Part 8a Object Oriented DB

8/15/2019 Part 8a Object Oriented DB

1/55

1

Object-Oriented Databases

based on

Fundamentals of Database Systems

Elmasri and Navathe

Minor corrections/modifications made by H. Hakimzadeh, 2005

Acknowledgement: Fariborz Farahmand


2/55

2

Outline

n Overview of O-O Concepts

n O-O Identity, Object Structure and Type

Constructorsn Encapsulation of Operations, Methods and

Persistence

n Type Hierarchies and Inheritance

n Complex Objectsn Other O-O Concepts


3/55

3

Introduction

n Reasons for creation of Object

Oriented Databases

n Need for more complex applications

n

Need for additional data modeling featuresn Increased use of object-oriented programming

languages


4/55

4

Public Domain and Commercial Systems

n Research OODB systems: Orion, Open-OODB at T.I.,

ODE at ATT Bell labs, Postgres - at UCB, etc.

n Commercial OODB products: Ontos, Gemstone,

Objectivity, Objectstore, Versant, etc.

n For more see:n http://www.service-architecture.com/products/object-oriented_databases.html


5/55

5

The Main Claim

n OO databases try to maintain a direct

correspondence between real-world and

database objects so that objects do not losetheir integrity and identity and can easily be

identified and operated upon

n OODBs handle complex data types better

n OODBs handle complex operations better


6/55


7/55


8/55

8

Instance Variables vs. Attributes

n The internal structure of an object in OOPLsincludes the specification of instance

variables, which hold the values that define

the internal state of the object.

n Hence, an instance variable is similar to theconcept of an attribute, except that instance

variables may be encapsulated within theobject and thus are not necessarily visible toexternal users.


9/55

9

Object-Oriented Concept

n Some OO models insist that all operations auser can apply to an object must bepredefined. This forces a completeencapsulation of objects.

n To encourage encapsulation, an operation isdefined in two parts:

n signature or interface of the operation, specifies

the operation name and arguments (orparameters).

n method or body, specifies the implementation ofthe operation.


10/55

10

Encapsulation in OODBs

n Operations can be invoked by passing a message to

an object, which includes the operation name and the

parameters.

n The object then executes the method for that

operation.

n Encapsulation permits modification of the internalstructure of an object, as well as the implementation of

its operations, without the need to disturb the external

programs that invoke these operations


11/55

11

Versioning…

n Some OO systems provide capabilities fordealing with multiple versions of the same

object (a feature that is essential in design and

engineering applications).

n For example, an old version of an object thatrepresents a tested and verified design should

be retained until the new version is tested andverified:

- very crucial for designs in manufacturing process

control, architecture, software systems …


12/55

12

Overloading in OODBs

n Operator polymorphism: It refers to anoperation’s ability to be applied to different

types of objects; This feature is also called

operator overloading


13/55

13

Object Identity

n The OODB provides a unique identity to eachindependent object stored in the database.

n

This unique identity is typically implemented via aunique system-generated object identifier, or OID.

n The main property required of an OID is that it beimmutable; that is, the OID value of a particular

object should not change over the life of the object.This preserves the identity of the real-world objectbeing represented.


14/55


15/55

15

Type Constructors

n The three most basic type constructors (types) areatom, tuple, and set.

n Other commonly used type constructors include list,bag, and array.


16/55

16

Example of an object

n The atom constructor is used to represent all basicatomic values, such as integers, real numbers,character strings, booleans, and any other basic datatypes that the system supports directly.

We use i 1, i 2, i 3, . . . to stand for unique system-generated object identifiers.

o1 = (i 1, atom, ‘Houston’)

o2 = (i 4, atom, 5)

o3 = (i 7, set, {i 1, i 2, i 3})


17/55

17

n Recall the COMPANY schema, visualize the Research

department and its components using the relational model.

