The problem with aliases is that if half of the neighborhood points
to one URI for a given resource, and the other half points to a
second, different URI for that same resource, the neighborhood is
divided. Not only is the aliased resource undervalued because of
this split, the entire neighborhood of resources loses value
because of the missing second-order relationships that should have
existed among the referring resources by virtue of their references
to the aliased resource.
A URI owner SHOULD NOT associate arbitrarily different URIs with
the same resource.
URI consumers also have a role in ensuring URI consistency. For
instance, when transcribing a URI, agents should not gratuitously
percent-encode characters. The term "character" refers to URI
characters as defined in section 2 of [[!URI]]; percent-encoding is
discussed in section 2.1 of that specification.
An agent that receives a URI SHOULD refer to the associated
resource using the same URI, character-by-character.
When a URI alias does become common currency, the URI owner should use protocol techniques such
as server-side redirects to relate the two resources. The community
benefits when the URI owner supports redirection of an aliased URI
to the corresponding "official" URI. For more information on
redirection, see section 10.3, Redirection, in [[!HTTP11]]. See
also [[!CHIPS]] for a discussion of some best practices for server
administrators.
Representation reuse
URI aliasing only occurs when more than one URI is used to identify
the same resource. The fact that different resources sometimes have
the same representation does not make the URIs for those resources
aliases.
Story
Dirk would like to add a link from his Web site to the Oaxaca
weather site. He uses the URI http://weather.example.com/oaxaca
and labels his link “report on weather in Oaxaca on
1 August 2004”. Nadia points out to Dirk that he is
setting misleading expectations for the URI he has used. The
Oaxaca weather site policy is that the URI in question
identifies a report on the current weather in Oaxaca—on any
given day—and not the weather on 1 August. Of course, on the
first of August in 2004, Dirk's link will be correct, but the
rest of the time he will be misleading readers. Nadia points
out to Dirk that the managers of the Oaxaca weather site do
make available a different URI permanently assigned to a
resource reporting on the weather on 1 August 2004.
In this story, there are two resources: “a report on the current
weather in Oaxaca” and “a report on the weather in Oaxaca on
1 August 2004”. The managers of the Oaxaca weather site
assign two URIs to these two different resources. On
1 August 2004, the representations for these resources
are identical. That fact that dereferencing two different URIs
produces identical representations does not imply that the two URIs
are aliases.
URI Schemes
In the URI "http://weather.example.com/", the "http" that appears
before the colon (":") names a URI scheme. Each URI scheme has a
specification that explains the scheme-specific details of how scheme
identifiers are allocated and become associated with a resource. The
URI syntax is thus a federated and extensible naming system wherein
each scheme's specification may further restrict the syntax and
semantics of identifiers within that scheme.
Examples of URIs from various schemes include:
mailto:joe@example.org
ftp://example.org/aDirectory/aFile
news:comp.infosystems.www
tel:+1-816-555-1212
ldap://ldap.example.org/c=GB?objectClass?one
urn:oasis:names:tc:entity:xmlns:xml:catalog
While Web architecture allows the definition of new schemes,
introducing a new scheme is costly. Many aspects of URI processing
are scheme-dependent, and a large amount of deployed software already
processes URIs of well-known schemes. Introducing a new URI scheme
requires the development and deployment not only of client software
to handle the scheme, but also of ancillary agents such as gateways,
proxies, and caches. See [[!RFC2718]] for other considerations and
costs related to URI scheme design.
Because of these costs, if a URI scheme exists that meets the needs
of an application, designers should use it rather than invent one.
A specification SHOULD reuse an existing URI scheme (rather than
create a new one) when it provides the desired properties of
identifiers and their relation to resources.
Consider our travel scenario: should the
agent providing information about the weather in Oaxaca register a
new URI scheme "weather" for the identification of resources related
to the weather? They might then publish URIs such as
"weather://travel.example.com/oaxaca". When a software agent
dereferences such a URI, if what really happens is that HTTP GET is
invoked to retrieve a representation of the resource, then an "http"
URI would have sufficed.
URI Scheme Registration
The Internet Assigned Numbers Authority (IANA)
maintains a registry [[!IANASchemes]] of mappings between URI
scheme names and scheme specifications. For instance, the IANA
registry indicates that the "http" scheme is defined in
[[!HTTP11]]. The process for registering a new URI scheme is
defined in [[!RFC2717]].
Unregistered URI schemes SHOULD NOT be used for a number of
reasons:
There is no generally accepted way to locate the scheme
specification.
Someone else may be using the scheme for other purposes.
One should not expect that general-purpose software will do
anything useful with URIs of this scheme beyond URI comparison.
One misguided motivation for registering a new URI scheme is to
allow a software agent to launch a particular application when
retrieving a representation. The same thing can be accomplished at
lower expense by dispatching instead on the type of the
representation, thereby allowing use of existing transfer protocols
and implementations.
Even if an agent cannot process representation data in an unknown
format, it can at least retrieve it. The data may contain enough
information to allow a user or user agent to make some use of it.
When an agent does not handle a new URI scheme, it cannot retrieve
a representation.
It is tempting to guess the nature of a resource by inspection of a
URI that identifies it. However, the Web is designed so that agents
communicate resource information state through representations, not identifiers. In
general, one cannot determine the type of a resource representation
by inspecting a URI for that resource. For example, the ".html" at
the end of "http://example.com/page.html" provides no guarantee that
representations of the identified resource will be served with the
Internet media type "text/html". The publisher is free to allocate
identifiers and define how they are served. The HTTP protocol does
not constrain the Internet media type based on the path component of
the URI; the URI owner is free to configure the server to return a
representation using PNG or any other data format.
Resource state may evolve over time. Requiring a URI owner to publish
a new URI for each change in resource state would lead to a
significant number of broken references. For robustness, Web
architecture promotes independence between an identifier and the
state of the identified resource.
Agents making use of URIs SHOULD NOT attempt to infer properties of
the referenced resource.
In practice, a small number of inferences can be made because they
are explicitly licensed by the relevant specifications. Some of these
inferences are discussed in the details of retrieving a representation .
The example URI used in the travel scenario
("http://weather.example.com/oaxaca") suggests to a human reader that
the identified resource has something to do with the weather in
Oaxaca. A site reporting the weather in Oaxaca could just as easily
be identified by the URI "http://vjc.example.com/315". And the URI
"http://weather.example.com/vancouver" might identify the resource
"my photo album."
On the other hand, the URI "mailto:joe@example.com" indicates that
the URI refers to a mailbox. The "mailto" URI scheme specification
authorizes agents to infer that URIs of this form identify Internet
mailboxes.
Some URI assignment authorities document and publish their URI
assignment policies. For more information about URI opacity, see TAG
issues metaDataInURI-31
and siteData-36.
Fragment Identifiers
Story
When browsing the XHTML document that Nadia receives as a
representation of the resource identified by
"http://weather.example.com/oaxaca", she finds that the URI
"http://weather.example.com/oaxaca#weekend" refers to the part of
the representation that conveys information about the weekend
outlook. This URI includes the fragment identifier "weekend" (the
string after the "#").
The fragment identifier component of a URI allows indirect
identification of a secondary resource by reference to a
primary resource and additional identifying information. The
secondary resource may be some portion or subset of the primary
resource, some view on representations of the primary resource, or
some other resource defined or described by those representations.
The terms "primary resource" and "secondary resource" are defined in
section 3.5 of [[!URI]].
The terms “primary” and “secondary” in this context do not limit the
nature of the resource—they are not classes. In this context, primary
and secondary simply indicate that there is a relationship between
the resources for the purposes of one URI: the URI with a fragment
identifier. Any resource can be identified as a secondary resource.
It might also be identified using a URI without a fragment
identifier, and a resource may be identified as a secondary resource
via multiple URIs. The purpose of these terms is to enable discussion
of the relationship between such resources, not to limit the nature
of a resource.
See TAG issue abstractComponentRefs-37,
which concerns the use of fragment identifiers with namespace names
to identify abstract components.
Future Directions for Identifiers
There remain open questions regarding identifiers on the Web.
Internationalized identifiers
The integration of internationalized identifiers (i.e., composed of
characters beyond those allowed by [[!URI]]) into the Web
architecture is an important and open issue. See TAG issue IRIEverywhere-27
for discussion about work going on in this area.
Assertion that two URIs identify the same resource
Emerging Semantic Web technologies, including the "Web Ontology
Language (OWL)" [OWL10], define RDF properties
such as sameAs to assert that two URIs identify the
same resource or inverseFunctionalProperty to imply
it.
Interaction
Communication between agents over a network about resources involves
URIs, messages, and data. The Web's protocols (including HTTP, FTP,
SOAP, NNTP, and SMTP) are based on the exchange of messages. A
message may include data as well as metadata about a
resource (such as the "Alternates" and "Vary" HTTP headers), the
message data, and the message itself (such as the "Transfer-encoding"
HTTP header). A message may even include metadata about the message
metadata (for message-integrity checks, for instance).
