Encapsulation of operations, methods & persistence

Encapsulation of
Operations, Methods &
Persistence
BY PREM LAMSAL
31
BSC.CSIT 7TH SEMESTER

Encapsulation
Encapsulation is related to the concepts of abstract data types and information
hiding in programming languages.
Encapsulation is a mechanism of binding data with functions.

Encapsulation of Operations
The concepts of encapsulation is applied to database objects in ODBs by
defining the behavior of a type of object based on the operations that can be
externally applied to objects of that type.
Some operations may be used to create (insert) or destroy (delete)
The external users of the object are only made aware of the interface of the
operations, which defines the name and arguments (parameters) of each
operation.
The implementation is hidden from the external users.

Encapsulation of Operations
For database applications, the requirement that all objects be completely
encapsulated is too stringent.
One way to relax this requirement is to divide the structure of an object into
visible and hidden attributes (instance variables).
Visible attributes can be seen by and are directly accessible to the database
users and programmers via the query language.
The hidden attributes of an object are completely encapsulated and can be
accessed only through predefined operations.
Most ODMSs employ high-level query languages for accessing visible
attributes.

Persistence
The OOPL is used to specify the method (operation) implementations as well as
other application code.
Not all objects are meant to be stored permanently in the database.
Transient objects exist in the executing program and disappear once the
program terminates.
Persistent objects are stored in the database and persist after program
termination.
The typical mechanisms for making an object persistent are naming and
reachability.

Persistence
The naming mechanism involves giving an object a unique persistent name
within a particular database.
This persistence object name can be given via a specific statement or operation
in the program
The named persistent objects are used as entry points to the database through
which users and applications can start their database access.
It is not practical to give name to all object in large database in thousands of
objects so, some objects are made persistence using second mechanism called
reachability.

Persistence : Reachability
The reachability mechanism works by making the object reachable from some
other persistent object.
An object B is said to be reachable from an object A if a sequence of references
in the database lead from object A to object B.
If we first create a named persistent object N, whose state is a set (or possibly a
bag) of objects of some class C, we can make objects of C persistent by adding
them to the set, thus making them reachable from N.
Hence, N is a named object that defines a persistent collection of objects of
class C. In the object model standard, N is called the extent of C.

Thank you !
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