A Constraint defines a rule that restricts the state or behavior of one or more elements in the meta-model. When a Constraint is attached to a ModelElement, the rule it encodes applies to all relevant instances of the ModelElement in a model.

A Constraint rule, represented by the 'expression' attribute, may be encoded in any form. The 'language' attribute may be used to denote the language and encoding scheme used.

While some Constraints on a model may need to be treated as invariant, it is often convenient for other Constraints to be relaxed, for instance while a model is being edited. While, the 'evaluationPolicy' attribute is used to represent these two cases, this information is at best advisory, since the MOF specification does not currently state how and when Constraints should be enforced.

Note

Attributes
Class	evaluationPolicy	Each constraint can be defined as immediate or deferred. For immediate Constraints, the constraint violation will be detected and reported within an operation in the chain of operations between the operation initiated by the MOF user and the operation that caused the constraint violation. The effect of an operation that violates an immediate constraint on the state of the object or objects being altered is implementation specific, and possibly undefined. However, if possible, an implementation should reverse the effects of the operation. For deferred Constraints, the constraint violation can only be detected when the Constraint is explicitly evaluated. The MOF defines an operation for such constraint evaluation, the verify operation. When the verify operation is invoked on a Constraint's container, the constraint is evaluated and a constraint violation is detected, if present.
Primitive Type string	expression	The Constraint's expression attribute provides a representation of the constraint. The MOF has no specific requirement to interpret this expression, or to validate it against the language attribute. The specific handling of the expression will necessarily vary with the language used. However, it is expected that for any language an implementation accepts, it will enforce the constraints expressed in that language. The expression can be represented in any format, including text or a composition of objects.
Primitive Type string	language	The language representing this Constraint's expression is defined in this attribute. Since it is a string, most any language can be represented, including format variances in a language (e.g., OCL as text verses OCL as a parse tree).

 evaluationPolicy

public Class evaluationPolicy

Each constraint can be defined as immediate or deferred. For immediate Constraints, the constraint violation will be detected and reported within an operation in the chain of operations between the operation initiated by the MOF user and the operation that caused the constraint violation. The effect of an operation that violates an immediate constraint on the state of the object or objects being altered is implementation specific, and possibly undefined. However, if possible, an implementation should reverse the effects of the operation.

For deferred Constraints, the constraint violation can only be detected when the Constraint is explicitly evaluated. The MOF defines an operation for such constraint evaluation, the verify operation. When the verify operation is invoked on a Constraint's container, the constraint is evaluated and a constraint violation is
detected, if present.

Properties:

Aggregation	none
Alias
Association
Association End
Class	Class Constraint
Collections
Datatype
Default
Default Value
Is Composite	false
Is Derived	false
Is Derived Union	false
Is Leaf	false
Is Ordered	false
Is Read Only	false
Is Static	false
Is Unique	true
Keywords
Lower	1
Lower Value
Multiplicity	1
Name	evaluationPolicy
Name Expression
Opposite
Owned Template Signature
Owner	Class Constraint
Owning Association
Owning Parameter
Qualified Name	model1::model1::Constraint::evaluationPolicy
Stereotype
Template Parameter
Type	Class
Upper	1
Upper Value
Visibility	public

 expression

public Primitive Type string expression

The Constraint's expression attribute provides a representation of the constraint. The MOF has no specific requirement to interpret this expression, or to validate it against the language attribute. The specific handling of the expression will necessarily vary with the language used. However, it is expected that for any language an implementation accepts, it will enforce the constraints expressed in that language. The expression can be represented in any format, including text or a composition of objects.

Properties:

Aggregation	none
Alias
Association
Association End
Class	Class Constraint
Collections
Datatype
Default
Default Value
Is Composite	false
Is Derived	false
Is Derived Union	false
Is Leaf	false
Is Ordered	false
Is Read Only	false
Is Static	false
Is Unique	true
Keywords
Lower	1
Lower Value
Multiplicity	1
Name	expression
Name Expression
Opposite
Owned Template Signature
Owner	Class Constraint
Owning Association
Owning Parameter
Qualified Name	model1::model1::Constraint::expression
Stereotype
Template Parameter
Type	Primitive Type string
Upper	1
Upper Value
Visibility	public

 language

public Primitive Type string language

The language representing this Constraint's expression is defined in this attribute. Since it is a string, most any language can be represented, including format variances in a language (e.g., OCL as text verses OCL as a parse tree).

Properties:

Aggregation	none
Alias
Association
Association End
Class	Class Constraint
Collections
Datatype
Default
Default Value
Is Composite	false
Is Derived	false
Is Derived Union	false
Is Leaf	false
Is Ordered	false
Is Read Only	false
Is Static	false
Is Unique	true
Keywords
Lower	1
Lower Value
Multiplicity	1
Name	language
Name Expression
Opposite
Owned Template Signature
Owner	Class Constraint
Owning Association
Owning Parameter
Qualified Name	model1::model1::Constraint::language
Stereotype
Template Parameter
Type	Primitive Type string
Upper	1
Upper Value
Visibility	public

• A Constraint defines a rule that restricts the state or behavior of one or more elements in the meta-model. When a Constraint is attached to a ModelElement, the rule it encodes applies to all relevant instances of the ModelElement in a model.
  

  
  A Constraint rule, represented by the 'expression' attribute, may be encoded in any form. The 'language' attribute may be used to denote the language and encoding scheme used.
  

  
  While some Constraints on a model may need to be treated as invariant, it is often convenient for other Constraints to be relaxed, for instance while a model is being edited. While, the 'evaluationPolicy' attribute is used to represent these two cases, this information is at best advisory, since the MOF specification does not currently state how and when Constraints should be enforced.
  

  
  

  Note

  
  A Constraint cannot over-ride structural integrity rules defined by other parts of a meta-model (e.g., multiplicity specifications) or the integrity rules defined by a particular mapping of the meta-model to implementation technology.