webml

WebML (Web Modeling Language http://www.webml.org) is a visual notation for designing complex data-intensive Web applications. It provides graphical, yet formal, specifications, embodied in a complete design process, which can be assisted by visual design tools.

Overview

The main objectives of the WebML design process are:

expressing the structure of a Web application with a high-level description, which can be used for querying, evolution, and maintenance;
providing multiple views of the same content;
separating the information content from its composition into pages, navigation, and presentation, which can be defined and evolved independently;
storing the meta-information collected during the design process within a repository, which can be used during the lifetime of the application for dynamically generating Web pages;
modelling users and communities explicitly in the repository, to permit the specification of personalization policies and one-to-one applications;
enabling the specification of data manipulation operations for updating the site content or interacting with arbitrary external services.

The main models included in the WebML design methodology are:

Data model
Hypertext model (including composition and navigation)
Presentation model

The design is guided by a formally specified development process.

Development process

The WebML approach to the development of Web applications consists of different phases that must be applied in an iterative and incremental manner. The process undergoes several cycles, each cycle producing a prototype or a partial version of the application that allows conducting testing and evaluation since the early development phases.

Out of the entire process, data and hypertext design are the activities most influenced by the adoption of WebML. Indeed, data publishing and content management applications have some regularities and peculiarities, which can be exploited in the design of data. Recognizing them may help the data designer organize his work in a systematic way, which normally results into more consistent data schemas. Therefore, the method stresses the distinct roles played by objects, and use this distinction to propose a sequence of steps for assembling the data schema of a Web application. After data design, hypertext design proceeds top-down:

A first coarse design activity aims at producing a first high-level specification of site views that exploits an informal textual notation to express visibility level for site view areas (landmark, default or internal areas), and specify the area content in terms of entities and relationships of the data schema. In doing so, special attention is paid to the role played by the various data elements, which may be exploited for accessing information, for publishing the content of core objects, for interconnecting core objects, or for personalization purposes.
Detailed design is a top-down refinement of coarse design, in which the draft schemas of site views are progressively revised until they become collections of WebML pages and units compliant with the users requirements. Detailed design exploits WebML hypertext sub-schemas, which are "canonical" configurations of pages and units, built on top of the core, access, interconnection, and personalization sub-schemas.

Data model

The WebML data model is a suitable adaptation of conceptual models for data design, as already in use in other disciplines, such as database design, software engineering, and knowledge representation. It is compatible with the Entity-Relationship data model, used in conceptual database design, and with UML class diagrams, used in object-oriented modeling. The fundamental elements of data models are entities, defined as containers of data elements, and relationships, defined as semantic connections between entities. Entities have named properties, called attributes, with an associated type. Entities can be organized in generalization hierarchies and relationships can be restricted by means of cardinality constraints. Instances of entities are considered individually addressable by means of a unique identifier (OID). WebML OIDs are abstract concepts, which can be implemented in alternative ways in the underlying storage manager, e.g., primary keys in a relational data store or XML ID attributes in a XML data source.

Hypertext model

Hypertext modelling specifies composition and navigation of the site. Composition describes which pages compose the hypertext, and which content units make up a page. The pages of the Web site are the containers of information actually delivered to the reader. Units are atomic content elements used to publish the information described in the data model. Seven types of units are predefined in WebML to compose pages: data, multi-data, index (and its variants multichoice and hierarchical), entry, scroller. Each unit is associated to one underlying entity, from which the content of the unit is computed. The specification of the underlying entity dictates the object type from which the content of a unit is derived (e.g., albums, artists,...). When appropriate, units may be optionally associated to a selector, i.e., the specification of a set of restrctions that determine the actual instances of the underlying entity to be used as the content of the unit at runtime. Navigation of the site is specified thru links. Links can be defined between the units inside a single page, between units placed in different pages, and between pages. The information carried along a link is called navigation context, or simply context. Links that carry context information are called contextual links, whereas links that have no associated context information are called non-contextual links. Context information is typically necessary to ensure the computability of units. The figure below reports and example of WebML hypertext specification.

Presentation model

Presentation is the orthogonal task of defining the look and feel of pages in a site view. WebML includes a very simple presentation model that allows to place dynamic contents in the page and to apply a graphic rendition style for each of them. For stricly graphic issues, WebML leverages standard approaches from Web design, more familiar to graphic and communication experts. This Since WebML specifications can be represented using XML, presentation is considered like a document transformation mapping the WebML specification of a page into a page written in a concrete implementation language like JSP or ASP.NET. Consequently, presentation is addressed in WebML by attaching XSL style sheets to site views, pages, units and unit subelements. XSL style sheets take in input WebML specifications, coded as XML documents conforming to the WebML Document Type Definition, and output page templates embodying the required mark-up code and data access queries. An implementation of WebML may include several pre-defined presentation style sheets and the server-side components supporting the data access queries needed to populate the content of the page templates produced by the XSL style sheets.

Webml

Overview

Development process

Data model

Hypertext model

Presentation model

See also...