The World Wide Web is an information space of interrelated resources. This information space is the basis of, and is shared by, a number of information systems. Within each of these systems, people and software retrieve, create, display, analyze, relate, and reason about resources.
Web architecture defines the information space in terms of identification of resources, representation of resource state , and the protocols that support the interaction between agents and resources in the space. Web architecture is influenced by social requirements and software engineering principles. These lead to design choices
and constraints on
the behavior of systems that use
the Web in order to achieve desired properties of the shared
information space: efficiency, scalability, and the potential for
indefinite growth across languages, cultures, and media. Good
practice by agents in the system is also important to the success of the system. This document reflects the three bases of Web architecture: identification, interaction, and representation.
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 is the 16 August 2004 Last Call Working Draft
of "Architecture of the
World Wide Web, First Edition."
The Last Call review period ends 17 September 2004, at 23:59 EDT. Please
send Last Call review comments on this document before that date to
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Last Call Working Draft status is described in
section
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provide a separate email message for each distinct comment.
The TAG has used the last call
comments page to track the status and discussion of
comments on the 9 December 2003
Draft. This draft has been informed by a large number of comments
made on the on the 9 December 2003
Last Call Working Draft. Because the text has changed substantially,
it is not always clear whether or not comments made on the previous draft
still apply.
The TAG expects a future revision of
this document to become a W3C
Recommendation.
This document has been developed by W3C's Technical Architecture Group
(TAG) (charter).
A complete list of changes to
this document since the first public Working Draft is available on the
Web.
The TAG charter
describes a process for issue resolution by the TAG. In accordance
with those provisions, the TAG maintains a running issues list.
The First Edition of "Architecture of the World Wide Web" does not
address every issue that the TAG has accepted since it began work in
January 2002. The TAG has selected a subset of issues that the First
Edition does address to the satisfaction of the TAG; those issues are
identified in the TAG's issues list. The TAG intends to address the
remaining (and future) issues after publication of the First Edition
as a Recommendation.
This document uses the concepts and terms regarding URIs as defined
in draft-fielding-uri-rfc2396bis-06, preferring them to those defined
in RFC 2396. The IETF Internet Draft draft-fieldi
ng-uri-rfc2396bis-06
is expected to obsolete RFC 2396, which is the
current URI standard. The TAG is tracking the evolution of
draft-fielding-uri-rfc2396bis-06.
Publication as a Working Draft does not imply endorsement by the
W3C Membership. This is a draft document and may be updated, replaced
or obsoleted by other documents at any time. It is inappropriate to
cite this document as other than "work in progress."
The patent policy for this document is expected to become the
5 February 2004
W3C Patent Policy, pending the Advisory Committee review of
the revision to the W3C Technical Architecture Group Charter.
Patent disclosures relevant to this specification may be found on the
Technical Architecture
Group's patent disclosure page. An individual who has actual knowledge
of a patent which the individual believes contains Essential Claim(s)
with respect to this specification should disclose the information in
accordance with
section 6 of the W3C Patent Policy.
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 (on behalf of a person,
entity, or process) 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 use software to retrieve associated
information (in this case, about the weather, as advertised).
When Nadia enters the URI into her browser:
- 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 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. Protocols define the
syntax and semantics of messages exchanged by agents over a network.
Web agents communicate the information state of a resource using
protocols. By clicking on a hypertext link,
Nadia tells her browser to
request the information state of the resource referenced by the URI in
the link.
In this example,
the browser sends an HTTP GET request to the server at
"weather.example.com" and the server sends back a representation of the information state of the resource. In this example, the representation includes XHTML data and metadata such as the Internet media type of the data, "application/xhtml+xml". Note: In this document, the noun "representation" means "octets that encode resource state information". These octets do not necessarily describe the resource , or portray a likeness of the resource, or represent the resource in other senses of the word "represent".
- Formats. Representations are built from a
non-exclusive set of data formats, used separately or in combination
(including XHTML, CSS, PNG, XLink, RDF/XML, SVG, and SMIL
animation). In this scenario, the representation is built primarily
in XHTML. While interpreting the XHTML representation data, the
browser retrieves and displays weather maps identified by URIs
within the XHTML. Some of those maps are built in SVG.
The following illustration shows the relationship between
identifier, resource, and representation.
In the remainder of this document, we highlight important
architectural points regarding Web identifiers, protocols, and
formats. We also discuss some important
general architectural principles in the context
of the Web.
This document describes the properties we desire of the Web and the
design choices that have been made to achieve them. It
promotes re-use 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]. However, 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.