Story
Nadia follows a hypertext link labeled "satellite image" expecting
to retrieve a satellite photo of the Oaxaca region. The link to the
satellite image is an XHTML link encoded as <a
href="http://example.com/satimage/oaxaca">satellite
image</a>. Nadia's browser analyzes the URI and
determines that its scheme is "http". The
browser configuration determines how it locates the identified
information, which might be via a cache of prior retrieval actions,
by contacting an intermediary (such as a proxy server), or by
direct access to the server identified by a portion of the URI. In
this example, the browser opens a network connection to port 80 on
the server at "example.com" and sends a "GET" message as specified
by the HTTP protocol, requesting a representation of the resource.
The server sends a response message to the browser, once again
according to the HTTP protocol. The message consists of several
headers and a JPEG image. The browser reads the headers, learns
from the "Content-Type" field that the Internet media type of the
representation is "image/jpeg", reads the sequence of octets that
make up the representation data, and renders the image.
This section describes the architectural principles and constraints
regarding interactions between agents, including such topics as network
protocols and interaction styles, along with interactions between the
Web as a system and the people that make use of it. The fact that the
Web is a highly distributed system affects architectural constraints
and assumptions about interactions.
Using a URI to Access a Resource
Agents may use a URI to access the referenced resource; this is
called dereferencing the URI. Access may take many forms,
including retrieving a representation of the resource (for instance,
by using HTTP GET or HEAD), adding or modifying a representation of
the resource (for instance, by using HTTP POST or PUT, which in some
cases may change the actual state of the resource if the submitted
representations are interpreted as instructions to that end), and
deleting some or all representations of the resource (for instance,
by using HTTP DELETE, which in some cases may result in the deletion
of the resource itself).
There may be more than one way to access a resource for a given URI;
application context determines which access method an agent uses. For
instance, a browser might use HTTP GET to retrieve a representation
of a resource, whereas a hypertext link checker might use HTTP HEAD
on the same URI simply to establish whether a representation is
available. Some URI schemes set expectations about available access
methods, others (such as the URN scheme [[!URN]]) do not. Section
1.2.2 of [[!URI]] discusses the separation of identification and
interaction in more detail. For more information about relationships
between multiple access methods and URI addressability, see the TAG
finding "URIs,
Addressability, and the use of HTTP GET and POST".
Although many URI schemes are named after
protocols, this does not imply that use of such a URI will
necessarily result in access to the resource via the named protocol.
Even when an agent uses a URI to retrieve a representation, that
access might be through gateways, proxies, caches, and name
resolution services that are independent of the protocol associated
with the scheme name.
Many URI schemes define a default interaction protocol for attempting
access to the identified resource. That interaction protocol is often
the basis for allocating identifiers within that scheme, just as
"http" URIs are defined in terms of TCP-based HTTP servers. However,
this does not imply that all interaction with such resources is
limited to the default interaction protocol. For example, information
retrieval systems often make use of proxies to interact with a
multitude of URI schemes, such as HTTP proxies being used to access
"ftp" and "wais" resources. Proxies can also to provide enhanced
services, such as annotation proxies that combine normal information
retrieval with additional metadata retrieval to provide a seamless,
multidimensional view of resources using the same protocols and user
agents as the non-annotated Web. Likewise, future protocols may be
defined that encompass our current systems, using entirely different
interaction mechanisms, without changing the existing identifier
schemes. See also, principle of
orthogonal specifications.
Details of retrieving a representation
Dereferencing a URI generally involves a succession of steps as
described in multiple specifications and implemented by the agent.
The following example illustrates the series of specifications that
governs the process when a user agent is instructed to follow a
hypertext link that is part of an SVG
document. In this example, the URI is
"http://weather.example.com/oaxaca" and the application context
calls for the user agent to retrieve and render a representation of
the identified resource.
Since the URI is part of a hypertext link in an SVG document,
the first relevant specification is the SVG 1.1 Recommendation
[SVG11]. Section 17.1 of this
specification imports the link semantics defined in XLink 1.0
[XLink10]: "The remote resource (the
destination for the link) is defined by a URI specified by the
XLink href attribute on the 'a' element."
The SVG specification goes on to state that interpretation of an
a element involves retrieving a representation of a
resource, identified by the href attribute in the
XLink namespace: "By activating these links (by clicking with the
mouse, through keyboard input, voice commands, etc.), users may
visit these resources."
The XLink 1.0 [XLink10] specification,
which defines the href attribute in section 5.4,
states that "The value of the href attribute must be a URI
reference as defined in [IETF RFC 2396], or must result in a URI
reference after the escaping procedure described below is applied."
The URI specification [[!URI]] states that "Each URI begins
with a scheme name that refers to a specification for assigning
identifiers within that scheme." The URI scheme name in this
example is "http".
[[!IANASchemes]] states that the "http" scheme is defined by
the HTTP/1.1 specification (RFC 2616 [[!HTTP11]], section 3.2.2).
In this SVG context, the agent constructs an HTTP GET request
(per section 9.3 of [[!HTTP11]]) to retrieve the representation.
Section 6 of [[!HTTP11]] defines how the server constructs a
corresponding response message, including the 'Content-Type' field.
Section 1.4 of [[!HTTP11]] states "HTTP communication usually
takes place over TCP/IP connections." This example addresses
neither that step in the process nor other steps such as Domain
Name System (DNS) resolution.
The agent interprets the returned representation according to
the data format specification that corresponds to the
representation's Internet Media
Type (the value of the HTTP 'Content-Type') in the relevant
IANA registry [MEDIATYPEREG].
Precisely which representation(s) are retrieved depends on a number
of factors, including:
Whether the URI owner makes available any representations at
all;
Whether the agent making the request has access privileges for
those representations (see the section on linking and access control);
If the URI owner has provided more than one representation (in
different formats such as HTML, PNG, or RDF; in different languages
such as English and Spanish; or transformed dynamically according
to the hardware or software capabilities of the recipient), the
resulting representation may depend on negotiation between the user
agent and server.
The time of the request; the world changes over time, so
representations of resources are also likely to change over time.
Assuming that a representation has been successfully retrieved, the
expressive power of the representation's format will affect how
precisely the representation provider communicates resource state.
If the representation communicates the state of the resource
inaccurately, this inaccuracy or ambiguity may lead to confusion
among users about what the resource is. If different users reach
different conclusions about what the resource is, they may
interpret this as a URI collision .
Some communities, such as the ones developing the Semantic Web,
seek to provide a framework for accurately communicating the
semantics of a resource in a machine readable way. Machine readable
semantics may alleviate some of the ambiguity associated with
natural language descriptions of resources.
A representation is data that encodes information about
resource state. Representations do not necessarily describe the
resource, or portray a likeness of the resource, or represent the
resource in other senses of the word "represent".
Representations of a resource may be sent or received using
interaction protocols. These protocols in turn determine the form in
which representations are conveyed on the Web. HTTP, for example,
provides for transmission of representations as octet streams typed
using Internet media types [RFC2046].
Just as it is important to reuse existing URI schemes whenever
possible, there are significant benefits to using media typed octet
streams for representations even in the unusual case where a new URI
scheme and associated protocol is to be defined. For example, if the
Oaxaca weather were conveyed to Nadia's browser using a protocol
other than HTTP, then software to render formats such as
text/xhmtl+xml and image/png would still be usable if the new
protocol supported transmission of those types. This is an example of
the principle of orthogonal
specifications.
New protocols created for the Web SHOULD transmit representations
as octet streams typed by Internet media types.
The Internet media type mechanism does have some limitations. For
instance, media type strings do not support versioning or other parameters. See TAG issues
uriMediaType-9
and mediaTypeManagement-45
which concern aspects of the media type mechanism.
Representation types and fragment identifier semantics
The Internet Media Type defines the syntax and semantics of the
fragment identifier (introduced in Fragment
Identifiers), if any, that may be used in conjunction with a
representation.
Story
In one of his XHTML pages, Dirk creates a hypertext link to an
image that Nadia has published on the Web. He creates a
hypertext link with <a
href="http://www.example.com/images/nadia#hat">Nadia's
hat</a>. Emma views Dirk's XHTML page in her Web
browser and follows the link. The HTML implementation in her
browser removes the fragment from the URI and requests the
image "http://www.example.com/images/nadia". Nadia serves an
SVG representation of the image (with Internet media type
"image/svg+xml"). Emma's Web browser starts up an SVG
implementation to view the image. It passes it the original URI
including the fragment,
"http://www.example.com/images/nadia#hat" to this
implementation, causing a view of the hat to be displayed
rather than the complete image.
Note that the HTML implementation in Emma's browser did not need to
understand the syntax or semantics of the SVG fragment
(nor does the SVG implementation have to understand HTML, WebCGM,
RDF ... fragment syntax or semantics; it merely had to recognize
the # delimiter from the URI syntax [URI] and remove the fragment
when accessing the resource). This orthogonality is an important feature of
Web architecture; it is what enabled Emma's browser to provide a
useful service without requiring an upgrade.
The semantics of a fragment identifier are defined by the set of
representations that might result from a retrieval action on the
primary resource. The fragment's format and resolution are
therefore dependent on the type of a potentially retrieved
representation, even though such a retrieval is only performed if
the URI is dereferenced. If no such representation exists, then the
semantics of the fragment are considered unknown and, effectively,
unconstrained. Fragment identifier semantics are orthogonal to URI
schemes and thus cannot be redefined by URI scheme specifications.
