Specification of Reactive Hardware/Software Systems - Electronic ...

4.7 Identity and Object Identifiers 87
4.7 Identity and Object Identifiers
4.7.1 Identity
Any object that is instantiated has an identity based on the bare fact that it exists. Identity
is not just a property. The expression ’has an identity’ suggests this. The wording ’an
object is an identity’ is more pure. Objects are always distinct entities, even if they
cannot be distinguished by their behaviour or their internal values. Objects cannot be
identified by their behaviour or internal values. The same identity guarantees the same
behaviour, however the same behaviour does not imply the same identity.
4.7.2 Identifiers
Unique identifiers (or unique names) enable distinction between separate identities. Identity
and identifier are independent concepts. An identifier is an artificial construct that
enables identification. Objects may be defined as having a property called an identifier
(or name). This must be a unique value within the scope where an explicit distinction
between identities (objects) has to be made. Defining such property can be a modelling
decision during analysis. However, in many cases a predefined type of identifier is
imposed by the design environment or the language(s) to be used. Some execution environments
give unique numbers to objects, so that unique identification is guaranteed.
There is no agreement whether objects should have identifiers during analysis. Rumbaugh
et al. state that explicit object identifiers are not required in an object model
[R 91]. This may be true for a class-oriented approach, but instance based models
require identifiers in practice.
The practical use of identifiers leads to problems due to the artificial nature of identifiers.
An identifier cannot grasp the real essence of identity. The attempt to uniquely identify
objects is performed by implementations that are not powerful enough to overcome all
problems in space and time:
Space: A system may be distributed. Several system parts may be developed in
different design environments that do not know each others name spaces.
Time: Persisting objects may need unique identifiers, independent of the time they
are processed.
Uniqueness is related to the concept of name space. Wegner [Weg87] defines a ’unit
of naming’ in the context of distributed systems, as ’the unit that determines name space’.
A distributed system is built from units (grains), and within the units a name must be
unique. There are also other applications of the concept of name space. It can be useful
when a name space is known only by particular owners and users. Separate name
spaces can also be used for protection and hiding.
Models become even more complex when identities can change dynamically. For instance,
two objects might be a description of the same identity. In that case one may