This document details and deepens some of the most important conformance-related concepts evoked in the QA Specification Guidelines, developing some of the analysis axes that need to be considered while designing a specification and providing advanced techniques, particularly for dealing with conformance variability and complexity
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This document is the second public Working Draft of Variability in Specifications made available by the the QA Working Group of the W3C Quality Assurance (QA) Activity for discussion by W3C members and other interested parties. For more information about the QA Activity, please see the QA Activity statement.
This version includes a few additions from the previous version, moved from the November 2004 version of the Specification Guidelines. It also includes the planned structure of new sections that have not been completed yet. This document does not represent explicit consensus of the Working Group. There are a few open
issues, marked with a capitalized
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 QA Working Group does not expect this document to become a Recommendation. Rather, after further development, review and refinement, it will be published and maintained as a WG Note.
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This document analyzes how design decisions of the conformance model of a specification may affect its implementability and the interoperability of its implementations. To do so, it introduces the concept of variability - how much implementations conforming to a given specification may vary among themselves - and presents a set of well-known dimensions of variability.
Its goal is to raise awareness of the potential cost that some benign-looking decisions may have on interoperability and to provide guidance on how to avoid these pitfalls by better understanding the mechanisms induced by variability.
It completes and deepens the concepts evoked in the Specification Guidelines.
Like the Specification Guidelines,
the primary audience of this document is editors and authors
, however, it is
applicable to a broader audience including:
This document first introduces the concept of
dimensions of variability, and then analyzes
specific aspects related to some of these dimensions, more
classes of products, and the subdividing dimensions
profiles, modules and levels, . Several principles of the Specification Guidelines [SPECGL] address a way in which
the conformance model of a specification might allow
variation among conforming implementations. For
example, a specification might allow implementations to choose
between one of two well defined behaviors for a given
functionality, thus two conforming implementations might vary on
QA Working Group has identified seven ways in
which a specification can allow variability , that are
referred to as dimensions of variability
(DoV). The seven dimensions of variability
recognized by this document are: above are not necessarily
all orthogonal to one another. There are many possible
associations, dependencies, and interrelationships. As a general
policy, this document and the Specification Guidelines do not
attempt to legislate correct or proper relationships among the
they try to clarify the nature of each dimension and suggest that
make deliberate and well documented choices.
The dimensions of variability are one of the principal concepts
in the Specification Guidelines
to help organize, classify and
assess the conformance characteristics of W3C
specifications. The seven DoV get special attention
because they are at the core of the definition of a specification's
conformance model, and thus there is significant potential for
negative interoperability impacts if they are handled carelessly or
without careful deliberation.
As a general principle, variability complicates interoperability. In theory, interoperability is best when there are numerous identical, complete, correct implementations. However, in practice, the net effect of conformance variability is not necessarily negative in all cases, when compared to the alternatives. For example profiles — subdivisions of the technology targeted at specific applications communities — introduce variability among implementations. Some will implement Profile ABC, some will implement Profile XYZ, and the two might not intercommunicate well if ABC and XYZ are fairly different. However, if ABC and XYZ are subsets of a large monolithic specification — too large for many implementors to tackle in total -- and if they are well targeted at actual application sectors, then subdivision by profiles may actually enhance interoperability.
Different sorts of variability have different negative and
positive impacts. The principal danger is "excessive"
variability - variability which goes beyond that needed
for a positive interoperability trade-off, and which unnecessarily
complicates the conformance
Specification writers need to
carefully consider and justify any conformance
variability allowed .
do so by reference
to the project requirements and use cases, and
explicitly document the choices made.
It is even more important to take into account the
multiplicative effect on variability created by combining several
dimensions of variability; each pair of dimensions of
variability used in a specification needs to be assessed with
regard to the variability it creates; the
should document the limited ways an implementation can combine
two dimensions. For instance, deprecated features in
4.01 [HTML4] are allowed in the Transitional profile and
forbidden in the Strict profile.
Note that the variability addressed by the so called dimensions of variability is only considered with regard to conformance to a well-defined specification. As such, the changes introduced in the conformance requirements between two versions or two editions of the specification are not considered as dimensions of variability.
visible dimension of
variability is the the class es of
product s. ,
which separate the different kind of
implementations a specification may have; for instance,
SVG 1.1 [SVG11]
defines conformance for 6 classes of product: SVG document
fragments, SVG stand-alone files, SVG included documents fragments,
SVG generators, SVG interpreters, SVG viewiers.