Interpretation of the fragment identifier is performed solely by
the agent that dereferences a URI; the fragment identifier is not
passed to other systems during the process of retrieval. This means
that some intermediaries in Web architecture (such as proxies) have
no interaction with fragment identifiers and that redirection (in
HTTP [[!HTTP11]], for example) does not account for fragments.
Fragment identifiers and content negotiation
Content negotiation refers to the practice of making
available multiple representations via the same URI. Negotiation
between the requesting agent and the server determines which
representation is served (usually with the goal of serving the
"best" representation a receiving agent can process). HTTP is an
example of a protocol that enables representation providers to use
content negotiation.
Individual data formats may define their own rules for use of the
fragment identifier syntax for specifying different types of
subsets, views, or external references that are identifiable as
secondary resources by that media type. Therefore, representation
providers must manage content negotiation carefully when used with
a URI that contains a fragment identifier. Consider an example
where the owner of the URI
"http://weather.example.com/oaxaca/map#zicatela" uses content
negotiation to serve two representations of the identified
resource. Three situations can arise:
The interpretation of "zicatela" is defined consistently by
both data format specifications. The representation provider
decides when definitions of fragment identifier semantics are are
sufficiently consistent.
The interpretation of "zicatela" is defined inconsistently by
the data format specifications.
The interpretation of "zicatela" is defined in one data format
specification but not the other.
The first situation—consistent semantics—poses no problem.
The second case is a server management error: representation
providers must not use content negotiation to serve representation
formats that have inconsistent fragment identifier semantics. This
situation also leads to URI collision
.
The third case is not a server management error. It is a means by
which the Web can grow. Because the Web is a distributed system in
which formats and agents are deployed in a non-uniform manner, Web
architecture does not constrain authors to only use "lowest common
denominator" formats. Content authors may take advantage of new
data formats while still ensuring reasonable backward-compatibility
for agents that do not yet implement them.
In case three, behavior by the receiving agent should vary
depending on whether the negotiated format defines fragment
identifier semantics. When a received data format does not define
fragment identifier semantics, the agent should not perform
silent error recovery unless the
user has given consent; see [[!CUAP]] for additional suggested
agent behavior in this case.
Inconsistencies between Representation Data and Metadata
Successful communication between two parties depends on a reasonably
shared understanding of the semantics of exchanged messages, both
data and metadata. At times, there may be inconsistencies between a
message sender's data and metadata. Examples, observed in practice,
of inconsistencies between representation data and metadata include:
The actual character encoding of a representation (e.g.,
"iso-8859-1", specified by the encoding attribute in an
XML declaration) is inconsistent with the charset parameter in the
representation metadata (e.g., "utf-8", specified by the
'Content-Type' field in an HTTP header).
The namespace of the root element
of XML representation data (e.g., as specified by the "xmlns"
attribute) is inconsistent with the value of the 'Content-Type' field
in an HTTP header.
On the other hand, there is no inconsistency in serving HTML content
with the media type "text/plain", for example, as this combination is
licensed by specifications.
Receiving agents should detect protocol inconsistencies and perform
proper error recovery.
Agents MUST NOT ignore message metadata without the consent of the
user.
Thus, for example, if the parties responsible for
"weather.example.com" mistakenly label the satellite photo of Oaxaca
as "image/gif" instead of "image/jpeg", and if Nadia's browser
detects a problem, Nadia's browser must not ignore the problem (e.g.,
by simply rendering the JPEG image) without Nadia's consent. Nadia's
browser can notify Nadia of the problem or notify Nadia and take
corrective action.
Furthermore, representation providers can help reduce the risk of
inconsistencies through careful assignment of representation metadata
(especially that which applies across representations). The section
on media types for XML presents an
example of reducing the risk of error by providing no metadata about
character encoding when serving XML.
The accuracy of metadata relies on the server administrators, the
authors of representations, and the software that they use.
Practically, the capabilities of the tools and the social
relationships may be the limiting factors.
The accuracy of these and other metadata fields is just as important
for dynamic Web resources, where a little bit of thought and
programming can often ensure correct metadata for a huge number of
resources.
Often there is a separation of control between the users who create
representations of resources and the server managers who maintain the
Web site software. Given that it is generally the Web site software
that provides the metadata associated with a resource, it follows
that coordination between the server managers and content creators is
required.
Server managers SHOULD allow representation creators to control the
metadata associated with their representations.
In particular, content creators need to be able to control the
content type (for extensibility) and the character encoding (for
proper internationalization).
The TAG finding "Authoritative
Metadata" discusses in more detail how to handle
data/metadata inconsistency and how server configuration can be used
to avoid it.
Safe Interactions
Nadia's retrieval of weather information (an example of a read-only
query or lookup) qualifies as a "safe" interaction; a safe
interaction is one where the agent does not incur any
obligation beyond the interaction. An agent may incur an obligation
through other means (such as by signing a contract). If an agent does
not have an obligation before a safe interaction, it does not have
that obligation afterwards.
Other Web interactions resemble orders more than queries. These
unsafe interactions may cause a change to the state of a
resource and the user may be held responsible for the consequences of
these interactions. Unsafe interactions include subscribing to a
newsletter, posting to a list, or modifying a database.
Note: In this context, the word "unsafe" does not
necessarily mean "dangerous"; the term "safe" is used in section
9.1.1 of [[!HTTP11]] and "unsafe" is the natural opposite.
Story
Nadia decides to book a vacation to Oaxaca at
"booking.example.com." She enters data into a series of online
forms and is ultimately asked for credit card information to
purchase the airline tickets. She provides this information in
another form. When she presses the "Purchase" button, her browser
opens another network connection to the server at
"booking.example.com" and sends a message composed of form data
using the POST method. This is an unsafe interaction; Nadia wishes to
change the state of the system by exchanging money for airline
tickets.
The server reads the POST request, and after performing the
booking transaction returns a message to Nadia's browser that
contains a representation of the results of Nadia's request. The
representation data is in XHTML so that it can be saved or
printed out for Nadia's records.
Note that neither the data transmitted with the POST nor the data
received in the response necessarily correspond to any resource
identified by a URI.
Safe interactions are important because these are interactions where
users can browse with confidence and where agents (including search
engines and browsers that pre-cache data for the user) can follow
hypertext links safely. Users (or agents acting on their behalf) do
not commit themselves to anything by querying a resource or following
a hypertext link.
Agents do not incur obligations by retrieving a representation.
For instance, it is incorrect to publish a URI that, when followed as
part of a hypertext link, subscribes a user to a mailing list.
Remember that search engines may follow such hypertext links.
The fact that HTTP GET, the access method most often used when
following a hypertext link, is safe does not imply that all safe
interactions must be done through HTTP GET. At times, there may be
good reasons (such as confidentiality requirements or practical
limits on URI length) to conduct an otherwise safe operation using a
mechanism generally reserved for unsafe operations (e.g., HTTP POST).
Nadia pays for her airline tickets online (through a POST
interaction as described above). She receives a Web page with
confirmation information and wishes to bookmark it so that she
can refer to it when she calculates her expenses. Although
Nadia can print out the results, or save them to a file, she
would also like to bookmark them.
Transaction requests and results are valuable resources, and like
all valuable resources, it is useful to be able to refer to them
with a persistent URI. However, in
practice, Nadia cannot bookmark her commitment to pay (expressed
via the POST request) or the airline company's acknowledgment and
commitment to provide her with a flight (expressed via the response
to the POST).
There are ways to provide persistent URIs for transaction requests
and their results. For transaction requests, user agents can
provide an interface for managing transactions where the user agent
has incurred an obligation on behalf of the user. For transaction
results, HTTP allows representation providers to associate a URI
with the results of an HTTP POST request using the
"Content-Location" header (described in section 14.14 of
[[!HTTP11]]).
Representation Management
Story
Since Nadia finds the Oaxaca weather site useful, she emails a
review to her friend Dirk recommending that he check out
'http://weather.example.com/oaxaca'. Dirk clicks on the resulting
hypertext link in the email he receives and is frustrated by a
404 (not found). Dirk tries again the next day and receives a
representation with "news" that is two-weeks old. He tries one
more time the next day only to receive a representation that
claims that the weather in Oaxaca is sunny, even though his
friends in Oaxaca tell him by phone that in fact it is raining.
Dirk and Nadia conclude that the URI owners are unreliable or
unpredictable. Although the URI owner has chosen the Web as a
communication medium, the owner has lost two customers due to
ineffective representation management.
A URI owner may supply zero or more authoritative representations of
the resource identified by that URI. There is a benefit to the
community in providing representations.
An application developer or specification author SHOULD NOT require
networked retrieval of representations each time they are
referenced.
Dereferencing a URI has a (potentially significant) cost in computing
and bandwidth resources, may have security implications, and may
impose significant latency on the dereferencing application.
Dereferencing URIs should be avoided except when necessary.
As is the case with many human interactions, confidence in
interactions via the Web depends on stability and predictability.
For an information resource, persistence depends on the consistency
of representations. The representation provider decides when
representations are sufficiently consistent (although that
determination generally takes user expectations into account).
Although persistence in this case is observable as a result of
representation retrieval, the term URI persistence is
used to describe the desirable property that, once associated with
a resource, a URI should continue indefinitely to refer to that
resource.
A URI owner SHOULD provide representations of the identified
resource consistently and predictably.
