Concepts for Object Oriented Databases.ppt

Chapter 1
Concepts for Object Oriented
Databases
By: Melaku Y

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
2

Introduction
• Data model is used to describe the structure of the database
• Traditional Data Models:
 Hierarchical (1960)
 Network (since mid-60’s)
 Entity-relationship Model
 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
• Give the designer to specify both the structure of complex objects
and the operations
 Increased use of object-oriented programming languages
• Difficult to integrate with traditional database
 Need for additional data modeling features 3

1.1 Overview of Object-Oriented Concepts(1)
• As commercial object DBMSs became available, the need for a
standard model and language was recognized.
• Main Claim: OO databases try to maintain a direct correspondence
between:
 real-world and database objects so that objects do not lose their
integrity and identity and can easily be identified and operated upon
• Object: A uniquely identifiable entity
 That contains both the attributes that describe the state of a ‘real
world’ object and the actions that are associated with it. (Simula
1960s)
• Object: has two components:
 state (value) and behavior (operations)
Similar to program variable in programming language, except that it
will typically have a complex data structure as well as specific
operations defined by the programmer 4

Overview of Object-Oriented Concepts (2)
• In OO databases,
 objects may have an object structure of arbitrary complexity in order to
contain all of the necessary information that describes the object.
• In contrast, 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.
• Persistent vs transient object
 Transient object: exist only during program execution
 Persistent object: exist beyond program termination
Stored by OO databases permanently in secondary storage
 Allow the sharing objects among multiple programs and applications.
 What needed: indexing and concurrency(DBMS Features)
5

• 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.
 Issues: users required to know attribute name to retrieve specific
objects and any simple retrieval requires a predefined operation
6

• 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.
 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
7

• Type and Class hierarchies and Inheritance
 permits specification of new types or classes that inherit much of their
structure and/or operations from previously defined types or classes.
 this makes it easier to develop the data types of a system incrementally
and to reuse existing type definitions when creating new types of
objects.
 Operator overloading(operator polymorphism)
 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 object it is applied to.
8

1.2 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(OID)
the main property of OID
immutable (should not change). this preserves the identity of the real-
world object being represented.
used only once, even if an object is removed from the database, its OID
should not be assigned to another object
Object may given one or more names meaningful to the user
identifies a single object within a database.
intended to act as ‘root’ objects that provide entry points into the
database. 9

Object Identity, Object Structure, and Type
Constructors (3)
• Type Constructors:
 In ODBs, a complex type may be constructed from other types by
nesting of type constructors.
• The three most basic constructors are:
 atom (basic built-in data types)
 tuple (compound or composite type)
• struct Name<FirstName: string, MiddleInitial: char, LastName:
string>
• struct CollegeDegree<Major: string, Degree: string, Year: date>
 collection (multivalued) => set, array, list, bag, dictionary
 The atom constructor is used to represent all basic atomic values
 integers, real numbers, character strings, Booleans, other
10

Constructors (4)
• Tuple constructor
 create structured values and objects of the form <a1:i1, a2:i2, … ,
an:in>
• Set constructor
 create objects or literals that are a set of distinct elements {i1, i2, … ,
in}, all of the same type
• Bag constructor
 Same as set but elements need not be distinct
• List constructor
 create an ordered list [i1, i2, … , in]
 Array constructor
 create a single-dimensional array of elements of the same type
 Dictionary constructor
 creates a collection of key-value pairs (K, V) 11

Constructors (5)
12

1.3 Encapsulation of Operations, Persistence of
Objects(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
 In traditional database models and systems this concept was not
applied
 since it is customary to make the structure of database objects
visible to users and external programs
The relation and its attributes can be accessed using generic
operations.
 The concept of encapsulation means that
 Object contains both data structure and the set of operations used to
manipulate it.
13

1.3 Encapsulation of Operations,
Methods, and Persistence (2)
• The concept of information hiding means that
 external aspects of an object is separated from its internal details,
which are hidden from the outside world.
 The external users of the object are only made aware of the
interface (signature) of the operation
• Specifying Object Behavior via Class Operations (methods):
 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
14

Encapsulation of Operations, Methods, and
Persistence (4)
 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).
 An operation is typically applied to an object by using the
dot notation.
15

Persistence (6)
• Specifying Object Persistence via Naming and Reachability:
 Transient objects
• exist in the executing program and disappear when program
terminates.
 Persistent objects
• stored in the database and persist after program termination.
• Mechanisms to make an object persistent
 Naming Mechanism:
• name can be given to an object via a specific statement or operation in the
program
• the named objects are used as entry points to the database through which
users and applications can start their database access
 Reachability Mechanism:
• Make the object reachable from some other 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.
17

Persistence (6)
18

1.4 Type and Class Hierarchies and Inheritance (1)
• Type (class) Hierarchy
 A type is defined by
 assigning type name and defining attributes (instance variables) and
operations (methods).
has a type name and a list of visible (public) functions
• Specifications: TYPE_NAME: function, function, . . . , function
Example:
• PERSON: Name, Address, Birthdate, Age, SSN
• Subtype:
 When the designer or user must create a new type
 that is similar but not identical to an already defined type
 inherits all the functions of supertype
19

Type and Class Hierarchies and 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
20

Type and Class Hierarchies and Inheritance (4)
• Example (2):
 GEOMETRY_OBJECT: Shape, Area, ReferencePoint
 RECTANGLE subtype-of GEOMETRY_OBJECT: Width, Height
 TRIANGLE subtype-of GEOMETRY_OBJECT: Side1, Side2, Angle
 CIRCLE subtype-of GEOMETRY_OBJECT: Radius
21

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