Research

Dept


18/55

18

Example 1:


19/55

19

Example 1 (cont.):


20/55


21/55

21

Now visualize the Research Department

in Object Oriented Model


22/55

22

Object Identity, Object Structure, and

Type Constructorsn Example 1

We use i 1, i 2, i 3, . . . to stand for unique system-generated object identifiers. Consider the followingobjects:

o1 = (i 1, atom, ‘Houston’)

o2 = (i 2, atom, ‘Bellaire’)

o3 = (i 3, atom, ‘Sugarland’)

o4 = (i 4, atom, 5)o

5= (i

5, atom, ‘Research’)

o6 = (i 6, atom, ‘1988-05-22’)

o7

= (i 7, set, {i

1, i

2, i

3})


23/55

23

Object Identity, Object Structure

Example 1 (cont.)

n The first six objects listed in this example

represent atomic values.

n Object 7 is a set-valued object that represents theset of locations for department 5; the set refers to

the atomic objects with values {‘Houston’,

‘Bellaire’, ‘Sugarland’}.


24/55

24

Object Identityn Example 1 (cont.)

o8 = (i 8, tuple, )

o9 = (i 9, tuple, )

o10 = (i 10, set, {i 12, i 13, i 14})

o11

= (i 11

, set {i 15

, i 16

, i 17

})

o12 = (i 12, tuple, )

. . .


25/55

25

Object Identity, Object Structure

Example 1 (cont.)

n Object 8 is a tuple-valued object that represents

department 5 itself, and has the attributesDNAME, DNUMBER, MGR, LOCATIONS, and so

on.


26/55

26

Equal vs. Identical Objects

n Two objects are said to have identical values, if

the graphs representing their values are identical

in every respect, including the OIDs at every level.

n Two object are said to have equal values if their

graph structure is the same, all the corresponding

atomic values should also be the same (same

states at the atomic level) but their OID do nothave to be the same.

n Equal is a weaker notion.


27/55

27

Equal vs. Identical Objects

Example 2:

This example illustrates the differencebetween the two definitions for comparing

object states for equality.o

1= (i

1, tuple, )

o2

= (i 2, tuple, )

o3

= (i 3, tuple, )

o4

= (i 4, atom, 10)

o5

= (i 5, atom, 10)

o6

= (i 6, atom, 20)


28/55

28

Equal Object States

Example 2 (cont.):

n In this example, The objects o1

and o2 have

equal states, since their states at the atomiclevel are the same but the values are reached

through distinct objects o4 and o5 .


29/55

29

Identical Object States

Example 2 (cont.):

n However, the states of objects o1

and o3 areident ical , even though the objects themselves

are not because they have distinct OIDs.

n Similarly, although the states of o4 and o5 are

identical, the actual objects o4 and o5 are

equal but not identical, because they havedistinct OIDs.


30/55

30

Graphical Representation of a Complex

ObjectResearch Department


31/55


32/55

32

Encapsulation of Operations, Methods, and

Persistence

Encapsulation

n One of the main characteristics of OO languages and systems

n related to the concepts of abstract data types and information

hiding in programming languages


33/55

33

Encapsulation of Operations, Methods,

and Persistence (cont.)

Specifying Object Behavior via Class Operations:

n The main idea is to define the behavior of a type of

object based on the operations that can beexternally applied to objects of that type.


34/55

34

Encapsulation of Operations, Methods,

and Persistence (cont.)

Specifying Object Behavior via Class Operations

(cont.):

n For database applications, the requirement that all

objects be completely encapsulated is too stringent.

n One way of relaxing this requirement is to divide the

structure of an object into visible and hidden attributes

(instance variables).


35/55


36/55

36

11.4 Type Hierarchies and Inheritancen Type (class) Hierarchy

n A type in its simplest form can be defined by it’s type

name and a listing of the names of its visible/ public functions.

TYPE_NAME: function, function, . . . , function

Example:

PERSON: Name, Address, Birthdate, Age, SSN


37/55

37

Type Hierarchies and Inheritance (cont.)

n Subtype: when the designer or user mustcreate a new type that is similar but notidentical to an already defined type

n Subtype: inherits all the functions of thepredefined type (also known as thesupertype)


38/55


39/55

39


Example (2):

consider a type that describes objects in plane geometry, which

may be defined as follows:

GEOMETRY_OBJECT type:

Shape, Area, ReferencePoint


40/55

40


n Example (2) (cont.):

Now suppose that we want to define a number of subtypes forthe GEOMETRY_OBJECT type, as follows:

