Object Database Management Systems

1
Object Database Management Systems

• Introduction

2
Chapters Covered

• Third edition
• Chapter 24 except 24.4, 24.5
• Chapter 25 except 25.2.1, 25.3, 25.4.4-25.6
• Fourth edition
• Chapter 25 except 25.4, 25.5, 25.7
• Chapter 26 except 26.1.2, 26.1.4, 26.3, 26.4.4
  26.5

3
The Necessity of a New Data Model

• Advanced applications require
• Complex data types
• New operations
• Recursive queries
• Impedance mismatch
• RDBMS problems
• Evolution of computer applications

4
Recursive Query

• Recursion in OO programming language
• Static long fact(int n)
• Assert.notFalse((ngt0 (nlt10))
• return nfact(n-1)
• Recursion in relational algebra and SQL
• The need to know the number of levels

5
Impedance Mismatch

• Mixing different programming paradigms
• Declarative and procedural programming languages
• Different data representation
• Data conversion
• Difficulties with automatic code checking
• The necessity to provide seamless integration
  between application program and a database

6
Object-Oriented Concepts

• Abstraction.
• Objects and attributes.
• Object identity.
• Methods and messages.
• Classes and inheritance.
• Polymorphism and dynamic binding.

7
Abstraction

• 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.
• Main aspects
• Encapsulation
• Information hiding

8
Encapsulation and Information Hiding

• Encapsulation - Object contains both data
  structure and set of operations used to
  manipulate it.
• Information hiding
• External aspects
• Internal details
• Logical data independence

9
Object

• Uniquely identifiable entity
• Object state
• Object attributes (instance variables)
• Simple attribute
• Complex attribute
• Collection attribute
• Reference attribute

10
Object Identity - OID

• System-generated
• Unique to the object
• Invariant during the object lifetime
• Independent of the object state
• Invisible for users

11
Object Identity

• Objects are identical if3 they have the same OIDs
• Objects equality
• Shallow equality all attributes except
  references contain the same values
• Deep equality Object states contain the same
  values and related object states also contain the
  same values

12
Object Behavior

• Methods operations with object attributes
• Method components
• Method name
• Block of code that implements required operations

13
Communication of Objects

• Message a request from one object to another
  asking second object to execute one of its
  methods.
• An object can send a message to itself

14
Abstract Data Type (ADT)

• User-defined data type that describes similar
  objects
• Contains
• Data structure - attributes
• Set of methods that realize the interface

15
Classes

• Objects that have the same attributes and respond
  to the same messages form a class
• An object is an instance of its class
• A class may have
• Class attributes
• Class methods

16
Inheritance

• Allows one class of objects to be defined as a
  special case of a more general class.
• Superclasses and subclasses
• Subclass inherits all properties of its
  superclass and can define its own unique
  properties.
• Subclass can redefine inherited methods.

17
Class Hierarchy

• A class can be a superclass and a subclass at the
  same time
• Inheritance
• Single inheritance
• Multiple inheritance
• Conflicts
• No single approach to conflict resolution
• Method overriding

18
Polymorphism

• Allows different objects to respond to same
  messages in different ways
• Inclusion polymorphism
• Operation polymorphism overloading
• Parametric polymorphism
• Binding - process of selecting appropriate method
  based on an objects type.
• Dynamic (late) binding

19
The Object Data Model

• Basic modeling constructs objects and literals
• Objects and literal are categorized into types.
• Object behavior is defined by a set of methods
  (operations)
• Object state is defined by the values of objects
  set of properties
• An ODMS stores objects based on a schema defined
  in the ODL

20
Object Properties

• Properties (attributes)
• Simple (atomic)
• Complex
• Composite
• Multi-valued
• Relationships
• Type constructors

21
Object-oriented vs. Relational Data Models
OODM RDM Difference
Object Entity Object includes behavior
Attribute Attribute Complex attributes
Association Relationship -
Message N/A -
Class Entity Set Class attributes and methods
Instance Entity (instance) -
Encapsulation N/A -
OID Primary key OID is state independent
22
ODM Diagrammatic Representation

• Class diagram
• Class representation
• Class name
• Properties
• Methods operations
• Associations
• Object diagram

23
Classes

• Generalization
• Specialization
• Inheritance
• Abstract class a class that has no direct
  instances
• Concrete class a class that can have direct
  instances

24
Inheritance Constraints

• Complete no additional subclasses are expected
• Incomplete additional subclasses may be
  required

25
Relationships in ODM

• Inheritance (Is a)
• Other types of relationships
• Association
• Aggregation
• Composition
• Inverse relationships

26
Associations

• Named relationships between or among object
  classes
• Association role the end of association where
  it connects to the class
• Each association has two roles
• Degree of an association
• Association implementation

27
Representation of Association

• ODM supports only binary relationships
• Cardinality (connectivity) the number of
  possible objects playing one role that may relate
  to a single object playing another role
• 11
• 1
• Associations of a degree higher than two

28
Intermediate (Association) Class

• Cardinality is determined as
• Association has some attributes
• Association itself can participate in a
  relationship with other class
• Representing associations of a degree higher than
  two
• Intermediate class contains references to
  corresponding classes and descriptive attributes

29
Aggregation and Composition (Has a)

• Aggregation - a part-of relationship between a
  component object and an aggregate object
• Aggregation vs. inheritance
• Composition an aggregation in which a part
  object belongs to only one whole object and the
  part object lifetime is the same as lifetime of
  the whole object

30
Behavioral Design

• State vs. behavior
• Public and private methods
• Main types of methods
• Constructors and destructors
• Access query operation
• Transform update operation
• Derived attributes in ERD

31
Implementation of Object Data Model

• OODM data model that expresses the semantics of
  objects in OO programming language
• OODB persistent and sharable collection of
  objects defined by OODM
• OODBMS software that manages an OODB

32
Two Main Approaches

• Include persistence into object-oriented
  programming languages
• Persistent objects
• Transient objects.
• Extend standards of relational DBMSs to include
  some of object-oriented concepts
• ORDBMS

33
Persistent Programming Language

• Preserves data across successive executions of a
  program
• Data is independent of any program
• Data exists beyond the execution and lifetime of
  the code that created it
• Eliminates impedance mismatch by extending
  programming language with database features.

34
Two-Level Storage Model
35
Single-Level Storage Model
36
OID in Single-Level Storage Model

• OID in C is physical address in the process
  memory space
• Logical OID independent of physical location of
  the object
• Physical OID describes the location of the
  object
• Pointer swizzling the conversion of OIDs to
  main memory pointers, and back

37
Accessing an Object (Entity) in RDBMS
38
Accessing an Object in OODBMS
39
Data Management Aspects in OODBMS

• Long-Duration Transactions in OODBMS
• An object is the unit of concurrency control
• An object is a unit of recovery
• Advanced transaction models
• Multiversion concurrency control protocols

40
Versions

• Identifiable state of the object
• Version management
• Version history
• Object references always point to correct object
  version.

41
Schema Evolution (Modification)

• Changes to class definition
• Modifying Attributes.
• Modifying Methods.
• Changes to class hierarchy
• Making class S a superclass of class C.
• Removing S from list of superclasses of C.
• Modifying the order of superclasses of C.
• Changes to set of classes
• Creating and deleting classes
• Modifying class names