Separation of Content, Presentation, and Interaction The web is a heterogeneous environment where a wide variety of agents provide access to content to users with a wide variety of capabilities. It is good practice for authors to create content that can reach the widest possible audience, including users with graphical desktop computers, hand-held devices and mobile phones, users with disabilities who may require speech synthesizers, and devices not yet imagined. Furthermore, authors cannot predict in some cases how an agent will display or process their content. Experience shows that the separation of content, presentation, and interaction promotes the reuse and device-independence of content; this follows from the principle of orthogonal specifications (5.1). 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.
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See.3 Extensibility and Extensions. Composition of data formats Many modern data format include mechanisms for composition. For example: It is possible to embed text comments in some image formats, such as jpeg/jfif. Although these comments are embedded in the containing data, they are not intended to affect the display of the image. There are container formats such as soap which fully expect content from multiple namespaces but which provide an personal overall semantic relationship of message envelope and payload. The semantics of combining rdf documents containing multiple vocabularies are well-defined. In principle, these relationships can be mixed and nested arbitrarily. A soap message, for example, can contain an svg image that contains an rdf comment which refers to a vocabulary of terms for describing the image. Note however, that for general xml there is no semantic model that defines the interactions within xml documents with elements and/or attributes from a variety of namespaces. Each application must define how namespaces interact and what effect the namespace of an element has on the element's ancestors, siblings, and descendants. See tag issues mixeduixmlnamespace-33 (concerning the meaning of a document composed of content in multiple namespaces xmlFunctions-34 (concerning one approach for managing xml transformation and composability and rdfinxhtml-35 (concerning the interpretation of rdf when embedded in an xhtml document).
Additional strategies include prompting the user for resume more input and automatically retrieving data from available hypertext links. More complex strategies are also possible, including mixing strategies. For instance, a language can include mechanisms for overriding standard behavior. Thus, a data format can specify "must ignore" semantics but also allow for extensions that override that semantics in light of application needs (for instance, with "must understand" semantics for a particular extension). Extensibility is not free. Providing hooks for extensibility is one of many requirements to be factored into the costs of language design. Experience suggests that the long term benefits of a well-designed extensibility mechanism generally outweigh the costs.
However, languages that have no extensibility mechanisms may be extended in ad hoc ways that impact interoperability as well. One key criterion of the essay mechanisms provided by language designers is that they allow the extended languages to remain in conformance with the original specification, increasing the likelihood of interoperability. Good practice: Extensibility conformance Extensibility must not interfere with conformance to the original specification. Application needs determine the most appropriate extension strategy for a specification. For example, applications designed to operate in closed environments may allow specification designers to define a versioning strategy that would be impractical at the scale of the web. 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.
Designers can facilitate the transition process by making careful choices about extensibility during the design of a language or protocol specification. In making these choices, the designers must weigh the trade-offs between extensibility, simplicity, and variability. A language without extensibility mechanisms may be simpler and less variable, improving initial interoperability. However, it's likely that changes to that language will be more difficult, possibly more complex and more variable, than if the initial design had provided such mechanisms. This may decrease interoperability over the long term. Good practice: Extensibility mechanisms A specification should provide mechanisms that allow any party to create extensions. Extensibility introduces variability which has an impact on interoperability.
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If "a" and writing "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 An xml format specification should include information about change policies for xml namespaces. As an example of a change policy designed to reflect the variable stability of a namespace, consider the W3C namespace policy for documents on the W3c recommendation track. The policy sets expectations that the working Group responsible for the namespace may modify it in any way until a certain point in the process candidate recommendation at which point W3C constrains the set of possible changes to the namespace in order to promote stable. Note that since namespace names are uris, the owner of a namespace uri has the authority to decide the namespace change policy. Extensibility requirements change over time. Successful technologies are adopted and adapted by new users.
See tag issue xmlversioning-41, which concerns good practices for designing extensible xml languages and for handling versioning. See also "Web Architecture: Extensible languages" extlang. Versioning There is typically a (long) transition period during which multiple versions of a format, protocol, or agent are simultaneously in use. Good practice: A data format specification should provide for version information. Versioning and xml namespace policy Story nadia and Dirk are designing an xml data format to encode data about the film industry. They provide for extensibility by using xml namespaces and creating a schema that allows the inclusion, in certain places, of elements from any namespace.