URI persistence is a matter of policy and commitment on the part of
the URI owner. The choice of a
particular URI scheme provides no guarantee that those URIs will be
persistent or that they will not be persistent.
HTTP [[!HTTP11]] has been designed to help manage URI persistence.
For example, HTTP redirection (using the 3xx response codes)
permits servers to tell an agent that further action needs to be
taken by the agent in order to fulfill the request (for example, a
new URI is associated with the resource).
In addition, content negotiation also
promotes consistency, as a site manager is not required to define
new URIs when adding support for a new format specification.
Protocols that do not support content negotiation (such as FTP)
require a new identifier when a new data format is introduced.
Improper use of content negotiation can lead to inconsistent
representations.
For more discussion about URI persistence, see [Cool].
Linking and access control
It is reasonable to limit access to a resource (for commercial or
security reasons, for example), but merely identifying the resource
is like referring to a book by title. In exceptional circumstances,
people may have agreed to keep titles or URIs confidential (for
example, a book author and a publisher may agree to keep the URI of
page containing additional material secret until after the book is
published), otherwise they are free to exchange them.
As an analogy: The owners of a building might have a policy that
the public may only enter the building via the main front door, and
only during business hours. People who work in the building and who
make deliveries to it might use other doors as appropriate. Such a
policy would be enforced by a combination of security personnel and
mechanical devices such as locks and pass-cards. One would not
enforce this policy by hiding some of the building entrances, nor
by requesting legislation requiring the use of the front door and
forbidding anyone to reveal the fact that there are other doors to
the building.
Story
Nadia sends to Dirk the URI of the current article she is
reading. With his browser, Dirk follows the hypertext link and
is asked to enter his subscriber username and password. Since
Dirk is also a subscriber to services provided by
"weather.example.com," he can access the same information as
Nadia. Thus, the authority for "weather.example.com" can limit
access to authorized parties and still provide the benefits of
URIs.
The Web provides several mechanisms to control access to resources;
these mechanisms do not rely on hiding or suppressing URIs for
those resources. For more information, see the TAG finding
"'Deep Linking' in
the World Wide Web".
Supporting Navigation
It is a strength of Web Architecture that links can be made and
shared; a user who has found an interesting part of the Web can
share this experience just by republishing a URI.
Story
Nadia and Dirk want to visit the Museum of Weather Forecasting
in Oaxaca. Nadia goes to "http://maps.example.com", locates the
museum, and mails the URI
"http://maps.example.com/oaxaca?lat=17.065;lon=-96.716;scale=6"
to Dirk. Dirk goes to "http://mymaps.example.com", locates the
museum, and mails the URI
"http://mymaps.example.com/geo?sessionID=765345;userID=Dirk" to
Nadia. Dirk reads Nadia's email and is able to follow the link
to the map. Nadia reads Dirk's email, follows the link, and
receives an error message 'No such session/user'. Nadia has to
start again from "http://mymaps.example.com" and find the
museum location once more.
For resources that are generated on demand, machine generation of
URIs is common. For resources that might usefully be bookmarked for
later perusal, or shared with others, server managers should avoid
needlessly restricting the reusability of such URIs. If the
intention is to restrict information to a particular user, as might
be the case in a home banking application for example, designers
should use appropriate access control
mechanisms.
Interactions conducted with HTTP POST (where HTTP GET could have
been used) also limit navigation possibilities. The user cannot
create a bookmark or share the URI because HTTP POST transactions
do not typically result in a different URI as the user interacts
with the site.
Future Directions for Interaction
There remain open questions regarding Web interactions. The TAG
expects future versions of this document to address in more detail
the relationship between the architecture described herein, Web Services, peer-to-peer systems,
instant messaging systems (such as [[!RFC3920]]), streaming audio
(such as RTSP [[!RFC2326]]), and voice-over-IP (such as SIP
[[!RFC3261]]).
Data Formats
A data format specification (for example, for XHTML, RDF/XML, SMIL,
XLink, CSS, and PNG) embodies an agreement on the correct
interpretation of representation
data. The first data format used on the Web was HTML. Since then, data
formats have grown in number. Web architecture does not constrain which
data formats content providers can use. This flexibility is important
because there is constant evolution in applications, resulting in new
data formats and refinements of existing formats. Although Web
architecture allows for the deployment of new data formats, the
creation and deployment of new formats (and agents able to handle them)
is expensive. Thus, before inventing a new data format (or "meta"
format such as XML), designers should carefully consider re-using one
that is already available.
For a data format to be usefully interoperable between two parties, the
parties must agree (to a reasonable extent) about its syntax and
semantics. Shared understanding of a data format promotes
interoperability but does not imply constraints on usage; for instance,
a sender of data cannot count on being able to constrain the behavior
of a data receiver.
Below we describe some characteristics of a data format that facilitate
integration into Web architecture. This document does not address
generally beneficial characteristics of a specification such as
readability, simplicity, attention to programmer goals, attention to
user needs, accessibility, nor internationalization. The section on
architectural specifications includes
references to additional format specification guidelines.
Binary and Textual Data Formats
Binary data formats are those in which portions of the data are
encoded for direct use by computer processors, for example 32 bit
little-endian two's-complement and 64 bit IEEE double-precision
floating-point. The portions of data so represented include numeric
values, pointers, and compressed data of all sorts.
A textual data format is one in which the data is specified in a
defined encoding as a sequence of characters. HTML, Internet e-mail,
and all XML-based formats are textual.
Increasingly, internationalized textual data formats refer to the
Unicode repertoire [[!UNICODE]] for character definitions.
If a data format is textual, as defined in this section, that does
not imply that it should be served with a media type beginning with
"text/". Although XML-based formats are textual, many XML-based
formats do not consist primarily of phrases in natural language. See
the section on media types for XML for
issues that arise when "text/" is used in conjunction with an
XML-based format.
In principle, all data can be represented using textual formats. In
practice, some types of content (e.g., audio and video) are generally
represented using binary formats.
The trade-offs between binary and textual data formats are complex
and application-dependent. Binary formats can be substantially more
compact, particularly for complex pointer-rich data structures. Also,
they can be consumed more rapidly by agents in those cases where they
can be loaded into memory and used with little or no conversion.
Note, however, that such cases are relatively uncommon as such direct
use may open the door to security issues that can only practically be
addressed by examining every aspect of the data structure in detail.
Textual formats are usually more portable and interoperable. Textual
formats also have the considerable advantage that they can be
directly read by human beings (and understood, given sufficient
documentation). This can simplify the tasks of creating and
maintaining software, and allow the direct intervention of humans in
the processing chain without recourse to tools more complex than the
ubiquitous text editor. Finally, it simplifies the necessary human
task of learning about new data formats; this is called the "view
source" effect.
It is important to emphasize that intuition as to such matters as
data size and processing speed is not a reliable guide in data format
design; quantitative studies are essential to a correct understanding
of the trade-offs. Therefore, designers of a data format
specification should make a considered choice between binary and
textual format design.
In a perfect world, language designers would invent languages that
perfectly met the requirements presented to them, the requirements
would be a perfect model of the world, they would never change over
time, and all implementations would be perfectly interoperable
because the specifications would have no variability.
In the real world, language designers imperfectly address the
requirements as they interpret them, the requirements inaccurately
model the world, conflicting requirements are presented, and they
change over time. As a result, designers negotiate with users, make
compromises, and often introduce extensibility mechanisms so that it
is possible to work around problems in the short term. In the long
term, they produce multiple versions of their languages, as the
problem, and their understanding of it, evolve. The resulting
variability in specifications, languages, and implementations
introduces interoperability costs.
Extensibility and versioning are strategies to help manage the
natural evolution of information on the Web and technologies used to
represent that information. For more information about how these
strategies introduce variability and how that variability impacts
interoperability, see Variability in
Specifications.
See TAG issue XMLVersioning-41,
which concerns good practices for designing extensible XML languages
and for handling versioning. See also "Web Architecture: Extensible
Languages" [[!EXTLANG]].
Versioning
There is typically a (long) transition period during which multiple
versions of a format, protocol, or agent are simultaneously in use.
A data format specification SHOULD provide for version
information.
Versioning and XML namespace policy
Story
Nadia and Dirk are designing an XML data format to encode data
about the film industry. They provide for extensibility by
using XML namespaces and creating a schema that allows the
inclusion, in certain places, of elements from any namespace.
When they revise their format, Nadia proposes a new optional
lang attribute on the film element.
Dirk feels that such a change requires them to assign a new
namespace name, which might require changes to deployed
software. Nadia explains to Dirk that their choice of
extensibility strategy in conjunction with their namespace
policy allows certain changes that do not affect conformance of
existing content and software, and thus no change to the
namespace identifier is required. They chose this policy to
help them meet their goals of reducing the cost of change.
Dirk and Nadia have chosen a particular namespace change policy
that allows them to avoid changing the namespace name whenever they
make changes that do not affect conformance of deployed content and
software. They might have chosen a different policy, for example
that any new element or attribute has to belong to a namespace
other than the original one. Whatever the chosen policy, it should
set clear expectations for users of the format.
In general, changing the namespace name of an element completely
changes the element name. If "a" and "b" are bound to two different
URIs, a:element and b:element are as
distinct as a:eieio and a:xyzzy.
