Object Databases

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Object Databases

• February 2007

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Contents

• Data Model History (Revisited)
• Semantic Data Models
• Functional Data Model and DAPLEX
• Object Oriented Databases
• Origins, Strategies and Issues
• Object-Oriented Database System Manifesto
• Object Database Standard
• JDO
• P/FDM

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Introduction

• Relational Model is the most popular data model
  used in industry
• But it has certain failings due mainly to its
  limited modelling capabilities
• Many models appeared which attempted to
  represent the real world more closely
• These are known as semantic data models
• Can be seen as the basis of object data models
  which we discuss later in this session

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Semantic Data Models

• First semantic model was the E-R model by Chen
  in 1976
• However, this is not a true data model
• Why not?
• Data model structure rules manipulation
• Now a widely accepted modelling tool for
  relational database design
• But not a data model

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Semantic Data Models

• In 1979, Codd devised RM/T
• an extended version of the relational model
• no distinction between entities and relationships
• varying types of relations (one-to-many and
  subtyping)
• E-attributes, E-relations, P-relations
• Supports associations (e.g. many-to-many
  relationships) and subtyping
• Nested Relational Model
• Originally proposed by Roth as NF2 (Roth, 1988)
• Non first-normal form relations
• Requires two new operations added to relational
  algebra
• NEST and UNNEST

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Semantic Data Models

• Other semantic data models
• Logical Data Model (Kuper and Vardi, 1985, 1997)
• Semantic Data Model (Hammer and McLeod, 1981)
• Semantic Association Model (Su, 1983)
• Functional Data Model (Kerschberg and Pacheco,
  1976, Shipman, 1981)
• We will briefly look at Shipmans model

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The Functional Data Model

• Originally defined by Kerschberg and Pacheco 1976
• Most common version devised by Shipman in 1981
• Defined functional data model with manipulation
  language DAPLEX
• No original implementation, but used in research
  projects
• ADAPLEX language based on ADA
• EFDM by Atkinson et. al. in Glasgow
• P/FDM by Gray, et. al. in Aberdeen
• Iris Object-Oriented Database

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Introduction to DAPLEX

• DAPLEX is a database language which
  incorporates1
• A formulation of data in terms of entities
• A functional representation for both actual and
  virtual (derived) data relationships
• A rich collection of language constructs for
  expressing entity selection criteria
• A notion of subtype/supertype relationships among
  entity types

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Quote!

• The DAPLEX language is an attempt to provide a
  database system interface which allows the user
  to more directly model the way he thinks about
  the problems he is trying to solve.

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Object Data Models

• Object models are attempts to provide models
  which more closely model the real world
• An extension of semantic models but using more
  common object-oriented concepts
• Developed to address the increasing complexity of
  database applications, e.g. CAD/CAM
• DAPLEX could be seen as a query language for
  object databases
• Due to its navigational querying abilities
• And its support for complex objects and
  inheritance
• Object-oriented database system Iris uses form of
  DAPLEX as query language

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Origins of OO Data Model

• Traditional Database Systems (e.g. relational)
• persistence, sharing, transactions, concurrency
  control, recovery control, security, integrity,
  querying
• Semantic Data Models
• generalisation, aggregation, navigational
  querying
• Object-Oriented Programming
• object identity, encapsulation, inheritance,
  types and classes, methods, complex objects,
  polymorphism, extensibility
• Special Requirements
• versioning, schema evolution

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Persistent Programming Languages

• Definition
• a language that provides its users with the
  ability to (transparently) preserve data across
  successive executions of a program, and even
  allows such data to be used by many different
  programs
• Database Programming Language
• a language that integrates some ideas from the
  database programming model with traditional
  programming language features

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Database Programming Languages

• A significant influence on OODB development
• PPL independent of any program
• DBPL incorporates features such as transaction
  management, concurrency control, recovery
• SQL2 specifies that SQL can be embedded in a host
  language - impedance mismatch
• PPL research has had a significant influence on
  OODBMS
• eliminate impedance mismatch and maintain the
  same data representation in primary and secondary
  storage

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OODBMS Development Strategies

• Various approaches
• Extend an existing OO-PL with database
  capabilities
• Provide extensible OO DBMS libraries
• Embed OO database language constructs in a
  conventional host language
• Extend an existing database language with OO
  capabilities
• We will discuss SQL-3 later in the module
• Develop a novel data model/data language

