Unified Modeling Language

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In the field of software engineering, the Unified Modeling Language (UML) is a standardized specification language for object modeling. UML is a general-purpose modeling language that includes a graphical notation used to create an abstract model of a system, referred to as a UML model.

UML is officially defined at the Object Management Group (OMG) by the UML metamodel, a Meta-Object Facility metamodel (MOF). Like other MOF-based specifications, the UML metamodel and UML models may be serialized in XMI. UML was designed to specify, visualize, construct, and document software-intensive systems.

UML is not restricted to modeling software. UML is also used for business process modeling, systems engineering modeling, and representing organizational structures. The Systems Modeling Language (SysML) is a Domain-Specific Modeling language for systems engineering that is defined as a UML 2.0 profile.

UML has been a catalyst for the evolution of model-driven technologies, which include Model Driven Development (MDD), Model Driven Engineering (MDE), and Model Driven Architecture (MDA). By establishing an industry consensus on a graphic notation to represent common concepts like classes, components, generalization, aggregation, and behaviors, UML has allowed software developers to concentrate more on design and architecture.

UML models may be automatically transformed to other representations (e.g. Java) by means of QVT-like transformation languages, supported by the OMG.

UML is extensible, offering the following mechanisms for customization: profiles and stereotype. The semantics of extension by profiles has been improved with the UML 2.0 major revision.

Contents

[edit] History

After Rational Software Corporation hired James Rumbaugh from General Electric in 1994, the company became the source for the two most popular object-oriented modeling approaches of the day: Rumbaugh's OMT, which was better for object-oriented analysis (OOA), and Grady Booch's Booch method, which was better for object-oriented design (OOD). Together Rumbaugh and Booch attempted to reconcile their two approaches and started work on a Unified Method.

They were soon assisted in their efforts by Ivar Jacobson, the creator of the OOSE method. Jacobson joined Rational in 1995, after his company, Objectory, was acquired by Rational. The three methodologists were collectively referred to as the Three Amigos, since they were well known to argue frequently with each other regarding methodological preferences. (Jacobson: "What's the difference between a terrorist and a methodologist? You can negotiate with a terrorist".)

In 1996 Rational concluded that the abundance of modeling languages was slowing the adoption of object technology, so repositioning the work on a Unified Method, they tasked the Three Amigos with the development of a non-proprietary Unified Modeling Language. Representatives of competing Object Technology companies were consulted during OOPSLA '96, and were won over by Rumbaugh's a cappella rendition of his version of Joni Mitchell's "Clouds". (Indicating the victory of his OMT notation of using boxes for representing classes over Grady Booch's Booch method's notation which used cloud symbols).

Under the technical leadership of the Three Amigos, an international consortium called the UML Partners was organized in 1996 to complete the Unified Modeling Language (UML) specification, and propose it as a response to the OMG RFP. The UML Partners' UML 1.0 specification draft was proposed to the OMG in January 1997. During the same month the UML Partners formed a Semantics Task Force, chaired by Cris Kobryn and administered by Ed Eykholt, to finalize the semantics of the specification and integrate it with other standardization efforts. The result of this work, UML 1.1, was submitted to the OMG in August 1997 and adopted by the OMG in November 1997.[1]

As a modeling notation, the influence of the OMT notation dominates (e.g., using rectangles for classes and objects). Though the Booch "cloud" notation was dropped, the Booch capability to specify lower-level design detail was embraced. The use case notation from Objectory and the component notation from Booch were integrated with the rest of the notation, but the semantic integration was relatively weak in UML 1.1, and was not really fixed until the UML 2.0 major revision.

Concepts from many other OO methods were also loosely integrated with UML with the intent that UML would support all OO methods. For example CRC Cards (circa 1989 from Kent Beck and Ward Cunningham), and OORam were retained. UML is useful in a variety of engineering problems, from single process, single user applications to concurrent, distributed systems, making UML rich but large.

The Unified Modeling Language is an international standard:

ISO/IEC 19501:2005 Information technology -- Open Distributed Processing -- Unified Modeling Language (UML) Version 1.4.2.

UML has matured significantly since UML 1.1. Several minor revisions (UML 1.3, 1.4, and 1.5) fixed shortcomings and bugs with the first version of UML, followed by the UML 2.0 major revision, which is the current OMG standard.

The first part of UML 2.0, the Superstructure which describes the new diagrams and modeling elements available, was adopted by the OMG in October 2004. Other parts of UML 2, notably the infrastructure, the Object Constraint Language (OCL) and the diagram interchange were yet to be completed and ratified as of November 2005.

The final UML 2.0 specification has been declared available and has been added to OMG's formal specification library. The other parts of the UML specification, the UML 2.0 infrastructure, the UML 2.0 Diagram Interchange, and UML 2.0 OCL specifications have been adopted.

UML version 2.1 revision is being developed, and should be available in the form of an XMI 2.1 version of the UML 2.1 version. The corresponding XMI 2.1 file will be made available from the OMG ADTF group.

Most of the commercially successful UML tools now support most of UML 2.0, leaving only the rarely used features left to implement. Of course, it will take some time for the tools that are in the hands of the developers to reach this level of compliance.

[edit] Methods

UML is not a method by itself; however, it was designed to be compatible with the leading object-oriented software development methods of its time (for example OMT, Booch, Objectory). Since UML has evolved, some of these methods have been recast to take advantage of the new notation (for example OMT), and new methods have been created based on UML. Most well known is Rational Unified Process (RUP). There are many other UML-based methods like Abstraction Method, Dynamic Systems Development Method, and others, designed to provide more specific solutions, or achieve different objectives.

