Chapter 11

Object-Oriented DBMSs

Chapter 27, 28 & Appendix K in Textbook

Advanced Database Applications

• Computer-Aided Design (CAD).

• Computer-Aided Manufacturing (CAM).

• Computer-Aided Software Engineering (CASE).

• Network Management Systems.

• Office Information Systems (OIS) and Multimedia Systems.

• Digital Publishing.

• Geographic Information Systems (GIS).

• Interactive and Dynamic Web sites.

• Other applications with complex and interrelated objects and procedural data.

OODBMS 2

Weaknesses of RDBMSs

• Poor Representation of “Real World” Entities.

• Semantic Overloading.

• Poor Support for Integrity and Enterprise Constraints.

• Homogeneous Data Structure.

• Limited Operations.

• Difficulty Handling Recursive Queries.

• Impedance Mismatch.

• Other Problems with RDBMSs: o Transactions are generally short-lived and concurrency control protocols not suited for long-lived transactions.

o Schema changes are difficult. o RDBMSs are poor at navigational access.

OODBMS 3

OODBMS

Example - Recursive Query

S005 S004 S003 S002 S001 S004 S003 S002 S001 NULL StaffNo S005 S004 S003 S002 S001 S005 S005 S005 S004 S004 S003 MangrSNo S004 S003 S002 S001 NULL S003 S002 S001 S002 S001 S001 4

What is an OODBMS ?

OODBMS (Object-oriented Database Management System):

is a database with data stored in objects and collections NOT rows and tables.

Object Oriented Concepts:

      Abstraction, encapsulation, and information hiding.

Objects and attributes.

Object identity.

Methods and messages.

Classes, subclasses, superclasses, and inheritance.

Overriding, Overloading, Polymorphism and Dynamic Binding.

OODBMS 5

OODBMS

Traditional DBS

• Persistence • Sharing • Transactions • Concurrency Control • Recovery Control • Security • Integrity • Querying

Semantic Data Model

• Generalization • Aggregation

OO Programming

• OID • Encapsulation • Inheritance • Types & Classes • Methods • Complex objects • Polymorphism • Extensibility

Special Requirements

• Versioning • Schema Evolution

OODBMS

OODBMS 6

Abstraction

Abstraction

is the process of identifying essential aspects of an entity and ignoring unimportant properties.

Concentrate on what an object is and what it does, before deciding how to implement it.

7 OODBMS

Encapsulation & Information Hiding

Encapsulation:

means that an object contains both data structure and set of operations used to manipulate it.

Information Hiding:

means separating external aspects of an object from its internal details, which are hidden from outside.

• Allows internal details of an object to be changed without affecting applications that use it, provided external details remain same.

• Provides

data independence

.

OODBMS 8

Objects

Object

is a uniquely identifiable entity that contains both: • the

attributes

that describe the state of a real-world object, • and the

actions

associated with it.

Definition very similar to that of an entity, however,

encapsulates both

state

and

behavior

; an entity only models

state

.

Object

Persistent Objects vs. Transient Objects:

•

Transient:

programming object’s memory allocated language’s runtime system.

and •

Persistent:

object’s storage managed by OODBMS.

deallocated by OODBMS 9

Attributes

Attributes

contain current state of an object: • Attributes can be classified as

simple

or

complex

.

•

Simple attribute

can be a primitive type such as integer, string, etc., which takes on literal values.

•

Complex attribute

can contain collections and/or references.

•

Reference attribute

represents relationship.

• An object that contains one or more complex attributes is called a

complex object

.

OODBMS 10

Object Identity (OID)

Object identifier (OID)

assigned to object when it is created that is: • System-generated.

• Unique to that object.

• Invariant.

• Independent of the values of its attributes (that is, its state).

• Invisible to the user (ideally).

OODBMS 11

Object Identity (OID)

In RDBMS

, entity identity is value-based: primary key is used to provide uniqueness.

Primary keys do not provide type of object identity required in OO systems: • key only unique within a relation, not across entire system; • key generally chosen from attributes of relation, making it dependent on entity state.

12 OODBMS

Methods & Messages

Method:

Defines behavior of an object, as a set of encapsulated functions.

Message:

Request from one object to another asking second object to execute one of its methods.

13 OODBMS

OODBMS

Object Showing Attributes & Methods

Method 1 Method 2

Attributes

Method 4 Method 3

14

Classes

Classes

are blueprints for defining a set of similar objects.

• Objects in a class are called

instances

.

• Class is also an object with own

class attributes

and

class methods

.

• Object created from the same class share the same class attributes and methods.

15 OODBMS

Class Instance Share Attributes & Methods

BranchNo

= B005

Street

= 22 Deer Rd

City

= London

Postcode

= SW1 4EH

BRANCH

Attributes

branchNo street city postcode BranchNo

= B007

Street

= 16 Argyll St

City

= Aberdeen

Postcode

= AB2 3SU

Methods

print() getPostCode() numberOfStaff() BranchNo

= B003

Street

= 163 Main St

City

= Glasgow

Postcode

= G11 9QX 16 OODBMS

Subclasses, Superclasses, & Inheritance

Inheritance

allows one class of objects to be defined as a special case of a more general class.

• Special cases are

subclasses

and more general cases are

superclasses

.

• Process of forming a superclass is

specialization

.

generalization

; forming a subclass is •

Subclass

inherits all properties of its superclass and can define its own unique properties.

• Subclass can redefine inherited methods (

overriding

).

• Relationship between subclass and superclass known as A KIND OF (AKO) relationship.

OODBMS 17

Subclasses, Superclasses, & Inheritance

Types of inheritance:

single, multiple, and repeated.