Practically speaking, this means that deployed applications will
have to be upgraded in order to recognize the new language; the
cost of this upgrade may be very high.
It follows that there are significant tradeoffs to be considered
when deciding on a namespace change policy. If a vocabulary has no
extensibility points (that is, if it does not allow elements or
attributes from foreign namespaces or have a mechanism for dealing
with unrecognized names from the same namespace), it may be
absolutely necessary to change the namespace name. Languages that
allow some form of extensibility without requiring a change to the
namespace name are more likely to evolve gracefully.
An XML format specification SHOULD include information about
change policies for XML namespaces.
As an example of a change policy designed to reflect the variable
stability of a namespace, consider the W3C namespace policy for
documents on the W3C Recommendation track. The policy sets
expectations that the Working Group responsible for the namespace
may modify it in any way until a certain point in the process
("Candidate Recommendation") at which point W3C constrains the set
of possible changes to the namespace in order to promote stable
implementations.
Note that since namespace names are URIs, the owner of a namespace
URI has the authority to decide the namespace change policy.
Extensibility
Requirements change over time. Successful technologies are adopted
and adapted by new users. Designers can facilitate the transition
process by making careful choices about extensibility during the
design of a language or protocol specification.
In making these choices, the designers must weigh the trade-offs
between extensibility, simplicity, and variability. A language
without extensibility mechanisms may be simpler and less variable,
improving initial interoperability. However, it's likely that
changes to that language will be more difficult, possibly more
complex and more variable, than if the initial design had provided
such mechanisms. This may decrease interoperability over the long
term.
A specification SHOULD provide mechanisms that allow any party to
create extensions.
Extensibility introduces variability which has an impact on
interoperability. However, languages that have no extensibility
mechanisms may be extended in ad hoc ways that impact
interoperability as well. One key criterion of the mechanisms
provided by language designers is that they allow the extended
languages to remain in conformance with the original specification,
increasing the likelihood of interoperability.
Extensibility MUST NOT interfere with conformance to the original
specification.
Application needs determine the most appropriate extension strategy
for a specification. For example, applications designed to operate
in closed environments may allow specification designers to define
a versioning strategy that would be impractical at the scale of the
Web.
A specification SHOULD specify agent behavior in the face of
unrecognized extensions.
Two strategies have emerged as being particularly useful:
"Must ignore": The agent ignores any content it does not
recognize.
"Must understand": The agent treats unrecognized markup as an
error condition.
A powerful design approach is for the language to allow either form
of extension, but to distinguish explicitly between them in the
syntax.
Additional strategies include prompting the user for more input and
automatically retrieving data from available hypertext links. More
complex strategies are also possible, including mixing strategies.
For instance, a language can include mechanisms for overriding
standard behavior. Thus, a data format can specify "must ignore"
semantics but also allow for extensions that override that
semantics in light of application needs (for instance, with "must
understand" semantics for a particular extension).
Extensibility is not free. Providing hooks for extensibility is one
of many requirements to be factored into the costs of language
design. Experience suggests that the long term benefits of a
well-designed extensibility mechanism generally outweigh the costs.
Many modern data format include mechanisms for composition. For
example:
It is possible to embed text comments in some image formats,
such as JPEG/JFIF. Although these comments are embedded in the
containing data, they are not intended to affect the display of the
image.
There are container formats such as SOAP which fully expect
content from multiple namespaces but which provide an overall
semantic relationship of message envelope and payload.
The semantics of combining RDF documents containing multiple
vocabularies are well-defined.
In principle, these relationships can be mixed and nested
arbitrarily. A SOAP message, for example, can contain an SVG image
that contains an RDF comment which refers to a vocabulary of terms
for describing the image.
Note however, that for general XML there is no semantic model that
defines the interactions within XML documents with elements and/or
attributes from a variety of namespaces. Each application must
define how namespaces interact and what effect the namespace of an
element has on the element's ancestors, siblings, and descendants.
See TAG issues mixedUIXMLNamespace-33
(concerning the meaning of a document composed of content in
multiple namespaces), xmlFunctions-34
(concerning one approach for managing XML transformation and
composability), and RDFinXHTML-35
(concerning the interpretation of RDF when embedded in an XHTML
document).
Separation of Content, Presentation, and Interaction
The Web is a heterogeneous environment where a wide variety of agents
provide access to content to users with a wide variety of
capabilities. It is good practice for authors to create content that
can reach the widest possible audience, including users with
graphical desktop computers, hand-held devices and mobile phones,
users with disabilities who may require speech synthesizers, and
devices not yet imagined. Furthermore, authors cannot predict in some
cases how an agent will display or process their content. Experience
shows that the separation of content, presentation, and interaction
promotes the reuse and device-independence of content; this follows
from the principle of orthogonal
specifications.
This separation also facilitates reuse of authored source content
across multiple delivery contexts. Sometimes, functional user
experiences suited to any delivery context can be generated by using
an adaptation process applied to a representation that does not
depend on the access mechanism. For more information about principles
of device-independence, see [DIPRINCIPLES].
A specification SHOULD allow authors to separate content from both
presentation and interaction concerns.
Note that when content, presentation, and interaction are separated
by design, agents need to recombine them. There is a recombination
spectrum, with "client does all" at one end and "server does all" at
the other.
There are advantages to each approach. For instance when a client
(such as a mobile phone) communicates device capabilities to the
server (for example, using CC/PP), the server can tailor the
delivered content to fit that client. The server can, for example,
enable faster downloads by adjusting links to refer to lower
resolution images, smaller video or no video at all. Similarly, if
the content has been authored with multiple branches, the server can
remove unused branches before delivery. In addition, by tailoring the
content to match the characteristics of a target client, the server
can help reduce client side computation. However, specializing
content in this manner reduces caching efficiency.
On the other hand, designing content that that can be recombined on
the client also tends to make that content applicable to a wider
range of devices. This design also improves caching efficiency and
offers users more presentation options. Media-dependent style sheets
can be used to tailor the content on the client side to particular
groups of target devices. For textual content with a regular and
repeating structure, the combined size of the text content plus the
style sheet is typically less than that of fully recombined content;
the savings improve further if the style sheet is reused by other
pages.
In practice a combination of both approaches is often used. The
design decision about where on this spectrum an application should be
placed depends on the power on the client, the power and the load on
the server, and the bandwidth of the medium that connects them. If
the number of possible clients is unbounded, the application will
scale better if more computation is pushed to the client.
Of course, it may not be desirable to reach the widest possible
audience. Designers should consider appropriate technologies, such as
encryption and access control, for limiting
the audience.
Some data formats are designed to describe presentation (including
SVG and XSL Formatting Objects). Data formats such as these
demonstrate that one can only separate content from presentation (or
interaction) so far; at some point it becomes necessary to talk about
presentation. Per the principle of orthogonal specifications these data formats
should only address presentation issues.
See the TAG issues formattingProperties-19
(concerning interoperability in the case of formatting properties and
names) and contentPresentation-26
(concerning the separation of semantic and presentational markup).
Hypertext
A defining characteristic of the Web is that it allows embedded
references to other resources via URIs. The simplicity of creating
hypertext links using absolute URIs (<a
href="http://www.example.com/foo">) and relative URI
references (<a href="foo"> and <a
href="foo#anchor">) is partly (perhaps largely) responsible
for the success of the hypertext Web as we know it today.
When one resource (representation) refers to another resource with a
URI, this constitutes a link between the two resources.
Additional metadata may also form part of the link (see [[!XLINK10]],
for example). Note: In this document, the term
"link" generally means "relationship", not "physical connection".
A specification SHOULD provide ways to identify links to other
resources, including to secondary resources (via fragment
identifiers).
Formats that allow content authors to use URIs instead of local
identifiers promote the network effect: the value of these formats
grows with the size of the deployed Web.
A specification SHOULD allow content authors to use URIs without
constraining them to a limited set of URI schemes.
What agents do with a hypertext link is not constrained by Web
architecture and may depend on application context. Users of
hypertext links expect to be able to navigate among representations
by following links.
A data format SHOULD incorporate hypertext links if hypertext is
the expected user interface paradigm.
Data formats that do not allow content authors to create hypertext
links lead to the creation of "terminal nodes" on the Web.
URI references
Links are commonly expressed using URI references
(defined in section 4.2 of [[!URI]]), which may be combined with a
base URI to yield a usable URI. Section 5.1 of [[!URI]] explains
different ways to establish a base URI for a resource and
establishes a precedence among them. For instance, the base URI may
be a URI for the resource, or specified in a representation (see
the base elements provided by HTML and XML, and the
HTTP 'Content-Location' header). See also the section on links in XML.
Agents resolve a URI reference before using the resulting URI to
interact with another agent. URI references help in content
management by allowing content authors to design a representation
locally, i.e., without concern for which global identifier may
later be used to refer to the associated resource.
XML-Based Data Formats
Many data formats are XML-based, that is to say they
conform to the syntax rules defined in the XML specification
[[!XML10]] or [XML11]. This section discusses
issues that are specific to such formats. Anyone seeking guidance in
this area is urged to consult the "Guidelines For the Use of XML in
IETF Protocols" [IETFXML], which contains a
thorough discussion of the considerations that govern whether or not
XML ought to be used, as well as specific guidelines on how it ought
to be used. While it is directed at Internet applications with
specific reference to protocols, the discussion is generally
applicable to Web scenarios as well.