When they revise their format, nadia proposes a new optional lang attribute on the film element. Dirk feels that such a change requires them to assign a new namespace name, which might require changes to deployed software. Nadia explains to dirk that their choice of extensibility strategy in conjunction with their namespace policy allows certain changes that do not affect conformance of existing content and software, and thus no change to the namespace identifier is required. They chose this policy to help them meet their goals of reducing the cost of change. Dirk and Nadia have chosen a particular namespace change policy that allows them to avoid changing the namespace name whenever they make changes that do not affect conformance of deployed content and software. They might have chosen a different policy, for example that any new element or attribute has to belong to a namespace other than the original one. Whatever the chosen policy, it should set clear expectations for users of the format. In general, changing the namespace name of an element completely changes the element name.
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Therefore, designers of a data format specification should make a considered choice between binary and mother textual format design. See tag issue binaryxml-30. Versioning and Extensibility In a perfect world, language designers would invent languages that perfectly met the requirements presented to them, the requirements would be a perfect model of the world, they would never change over time, and all implementations would be perfectly interoperable because the. In the real world, language designers imperfectly address the requirements as they interpret them, the requirements inaccurately model the world, conflicting requirements are presented, and they change over time. As a result, designers negotiate with users, make compromises, and often introduce extensibility mechanisms so that it is possible to work around problems in the short term. In the long term, they produce multiple versions of their languages, as the problem, and their understanding of it, evolve. The resulting variability in specifications, languages, and implementations introduces interoperability costs. Extensibility and versioning are strategies to help manage the natural evolution of information on the web and technologies used to represent that information. For more information about how these strategies introduce variability and how that variability impacts interoperability, see variability in Specifications.
Also, they can be consumed more rapidly by agents in those cases best where they can be loaded into memory and used with little or no conversion. Note, however, that such cases are relatively uncommon as such direct use may open the door to security issues that can only practically be addressed by examining every aspect of the data structure in detail. Textual formats are usually more portable and interoperable. Textual formats also have the considerable advantage that they can be directly read by human beings (and understood, given sufficient documentation). This can simplify the tasks of creating and maintaining software, and allow the direct intervention of humans in the processing chain without recourse to tools more complex than the ubiquitous text editor. Finally, it simplifies the necessary human task of learning about new data formats; this is called the "view source" effect. It is important to emphasize that intuition as to such matters as data size and processing speed is not a reliable guide in data format design; quantitative studies are essential to a correct understanding of the trade-offs.
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 (4.5.7) 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.
Below we describe some characteristics of a data about format that facilitate integration into. 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 (7.1) includes references to additional format specification guidelines. Binary and, textual Data formats, binary data formats are those in which portions of the data are encoded for direct use by computer processors, for example 32 bit little-endian two's-complement and 64 bit ieee double-precision floating-point. The portions of data so represented include numeric values, pointers, and compressed data of all sorts. A textual data format is one in which the data is specified in a defined encoding as a sequence of characters. Html, internet e-mail, and all.
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A data format specification (for example, for xhtml, rdf/xml, smil, xlink, css, and png) embodies an agreement on the correct interpretation of representation data. The first data format used on the web was html. Since then, data formats have grown in number. Web architecture does not constrain which data formats content providers can use. This flexibility is important because there is constant evolution in applications, resulting in new data formats and refinements of existing formats. Although Web architecture allows for the deployment of new data formats, the creation and deployment of new formats (and agents surgery able to handle them) is expensive. Thus, before inventing a new data format (or "meta" format such as xml designers should carefully consider re-using one that is already available. For a data format to be usefully interoperable between two parties, the parties must agree (to a reasonable extent) about its syntax and semantics. Shared understanding of a data format promotes interoperability but does not imply constraints on usage; for instance, a sender of data cannot count on being able to constrain the behavior of a data receiver.