This excerpt from Chapter No. 2 "Basic Principles and Background" from UML 2.0 in Action A Project-Based Tutorial   by Patrick Grässle, Henriette Baumann, Philippe Baumann, is printed with permission from Packt Publishing , Copyright 2007.

In addition to the proposal of UML Infrastructure and UML Superstructure specifications, separate proposals were published for a new ObjectConstraint Language (OCL) as well as for Diagram Interchange. Together, they make up the complete UML 2.0 package, as shown in Figure 2.12: 

UML 2.0, as a whole, is more extensive and more complex than earlier versions. The extent of UML documentation has also further increased. While the documentation of UML 1.5, including OCL, comprised about 730 pages, the documentation of UML 2.0, also including OCL, contains approximately 1050 pages.

Even though part of the documentation doesn't concern the 'normal' UML user, for a member of a software development project, reading the complete work is unrealistic. This is not only due to the number of pages, but also because of the number and complexity of UML constructs. Because of this, reduction to the UML constructs necessary for everyday project work is even more necessary than with earlier versions.

From this follow two conclusions for our book UML20inActionAProject-Based . : Tutorial.

The concept of this book is to show a very simplified picture of UML. This is becoming even more important with the increasing scope of UML, since the accessibility of UML did not become any greater with version 2.0.

Fortunately, many of the new features of UML 2.0 have little or no influence at the level of detail used in this book. Consequently, there are only a few changes compared to the earlier German editions of UML20inAction . : A Project-BasedTutorial . The restricted scope of our book ensures stability towards the changes in new UML versions.

We would also like to point out a new possibility that UML 2.0 opens up. One of the goals of UML 2.0 was the definition of formal and completely defined semantics. If this new possibility is utilized for the development of models, corresponding systems can be generated from these models. This yields the following advantages:

• A model that was described with UML reflects the real system.
• It is possible to correct mistakes in the model early and continuously.
• Intermediate steps such as amending code outside of the model design are omitted.
• It is possible to make the same model executable on different platforms (hardware as well as software).

However, a price has to be paid for these advantages. It becomes necessary to acquire a deep and accurate understanding of UML and considerable effort has to be invested in the development of the models.

Effects on the Business System Model

Some changes made in performance modeling enhanced the possibilities for modeling business systems. First, we'll give examples of several of the changes and improvements.

Activity diagrams are no longer special cases of the statechart diagram. Initially, this fact was not relevant for the normal UML user. However, in addition to the new autonomy in the meta-model, several other changes and improvements were made:

Until now, the separate steps in the activity diagram were referred to as activities. Now the entire diagram is called an activity, whereas the steps previously called activities are now referred to as actions. An action can call a primary operation as well as another activity. This enables flexible modulation in the top-down view of models.

A division does not necessarily have to be re-synchronized.

An activity can have more than one initial state. With this, several events can be started at the same time.

Input and output parameters can be added to an activity.

One of the improvements made in the sequence diagram is the addition of so-called operators. These operators make it possible to package several actions/activities within a sequence diagram. For instance, operators can be used to refer to other sequence diagrams or individual sequences. Appropriate operators can also represent iterations. With the newly introduced operators, sequence diagrams now support a top-down view.

OCL is now an inherent part of UML. It can be used to describe agreements, invariants, preconditions, and post conditions within UML models, which enables more precise modeling of business systems and business processes.

Effects on the IT System Mode

The diagrams that we have used in this book in the different views of the IT system did not undergo any significant changes.

The biggest change occurred in the notation of the sequence diagram. Here, among other things, the interaction reference is available as a construct for modularization. However, nothing changed concerning the meaning and functionality of sequence diagrams at the level of detail used in this book. The same holds true for the class diagram and the case diagram.

Statechart diagrams underwent the most interesting changes for the modeling of IT systems: connection points allow, for example, better modulation of statechart diagrams. However, we decided not to use this language element in our simplified approach to UML.

Effects on the Systems Integration Model

Of course, the improvements in behavioral modeling also had an effect on the process view in the systems integration model. A significant improvement is the ability to add input and output parameters to activities (see Section 2.7.2, Effects on the Business System Model).

Hardly any changes were made in the area of static views, meaning the design of business objects with class diagrams.

In addition to the changes that were made within the framework of UML 2.0, the UML profile for Enterprise Application Integration (EAI) is of increasing importance in the field of system integration. Besides the basic operations needed in the field of system integration, it shows the data meta-models of various programming languages that are not object-oriented. However, this occurs at a more detailed level, which has no influence upon this text.

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