- The addition of a conformance section is not likely to
increase the utility of the 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
Readers will benefit from familiarity with the Requests for Comments
(RFC) series from the IETF, some of which define pieces of the
architecture discussed in this document.
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.
This document presents the general architecture of the Web. Other groups
inside and outside W3C also address specialized
aspects of Web architecture, including accessibility,
internationalization, device independence, and Web Services.
The section on Architectural Specifications
includes references to these related specifications.
This document strikes 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 also 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 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, [VOICEXML2]). 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.
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 design 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. Other design choices are more
fundamental; these are the focus of this document. Design
choices can 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 certain
properties of the system,
such as accessibility and global scope, and
non-functional properties, such as relative ease of evolution,
re-usability of components, 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.
In order to communicate internally, a community agrees (to a
reasonable extent) on a set of terms and their meanings.
Since its inception, one goal of the Web 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 global scope 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 [section 4.4]
).
Principle:
Global Identifiers
Global naming leads to global network effects.
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].
The choice of syntax for global identifiers is somewhat arbitrary; it
is their global scope which is important.
The Uniform Resource Identifier ([URI], currently being revised) 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. 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 a URI 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.
Good practice:
Identify with URIs
To benefit from and increase the value of the
World Wide Web, agents should provide URIs as identifiers for
resources.
Other resource identification systems (see the section on future directions for identifiers) may
expand the Web as we know it today. However, there are substantial
costs to creating a new identification system that has the same
properties as URIs.
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. A significant class of resources, information resources, are discussed in Information Resources and
Representations [section 3.1].
Constraint:
URIs Identify a Single Resource
Assign
distinct URIs to distinct resources.
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.
URI Scheme specifications define the protocols by which scheme specific URI are associated with resources and take on meaning. For example, the HTTP URI scheme (RFC2616) uses DNS so that names such as “http://example.com/somepath#someFrag” take on meaning by way of HTTP GET response messages from the domain holder (or an agent they delegate to).
While other communications may suggest meanings for such names, it's a local policy decision whether those suggestions should be heeded, whereas the result obtained through HTTP GET is, by Internet-wide agreement, authoritative.
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.
The following sections address other questions about the
relationship between URIs and resources, including:
To avoid URI collision, it is important to avoid assigning the same URI to different resources.
One way to avoid URI collision is through
delegated ownership.
It is useful for a URI scheme to establish a unique
relationship between a social entity and a URI; this is the case for
the "http", "mailto", "ftp", and "urn" schemes, for example. This
relationship is called URI ownership. In this document, the phrase
"authority responsible for domain X" indicates that the same entity
owns those URIs where the authority component is domain X. This
document does not address how the benefits and responsibilities of URI
ownership may be delegated to other parties, such as to a server
manager or to someone who has been delegated part of the URI space on
a given Web server.
The approach taken for the "http" URI scheme follows the pattern
whereby the Internet community delegates authority, via the IANA URI
scheme registry [IANASchemes] 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 URI owner may, upon request, provide representations of the
resource identified by the URI. For example, when a URI owner uses the
HTTP protocol to provide those representations, the HTTP origin server
(defined in [RFC2616]) is the software agent acting on behalf of the URI owner to provide the authoritative representations for the resource identified by that URI. The owner is also responsible for accepting or rejecting requests to modify the resource identified by that URI, for example, by configuring a server to accept or reject HTTP PUT data based on Internet media type, validity constraints, or other constraints.
Recall that Web architecture allows different URI owners to
create URI aliases. This means that
multiple parties may provide representations of the same resource,
depending on which URI is used for interaction.
There are social expectations for responsible representation management [section 3.6]
by URI
owners, discussed below. Additional social implications of
URI ownership are not discussed here. 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.
See TAG issue siteData-36,
which concerns the expropriation of naming authority.
Some schemes use techniques other than delegated ownership to avoid overloading. Some schemes have construction rules that work to guarantee uniqueness. Other schemes (such as "news:comp.text.xml") rely on a social process.
As discussed above, 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."
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 created" or "when the discussion forum about the movie was created."
The section below on URI assignment [section 2.2.1]
examines approaches for establishing the authoritative source of
information about what resource a URI identifies.
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 arbitrary entities. Globally adopted assignment policies make some URIs appealing as general-purpose identifiers. Local policy establishes what they indirectly identify.
For example, the URI "mailto:nadia@example.com" identifies an
Internet mailbox (as specified by the "mailto" URI scheme). Suppose
this particular URI identifies Nadia's Internet mailbox. The
organizers of a conference attended by Nadia might use
"mailto:nadia@example.com" to refer indirectly to her (e.g., using the
URI as a database key in their database of conference participants).
