Concept of Object-Oriented and Distributed Database Model

Object Oriented Database

Overview of Object Oriented Concepts:

OO databases try to maintain a direct correspondence between real-world and database objects so that the objects do not lose their integrity and identity and can easily be identified and operated upon. Object has two compounds and they are:

• State (value) and behavior (operations).

In an object-oriented database, objects may have an object structure of arbitrary complexity in order to contain all of the necessary information that is required describes the object. On the contrary in traditional database systems, information about a complex object is often scattered over many relations or records leading to loss of direct correspondence between a real-world object and its database representation. The internal structure of an object in OOPLs includes the specification of instance variables which holds the value that defines the internal state of the object. An instance variable is similar to the concept of an attribute except for the instance variables may be encapsulated within the object and thus are not necessarily visible to external users. Some OO models insist that all operations a user can apply to an object must be predefined. This forces to a complete encapsulation of objects. In order to encourage encapsulation an operation is defined in two parts:

• Signature or interface of the operation specifies the operation name and arguments or parameters.
• Method or body specifies the implementation of the operation.

Operation polymorphism refers to an operation’s ability to be applied to the different types of objects. In such situation, an operation name may refer to several distinct implementations depending on the type of object it is applied to. This feature is also called as operator overloading.

Object-oriented databases allow the designer to specify the structure of complex objects and the operations that can be applied to objects. It maintains a direct correspondence between the real-world and the database objects. A real-world object may have different names for key attributes in different relations in traditional database systems. For example: EMP_ID, SSN in different relations. An object-oriented database is known as a unique identity for each independent object stored in the database. The objects are created by a unique, system-generated object identifier or OID. The OID value of a particular object should not change.

Classes and objects:

A class is understood as an entity that has a well-defined role in the application domain. An object is referred to as a particular instance of a class. An object has structure or state (variables) and methods (behavior/operations). An object is described by four characters and they are:

• Identifier: A system-wide unique id for an object.
• Name: An object may also have a unique name in database which is optional.
• Lifetime: It determines if the object is persistent or transient.
• Structure: The construction of objects using type constructors.

Object Oriented Database Model: This model represents an entity as a class. A class captures the attributes as well as the behavior. Instances of the class are called objects. Within an object, the class attributes take on specific values which distinguish one object from another. Object Oriented Examples:

• Class: Cat.
• Attributes: Color, weight, breed.
• Behavior: Scratches, sleeps, purrs.

An instance of the cat class is an object with specific attributes.

Advantages of OODBMS:

• Easier Design-Reflect Applications
• Modularity and Reusability
• Incremental refinement and abstraction
• Multiple inheritance
• Support for multiple version and Alternatives
• Designer can specify the structure of objects and their behavior (methods).
• Better interaction with object-oriented languages such as Java and C++
• Definition of complex and user-defined types.
• Encapsulation of operations and user-defined methods.

Class Diagram: It shows the static structure of an object-oriented model such as object classes, internal structures, and relationships. Class diagram showing two classes is shown below:

Distributed Database System:

A distributed database system consists of loosely coupled sites that do not share any physical component. Database systems that run on each site are independent of each other. The transactions may access data at one or more sites. The data is spread over multiple machines which are also referred to as sites or nodes. Network interconnects the machines and the data is shared by users on multiple machines. The main difference between the centralized and distributed database system is that the data reside in on the single location whereas in the later the data reside in several locations. There are mainly two types of distributed database system:

• Heterogeneous Database
• Homogeneous Database

Homogeneous Database: In a homogeneous database all the sites have identical software. Homogeneous databases are aware of each other and agree to cooperate in processing user requests. Each site surrenders a part of its autonomy in terms of right to change schema or software. It appears to the user as a single system.

Heterogeneous distributed database: In heterogeneous database different sites may have used different schema and software. The difference in schema is a major problem for query processing. The difference in software is a major problem for transaction processing. The sites may not be aware of each other and may provide only limited facilities for cooperation in transaction processing.

References:

1. H.F.Korth and A. Silberschatz,"Database system concepts",McGraw Hill,2010
2. A.K.Majumdar and p, Bhatt acharya,"Database Management Systems",Tata McGraw Hill,India,2004
3. F.Korth, Henry. Database System Concepts. 6th edition.