The unified modeling language (UML) is a general-purpose visual modeling language that is intended to provide a standard way to visualize the design of a system.^[1]

UML provides a standard notation for many types of diagrams which can be roughly divided into three main groups: behavior diagrams, interaction diagrams, and structure diagrams.

The creation of UML was originally motivated by the desire to standardize the disparate notational systems and approaches to software design. It was developed at Rational Software in 1994–1995, with further development led by them through 1996.^[2]

In 1997, UML was adopted as a standard by the Object Management Group (OMG) and has been managed by this organization ever since. In 2005, UML was also published by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) as the ISO/IEC 19501 standard.^[3] Since then the standard has been periodically revised to cover the latest revision of UML.^[4]

In software engineering, most practitioners do not use UML, but instead produce informal hand drawn diagrams; these diagrams, however, often include elements from UML.^[5]: 536

UML has evolved since the second half of the 1990s and has its roots in the object-oriented programming methods developed in the late 1980s and early 1990s. The timeline (see image) shows the highlights of the history of object-oriented modeling methods and notation.

It is originally based on the notations of the Booch method, the object-modeling technique (OMT), and object-oriented software engineering (OOSE), which it has integrated into a single language.^[6]

Rational Software Corporation hired James Rumbaugh from General Electric in 1994 and after that, the company became the source for two of the most popular object-oriented modeling approaches of the day:^[7] Rumbaugh's object-modeling technique (OMT) and Grady Booch's method. They were soon assisted in their efforts by Ivar Jacobson, the creator of the object-oriented software engineering (OOSE) method, who joined them at Rational in 1995.^[2]

Under the technical leadership of those three (Rumbaugh, Jacobson, and Booch), a consortium called the UML Partners was organized in 1996 to complete the Unified Modeling Language (UML) specification and propose it to the Object Management Group (OMG) for standardization. The partnership also contained additional interested parties (for example HP, DEC, IBM, and Microsoft). The UML Partners' UML 1.0 draft was proposed to the OMG in January 1997 by the consortium. During the same month, the UML Partners formed a group, designed to define the exact meaning of language constructs, chaired by Cris Kobryn and administered by Ed Eykholt, to finalize 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.^[2][8]

After the first release, a task force was formed^[2] to improve the language, which released several minor revisions, 1.3, 1.4, and 1.5.^[9]

The standards it produced (as well as the original standard) have been noted as being ambiguous and inconsistent.^[10]

As with database Chen, Bachman, and ISO ER diagrams, class models are specified to use "look-across" cardinalities, even though several authors (Merise,^[11] Elmasri & Navathe,^[12] amongst others^[13]) prefer same-side or "look-here" for roles and both minimum and maximum cardinalities. Recent researchers (Feinerer^[14] and Dullea et al. ^[15]) have shown that the "look-across" technique used by UML and ER diagrams is less effective and less coherent when applied to n-ary relationships of order strictly greater than 2.

Feinerer says: "Problems arise if we operate under the look-across semantics as used for UML associations. Hartmann^[16] investigates this situation and shows how and why different transformations fail.", and: "As we will see on the next few pages, the look-across interpretation introduces several difficulties which prevent the extension of simple mechanisms from binary to n-ary associations."

UML 2.0 major revision replaced version 1.5 in 2005, which was developed with an enlarged consortium to improve the language further to reflect new experiences on the usage of its features.^[17]

Although UML 2.1 was never released as a formal specification, versions 2.1.1 and 2.1.2 appeared in 2007, followed by UML 2.2 in February 2009. UML 2.3 was formally released in May 2010.^[18] UML 2.4.1 was formally released in August 2011.^[18] UML 2.5 was released in October 2012 as an "In progress" version and was officially released in June 2015.^[18] The formal version 2.5.1 was adopted in December 2017.^[1]

There are four parts to the UML 2.x specification:

• The Superstructure that defines the notation and semantics for diagrams and their model elements
• The Infrastructure that defines the core metamodel on which the Superstructure is based
• The Object Constraint Language (OCL) for defining rules for model elements
• The UML Diagram Interchange that defines how UML 2 diagram layouts are exchanged

Until UML 2.4.1, the latest versions of these standards were:^[19]

• UML Superstructure version 2.4.1
• UML Infrastructure version 2.4.1
• OCL version 2.3.1
• UML Diagram Interchange version 1.0.