The discussion here should be seen as ancillary to the content of
[IETFXML]. Refer also to "XML Accessibility
Guidelines" [XAG] for help designing XML formats
that lower barriers to Web accessibility for people with
disabilities.
When to use an XML-based format
XML defines textual data formats that are naturally suited to
describing data objects which are hierarchical and processed in a
chosen sequence. It is widely, but not universally, applicable for
data formats; an audio or video format, for example, is unlikely to
be well suited to expression in XML. Design constraints that would
suggest the use of XML include:
Requirement for a hierarchical structure.
Need for a wide range of tools on a variety of platforms.
Need for data that can outlive the applications that currently
process it.
Ability to support internationalization in a self-describing
way that makes confusion over coding options unlikely.
Early detection of encoding errors with no requirement to "work
around" such errors.
A high proportion of human-readable textual content.
Potential composition of the data format with other XML-encoded
formats.
Desire for data easily parsed by both humans and machines.
Desire for vocabularies that can be invented in a distributed
manner and combined flexibly.
Links in XML
Sophisticated linking mechanisms have been invented for XML
formats. XPointer allows links to address content that does not
have an explicit, named anchor. [[!XLINK10]] is an appropriate
specification for representing links in hypertext XML applications. XLink allows links to
have multiple ends and to be expressed either inline or in "link
bases" stored external to any or all of the resources identified by
the links it contains.
Designers of XML-based formats may consider using XLink and, for
defining fragment identifier syntax, using the XPointer framework
and XPointer element() Schemes.
XLink is not the only linking design that has been proposed for
XML, nor is it universally accepted as a good design. See also TAG
issue xlinkScope-23.
XML namespaces
The purpose of an XML namespace (defined in [XMLNS]) is to allow the deployment of XML vocabularies
(in which element and attribute names are defined) in a global
environment and to reduce the risk of name collisions in a given
document when vocabularies are combined. For example, the MathML
and SVG specifications both define the set element.
Although XML data from different formats such as MathML and SVG can
be combined in a single document, in this case there could be
ambiguity about which set element was intended. XML
namespaces reduce the risk of name collisions by taking advantage
of existing systems for allocating globally scoped names: the URI
system (see also the section on URI allocation). When using XML namespaces,
each local name in an XML vocabulary is paired with a URI (called
the namespace URI) to distinguish the local name from local names
in other vocabularies.
The use of URIs confers additional benefits. First, each URI/local
name pair can be mapped to another URI, grounding the terms of the
vocabulary in the Web. These terms may be important resources and
thus it is appropriate to be able to associate URIs with them.
For flat namespaces, concatenation is one useful mapping. If
namespace URIs that end with a hash (“#”) are chosen, then simple
concatenation of the namespace URI and the local name creates a URI
for a secondary resource (the identified term). This technique is
used for many [[!RDFXML]] namespaces.
Other mappings are likely to be more suitable for hierarchical
namespaces; see the related TAG issue abstractComponentRefs-37.
Designers of XML-based data formats who declare namespaces thus
make it possible to reuse those data formats and combine them in
novel ways not yet imagined. Failure to declare namespaces makes
such reuse more difficult, even impractical in some cases.
A specification that establishes an XML vocabulary SHOULD place
all element names and global attribute names in a namespace.
Attributes are always scoped by the element on which they appear.
An attribute that is "global," that is, one that might meaningfully
appear on elements of many types, including elements in other
namespaces, should be explicitly placed in a namespace. Local
attributes, ones associated with only a particular element type,
need not be included in a namespace since their meaning will always
be clear from the context provided by that element.
The type attribute from the W3C XML Schema Instance
namespace "http://www.w3.org/2001/XMLSchema-instance" ([XMLSCHEMA], section 4.3.2) is an example of a
global attribute. It can be used by authors of any vocabulary to
make an assertion in instance data about the type of the element on
which it appears. As a global attribute, it must always be
qualified. The frame attribute on an HTML table is an
example of a local attribute. There is no value in placing that
attribute in a namespace since the attribute is unlikely to be
useful on an element other than an HTML table.
Applications that rely on DTD processing must impose additional
constraints on the use of namespaces. DTDs perform validation based
on the lexical form of the element and attribute names in the
document. This makes prefixes syntactically significant in ways
that are not anticipated by [[!XMLNS]].
Namespace documents
Story
Nadia receives representation data from "weather.example.com"
in an unfamiliar data format. She knows enough about XML to
recognize which XML namespace the elements belong to. Since the
namespace is identified by the URI
"http://weather.example.com/2003/format", she asks her browser
to retrieve a representation of the identified resource. She
gets back some useful data that allows her to learn more about
the data format. Nadia's browser may also be able to perform
some operations automatically (i.e., unattended by a human
overseer) given data that has been optimized for software
agents. For example, her browser might, on Nadia's behalf,
download additional agents to process and render the format.
Another benefit of using URIs to build XML namespaces is that the
namespace URI can be used to identify an information resource that
contains useful information, machine-usable and/or human-usable,
about terms in the namespace. This type of information resource is
called a namespace document. When a namespace URI owner
provides a namespace document, it is authoritative for the
namespace.
There are many reasons to provide a namespace document. A person
might want to:
understand the purpose of the namespace,
learn how to use the markup vocabulary in the namespace,
find out who controls it and associated policies,
request authority to access schemas or collateral material
about it, or
report a bug or situation that could be considered an error in
some collateral material.
A processor might want to:
retrieve a schema, for validation,
retrieve a style sheet, for presentation, or
retrieve ontologies, for making inferences.
In general, there is no established best practice for creating
representations of a namespace document; application expectations
will influence what data format or formats are used. Application
expectations will also influence whether relevant information
appears directly in a representation or is referenced from it.
The owner of an XML namespace name SHOULD make available material
intended for people to read and material optimized for software
agents in order to meet the needs of those who will use the
namespace vocabulary.
For example, the following are examples of data formats for
namespace documents: [[!OWL10]], [[!RDDL]], [[!XMLSCHEMA-1]], and
[[!XHTML11]]. Each of these formats meets different requirements
described above for satisfying the needs of an agent that wants
more information about the namespace. Note, however, issues related
to fragment identifiers and content
negotiation if content negotiation is used.
See TAG issues namespaceDocument-8
(concerning desired characteristics of namespace documents) and
abstractComponentRefs-37
(concerning the use of fragment identifiers with namespace names to
identify abstract components).
QNames in XML
Section 3 of "Namespaces in XML" [XMLNS]
provides a syntactic construct known as a QName for the compact
expression of qualified names in XML documents. A qualified name is
a pair consisting of a URI, which names a namespace, and a local
name placed within that namespace. "Namespaces in XML" provides for
the use of QNames as names for XML elements and attributes.
Other specifications, starting with [XSLT10],
have employed the idea of using QNames in contexts other than
element and attribute names, for example in attribute values and in
element content. However, general XML processors cannot reliably
recognize QNames as such when they are used in attribute values and
in element content; for example, the syntax of QNames overlaps with
that of URIs. Experience has also revealed other limitations to
QNames, such as losing namespace bindings after XML
canonicalization.
Because QNames are compact, some specification designers have
adopted the same syntax as a means of identifying resources. Though
convenient as a shorthand notation, this usage has a cost. There is
no single, accepted way to convert a QName into a URI or vice
versa. Although QNames are convenient, they do not replace the URI
as the identification system of the Web. The use of QNames to
identify Web resources without providing a mapping to URIs is
inconsistent with Web architecture.
A specification in which QNames serve as resource identifiers
MUST provide a mapping to URIs.
See XML namespaces for examples of
some mapping strategies.
See also TAG issues rdfmsQnameUriMapping-6
(concerning the mapping of QNames to URIs), qnameAsId-18
(concerning the use of QNames as identifiers in XML content), and
abstractComponentRefs-37
(concerning the use of fragment identifiers with namespace names to
identify abstract components).
XML ID semantics
Consider the following fragment of XML: <section
>. Does the section element have what the
XML Recommendation refers to as the ID foo (i.e.,
"foo" must not appear in the surrounding XML document more than
once)? One cannot answer this question by examining the element and
its attributes alone. In XML, the quality of "being an ID" is
associated with the type of an attribute, not its name. Finding the
IDs in a document requires additional processing.
Processing the document with a processor that recognizes DTD
attribute list declarations (in the external or internal subset)
might reveal a declaration that identifies the name
attribute as an ID. Note: This processing is not
necessarily part of validation. A non-validating, DTD-aware
processor can recognize IDs.
Processing the document with a W3C XML schema might reveal an
element declaration that identifies the name attribute
as an W3C XML Schema ID.
In practice, processing the document with another schema
language, such as RELAX NG [RELAXNG], might
reveal the attributes declared to be of ID in the XML Schema sense.
Many modern specifications begin processing XML at the Infoset
[INFOSET] level and do not specify
normatively how an Infoset is constructed. For those
specifications, any process that establishes the ID type in the
Infoset (and Post Schema Validation Infoset (PSVI)
defined in [XMLSCHEMA]) may usefully
identify the attributes of type ID.
In practice, applications may have independent means (such as
those defined in the XPointer specification, [XPTRFR]
section 3.2) of locating identifiers inside a document.
