Entity vs Datatype

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OMG UML Specification “A DataType is a kind of Classifier. DataType differs from Class in that instances of a DataType are identified only by their value. All instances of a DataType with the same value are considered to be equal instances. If a DataType has attributes (i.e., Properties owned by it and in its namespace) it is called a structured DataType. Instances of a structured DataType contain attribute values matching its attributes. Instances of a structured DataType are considered to be equal if and only if the structure is the same and the values of the corresponding attributes are equal.” http://www.omg.org/spec/UML/2.5.1

DDD Entity vs Datatype Summary Characteristic Entity Datatype Mutability Mutable attribute values Immutable attribute values Identity Immutable identifier added (Immutable) Attribute values # Equality Compare (immutable) identifier values Compare (immutable) attribute values # Uniqueness Unique identifier means every Entity can be distinguished Many datatypes may have the same attribute values and hence there is no real notion of uniqueness Changing an attribute value mutate in place create a new datatype with the required value and update the reference to point to it Valid as an Entity attribute type No Yes Valid as a Datatype attribute type Can be linked via associations Not incoming to a Datatype. Outgoing from a Datatype is possible to an Entity # - the default would be for every attribute value to be part of the identity. In some cases attributes such as descriptions may be excluded by explicitly defining which attributes form the identity