Object Oriented Databases (OODBs)
• Relational and OO data models.
• Advantages and Disadvantages of OO as compared with relational databases.
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A Database of Students and Modules
Student
0..*
Student Number {PK}
takes
0..*
Module
Module Code {PK}
2
Relational Model
Student
Module
Student Number {PK}
Student Name
Module Code {PK}
Module Title
1..1
1..1
Takes
Student Number {PK}
0..*
Module Code {PK}
0..*
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Relational Schema
Student {Student Number, Student Name}
Module {Module Code, Module Title}
Takes {Student Number∗, Module Code∗}
4
Relational Model
Each of these relations is implemented as a two dimensional
table, whose columns are called attributes, and whose rows are
called tuples or instances.
Student
Number
12486
43292
42403
52212
77123
Name
Fred Jones
Clare Adams
Priscilla White
J. Bigshott
Clare Adams
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Relational Model
Module
Code
C66
C80
C89
...
...
...
Title
Computing is Fun
Extremely Difficult Computing
Practical Business Computing
...
...
...
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Relational Model
Takes
Code
C66
C89
C66
C80
C40
C89
C66
C40
C42
C35
C42
C89
Number
43292
52212
12486
52212
43292
12486
52212
52212
77123
52212
52212
43292
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Relational Model
Queries can be written using relational algebra, relational calculus or SQL.
Suppose mynumber is my student number. Then the query ”list
the codes and titles of my modules” might look like this in SQL:
SELECT
FROM
WHERE
AND
Module.code, Module.title
Takes, Module
Takes.number = mynumber
Module.code = Takes.code
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Relational Model
or like this:
SELECT Module.code, Module.title
FROM
Modules
WHERE Module.code IN
SELECT Takes.code
FROM Takes
WHERE Takes.number = mynumber
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Relational Model – Terminology
• Entity instance: represents an object of interest. For example, a student record is an entity instance that represents
an individual student.
• Entity class: a collection of entity instances with a common
structure. For example, a whole collection of student records
forms an entity class.
• Attribute: a piece of interesting information, about the instances of an entitity class. For example, ’name’ is a fact
about students that is represented as an attribute of all the
instances of the class Student.
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• In a relational data model, the identity of an entity instance is
defined by the values of its attributes; no two entity instances
can have the same attribute values.
• Relationship: an association between entities. Entities are
often identified by nouns in a requirements specification, and
relationships by verbs. For example, ’takes’ indicates a relationship between Student and Module. Relationships can be
between entity classes, or between entity instances.
• Mathematically, a relation has a heading, which is a subset
of the Cartesian product of a set of (attribute name, domain)
pairs, and a body, which contains (attribute name, value)
pairs.
For example, Student could be represented as a heading,
(student number, integer), (student name, text) and a body
containing values like (student number, 12486),(student name,
Fred Jones)
• A relation is implemented as a table in a relational database.
Object Model
Student
0..*
takes
0..*
Module
Student Number {PK}
Student Name
Module Code {PK}
Module Title
set Student Name
get Student Name
take Module
lookup Modules
set Title
get Title
print Register
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Classes and Instances in the Object Data Model
• An object oriented data model (OODM) is a logical data
model that captures the meaning of objects of the kind used
in object oriented programming.
• It defines object classes, like Student and Module, and object
instances that have the attributes defined by the object class
but with particular values for each instance.
• Instances of Student have instance variables studentNumber
and studentName, with values like 12486 and Fred Jones respectively.
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Object Identity
• The object instances in an OODM have an identity that is
independent of the values of their attributes (contrast with
the relational model).
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Complex Objects, Implementing relationships
• The relationship, takes, could be represented as an instance
variable of Student. This could be a set, bag, list or other
container of references to instances of Module. The container would then be accessed using the message (method
invocation) lookupModules.
• Notice how an object oriented data model allows complex
objects (contrast with the relational model).
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Bidirectional relationships
• Instances of Module have state variables moduleCode and moduleTitle.
• Instances of Module could also implement taken by, the reverse of the relationship Takes, by means of a container of
references to instances of student.