URI that are identical , character-by-character, refer to the same resource. Since Web Architecture allows the association of multiple URI with a given resource, two URI that are not character-by-character identical may still refer to the same resource. Different URI do not necessarily refer to different resources but there is generally a higher computational cost to determine that multiple different URI 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), certain specifications specify equivalence
tests in addition to character-by-character comparison. For example,
for "http" URIs, the authority component (the part after "//" and
before the next "/") is defined to be case-insensitive. Thus, the
"http" URI specification allows agents to conclude that
authority components in two "http" URIs identify the same resource
when those strings are character-by-character equivalent or differ only by
case. Agents that reach conclusions based on comparisons that are not
specified by relevant specifications take responsibility for any
problems that result; see the section on
error handling [section 5.3]
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.
See the section below on approaches other than string comparison
that allow different agents to conclude that two URIs identify the same
resource [section 2.7.2]
.
Although there are benefits (such as naming flexibility) to URI
aliases, there are also costs.
URI aliases are harmful when they cause bifurcation in 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
that link to it (the network neighborhood of the measured resource).
This type of valuation is commonly used to rank the relative
value of search results because people tend to
create links relating a given topic to those resources that
they feel best reflect that topic, and hence the number of
inbound references are a reflection of the degree to which
the community values a resource.
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.
Good practice:
Avoiding URI aliases
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.
Good practice:
Consistent URI usage
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 RFC2616.
Story
Dirk notices that on 03 August 2004, the representation of the resource identified by "http://weather.example.com/oaxaca" is exactly the same as the representation of the resource identified by "http://weather.example.com/2004/08/03/oaxaca". He asks Nadia if it matters which weather forecast bookmarks. Nadia explains that it certainly does, one URI identifies the resource "today's weather," the other identifies the resource "weather on 03 August 2004," even though they may have the same representation today.
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. This occurs most commonly when a URI owner provides a URI for both the "current version" of something and a URI for the "version at a point in time," as the Oaxaca weather Web site is doing, but it can occur in other ways as well.
The distinguishing characteristic of representation reuse, as opposed to aliasing, is that the underlying resources are different.
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 significant 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.
Good practice: Reuse URI schemes
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.
If the motivation behind registering a new scheme is to allow a software agent to launch a particular application when retrieving a representation, such dispatching can be accomplished at lower expense via Internet media types. When designing a new data format, the appropriate mechanism to promote its deployment on the Web is the Internet media type. Media types also provide a means for building new information space applications [section 4.6] , described below.
Note that 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.
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 [RFC2616]. 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; the network effect is lost.
Note: Some URI scheme specifications (such as the "ftp" URI scheme specification) use the term "designate" where the current document uses "identify."
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 Internet media type of
representations of a resource 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 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.
Good practice:
URI
opacity
Agents making use of URIs SHOULD NOT attempt to infer
properties of the referenced resource except as specified by relevant
specifications.
The example URI used in the travel scenario
("http://weather.example.com/oaxaca") suggests 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 issue
metaDataInURI-31.
Story
When navigating within the XHTML data that Nadia receives as a
representation of the resource identified by
"http://weather.example.com/oaxaca", Nadia finds that the
URI "http://weather.example.com/oaxaca#tom" refers to
the part of the representation that conveys
information about tomorrow's weather in Oaxaca. This URI includes the
fragment identifier "tom" (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 interpretation of fragment identifiers is
discussed in the section on media types
and fragment identifier semantics.
See TAG issue
abstractComponentRefs-37,
which concerns the use of fragment identifiers with namespace names
to identify abstract components.
There remain open questions regarding identifiers on the Web.
The following sections identify a few areas of future work in the
Web community.
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.
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.
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). Two important
classes of message are those that request a representation of an Information Resource, and those that
return the result of such a request.
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.
The term
Information Resource refers to resources that convey information. Any resource that has a representation is an information resource. A representation
consists
logically of two parts: data (expressed in one or more formats used separately or in combination) and
metadata (such as the Internet media
type of the data).
The Information Resource provides the foundation for the familiar
hypertext Web, where agents use representations to modify as well as
retrieve information state. Much of this document describes
architecture specific to Information Resources. For instance, the
techniques of caching and content negotiation, and the social processes of publishing, apply to Information Resources.
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 [RFC 2141]) 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.
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 are involved when a user instructs a user agent 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 [RFC2616], section 3.2.2).
- In this SVG context, the agent constructs an HTTP GET request (per section 9.3 of [RFC2616]) to retrieve the representation.
- Section 6 of [RFC2616] defines how the server constructs a corresponding response message, including the 'Content-Type' field.