Defining these classes of products is thus one of the most
important step in the design of a conformance model for a
; this section
tries and gives advice s
on how to do this design , introducing to do so
the concept of specification categories.
From this categorization of specifications, a Working Group can identify the class of products that are affected by the specification. Classes of products can be generalized as either producers or consumers, or as content itself.
For example, identifying which are producers and consumers is clear for a protocol-type specification: the two parties to the dialog are the targets. For a processor-type specification, the processor is the consumer of an XML vocabulary defined in the specification. For content-type specifications, there may be one or more consumers that take the content and 'play' or 'read' it in some way.
The following is a list of the most common classes of products for W3C specifications:
This list does not exhaust all possibilities. Specifications may have to define their own classes of product if none of these fits.
ISSUE: needs to explain the different categories, how to actually make a specification category analysis
To answer the question "what needs to conform?" it
help to first look at the nature of the
specification and categorize it
and then look at the types of
products that would implement the specification.
Categorizing the specification provides a
basis for classifying the software that may be affected by the
specification. The specification category is the
generic name for the type of specification and the technology it
The following is a list of some of the most common specification categories:
The categories indicate what the specification
describes. One specification could potentially fall into more than
one category. This list does not exhaust all possibilities.
Specifications may have to define their own specification
category if none of these fits.
Profiles, modules and levels are three ways to subdivide a specification into related groups of conformance requirements. Because these three dimensions of variability define subsets of a technology, they share some characteristics in the way they affect conformance and interoperability.
A profile is a subset of the technology that supports a particular functional objective or a subset of a set of technologies defining how they are required to operate together (e.g., XHTML plus MathML plus SVG).
Profiles can be based on hardware considerations associated with target product classes — for example, SVG Tiny is aimed at mobile phones — or they may be driven by other functional requirements of their target constituencies — for example, a graphical profile tailored for technical illustrations in aircraft maintenance manuals.
The use of profiles to divide the technology is described in the specification, and may or may not be reflected and paralleled by the structure and organization of the specification.
Specifications may define individual profiles, or may define rules for profiles, or both. An individual profile defines the requirements for classes of products that conform to that profile. Rules for profiles define validity criteria for profiles themselves — i.e., if others (users, applications, or other standards) define their own profiles of the standard (called derived profiles of the specification), then rules for profiles define the constraints that those derived profiles must satisfy in order to be considered valid profiles of the specification.
For example, XHTML Modularization ([XHTML-MOD], section 3) and Synchronized Multimedia Integration Language (SMIL 2.0), [SMIL20] specifications both define rules for profiles -- what constraints must a profile meet in order to be classified as a "Host Language Profile" or an "Integration Set Profile." SMIL further defines some specific profiles, using the modularization. Separate recommendations -- XHTML Basic [XHTML-BASIC] and XHTML 1.1 [XHTML11] — define specific profiles based on the XHTML modularization.
Modules are discrete divisions or functional groupings of the technology and do not necessarily fit in a simple hierarchical structure.
Modules generally can be implemented independently of one another — e.g., audio vs. video module. That notwithstanding, it is possible for one module's definition (and therefore implementation) to have explicit dependency upon another module. It is not only possible, but common to implement multiple modules.
Functional levels — or in common usage, simply levels — are used to group functionality into nested subsets, ranging from minimal or core functionality to full or complete functionally. Level 1 is the minimum or core of the technology. Level 2 includes all of level 1 plus additional functionality. This nesting continues until level n, which consists of the entire technology.
Levels may result from progressive historical development and enrichment of the technology in a series of specifications, as in the case of CSS and DOM. Levels could also be defined explicitly in a single edition of the specification, as in the Web Content Accessibility Guidelines.
Sometimes, the nesting goal of levels is achieved through profiles. For example, SVG 1.1 [SVG11] together with SVG Mobile [Mobile [SVG-MOBILE] define three nested profiles — Tiny, Basic, Full — which are each targeted at a specific graphics hardware community (mobile phone, hand-held computer, desktop computer).
ISSUE: needs to address discretionary items, and more largely, the remaining DoV.
The following QA Working Group and Interest Group participants have contributed significantly to the content of this document:
These references are informative.