MODELING AND DESIGN IN OODBMS

Basically, an OODBMS is an object database that provides DBMS capabilities to objects thathave been created using an object-oriented programming language (OOPL). The basic principle is to add persistence to objects and to make objects persistent.

Consequently application programmers who use OODBMSs typically write programs in a native OOPL such as Java, C++ or Smalltalk, and the language has some kind of Persistent class, Database class, Database Interface, or Database API that provides DBMS functionality as, effectively, an extension of the OOPL.

Object-oriented DBMSs, however, go much beyond simply adding persistence to any one object-oriented programming language. This is because, historically, many object-oriented DBMSs were built to serve the market for computer-aided design/computer-aided manufacturing (CAD/CAM) applications in which features like fast navigational access,

versions, and long transactions are extremely important.

Object-oriented DBMSs, therefore, support advanced object-oriented database applications with features like support for persistent objects from more than one programming language, distribution of data, advanced transaction models, versions, schema evolution, and dynamic generation of new types.

Object data modeling

An object consists of three parts: structure (attribute, and relationship to other objects like aggregation, and association), behavior (a set of operations) and characteristic of types (generalization/serialization). An object is similar to an entity in ER model; therefore we begin with an example to demonstrate the structure and relationship.

    

Attributes are like the fields in a relational model. However in the Book example we have,for attributes publishedBy and writtenBy, complex types Publisher and Author,which are also objects. Attributes with complex objects, in RDNS, are usually other tableslinked by keys to the employee table.

Relationships: publish and writtenBy are associations with I:N and 1:1 relationship; composed_of is an aggregation (a Book is composed of chapters). The 1:N relationship is usually realized as attributes through complex types and at the behavioral level. For example,

Generalization/Serialization is the is_a relationship, which is supported in OODB through class hierarchy. An ArtBook is a Book, therefore the ArtBook class is a subclass of Book class. A subclass inherits all the attribute and method of its superclass.

Message: means by which objects communicate, and it is a request from one object to another to execute one of its methods. For example:

Publisher_object.insert (”Rose”, 123,…)  i.e. request to execute the insert method on a Publisher object )

Method: defines the behavior of an object. Methods can be used

. to change state by modifying its attribute values . to query the value of selected attributes The method that responds to the message example is the method insert defied in the Publisher class.

The main differences between relational database design and object oriented database design include:

• Many-to-many relationships must be removed before entities can

be translated into relations. Many-to-many relationships can be implemented directly in an object-oriented database.

• Operations are not represented in the relational data model.

Operations are one of the main components in an object-oriented

database.

• In the relational data model relationships are implemented by

primary and foreign keys. In the object model objects communicate through their interfaces. The interface describes the data (attributes) and operations (methods) that are visible to other objects.