Single Inheritance Multiple Inheritance Repeated Inheritance

18 OODBMS

Overriding, Overloading & Polymorphism

Overriding

is the process of redefining a property within a subclass.

Overloading

allows name of a method to be reused with a class or across classes.

Polymorphism

means ‘

many forms

’.

Three types:

operation, inclusion, and parametric.

The process of selecting the appropriate method based on an called

binding.

object’s type is • If the determination of an object’s type can be deferred until runtime (rather than compile time), the selection is called

dynamic binding.

OODBMS 19

Complex Objects

A

Complex object

is something that can be viewed as a single object in the real world but it actually consists of many sub-objects.

Two types of complex objects:

• Unstructured complex objects: • Their structure hard to determine.

• Requires a large amount of storage.

• BLOB (Binary Large Objects): images & long test strings.

• Structured complex objects: • Clear structure.

• Sub-objects in A PART-OF (APO) relationship.

OODBMS 20

Structured Complex Objects

The contained object can be handled in one of two ways:

• Contained object can be encapsulated within complex object, accessed by complex object’s methods.

Obj 1

M1 M2

Obj 2

M4 M3 • Or have its own independent existence, and only an OID is stored in complex object.

Obj 1 Obj 2 Obj 3

M1 M2 M1 M2

OID 2 OID 2

M4 M3 M4 M3 OODBMS 21

Collections

Contains a number of unnamed, homogeneous elements; each can be instance of atomic type, another collection, or a literal type.

Types of Constructors:

Set: unordered collection of objects without duplicates .

Bag: unordered collection of objects that allows duplicates .

List: ordered collection of objects that allows duplicates .

Array: ordered collection of objects without duplicates .

Dictionary: unordered sequence of key-value pairs without duplicate keys .

OODBMS 22

Commercial OODBMSs

• GemStone from Gemstone Systems Inc., • Itasca from Ibex Knowledge Systems SA, • Objectivity/DB from Objectivity Inc., • ObjectStore from eXcelon Corp., • Ontos from Ontos Inc., • Poet from Poet Software Corp., • Jasmine from Computer Associates/Fujitsu, • Versant from Versant Object Technology.

OODBMS 23

Object Data Management Group (ODMG)

www.odmg.org

ODMG

is an international consortium founded to address object standards.

Major components of ODMG architecture for an OODBMS are:

• Unified Modeling Language (UML).

• Object Model (OM).

• Object Definition Language (ODL).

• Object Query Language (OQL).

• C++, Smalltalk, and Java Language Binding.

24 OODBMS

Unified Modeling Language (UML)

UML

is a standard language for specifying, constructing, visualizing, and documenting the artifacts of a software system.

• Include many structural diagrams (Class, Object diagrams…) and behavioral diagrams (UseCase, Sequence diagrams …).

• Used to model objects and object relationships.

Class Name Attribute Method MANAGER

StaffNo sex DOB salary increasesalary()

Association

1..1 manage 1.1

BRANCH

branchNo street city postcode print() getPostCode() numberOfStaff() 1..1 offer 1.* offered-by

PROPERTY

PropertyNo street city postcode rooms type OODBMS 25

Unified Modeling Language (UML)

PERSON

Name FName LName

MANAGER 1 STAFF

StaffNo position DOB salary Manages ManagedBy

SALESTAFF M

Has

POWNER

OwnerNo address OwnedBy

1

Owns

CLIENT

ClientNo telNO prefType MaxRent ViewedBy

N

Views

1

WorksAt

1 BRANCH

BranchNo address

M PROPERTY

PropertyNo rooms rent

M M 1

Offers IsOfferedBy OODBMS 26

Object Model (OM)

Object Model (OM)

provides the data type, type constructors & other concepts utilized in the ODL to specify the object schema.

• Basic building blocks are

object

and

literal

.

• Only an

object

has a unique identifier and state (current value).

•

Literal

is a constant values. e.g.

“Ahmed”, “123 Main St, London” .

• Behavior defined by set of

operations

that can be performed on or by object.

• State defined by values objects carry for a set of

attributes

of object or

relationships

between object and one or more other objects.

OODBMS 27

Object Definition Language (ODL)

Object Definition Language (ODL)

is a language for defining the specification of object types for OODBMS.

e.g. of ODL definition for DreamHome:

{

module

DreamHome {

Class

Branch (

extent

branchOffices

key

branchNo)

attribute

string branchNo;

attribute struct

BranchAddress {string street, string city, string postcode} address;

relationship

Manager ManagedBy

inverse

Manager::Manages;

relationship

set Has

inverse

SalesStaff::WorksAt;

relationship

set Offers

inverse

PropertyForRent::IsOfferedBy; void takeOnPropertyForRent(in string propertyNo)

raises

(propertyAlreadyForRent); }; }; OODBMS 28

Object Query Language (OQL)

Object Query Language OQL

is a query language for OODBMS.

• Does not provide explicit update operators - leaves this to operations defined on object types.

OQL can be used for both associative and navigational access: •

Associative query

returns collection of objects (like SQL).

•

Navigational query

accesses individual objects and object relationships used to navigate from one object to another.

OODBMS 29

Object Query Language (OQL)

OQL vs. SQL A Simple Example:

List the names of the children of employees working in the sales department.

OQL select

c.fname, c.lname

from

Department d, d.Employee e, e.Children c

where

d.name = “Sales”;

SQL select

c.fname, c.lname

from

Department d, Employee e, Children c

where

d.name = “Sales”

and

d.deptID = e.deptID

and

c.parentID = e.empID; OODBMS 30