- Section 1.4 of [RFC2616] states "HTTP communication usually takes place over TCP/IP connections." This example does not address that step in the process, or 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; information changes over time, and so representations of that information are also likely to change.
Note also that the choice and expressive power of a format can affect how precisely a representation provider communicates resource state. The use of natural language to communicate information may lead to ambiguity about what the associated resource is, which in turn can lead to URI collision.
The Internet media type [RFC2046]) of a
representation determines which data format specification(s) provide the
authoritative interpretation of the representation data (including fragment identifier syntax and
semantics, if any). The IANA registry [MEDIATYPEREG] maps media types to data formats.
The TAG finding "Internet media type
registration, consistency of use" provides more information
to W3C groups about media type registration.
The Internet media type mechanism does have its limitations. For
instance, media type strings do not support versioning or other parameters. See TAG issue
mediaTypeManagement-45,
which concern the appropriate level of granularity of the media type
mechanism.
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 requesting 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 media type [RFC2046] 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 [RFC2616], for example) does not account for
fragments.
As with any URI, use of a fragment identifier component does not
imply that a retrieval action will take place. A URI with a fragment
identifier may be used to refer to the secondary resource without any
implication that the primary resource is accessible or will ever be
accessed. One may compare URIs with fragment identifiers without a
retrieval action. Parties that draw conclusions about the
interpretation of a fragment identifier based solely on a syntactic
analysis of all or part of a
URI do so at their own risk; such interpretations are not
authoritative because they are not licensed by specification (specifically
[URI]).
Please note the following about primary and secondary
resources:
- A resource may be both a primary and secondary resource since
more than one URI may identify the resource.
- One cannot carry out an HTTP POST operation
using a URI that identifies a secondary resource.
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 restrictions on, or
structure within, 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.
See related TAG issue
RDFinXHTML-35.
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. For
instance, examples that have been 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 specification.
Receiving agents should detect protocol inconsistencies and perform
proper error recovery.
Principle:
Data-metadata inconsistency
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.
In all cases 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.
Good practice:
Metadata association
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 this
type of inconsistency and how server configuration can be used to
avoid it.
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
[RFC2616] 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.
Principle:
Safe
retrieval
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 URI retrieval is safe does not imply that all safe
interactions must be done through URI retrieval.
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).
For more information about safe and unsafe operations using HTTP
GET and POST, and handling security concerns around the use of HTTP
GET, see the TAG finding "URIs,
Addressability, and the use of HTTP GET and POST".
Story
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 improve the situation. 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 [RFC2616]).
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 it in fact it is raining (and he trusts them more
than he trusts the Web site in question).
Dirk and Nadia conclude that the URI owners are unreliable or
unpredictable. Although the URI owner has chosen the
Web as a communication medium, they have lost two customers due to
ineffective resource 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.
Good practice:
Available representation
A
URI owner SHOULD provide representations of the identified
resource.
For example, owners of XML namespace URIs should use them
to identify a namespace
document.
A corollary to the good practice for URI owners applies to application
developers.
Principle:
Reference does not imply dereference
An application developer or specification author SHOULD NOT require
networked retrieval to representations each time they are referenced.
Dereferencing a URI has a cost, potentially a significant cost,
perhaps in terms of security, network latency, or other factors.
Attempting to retrieve representations of resources when such
retrieval is not necessary should be avoided.
The following sections discuss some aspects of representation
management, including promoting URI
persistence, managing access to
resources, and
supporting navigation.
As is the case with many human interactions, confidence in
interactions via the Web depends on stability and predictability. For
an Information Resource,
persistence generally depends directly on the consistency of
information conveyed by a series 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.
Good practice:
Consistent representation
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 [RFC2616] 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].
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, 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".
It is a strength of Web Architecture that links can be made and
shared; once one user has found an interesting part of the Web, they
can share this experience just by exchanging 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 applicability 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 with others since the URI seen by the user
does not change as the user moves from page to page.
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 [XMPP]), streaming audio (such as
RTSP [RFC2326]),
and voice-over-IP (such as SIP
[RFC3261]).
A data format (including XHTML, RDF/XML, SMIL, XLink, CSS,
and PNG) specifies the 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 data sender 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 data formats are those in which portions of the data are
encoded for direct use by computer processors, for example thirty-two
bit little-endian two's-complement and sixty-four 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 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.
Text (i.e., a sequence of characters from a
repertoire) is distinct from serving data 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.
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.
See TAG issue binaryXML-30.
Extensibility and versioning are strategies to help manage the
natural evolution of information on the Web and technologies used to
represent that information.
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].
