WebML

Web Modeling Language, (WebML) is a visual notation and methodology for the design of a data intensive web applications. It provides a graphical means to define the specifics of web application design within a structured design process. This process can be enhanced with the assistance of visual design tools. In 2013, WebML was expanded to encompass a broader range of front end interfaces, leading to the development of the Interaction Flow Modeling Language (IFML). IFML has since been recognized as a standard by the Object Management Group (OMG). This approach involves the development of five distinct models: structure, derivation, composition, navigation, and presentation. These models are refined through an iterative process.

WebML enables designers to express the core features of a site at a high level without committing to detailed architectural details. WebML concepts are associated with a graphic representation, which can be supported by CASE tools and communicated to non technical members of a site development team (e. g., with graphic designers and content producers). WebML also supports an XML syntax, which can be fed to software generators for automatically producing the implementation of a website. The specification of a site in WebML consists of four orthogonal perspectives:

Structural Model: expresses the data content of the site, in terms of the relevant entities and relationships. WebML does not propose yet another language for data modeling, but is compatible with classical notations like the E/R model, the ODMG object oriented model, and UML class diagrams. Hypertext Model: describes one or more hypertexts that can be published in the site. Each different hypertext defines a so called site view. Site view descriptions in turn consist of two sub models. Composition Model: specifies which pages compose the hypertext, and which content units make up a page. Navigation Model: expresses how pages and content units are linked to form the hypertext. Links are either non contextual, when they connect semantically independent pages (e. g., the page of an artist to the home page of the site), or contextual, when the content of the destination unit of the link depends on the content of the source unit. Presentation Model: expresses the layout and graphic appearance of pages, independently of the output device and of the rendition language, by means of an abstract XML syntax. Presentation specifications are either page specific or generic. Personalization Model: users and user groups are explicitly modeled in the structure schema in the form of predefined entities called User and Group. The features of these entities can be used for storing group specific or individual content, like shopping suggestions, list of favorites, and resources for graphic customization.

The fundamental elements of WebML structure model are entities, which are containers of data elements, and relationships, which enable the semantic connection of entities. Entities have named attributes, with an associated type; properties with multiple occurrences can be organized by means of multi valued components, which corresponds to the classical part of relationship. Entities can be organized in generalization hierarchies. Relationships may be given cardinality constraints and role names.

Units and pages do not exist in isolation, but must be connected to form a hypertext structure. The purpose of navigation modeling is to specify the way in which the units and pages are linked to form a hypertext. To this purpose, WebML provides the notion of link. There are two variants of links:

Contextual links (connect units in a way coherent to the semantics expressed by the structure schema of the application. Carries some information (called context) from the source unit to the destination unit. Context is used to determine the actual object or set of objects to be shown in the destination unit). Non contextual links (connect pages in a totally free way, i. e., independently of the units they contain and of the semantic relations between the structural concepts included in those units. Syntactically, contextual and non contextual links are denoted by element INFOLINK and HYPERLINK, respectively nested within units and pages).

Personalization is the definition of content or presentation style based on user profile data. In WebML, units, pages, their presentation styles, and site views can be defined so to take user or group specific data into account. This can be done in two complementary ways:
Declarative personalization: the designer defines derived concepts (e. g., entities, attributes, multi valued components) whose definition depends on user specific data. In this way, customization is specified declaratively; the system fills in the information relative to each user when computing the content of units. Procedural personalization: WebML includes an XML syntax for writing business rules that compute and store user specific information. A business rule is a triple event condition action, which specifies the event to be monitored, the precondition to be checked when the event occurs, and the action to be taken when the condition is found true. Typical tasks performed by business rules are the assignment of users to user groups based on dynamically collected information, the notification of messages to users upon the update of the information base (push technology), the logging of user actions into user specific data structures, and so on.

Presentation modeling is concerned with the look and feel of the pages identified by composition modeling. WebML pages are rendered according to a style sheet. A style sheet dictates the layout of pages and the content elements to be inserted into such layout, and is independent of the actual language used for page rendition. For better reusability, two categories of style sheets are provided: untyped style sheets (also called models) describe the page layout independently of its content, and thus can be applied regardless of the mapping of the page to a given concept; typed style sheets are specified at a finer granularity and thus apply only to pages describing specific concepts.