Since version 2.5, the UML Specification has been simplified (without Superstructure and Infrastructure), and the latest versions of these standards are now:^[20]

• UML Specification 2.5.1
• OCL version 2.4

It continues to be updated and improved by the revision task force, who resolve any issues with the language.^[21]

UML offers a way to visualize a system's architectural blueprints in a diagram, including elements such as:^[6]

• any activities (jobs);
• individual components of the system;
  • and how they can interact with other software components;
• how the system will run;
• how entities interact with others (components and interfaces);
• external user interface.

Although originally intended for object-oriented design documentation, UML has been extended to a larger set of design documentation (as listed above),^[22] and has been found useful in many contexts.^[23]

UML is not a development method by itself;^[24] however, it was designed to be compatible with the leading object-oriented software development methods of its time, for example, OMT, Booch method, Objectory, and especially RUP it was originally intended to be used with when work began at Rational Software.

It is important to distinguish between the UML model and the set of diagrams of a system. A diagram is a partial graphic representation of a system's model. The set of diagrams need not completely cover the model and deleting a diagram does not change the model. The model may also contain documentation that drives the model elements and diagrams (such as written use cases).

UML diagrams represent two different views of a system model:^[25]

• Static (or structural) view: emphasizes the static structure of the system using objects, attributes, operations and relationships. It includes class diagrams and composite structure diagrams.
• Dynamic (or behavioral) view: emphasizes the dynamic behavior of the system by showing collaborations among objects and changes to the internal states of objects. This view includes sequence diagrams, activity diagrams and state machine diagrams.

UML models can be exchanged among UML tools by using the XML Metadata Interchange (XMI) format.

In UML, one of the key tools for behavior modeling is the use-case model, caused by OOSE. Use cases are a way of specifying required usages of a system. Typically, they are used to capture the requirements of a system, that is, what a system is supposed to do.^[26]

UML 2 has many types of diagrams, which are divided into two categories.^[6] Some types represent structural information, and the rest represent general types of behavior, including a few that represent different aspects of interactions. These diagrams can be categorized hierarchically as shown in the following class diagram:^[6]

These diagrams may all contain comments or notes explaining usage, constraint, or intent.

Structure diagrams represent the static aspects of the system. It emphasizes the things that must be present in the system being modeled. Since structure diagrams represent the structure, they are used extensively in documenting the software architecture of software systems. For example, the component diagram describes how a software system is split up into components and shows the dependencies among these components.

• 
  Component diagram
• 
  Class diagram

Behavior diagrams represent the dynamic aspect of the system. It emphasizes what must happen in the system being modeled. Since behavior diagrams illustrate the behavior of a system, they are used extensively to describe the functionality of software systems. As an example, the activity diagram describes the business and operational step-by-step activities of the components in a system.

• 
  Activity diagram
• 
  Use case diagram

Visual Representation: Staff User → Complaints System: Submit Complaint Complaints System → HR System: Forward Complaint HR System → Department: Assign Complaint Department → Complaints System: Update Resolution Complaints System → Feedback System: Request Feedback Feedback System → Staff User: Provide Feedback Staff User → Feedback System: Submit Feedback. This description can be used to draw a sequence diagram using tools like Lucidchart, Draw.io, or any UML diagram software. The diagram would have actors on the left side, with arrows indicating the sequence of actions and interactions between systems and actors as described. Sequence diagrams should be drawn for each use case to show how different objects interact with each other to achieve the functionality of the use case.

In UML, an artifact^[1] is the "specification of a physical piece of information that is used or produced by a software development process, or by deployment and operation of a system."^[1]

"Examples of artifacts include model files, source files, scripts, and binary executable files, a table in a database system, a development deliverable, a word-processing document, or a mail message."^[1]

Artifacts are the physical entities that are deployed on Nodes^[1] (i.e. Devices and Execution Environments). Other UML elements such as classes and components are first manifested into artifacts and instances of these artifacts are then deployed. Artifacts can also be composed of other artifacts.

