Topic 6 Concepts for Object Databases.ppt

Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe
Chapter 20
Concepts for
Object Databases

Chapter Outline
 1 Overview of O-O Concepts
 2 O-O Identity, Object Structure and Type
Constructors
 3 Encapsulation of Operations, Methods and
Persistence
 4 Type and Class Hierarchies and Inheritance
 5 Complex Objects
 6 Other O-O Concepts
 7 Summary & Current Status

Introduction
 Traditional Data Models:
 Hierarchical
 Network (since mid-60’s)
 Relational (since 1970 and commercially since 1982)
 Object Oriented (OO) Data Models since mid-90’s
 Reasons for creation of Object Oriented Databases
 Need for more complex applications
 Need for additional data modeling features
 Increased use of object-oriented programming languages
 Commercial OO Database products –
 Several in the 1990’s, but did not make much impact on
mainstream data management

History of OO Models and Systems
 Languages:
 Simula (1960’s)
 Smalltalk (1970’s)
 C++ (late 1980’s)
 Java (1990’s and 2000’s)

(contd.)
 Experimental Systems:
 Orion at MCC
 IRIS at H-P labs
 Open-OODB at T.I.
 ODE at ATT Bell labs
 Postgres - Montage - Illustra at UC/B
 Encore/Observer at Brown

(contd.)
 Commercial OO Database products:
 Ontos
 Gemstone
 O2 ( -> Ardent)
 Objectivity
 Objectstore ( -> Excelon)
 Versant
 Poet
 Jasmine (Fujitsu – GM)

Overview of Object-Oriented Concepts
 The internal structure of an object in OOPLs
includes the specification of instance variables,
which hold the values that define the internal
state of the object.
 An instance variable is similar to the concept of
an attribute, except that 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 a complete encapsulation of objects.
 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.

Overview of Object-Oriented Concepts (5)
 Operations can be invoked by passing a
message to an object, which includes the
operation name and the parameters.
 The object then executes the method for that
operation.
 This encapsulation permits modification of the
internal structure of an object, as well as the
implementation of its operations, without the need
to disturb the external programs that invoke these
operations

 Some OO systems provide capabilities for
dealing with multiple versions of the same object
(a feature that is essential in design and
engineering applications).
 For example, an old version of an object that
represents a tested and verified design should be
retained until the new version is tested and
verified:
 very crucial for designs in manufacturing process
control, architecture , software systems …..

 Operator polymorphism:
 This refers to an operation’s ability to be applied to
different types of objects; in such a situation, an
operation name may refer to several distinct
implementations, depending on the type of objects
it is applied to.
 This feature is also called operator overloading

20.2 Object Identity, Object Structure, and
Type Constructors (1)
 Unique Identity:
 An OO database system provides a unique identity
to each independent object stored in the database.

This unique identity is typically implemented via a
unique, system-generated object identifier, or OID
 The main property required of an OID is that it be
immutable
 Specifically, the OID value of a particular object
should not change.
 This preserves the identity of the real-world object
being represented.

Object Identity, Object Structure, and
 Type Constructors:
 In OO databases, the state (current value) of a complex
object may be constructed from other objects (or other
values) by using certain type constructors.
 The three most basic constructors are atom, tuple, and
set.
 Other commonly used constructors include list, bag, and
array.
 The atom constructor is used to represent all basic atomic
values, such as integers, real numbers, character strings,
Booleans, and any other basic data types that the system
supports directly.

 Example 1
 One possible relational database state
corresponding to COMPANY schema

 Example 1 (contd.):

 Example 1 (contd.)