There is typically a (long) transition period during which multiple
versions of a format, protocol, or agent are simultaneously in
use.
Good practice:
Version information
A data format specification SHOULD provide for version information.
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.
For almost all applications, 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.
Good practice:
Namespace policy
A 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.
Designers can facilitate the transition process by
making careful choices about extensibility during the design of a
language or protocol specification.
Good practice:
Extensibility mechanisms
A
specification
SHOULD provide mechanisms that allow any party to create extensions
that do 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.
Good practice:
Unknown extensions
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 extensibility
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 have little or no effect on the display of the image.
- There are container formats such as SOAP which fully expect to be
composed from multiple namespaces but which provide an overall
semantic relationship of message envelope and payload.
- The semantics of combining RDF documents containing multiple
vocabularies is well-defined.
These relationships can be mixed and nested arbitrarily. In
principle, a SOAP message 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).
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].
Good practice:
Separation of content, presentation, interaction
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: recombination on the server
allows the server to send out generally smaller amounts of data that
can be tailored to specific devices (such as mobile phones). However,
such data will not be readily reusable by other clients and may not
allow client-side agents to perform useful tasks unanticipated by
the author. When a client does the work of recombination, content is
likely to be more reusable by a broader audience and more
robust. However, such data may be of greater size and may require more
computation by the client.
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 be desirable to limit the 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).
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 birth 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".
Good practice:
Link
identification
A specification SHOULD
provide ways to identify links to other resources and to
portions of representation data (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.
Good practice:
Web
linking
A specification SHOULD
allow Web-wide linking, not just internal document linking.
Good practice:
Generic
URIs
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 links among
representations.
Good practice:
Hypertext 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.
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 [section 4.5.2]
.
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.
Many data formats are XML-based, that is to say they conform to the
syntax rules defined in the XML specification [XML10]. 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.
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.
- Immediate 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.
Sophisticated linking mechanisms have been invented for XML
formats. XPointer allows links to address content that does not have
an explicit, named anchor. XLink 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 should consider using XLink and, for
defining fragment identifier syntax, using the XPointer framework and
XPointer element() Schemes.
See TAG issue xlinkScope-23.
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 assignment [section 2.2.1]
). 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. All of the globally grounded terms in an XML
namespace share the same syntactic prefix: the namespace URI.
The use of URIs confers additional benefits. First, each local
name / URI 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. One
particularly useful mapping in the case of flat namespaces (specified,
for example, in [RDF10]) is to combine the
namespace URI, a hash ("#"), and the local name, thus creating a URI
for a secondary resource (the identified term). 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
re-use more difficult, even impractical in some cases.
Good practice:
Namespace adoption
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 any type, 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].
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.
Good practice:
Namespace documents
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], 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).
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.
Good practice:
QNames Indistinguishable from URIs
A specification in which QNames represent URI/local-name pairs
SHOULD NOT allow both Qnames and URIs in attribute values or element
content, where they would be indistinguishable.
For more information, see the TAG finding "Using QNames as
Identifiers in Content".
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.
Good practice:
QName
Mapping
A specification in which QNames serve as resource
identifiers MUST provide a mapping to URIs.
One particularly useful mapping in the case of flat namespaces is
to combine the namespace URI, a hash ("#"), and the local name; see
the section on XML namespaces for more
examples.
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).
Consider the following fragment of XML: <section
name="foo">. 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 perform ID
assignment.
- 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.
The TAG expects to continue to work with other groups to help
resolve open questions about establishing "ID-ness" in XML
formats; this is the subject of TAG issue xmlIDSemantics-32.
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".
These Internet media types create two problems: 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.
Good practice:
XML and
"text/*"
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.
Good practice:
XML and
character encodings
In general, a representation
provider SHOULD NOT
specify the character encoding for XML data in protocol headers
since the data is self-describing.
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.
See TAG issue fragmentInXML-28.
Because of their role in defining fragment identifier semantics,
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 [RDF10]. This
document does not discuss the Semantic Web in detail; the TAG expects
that future editions of this document will. See the related TAG issue
httpRange-14.
A number of general architecture principles apply
to all three bases of Web architecture.
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.
Principle:
Orthogonality
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 advance into
territory rightfully governed by another
specification.
The information in the Web and the technologies used to represent
that information change over time. Extensibility describes the
property of a technology that promotes both evolution and
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.
For further discussion, see the section on versioning and extensibility. See also TAG
issue xmlProfiles-29
and HTML Dialects.
Errors occur in networked information systems. An error condition
can be well-specified (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.
Principle:
Error recovery
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 [section 4.2]
.
- 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.
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.
