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Several approaches have been proposed to make the objects persistent.
persistence by class.
Declare class to be persistent: all objects of the class are then
persistent objects. Simple, not flexible since it is often useful
to have both transient and persistent objects in a single class.
In many OODB systems, declaring a class to be persistent is interpreted as
``persistable'' -- objects in the class potentially can be made persistent.
persistence by creation.
Introduce new syntax to create persistent objects.
persistence by marking.
Mark an object persistent after it is created (and before the program
persistence by reference.
One or more objects are explicitly declared as (root) persistent objects.
All other objects are persistent iff they are referred, directly or
indirectly, from a root persistent object.
It is easy to make the entire data structure persistent by merely
declaring the root of the structure as persistent, but is expensive
to follow the chains in detection for a database system.
Mon Jun 29 17:30:13 PDT 1998