We use i1, i2, i3, . . . to stand for unique system-
generated object identifiers. Consider the following
objects:

o1 = (i1, atom, ‘Houston’)

o2 = (i2, atom, ‘Bellaire’)

o3 = (i3, atom, ‘Sugarland’)

o4 = (i4, atom, 5)

o5 = (i5, atom, ‘Research’)

o6 = (i6, atom, ‘1988-05-22’)

o7 = (i7, set, {i1, i2, i3})

 Example 1(contd.)
 o8 = (i8, tuple, <dname:i5, dnumber:i4, mgr:i9,
locations:i7, employees:i10, projects:i11>)
 o9 = (i9, tuple, <manager:i12, manager_start_date:i6>)
 o10 = (i10, set, {i12, i13, i14})
 o11 = (i11, set {i15, i16, i17})
 o12 = (i12, tuple, <fname:i18, minit:i19, lname:i20, ssn:i21, .
. ., salary:i26, supervi
sor:i27, dept:i8>)

. . .

 The first six objects listed in this example
represent atomic values.
 Object seven is a set-valued object that represents
the set of locations for department 5; the set refers
to the atomic objects with values {‘Houston’,
‘Bellaire’, ‘Sugarland’}.
 Object 8 is a tuple-valued object that represents
department 5 itself, and has the attributes
DNAME, DNUMBER, MGR, LOCATIONS, and so
on.

 Example 2:
 This example illustrates the difference between the
two definitions for comparing object states for
equality.

o1 = (i1, tuple, <a1:i4, a2:i6>)

o2 = (i2, tuple, <a1:i5, a2:i6>)

o3 = (i3, tuple, <a1:i4, a2:i6>)

o4 = (i4, atom, 10)

o5 = (i5, atom, 10)

o6 = (i6, atom, 20)

 Example 2 (contd.):
 In this example, The objects o1 and o2 have equal
states, since their states at the atomic level are the
same but the values are reached through distinct
objects o4 and o5.
 However, the states of objects o1 and o3 are
identical, even though the objects themselves are
not because they have distinct OIDs.
 Similarly, although the states of o4 and o5 are
identical, the actual objects o4 and o5 are equal
but not identical, because they have distinct OIDs.

20.3 Encapsulation of Operations,
Methods, and Persistence (1)
 Encapsulation
 One of the main characteristics of OO languages
and systems
 Related to the concepts of abstract data types
and information hiding in programming
languages

Encapsulation of Operations, Methods,
and Persistence (2)
 Specifying Object Behavior via Class
Operations:
 The main idea is to define the behavior of a type
of object based on the operations that can be
externally applied to objects of that type.
 In general, the implementation of an operation
can be specified in a general-purpose
programming language that provides flexibility and
power in defining the operations.

and Persistence (3)
 Specifying Object Behavior via Class Operations
(contd.):
 For database applications, the requirement that all
objects be completely encapsulated is too
stringent.
 One way of relaxing this requirement is to divide
the structure of an object into visible and hidden
attributes (instance variables).

and Persistence (4)

and Persistence (5)
 Specifying Object Persistence via Naming and
Reachability:
 Naming Mechanism:

Assign an object a unique persistent name through which it
can be retrieved by this and other programs.
 Reachability Mechanism:

Make the object reachable from some persistent object.

An object B is said to be reachable from an object A if a
sequence of references in the object graph lead from object A
to object B.

and Persistence (6)
 Specifying Object Persistence via Naming and
Reachability (contd.):
 In traditional database models such as relational
model or EER model, all objects are assumed to
be persistent.
 In OO approach, a class declaration specifies only
the type and operations for a class of objects. The
user must separately define a persistent object of
type set (DepartmentSet) or list (DepartmentList)
whose value is the collection of references to all
persistent DEPARTMENT objects

and Persistence (7)

20.4 Type and Class Hierarchies and
Inheritance (1)
 Type (class) Hierarchy
 A type in its simplest form can be defined by giving
it a type name and then listing the names of its
visible (public) functions
 When specifying a type in this section, we use the
following format, which does not specify
arguments of functions, to simplify the discussion:

TYPE_NAME: function, function, . . . , function
 Example:

PERSON: Name, Address, Birthdate, Age, SSN

Type and Class Hierarchies and
Inheritance (2)
 Subtype:
 When the designer or user must create a new type
that is similar but not identical to an already
defined type
 Supertype:
 It inherits all the functions of the subtype