To further complicate matters, DTDs establish the ID type in the
Infoset whereas W3C XML Schema produces a PSVI but does not modify
the original Infoset. This leaves open the possibility that a
processor might only look in the Infoset and consequently would
fail to recognize schema-assigned IDs.
See the TAG issue xmlIDSemantics-32
for additional background information and [XML-ID] for a solution under development.
Media types for XML
RFC 3023 defines the Internet media types "application/xml" and
"text/xml", and describes a convention whereby XML-based data
formats use Internet media types with a "+xml" suffix, for example
"image/svg+xml".
There are two problems associated with the “text” media types:
First, for data identified as "text/*", Web intermediaries are
allowed to "transcode", i.e., convert one character encoding to
another. Transcoding may make the self-description false or may
cause the document to be not well-formed.
In general, a representation provider SHOULD NOT assign Internet
media types beginning with "text/" to XML representations.
Second, representations whose Internet media types begin with
"text/" are required, unless the charset parameter is
specified, to be considered to be encoded in US-ASCII. Since the
syntax of XML is designed to make documents self-describing, it is
good practice to omit the charset parameter, and since
XML is very often not encoded in US-ASCII, the use of "text/"
Internet media types effectively precludes this good practice.
In general, a representation provider SHOULD NOT specify the
character encoding for XML data in protocol headers since the
data is self-describing.
Fragment identifiers in XML
The section on media types and
fragment identifier semantics discusses the interpretation of
fragment identifiers. Designers of an XML-based data format
specification should define the semantics of fragment identifiers
in that format. The XPointer Framework [XPTRFR] provides an interoperable starting point.
When the media type assigned to representation data is
"application/xml", there are no semantics defined for fragment
identifiers, and authors should not make use of fragment
identifiers in such data. The same is true if the assigned media
type has the suffix "+xml" (defined in "XML Media Types" [RFC3023]), and the data format specification does
not specify fragment identifier semantics. In short, just knowing
that content is XML does not provide information about fragment
identifier semantics.
Many people assume that the fragment identifier #abc,
when referring to XML data, identifies the element in the document
with the ID "abc". However, there is no normative support for this
assumption. A revision of RFC 3023 is expected to address this.
Data formats enable the creation of new applications to make use of
the information space infrastructure. The Semantic Web is one such
application, built on top of RDF [RDFXML]. This
document does not discuss the Semantic Web in detail; the TAG expects
that future volumes of this document will. See the related TAG issue
httpRange-14.
General Architecture Principles
A number of general architecture principles apply to all three bases of
Web architecture.
Orthogonal Specifications
Identification, interaction, and representation are orthogonal
concepts, meaning that technologies used for identification,
interaction, and representation may evolve independently. For
instance:
Resources are identified with URIs. URIs can be published without
building any representations of the resource or determining whether
any representations are available.
A generic URI syntax allows agents to function in many cases
without knowing specifics of URI schemes.
In many cases one may change the representation of a resource
without disrupting references to the resource (for example, by using
content negotiation).
When two specifications are orthogonal, one may change one without
requiring changes to the other, even if one has dependencies on the
other. For example, although the HTTP specification depends on the
URI specification, the two may evolve independently. This
orthogonality increases the flexibility and robustness of the Web.
For example, one may refer by URI to an image without knowing
anything about the format chosen to represent the image. This has
facilitated the introduction of image formats such as PNG and SVG
without disrupting existing references to image resources.
Orthogonal abstractions benefit from orthogonal specifications.
Experience demonstrates that problems arise where orthogonal concepts
occur in a single specification. Consider, for example, the HTML
specification which includes the orthogonal x-www-form-urlencoded
specification. Software developers (for example, of [CGI] applications) might have an easier time finding the
specification if it were published separately and then cited from the
HTTP, URI, and HTML specifications.
Problems also arise when specifications attempt to modify orthogonal
abstractions described elsewhere. An historical
version of the HTML specification added a "Refresh"
value to the http-equiv attribute of the
meta element. It was defined to be equivalent to the
HTTP header of the same name. The authors of the HTTP specification
ultimately decided not to provide this header and that made the two
specifications awkwardly at odds with each other. The W3C HTML
Working Group eventually removed the "Refresh" value.
A specification should clearly indicate which features overlap with
those governed by another specification.
Extensibility
The information in the Web and the technologies used to represent
that information change over time. Extensibility is the property of a
technology that promotes evolution without sacrificing
interoperability. Some examples of successful technologies designed
to allow change while minimizing disruption include:
the fact that URI schemes are orthogonally specified;
the use of an open set of Internet media types in mail and HTTP
to specify document interpretation;
the separation of the generic XML grammar and the open set of XML
namespaces for element and attribute names;
extensibility models in Cascading Style Sheets (CSS), XSLT 1.0,
and SOAP;
user agent plug-ins.
An example of an unsuccessful extension mechanism is HTTP mandatory
extensions [HTTPEXT]. The community has sought
mechanisms to extend HTTP, but apparently the costs of the mandatory
extension proposal (notably in complexity) outweighed the benefits
and thus hampered adoption.
Below we discuss the property of "extensibility," exhibited by URIs,
some data formats, and some protocols (through the incorporation of
new messages).
Subset language: one language is a subset (or "profile")
of a second language if any document in the first language is also a
valid document in the second language and has the same interpretation
in the second language.
Extended language: If one language is a subset of another,
the latter superset is called an extended language; the difference
between the languages is called the extension. Clearly, extending a
language is better for interoperability than creating an incompatible
language.
Ideally, many instances of a superset language can be safely and
usefully processed as though they were in the subset language.
Languages that can evolve this way, allowing applications to provide
new information when necessary while still interoperating with
applications that only understand a subset of the current language,
are said to be "extensible." Language designers can facilitate
extensibility by defining the default behavior of unknown
extensions—for example, that they be ignored (in some defined way) or
should be considered errors.
For example, from early on in the Web, HTML agents followed the
convention of ignoring unknown tags. This choice left room for
innovation (i.e., non-standard elements) and encouraged the
deployment of HTML. However, interoperability problems arose as well.
In this type of environment, there is an inevitable tension between
interoperability in the short term and the desire for extensibility.
Experience shows that designs that strike the right balance between
allowing change and preserving interoperability are more likely to
thrive and are less likely to disrupt the Web community. Orthogonal specifications help reduce the
risk of disruption.
Errors occur in networked information systems. An error condition can
be well-characterized (e.g., well-formedness errors in XML or 4xx
client errors in HTTP) or arise unpredictably. Error
correction means that an agent repairs a condition so that
within the system, it is as though the error never occurred. One
example of error correction involves data retransmission in response
to a temporary network failure. Error recovery means that
an agent does not repair an error condition but continues processing
by addressing the fact that the error has occurred.
Agents frequently correct errors without user awareness,
sparing users the details of complex network communications. On the
other hand, it is important that agents recover from error
in a way that is evident to users, since the agents are acting on
their behalf.
Agents that recover from error by making a choice without the
user's consent are not acting on the user's behalf.
An agent is not required to interrupt the user (e.g., by popping up a
confirmation box) to obtain consent. The user may indicate consent
through pre-selected configuration options, modes, or selectable user
interface toggles, with appropriate reporting to the user when the
agent detects an error. Agent developers should not ignore usability
issues when designing error recovery behavior.
To promote interoperability, specification designers should identify
predictable error conditions. Experience has led to the following
observations about error-handling approaches.
Protocol designers should provide enough information about an
error condition so that an agent can address the error condition. For
instance, an HTTP 404 status code (not found) is useful because it
allows user agents to present relevant information to users, enabling
them to contact the representation provider in case of problems.
Experience with the cost of building a user agent to handle the
diverse forms of ill-formed HTML content convinced the designers of
the XML specification to require that agents fail upon encountering
ill-formed content. Because users are unlikely to tolerate such
failures, this design choice has pressured all parties into
respecting XML's constraints, to the benefit of all.
An agent that encounters unrecognized content may handle it in a
number of ways, including by considering it an error; see also the
section on extensibility and
versioning.
Error behavior that is appropriate for a person may not be
appropriate for software. People are capable of exercising judgement
in ways that software applications generally cannot. An informal
error response may suffice for a person but not for a processor.
See the TAG issue contentTypeOverride-24,
which concerns the source of authoritative metadata.
Protocol-based Interoperability
The Web follows Internet tradition in that its important interfaces
are defined in terms of protocols, by specifying the syntax,
semantics, and sequencing constraints of the messages interchanged.
Protocols designed to be resilient in the face of widely varying
environments have helped the Web scale and have facilitated
communication across multiple trust boundaries. Traditional
application programming interfaces (APIs) do not always
take these constraints into account, nor should they be required to.
One effect of protocol-based design is that the technology shared
among agents often lasts longer than the agents themselves.
It is common for programmers working with the Web to write code that
generates and parses these messages directly. It is less common, but
not unusual, for end users to have direct exposure to these messages.
It is often desirable to provide users with access to format and
protocol details: allowing them to “view source,” whereby they may
gain expertise in the workings of the underlying system.
The practice of providing multiple
representations available via the same URI. Which representation is
served depends on negotiation between the requesting agent and the
agent serving the representations.