RECTANGLE subtype-of GEOMETRY_OBJECT:Width, Height

TRIANGLE subtype-of GEOMETRY_OBJECT:

Side1, Side2, Angle

CIRCLE subtype-of GEOMETRY_OBJECT: Radius


41/55

41


n Example (2) (cont.): An alternative way of declaring these three subtypes is tospecify the value of the Shape attribute as a condition that mustbe satisfied for objects of each subtype:

RECTANGLE subtype-of GEOMETRY_OBJECT(Shape=‘rectangle’): Width, Height

TRIANGLE subtype-of GEOMETRY_OBJECT(Shape=‘triangle’): Side1, Side2, Angle

CIRCLE subtype-of GEOMETRY_OBJECT(Shape=‘circle’): Radius


42/55

42


n Extents: collections of objects of the same type.


43/55

43


n Persistent Collection: It holds a collection of objectsthat is stored permanently in the database and hencecan be accessed and shared by multiple programs


44/55


45/55

45

11.5 Complex Objects

n Unstructured complex object:

n Provided by a DBMS

n Permits the storage and retrieval of large objects that

are needed by the database application.

n Examples: of such objects are bi tmap images and

l ong tex t s t r ings (such as documents); they are also

known as binary large objects, or BLOBs for short.


46/55

46

Complex Objects (cont.)

n Structured complex object:

n Differs from an unstructured complex object in that the

object’s structure is defined by repeated application of

the type constructors provided by the OODBMS.

Hence, the object structure is defined and known to the

OODBMS.


47/55

47

Other Objected-Oriented Concepts

n Polymorphism (Operator Overloading):

This concept allows the same operator name or symbol to be

bound to two or more different implementations of the operator,

depending on the type of objects to which the operator is

applied


48/55

48

11.6 Other Objected-Oriented Concepts

(cont.)

n Multiple Inheritance and Selective

Inheritance

n Multiple inheritance in a type hierarchy occurs when a

certain subtype T is a subtype of two (or more) typesand hence inherits the functions (attributes and

methods) of both supertypes.

n Example, we may create a subtype

ENGINEERING_MANAGER that is a subtype of bothMANAGER and ENGINEER. This leads to the

creation of a type lattice rather than a type hierarchy.


49/55

49


(cont.)

Versions and Configurations

n Many database applications that use OO systems

require the existence of several versions of the same

object

n There may be more than two versions of an object.


50/55

50


(cont.)

n Configuration: A configuration of the complex object

is a collection consisting of one version of each

module arranged in such a way that the module

versions in the configuration are compatible and

together form a valid version of the complex object.


51/55

51

Summary

n Object ident i ty : Objects have unique identities thatare independent of their attribute values.

n Type con structors : Complex object structures can

be constructed by recursively applying a set of basicconstructors, such as tuple, set, list, and bag.

n Encapsulat ion of operat ions : Both the objectstructure and the operations that can be applied toobjects are included in the object class definitions.


52/55

52

Summary (cont.)

n Programm ing langu age com patib i l ity : Bothpersistent and transient objects are handleduniformly. Objects are made persistent bybeing attached to a persistent collection.

n Type hierarch ies and inher i tance : Objecttypes can be specified by using a typehierarchy, which allows the inheritance ofboth attributes and methods of previouslydefined types.


53/55

53

Summary (cont.)

n Extents : All persistent objects of a particular type canbe stored in an extent. Extents corresponding to atype hierarchy have set/subset constraints enforcedon them.

n Support for complex objects : Both structured andunstructured complex objects can be stored andmanipulated.

n Polymo rphism and operator over loading :Operations and method names can be overloaded toapply to different object types with differentimplementations.


54/55

54

Summary (cont.)

n Versioning : Some OO systems provide support for

maintaining several versions of the same object.


55/55

55

Current Status

n OODB market not growing much per se

n O-O ideas are being used in a large number ofapplications, without explicitly using the OODB

platform to store data.

n Growth: O-O tools for modeling and analysis, O-OProgramming Languages like Java and C++

n Compromise Solution Proposed: Object RelationalDB Management (Informix Universal Server, Oracle9i, IBM’s UDB, …)

Documents

Part 8a Object Oriented DB