[edit] Modeling

It is very important to distinguish between the UML model and the set of diagrams of a system. A diagram is a partial graphical representation of a system's model. The model also contains a "semantic backplane" — documentation such as written use cases that drive the model elements and diagrams.

There are three prominent parts of a system's model:

Functional Model

Showcases the functionality of the system from the user's Point of View.
Includes Use case diagrams.

Object Model

Showcases the structure and substructure of the system using objects, attributes, operations, and relationships.
Includes Class Diagrams.

Dynamic Model

Showcases the internal behavior of the system.
Includes sequence diagrams, activity diagrams and state machine diagrams.

Models can be exchanged among UML tools by using the XMI format.

[edit] Diagrams

In UML 2.0 there are 13 types of diagrams. To understand them, it is sometimes useful to categorize them hierarchically, as shown in the hierarchy chart on the right.

Hierarchy of UML 2.0 Diagrams, shown as a class diagram

Structure Diagrams emphasize what things must be in the system being modeled:

Behavior Diagrams emphasize what must happen in the system being modeled:

Interaction Diagrams, a subset of behavior diagrams, emphasize the flow of control and data among the things in the system being modeled:

The Protocol State Machine is a sub-variant of the State Machine. It may be used to model network communication protocols.

UML does not restrict UML element types to a certain diagram type. In general, every UML element may appear on almost all types of diagrams. This flexibility has been partially restricted in UML 2.0.

Diagrams can be exchanged among UML tools by XMI (also DI, XMI[DI]).

In keeping with the tradition of engineering drawings, a comment or note explaining usage, constraint, or intent is always allowed in a UML diagram.

[edit] Concepts

UML uses many concepts from many sources. For a definitive list, consult the glossary of Unified Modeling Language terms. Notable concepts are listed here.

For Structure

Actor, Attribute, Class, Component, Interface, Object, Package.

For Behavior

Activity, Event, Message, Method, Operation, State, use case.

For Relationships

Aggregation, Association, Composition, Depends, Generalization (or Inheritance).

Other Concepts

  • Stereotype. It qualifies the symbol it is attached to.

Multiplicity notation which corresponds to Database modeling cardinality, e.g., 1, 0..1, 1..*

[edit] Criticisms

Although UML is a widely recognized and used modeling standard, it is frequently criticized for the following deficiencies:

  • Language bloat. UML is often criticized as being gratuitously large and complex. It contains many diagrams and constructs that are redundant or infrequently used. This criticism is more frequently directed at UML 2.0 than UML 1.0, since newer revisions include more design-by-committee compromises.
  • Imprecise semantics. Since UML is specified by a combination of itself (abstract syntax), OCL (well-formedness rules) and English (detailed semantic), it lacks the rigor of a language precisely defined using formal language techniques. In some cases, the UML abstract syntax, OCL and English are contradictory, in other cases they are incomplete. The imprecision of the UML specification has consequences for users and tool vendors alike, such as causing incompatibilities among tools due to unique interpretations of specs.
  • Problems in learning and adopting. The problems cited above make learning and adopting UML problematic, especially when management forces UML upon engineers lacking the prerequisite skills. (See the ACM article for an amusing account of such issues.)
  • Only the code is in sync with the code. Another perspective holds that it is working systems that are important, not beautiful models. As Jack Reeves succinctly put it, "The code is the design." [1][2] Pursuing this notion leads to the need for better ways of writing software; UML has value in approaches that compile the models to generate source or executable code. This however, may still not be sufficient since UML as a language does not exhibit Turing completeness, and any generated source or executable code would be limited to what a UML interpreting tool can discern or assume.
  • Cumulative Impedance/Impedance mismatch. As with any notational system, UML is able to represent some systems more concisely or efficiently than others. Thus a developer is influenced toward solutions that most comfortably inhabit the intersection of the strengths between UML and the coding language. This problem is particularly pronounced if the coding implementation language does not adhere to orthodox object-oriented doctrine.
  • Tries to be all things to all people. UML is a general purpose modeling language, which tries to achieve compatibility with every possible implementation language. In the context of a specific project, the most applicable features of UML must be delimited for use by the design team to accomplish the specific goal. Additionally, the means of restricting the scope of UML to a particular domain is through a formalism that is not completely formed, and is itself the subject of criticism.

A paper by prof. Brian Henderson-Sellers at the MoDELS/UML conference in Genova, Italy, October 2006 details flaws in the definition of the UML 2.0 language.

[edit] Extensions

When it is necessary to introduce new notations or terminologies, UML provides user-defined extensions through the use of stereotypes, tagged values and constraints. Currently there are two extensions defined, namely Business and Objectory Process extensions.

  • Magnus Penker and Hans-Erik Eriksson (2000) describe Business Extensions in Business Modeling with UML.
  • Ovidiu S. Noran at the Griffith University compares UML and IDEF in "Business Modelling: UML vs. IDEF". [3]
  • Peter Coad et al. (1999) have also suggested a small set of UML colors.
  • Scott Ambler has done extensive work defining how to data model with the UML.

[edit] See also

[edit] Notes

  1. ^ UML Specification v. 1.1 (OMG document ad/97-08-11)

[edit] References

  • Penker, Magnus; Hans-Erik Eriksson (2000). Business Modeling with UML. John Wiley & Sons. ISBN 0-471-29551-5. 
  • Henderson Sellers, Brian (2006). about UML profiles. Springer Verlag. MoDELS'2006 Conference, Genova. 

[edit] External links

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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