Object Identity

1. Object identity: An object retains its identity even if some or all of the values of variables or definitions of methods change over time.

   This concept of object identity is necessary in applications but doe not apply to tuples of a relational database.

2. Object identity is a stronger notion of identity than typically found in programming languages or in data models not based on object orientation.
3. Several forms of identity:
   • value: A data value is used for identity (e.g., the primary key of a tuple in a relational database).
   • name: A user-supplied name is used for identity (e.g., file name in a file system).
   • built-in: A notion of identity is built-into the data model or programming languages, and no user-supplied identifier is required (e.g., in OO systems).
4. Object identity is typically implemented via a unique, system-generated OID. The value of the OID is not visible to the external user, but is used internally by the system to identify each object uniquely and to create and manage inter-object references.
5. There are many situations where having the system generate identifiers automatically is a benefit, since it frees humans from performing that task. However, this ability should be used with care. System-generated identifiers are usually specific to the system, and have to be translated if data are moved to a different database system. System-generated identifiers may be redundant if the entities being modeled already have unique identifiers external to the system, e.g., SIN#.