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This excerpt from Modeling XML Applications with UML
explains behavior diagrams, structure diagrams and how to model XML
vocabularies with UML.
Distributed, inter-enterprise business systems are very complex beasts. Few people can fully comprehend all aspects of the system at one time. Instead, they must approach the problem as a set of subsystems, each of which is further decomposed into a set of alternate models and views. Each model deliberately ignores aspects of the system that are not relevant to its purpose. Building these models is similar to the way we cope with the complexity of everyday life by ignoring irrelevant details.
| David Carlson |
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The Unified Modeling Language (UML) defines a standard language and graphical
notation for creating models of business and technical systems. Contrary to popular opinion, UML is not only for programmers. In fact, UML defines several model types that span a range from functional requirements and activity workflow models, to class structure design and component diagrams. This book describes how models are used when specifying XML within the context of an e-business system. These models, and a development process that uses them, improve and simplify communication among an application’s many diverse stakeholders.
The UML’s standard graphical notation defines the following types of diagrams:
Standard diagrams in the UML
Structure Diagrams
Class (static structure) -
Class diagrams illustrate the static design view of a system, including
packages, classes, interfaces, collaborations, and their relationships.
Object - Object diagrams
show static snapshots of instances of things found in the class
diagrams, especially from the perspective of real or prototypical cases.
Component - Component
diagrams illustrate the static implementation view of a system, showing
a set of components and their relationships. A component represents a
physical implementation of the logical abstractions in a model, such as
classes and their interactions.
Deployment - Deployment
diagrams illustrate the connectivity of physical nodes in an
architectural view of the system. A node is a computational resource
that provides a physical operating environment for executing one or more
components.
Behavior Diagrams
Use Case - Use Case diagrams
model the behavior of a system, subsystem or a class by illustrating the
relationships among a set of use cases and their actors.
Activity - Activity diagrams
show the flow of activity within a system, including the sequential or
branching flow from activity to activity and the objects that act or are
acted upon by those activities.
Statechart - Statechart
diagrams illustrate a state machine, consisting of states, transitions,
events, and activities. These diagrams are most often used to model the
event-ordered behavior of an object.
Sequence - Sequence diagrams
illustrate the interaction among objects by emphasizing the time
ordering sequence of messages. The objects are typically instances of
classes, but may also represent other classifiers such as actors,
components, or nodes.
Collaboration -
Collaboration diagrams also show the interaction among objects, but
emphasize the structural organization of the objects that send and
receive messages.
Note: Sequence and collaboration diagrams are
alternative representations for the same model elements, so you can
convert one to the other without loss of information.
But how are these diagrams used to improve our analysis and
design of XML applications? I’ll illustrate the use of two diagrams in a small
case study. The Rich Site Summary (RSS) is a simple XML vocabulary used for
exchanging news headline information, without presentation markup. Netscape
originally developed the RSS vocabulary for use on My Netscape Network, a
customizable portal home page for Netcenter. But use of RSS has grown far beyond
its original application at Netscape.
In portal applications, XML enables communication of
information content without limiting the choice of its presentation.
Web applications based on RSS newsfeeds recombine the XML information content
with a presentation that is appropriate for the viewing device and the user's
requirements.
A portal is not necessarily viewed from a PC desktop, nor is
it necessarily rendered from HTML. A portal provides a view into the World Wide
Web of distributed information content, and it presents that content and
navigation in a way that is most appropriate for the user’s current context.
XML content such as RSS enables the convergence of wireless Web applications
with more conventional wired desktop browsers. As you read through the following
UML diagrams, keep your mind's eye on this vision of global content accessed
from many devices.
Use Case Analysis
There are two primary components to the Web applications
discussed in this book: content management and portal design. These requirements
are most easily illustrated in a UML use case diagram, as shown in Figure 1.
Figure 1: Portal
design use case diagram
The use cases for content management, illustrated with gray
shading, describe the requirements for business analysts and content developers.
These two stakeholders are responsible for defining the business vocabularies
that determine the structure of XML documents created as an application's
content. In addition, many Web applications depend greatly on the metadata
assigned to content that will be used to personalize the portal presentation.
Personalization rules evaluate the relevance of content by matching its metadata
with the profile attributes of the current user.
The use cases shown for portal design are not exhaustive, but
they illustrate the principal requirements, including the dependencies on
content management. It is becoming common to refer to the sub-panes of a portal
application interface as portlets, obviously extending the use of terms
such as applets and servlets used in Web application design. A portlet often
includes a template for its presentation, along with an XSLT stylesheet used to
format the underlying XML data. Most portal applications also allow users to
customize the portal layout and portlet selection in their personalized view.
XML Vocabulary Design
A UML class diagram can be constructed to
visually represent the elements, relationships, and constraints of an
XML vocabulary. With a little initial coaching, class diagrams allow
complex vocabularies to be shared with non-technical business
stakeholders. A subset of the standard RSS DTD was manually
reverse-engineered into the class diagram shown in Figure 2.
Figure 2: RSS
vocabulary design in UML
UML models can capture the semantics for
an application domain model, and the definitions can be directly
processed by e-business systems. The UML also standardizes the graphical
notation for use by human readers. Four chapters in Part II of this book
step through the concepts and standards for translating UML class and
object models into XML vocabularies and documents. Chapters 6 and 7
concentrate on defining a vocabulary's structure and relationships and
focus on the decomposition of UML object instance diagrams into XML
documents. Chapter 8 reviews the current standards for defining schemas
used to validate XML documents, and Chapter 9 reviews the process for
automatic generation of XML DTDs and Schemas from UML class models.
About the author
David Carlson has created models for a variety of XML schemas, including UDDI, RSS,
CatML, RosettaNet, and XHTML. This article is an excerpt from Chapters 3 and 5 of
David's Modeling XML Applications with UML (Addison-Wesley, ISBN
0-201-70915-5). David is also the author of eclipse
Distilled (Addison Wesley, ISBN 0-321-28815-7).
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