The Object Management Group (OMG) has developed a metamodeling architecture to define the UML, called the Meta-Object Facility.^[27] MOF is designed as a four-layered architecture, as shown in the image at right. It provides a meta-meta model at the top, called the M3 layer. This M3-model is the language used by Meta-Object Facility to build metamodels, called M2-models.

The most prominent example of a Layer 2 Meta-Object Facility model is the UML metamodel, which describes the UML itself. These M2-models describe elements of the M1-layer, and thus M1-models. These would be, for example, models written in UML. The last layer is the M0-layer or data layer. It is used to describe runtime instances of the system.^[28]

The meta-model can be extended using a mechanism called stereotyping. This has been criticized as being insufficient/untenable by Brian Henderson-Sellers and Cesar Gonzalez-Perez in "Uses and Abuses of the Stereotype Mechanism in UML 1.x and 2.0".^[29]

In 2013, UML had been marketed by OMG for many contexts, but aimed primarily at software development with limited success.^[23][30]

It has been treated, at times, as a design silver bullet, which leads to problems. UML misuse includes overuse (designing every part of the system with it, which is unnecessary) and assuming that novices can design with it.^[31]

It is considered a large language, with many constructs. Some people (including Jacobson) feel that UML's size hinders learning and therefore uptake.^[32]

MS Visual Studio dropped support for UML in 2016 due to lack of usage.^[33]

According to Google Trends, UML has been on a steady decline since 2004.^[34]

• Applications of UML
• Business Process Model and Notation (BPMN)
• C4 model
• Department of Defense Architecture Framework
• DOT (graph description language)
• List of Unified Modeling Language tools
• MODAF
• Model-based testing
• Model-driven engineering
• Object-oriented role analysis and modeling
• Process Specification Language
• Systems Modeling Language (SysML)

• Ambler, Scott William (2004). The Object Primer: Agile Model Driven Development with UML 2. Cambridge University Press. ISBN 0-521-54018-6. Archived from the original on 31 January 2010. Retrieved 29 April 2006.
• Chonoles, Michael Jesse; James A. Schardt (2003). UML 2 for Dummies. Wiley Publishing. ISBN 0-7645-2614-6.
• Fowler, Martin (2004). UML Distilled: A Brief Guide to the Standard Object Modeling Language (3rd ed.). Addison-Wesley. ISBN 0-321-19368-7.
• Jacobson, Ivar; Grady Booch; James Rumbaugh (1998). The Unified Software Development Process. Addison Wesley Longman. ISBN 0-201-57169-2.
• Martin, Robert Cecil (2003). UML for Java Programmers. Prentice Hall. ISBN 0-13-142848-9.
• Noran, Ovidiu S. "Business Modelling: UML vs. IDEF" (PDF). Retrieved 14 November 2022.
• Horst Kargl. "Interactive UML Metamodel with additional Examples".
• Penker, Magnus; Hans-Erik Eriksson (2000). Business Modeling with UML. John Wiley & Sons. ISBN 0-471-29551-5.
• Douglass, Bruce Powel. "Bruce Douglass: Real-Time Agile Systems and Software Development" (web). Retrieved 1 January 2019.
• Douglass, Bruce (2014). Real-Time UML Workshop 2nd Edition. Newnes. ISBN 978-0-471-29551-8.
• Douglass, Bruce (2004). Real-Time UML 3rd Edition. Newnes. ISBN 978-0321160768.
• Douglass, Bruce (2002). Real-Time Design Patterns. Addison-Wesley Professional. ISBN 978-0201699562.
• Douglass, Bruce (2009). Real-Time Agility. Addison-Wesley Professional. ISBN 978-0321545497.
• Douglass, Bruce (2010). Design Patterns for Embedded Systems in C. Newnes. ISBN 978-1856177078.

• Official website
• Current UML specification: Unified Modeling Language 2.5.1. OMG Document Number formal/2017-12-05. Object Management Group Standards Development Organization (OMG SDO). December 2017.