• ObjectStore, an OODBMS described in C&B, implements
relationships like Takes by bidirectional links. The DBMS
automatically maintains referential integrity between the two.
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Queries in an Object Oriented Data Model
• Queries are responses to messages, and are implemented as
method invocations. For example, if aStudent is an instance
of Student, then an invocation like aStudent.lookupModules()
queries the modules taken by Student.
• Queries are intimately bound up with object classes; object
instances have their own queries, which are defined by their
class. (Contrast with SQL queries and the relational model).
• The object query language (OQL) uses an SQL-like syntax
to access the database; updates are performed by the methods defined in the classes.
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Encapsulation and Inheritance
• The object classes in the OODM allow for encapsulation.
• For example, the implementation of takes is hidden by Student and Module, and can only be accessed by invoking the
relevant methods.
• The object classes in the OODM allow for inheritance.
• This means that attributes that are shared by different kinds
of entity can be represented once, using superclasses or generalization. Subclasses, or specializations of a superclass can
inherit these shared attributes.
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• For example, we could define a class Person, of which Student and e.g. staff member were specializations.
• This reduces redundancy in the database schema (contrast
with the relational model . . . though table inheritance is supported by modern DBMSs).
Object Oriented Database
• An object oriented database (OODB) is a persistent and
shareable collection of objects modelled by an OODM.
• An object oriented database management system (OODBMS)
is a software system that provides the functionality that is
normally required of a database management system, but
whose underlying model is an OODM, rather than, for example, the relational data model.
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Applications
• Object oriented database management systems were developed (largely) in response to engineering needs.
• Applications include computer aided design, and integrated
software development environments (IDEs).
• Relational database systems were developed (largely) in response to commercial needs,
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Requirements on Commercial Databases
• information can largely be described before the database is
implemented
• infrequent schema update, controlled by DBA
• atomic, fixed-length data
• few entity types, with many instances
• large initial data load, with slow constant growth thereafter
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• single value for each data item; updates in situ
• short transactions can be used as a basis for concurrency
control
Requirements on Engineering Databases
• continuous changes to the schema, made by different users
• large, complex, variable length data items
• many entity types, few instances, complex relationships
• small initial data load, rapid early growth, slowing after design
• need for configuration and version management
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• long transactions, periods of inconsistency, problems with
concurrency control
Advantages of Object Oriented compared with Relational
Databases
• Object model intuitively closer to real world
• Extensibility – inheritance
• Complex values
• Removal of impedance mismatch
• More expressive query language
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• Tight coupling between data and applications allows schema
to capture more of the meaning of applications
• Support for long transactions
• Better support for applications like software engineering or
computer aided design (CAD)
• Arguably better performance, though benchmarks have mainly
been applied in areas like engineering support to which OODBMS
are better suited.
Disadvantages of Object Oriented compared with Relational Databases
• Lack of a theoretical foundation, so exact meaning of OODM
is not well defined (though this is mitigated by the ODMG
model)
• More difficult to get staff experienced with OODBMSs
• Lack of standards (mitigated by ODMG’s OQL)
• Competition from relational and object-relational DBMS’s
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• Encapsulation compromised to optimize queries (but in RDBMS’s,
normalization may be compromised to improve performance).
• OODBMS’s usually control concurrency by locking; locking
an inheritance hierarchy is difficult and may affect performance.
• OODM is inherently more complex than relational data model;
OODBMS provides more complex functionality than RDBMS;
complexity leads to higher implementation and maintenance
costs.
• Lack of views; but are views necessary with an object model?
• OODBMS’s usually provide for coarse-grained access control; finer grained security mechanisms are needed for most
commercial applications.
Trends
• Some OODBMS have emerged and seem likely to continue
in use (e.g. ObjectStore)
• The Object Data Management Group have proposed a standard covering an obect data model, an object data definition
language (ODL) an object query language (OQL) an an object manipulation language (OML)
• Currently, though the object relational data model seems to
be more widely used
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• and for the future, it looks as though some form of e.g.
Java persistence will become the norm for persistent object
oriented data.