Inheritance (3)
 Example (1):
 PERSON: Name, Address, Birthdate, Age, SSN
 EMPLOYEE: Name, Address, Birthdate, Age,
SSN, Salary, HireDate, Seniority
 STUDENT: Name, Address, Birthdate, Age, SSN,
Major, GPA
 OR:
 EMPLOYEE subtype-of PERSON: Salary,
HireDate, Seniority
 STUDENT subtype-of PERSON: Major, GPA

Inheritance (4)
 Example (2):
 Consider a type that describes objects in plane geometry,
which may be defined as follows:

GEOMETRY_OBJECT: Shape, Area, ReferencePoint
 Now suppose that we want to define a number of subtypes
for the 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

Inheritance (5)
 Example (2) (contd.):
 An alternative way of declaring these three
subtypes is to specify the value of the Shape
attribute as a condition that must be 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

Inheritance (6)
 Extents:
 In most OO databases, the collection of objects in an extent
has the same type or class.
 However, since the majority of OO databases support types,
we assume that extents are collections of objects of the
same type for the remainder of this section.
 Persistent Collection:
 This holds a collection of objects that is stored permanently
in the database and hence can be accessed and shared by
multiple programs
 Transient Collection:
 This exists temporarily during the execution of a program
but is not kept when the program terminates

20.5 Complex Objects (1)
 Unstructured complex object:
 These is provided by a DBMS and permits the storage and
retrieval of large objects that are needed by the database
application.
 Typical examples of such objects are bitmap images and
long text strings (such as documents); they are also
known as binary large objects, or BLOBs for short.
 This has been the standard way by which Relational
DBMSs have dealt with supporting complex objects,
leaving the operations on those objects outside the
RDBMS.

Complex Objects (2)
 Structured complex object:
 This 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.
 The OODBMS also defines methods or operations
on it.

20.6 Other Objected-Oriented Concepts
(1)
 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
 For example + can be:

Addition in integers

Concatenation in strings (of characters)

Other Objected-Oriented Concepts (2)
 Multiple Inheritance and Selective Inheritance
 Multiple inheritance in a type hierarchy occurs
when a certain subtype T is a subtype of two (or
more) types and hence inherits the functions
(attributes and methods) of both supertypes.
 For example, we may create a subtype
ENGINEERING_MANAGER that is a subtype of
both MANAGER and ENGINEER.

This leads to the creation of a type lattice rather
than a type hierarchy.

Other Objected-Oriented Concepts (3)
 Versions and Configurations
 Many database applications that use OO systems require
the existence of several versions of the same object
 There may be more than two versions of an object.
 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.

20.7 Summary (1)
 Object identity:
 Objects have unique identities that are
independent of their attribute values.
 Type constructors:
 Complex object structures can be constructed by
recursively applying a set of basic constructors,
such as tuple, set, list, and bag.
 Encapsulation of operations:
 Both the object structure and the operations that
can be applied to objects are included in the object
class definitions.

Summary (2)
 Programming language compatibility:
 Both persistent and transient objects are handled uniformly.
Objects are made persistent by being attached to a
persistent collection.
 Type hierarchies and inheritance:
 Object types can be specified by using a type hierarchy,
which allows the inheritance of both attributes and methods
of previously defined types.
 Extents:
 All persistent objects of a particular type can be stored in an
extent. Extents corresponding to a type hierarchy have
set/subset constraints enforced on them.

Summary (3)
 Support for complex objects:
 Both structured and unstructured complex objects
can be stored and manipulated.
 Polymorphism and operator overloading:
 Operations and method names can be overloaded
to apply to different object types with different
implementations.
 Versioning:
 Some OO systems provide support for maintaining
several versions of the same object.

Current Status
 OODB market growing very slowly these days.
 O-O ideas are being used in a large number of
applications, without explicitly using the OODB platform to
store data.
 Growth:
 O-O tools for modeling and analysis, O-O Programming
Languages like Java and C++
 Compromise Solution Proposed:
 Object Relational DB Management (Informix Universal
Server, Oracle 10i, IBM’s UDB, DB2/II …)

Topic 6 Concepts for Object Databases.ppt

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