One language is a subset of a second language
if any document in the first language is also a valid document in the
second language and has the same interpretation in the second
language.
Cool URIs
don't change T. Berners-Lee, W3C, 1998 Available at
http://www.w3.org/Provider/Style/URI. Note that the title is
somewhat misleading. It is not the URIs that change, it is what
they identify.
Mandatory Extensions in HTTP, H. Frystyk Nielsen, P.
Leach, S. Lawrence, 20 January 1998. This expired IETF Internet
Draft is available at
http://www.w3.org/Protocols/HTTP/ietf-http-ext/draft-frystyk-http-mandatory.
IETF Guidelines
For The Use of XML in IETF Protocols, S. Hollenbeck, M.
Rose, L. Masinter, eds., 2 November 2002. This IETF Internet Draft
is available at
http://www.imc.org/ietf-xml-use/xml-guidelines-07.txt. If this
document is no longer available, refer to the ietf-xml-use mailing
list.
XML Information Set
(Second Edition), R. Tobin, J. Cowan, Editors, W3C
Recommendation, 04 February 2004,
http://www.w3.org/TR/2004/REC-xml-infoset-20040204. Latest version available at
http://www.w3.org/TR/xml-infoset.
OWL Web Ontology Language
Reference, M. Dean, G. Schreiber, Editors, W3C Recommendation,
10 February 2004,
http://www.w3.org/TR/2004/REC-owl-ref-20040210/. Latest version available at
http://www.w3.org/TR/owl-ref/.
RDF/XML Syntax
Specification (Revised), D. Beckett, Editor, W3C
Recommendation, 10 February 2004,
http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/.
Latest
version available at http://www.w3.org/TR/rdf-syntax-grammar.
Representational State Transfer (REST), Chapter 5 of
"Architectural Styles and the Design of Network-based Software
Architectures", Doctoral Thesis of R. T. Fielding, 2000. Designers
of protocol specifications in particular should invest time in
understanding the REST model and the relevance of its principles to
a given design. These principles include statelessness, clear
assignment of roles to parties, uniform address space, and a
limited, uniform set of verbs. Available at
http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm.
IETF RFC 2718:
Guidelines for new URL Schemes, L. Masinter, H.
Alvestrand, D. Zigmond, R. Petke, November 1999. Available at:
http://www.ietf.org/rfc/rfc2718.txt.
SOAP Version 1.2 Part
1: Messaging Framework, J. Moreau, N. Mendelsohn, H. Frystyk
Nielsen, et. al., Editors, W3C Recommendation,
24 June 2003,
http://www.w3.org/TR/2003/REC-soap12-part1-20030624/. Latest version available
at http://www.w3.org/TR/soap12-part1/.
Uniform Resource Identifiers (URI): Generic Syntax (T.
Berners-Lee, R. Fielding, L. Masinter, Eds.) is currently being
revised. Citations labeled [[!URI]] refer to "Uniform
Resource Identifier (URI): Generic Syntax."
XHTML™ 1.1 - Module-based
XHTML, S. McCarron, M. Altheim, Editors, W3C Recommendation,
31 May 2001,
http://www.w3.org/TR/2001/REC-xhtml11-20010531. Latest version available at
http://www.w3.org/TR/xhtml11/.
XML Linking Language (XLink)
Version 1.0, E. Maler, S. DeRose, D. Orchard, Editors, W3C
Recommendation, 27 June 2001,
http://www.w3.org/TR/2001/REC-xlink-20010627/. Latest version available at
http://www.w3.org/TR/xlink/.
xml:id Version 1.0, D.
Veillard, J. Marsh, Editors, W3C Working Draft (work in progress),
07 April 2004,
http://www.w3.org/TR/2004/WD-xml-id-20040407. Latest version available at
http://www.w3.org/TR/xml-id/.
Extensible Markup Language
(XML) 1.1, J. Paoli, C. M. Sperberg-McQueen, J. Cowan, et.
al., Editors, W3C Recommendation, 04 February 2004,
http://www.w3.org/TR/2004/REC-xml11-20040204/. Latest version available at
http://www.w3.org/TR/xml11/.
Namespaces in XML
1.1, R. Tobin, D. Hollander, A. Layman, et. al.,
Editors, W3C Recommendation, 04 February 2004,
http://www.w3.org/TR/2004/REC-xml-names11-20040204. Latest version available at
http://www.w3.org/TR/xml-names11/.
XML Schema Part 1:
Structures, D. Beech, M. Maloney, H. S. Thompson, et.
al., Editors, W3C Recommendation, 02 May 2001,
http://www.w3.org/TR/2001/REC-xmlschema-1-20010502/. Latest version available at
http://www.w3.org/TR/xmlschema-1/.
XPointer
Framework, E. Maler, N. Walsh, P. Grosso, et. al.,
Editors, W3C Recommendation, 25 March 2003,
http://www.w3.org/TR/2003/REC-xptr-framework-20030325/. Latest version available
at http://www.w3.org/TR/xptr-framework/.
Authoring Tool
Accessibility Guidelines 1.0, C. McCathieNevile, I. Jacobs,
J. Treviranus, et. al., Editors, W3C Recommendation,
03 February 2000,
http://www.w3.org/TR/2000/REC-ATAG10-20000203. Latest version available at
http://www.w3.org/TR/ATAG10.
Device Independence
Principles, R. Gimson, Editor, W3C Note,
01 September 2003,
http://www.w3.org/TR/2003/NOTE-di-princ-20030901/. Latest version available at
http://www.w3.org/TR/di-princ/.
Web
Architecture: Extensible Languages, T. Berners-Lee, D.
Connolly, 10 February 1998. This W3C Note is available at
http://www.w3.org/TR/1998/NOTE-webarch-extlang-19980210.
Principled
Design of the Modern Web Architecture, R.T. Fielding
and R.N. Taylor, UC Irvine. In Proceedings of the 2000
International Conference on Software Engineering (ICSE 2000),
Limerick, Ireland, June 2000, pp. 407-416. This document is
available at
http://www.ics.uci.edu/~fielding/pubs/webarch_icse2000.pdf.
QA Framework:
Specification Guidelines, D. Hazaël-Massieux, L. Rosenthal,
L. Henderson, et. al., Editors, W3C Working Draft (work
in progress), 30 August 2004,
http://www.w3.org/TR/2004/WD-qaframe-spec-20040830/. Latest version available
at http://www.w3.org/TR/qaframe-spec/.
Variability in
Specifications, L. Rosenthal, D. Hazaël-Massieux, Editors,
W3C Working Draft (work in progress), 30 August 2004,
http://www.w3.org/TR/2004/WD-spec-variability-20040830/. Latest version
available at http://www.w3.org/TR/spec-variability/.
User Agent Accessibility
Guidelines 1.0, J. Gunderson, I. Jacobs, E. Hansen, Editors,
W3C Recommendation, 17 December 2002,
http://www.w3.org/TR/2002/REC-UAAG10-20021217/. Latest version available at
http://www.w3.org/TR/UAAG10/.
Web Content Accessibility
Guidelines 2.0, W. Chisholm, J. White, B. Caldwell, et.
al., Editors, W3C Working Draft (work in progress),
30 July 2004,
http://www.w3.org/TR/2004/WD-WCAG20-20040730/. Latest version available at
http://www.w3.org/TR/WCAG20/.
Web Services
Architecture, D. Booth, F. McCabe, E. Newcomer, et.
al., Editors, W3C Note, 11 February 2004,
http://www.w3.org/TR/2004/NOTE-ws-arch-20040211/. Latest version available at
http://www.w3.org/TR/ws-arch/.
XML Accessibility
Guidelines, S. B. Palmer, C. McCathieNevile, D. Dardailler,
Editors, W3C Working Draft (work in progress),
03 October 2002,
http://www.w3.org/TR/2002/WD-xag-20021003. Latest version available at
http://www.w3.org/TR/xag.
Acknowledgments
This document was authored by the W3C Technical Architecture Group
which included the following participants: Tim Berners-Lee (co-Chair,
W3C), Tim Bray (Antarctica Systems), Dan Connolly (W3C), Paul Cotton
(Microsoft Corporation), Roy Fielding (Day Software), Mario Jeckle
(Daimler Chrysler), Chris Lilley (W3C), Noah Mendelsohn (IBM), David
Orchard (BEA Systems), Norman Walsh (Sun Microsystems), and Stuart
Williams (co-Chair, Hewlett-Packard).
The TAG appreciates the many contributions on the TAG's public mailing
list, www-tag@w3.org (archive), which have
helped to improve this document.
In addition, contributions by David Booth, Erik Bruchez, Kendall Clark,
Karl Dubost, Bob DuCharme, Martin Duerst, Olivier Fehr, Al Gilman, Tim
Goodwin, Elliotte Rusty Harold, Tony Hammond, Sandro Hawke, Ryan Hayes,
Dominique Hazaël-Massieux, Masayasu Ishikawa, David M. Karr, Graham
Klyne, Jacek Kopecky, Ken Laskey, Susan Lesch, Håkon Wium Lie, Frank
Manola, Mark Nottingham, Bijan Parsia, Peter F. Patel-Schneider, David
Pawson, Michael Sperberg-McQueen, Patrick Stickler, and Yuxiao Zhao are
gratefully acknowledged.