The Object Constraint Language (OCL) is a

declarative language In computer science, declarative programming is a programming paradigm—a style of building the structure and elements of computer programs—that expresses the logic of a computation without describing its control flow. Many languages that a ...

describing rules applying to

Unified Modeling Language The Unified Modeling Language (UML) is a general-purpose, developmental modeling language in the field of software engineering that is intended to provide a standard way to visualize the design of a system. The creation of UML was originally m ...

(UML) models developed at IBM and is now part of the UML standard. Initially, OCL was merely a formal specification language extension for UML. OCL may now be used with any

Meta-Object Facility The Meta-Object Facility (MOF) is an Object Management Group (OMG) standard for model-driven engineering. Its purpose is to provide a type system for entities in the CORBA architecture and a set of interfaces through which those types can be creat ...

(MOF)

Object Management Group The Object Management Group (OMG) is a computer industry standardization, standards consortium. OMG Task Forces develop enterprise integration standards for a range of technologies. Business activities The goal of the OMG was a common portabl ...

(OMG) meta-model, including UML. The Object Constraint Language is a precise text language that provides constraint and object query expressions on any MOF model or meta-model that cannot otherwise be expressed by diagrammatic notation. OCL is a key component of the new OMG standard recommendation for transforming models, the Queries/Views/Transformations (

QVT QVT (Query/View/Transformation) is a standard set of languages for model transformation defined by the Object Management Group. Overview Model transformation is a key technique used in model-driven architecture. As the name QVT indicates, the ...

) specification.

Description

OCL is a descendant of Syntropy, a second-generation object-oriented analysis and design method. The OCL 1.4 definition specified a constraint language. In OCL 2.0, the definition has been extended to include general object query language definitions. OCL statements are constructed in four parts: # a context that defines the limited situation in which the statement is valid # a property that represents some characteristics of the context (e.g., if the context is a class, a property might be an attribute) # an operation (e.g., arithmetic, set-oriented) that manipulates or qualifies a property, and # keywords (e.g., if, then, else, and, or, not, implies) that are used to specify conditional expressions.

Relation

OCL and UML

OCL supplements

UML The Unified Modeling Language (UML) is a general-purpose, developmental modeling language in the field of software engineering that is intended to provide a standard way to visualize the design of a system. The creation of UML was originally m ...

by providing expressions that have neither the ambiguities of natural language nor the inherent difficulty of using complex mathematics. OCL is also a navigation language for graph-based models.

OCL and MOF

OCL makes a

model more precise by associating assertions with its meta-elements.

OCL and QVT

Of particular importance to

Model Driven Engineering Model-driven engineering (MDE) is a software development methodology that focuses on creating and exploiting domain models, which are conceptual models of all the topics related to a specific problem. Hence, it highlights and aims at abstract r ...

(MDE) or

model-driven architecture Model Driven Architecture (MDA) is a software design approach for the development of software systems. It provides a set of guidelines for the structuring of specifications, which are expressed as models. Model Driven Architecture is a kind of doma ...

is the notion of

Model transformation A model transformation, in model-driven engineering, is an automated way of modifying and creating models. An example use of model transformation is ensuring that a family of Computer model, models is consistent, in a precise sense which the softw ...

. The OMG has defined a specific standard for model transformation called MOF/QVT or in short

. Several model transformation languages like

GReAT Great may refer to: Descriptions or measurements * Great, a relative measurement in physical space, see Size * Greatness, being divine, majestic, superior, majestic, or transcendent People * List of people known as "the Great" *Artel Great (born ...

VIATRA VIATRA is an open-source model transformation framework based on the Eclipse Modeling Framework (EMF) and hosted by the Eclipse Foundation. VIATRA supports the development of model transformations with specific focus on event-driven, reactive t ...

, or

Tefkat Tefkat is a model transformation language and a model transformation engine. The language is based on F-logic and the theory of stratified logic programs. The engine is an Eclipse plug-in for the Eclipse Modeling Framework (EMF). History T ...

are presently available, with different levels of compliance with the QVT standard. Many of these languages are built on top of OCL, which is the main part of the

-compliance.

Alternatives

Being a rule-based validation language,

Schematron Schematron is a rule-based validation language for making assertions about the presence or absence of patterns in XML trees. It is a structural schema language expressed in XML using a small number of elements and XPath. In many implementations ...

may be considered an alternative to OCL. However Schematron works for Extensible Markup Language (XML) trees while OCL makes it possible to navigate MOF-based models and meta-models (i.e. XML Metadata Interchange ( XMI) trees). In other words, OCL relates to

or MOF similarly to how Schematron relates to

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable ...

. (Note that Schematron uses

XPath XPath (XML Path Language) is an expression language designed to support the query or transformation of XML documents. It was defined by the World Wide Web Consortium (W3C) and can be used to compute values (e.g., strings, numbers, or Boolean v ...

to navigate inside the XML trees.)
Being a model specification language permitting designers to decorate a model or a meta-model with side-effect-free annotations, OCL could be replaced by languages like

Alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

. Automated OCL generation is in principle possible from natural language.

Extensions

Eclipse OCL extends Standard OCL with additional operators and a type for Maps. AgileUML extends Standard OCL with Map and Function types. These extensions are consistent with the map and function types present in modern programming languages such as Python and Swift.

References

External links

OMG OCL specification

OCL Portal - The center for OCL related information

(brief OCL 2.0 syntax)
Octopus: OCL Tool for Precise Uml Specifications
(OCL checker)
Dresden OCL Toolkit
(OCL Toolkit, various OCL related publications)
HOL-OCL
(An interactive theorem proof environment for OCL, various OCL related publications)
OCL for Java tutorial on ParlezUML

USE (UML-based Specification Environment)
(OCL Tool for model validation, various OCL related publications)
OCL tutorial

NL2OCL
(OCL Tool for invariant generation from NL) {{ISO standards Unified Modeling Language Formal specification languages ISO standards