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OODB Issues

• Transactions
• transactions on complex objects can be lengthy
• need to support long duration transactions
• unit of concurrency and recovery control is
  generally an object
• normally use locking-based protocols, but
  unacceptable if long duration transaction aborts
  due to a lock conflict

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OODB Issues (2)

• Versions
• many applications may need to access a previous
  state of an object
• version management is the process of maintaining
  the evolution of objects
• object version represents an identifiable state
  of an object
• version history represents the evolution of an
  object

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OODB Issues (3)

• Schema Evolution
• dynamically defining and modifying the database
  schema
• Changes to schema must not leave it in an
  inconsistent state
• Typical changes to schema
• changes to class definition - modifying
  attributes, modifying methods
• changes to the inheritance hierarchy, such as
  making a class a superset of another, removing a
  class from a list of superclasses, modifying
  superclass order
• changes to the set of classes, such as creating
  and deleting classes and modifying class names

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OODB Issues (4)

• Architecture
• 3 basic client-server architectures
• object server
• page server
• database server
• Methods - two approaches to handling methods
• store the methods in external files
• store the methods in the database

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Object Oriented Database System Manifesto (1989)

• Atkinson devised 13 mandatory features for an
  OODBMS
• based on two criteria
• should be an OO system
• should be a DBMS

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Object-Oriented Database System Manifesto

• OO Criteria
• 1. Complex objects must be supported
• 2. Object identity must be supported
• 3. Encapsulation must be supported
• 4. Types or classes must be supported
• 5. Types or classes must be able to inherit from
  their ancestors
• 6. Dynamic binding must be supported
• 7. The DML must be computationally complete
• 8. The set of data types must be extensible

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Object-Oriented Database System Manifesto

• DBMS Criteria
• 9. Data persistence must be provided
• 10. The DBMS must be capable of managing very
  large databases
• 11. The DBMS must support concurrent users
• 12. The DBMS must be capable of recovery from
  hardware and software
• 13. The DBMS must provide a simple way of
  querying data

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OODB Advantages

• Enriched modelling capabilities
• Extensibility
• Removal of impedance mismatch
• More expressive query language
• Support for schema evolution
• Support for long duration transactions
• Applicability to advanced database applications
• improved performance

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OODB Disadvantages

• Lack of universal data model
• Lack of experience
• Lack of standards
• Query optimisation compromises encapsulation
• Locking at object level may impact performance
• Complexity
• Lack of support for views
• Lack of support for security

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And most importantly

• What about integrity?

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Object Database Standard

• Standard for Object-Oriented Data Model proposed
  by Object Data Management Group
• ODMG Object model is a superset of OMG object
  model
• Consists of
• Object model (OM)
• Object Definition Language (ODL)
• Object Interchange Format (OIF)
• Object Query Language (OQL)
• Language bindings C, Smalltalk, Java

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Object Model

• Basic Constructs
• object
• literals
• both characterised by types
• objects
• attributes
• relationships
• operations

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Types

• Interface Definition
• defines abstract behaviour of object type
• e.g. interface Employee..
• Class
• defines abstract behaviour and abstract state
• extended interface with information for ODMS
  schema definition
• objects are class instances
• e.g. class Person..
• Literal
• abstract state of a literal type
• e.g. struct Complex float ie, float im

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Types (cont)

• Inheritance
• applied to both interfaces and classes
• inheritance of behaviour between object types
• Extend
• applied to object types only
• inheritance of state and behaviour
• Extent
• set of all instances of a class
• extension
• must have an unique key

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Objects

• Instances of a class
• Have an unique object identifier
• remains for lifetime of object
• Names
• equivalent to global variables
• Lifetime can be
• transient
• persistent
• type and lifetime are independent

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Objects

• Collections
• Set, Bag, List, Array
• Dictionary - sequenced key-value pairs
• Structured objects
• Date, Interval, Time, Timestamp
• Literals
• Atomic, Collection, Structured

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Example ODL Schema

• class Student
• (extent students)
• attribute short id
• attribute string name
• attribute string address
• attribute date dob
• relationship setltModulegt takes
• inverse Module takenby
• short age()

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Example ODL Schema

• class Module
• (extent modules)
• attribute string title
• attribute short semester
• relationship setltStudentgt takenby
• inverse Student takes
• class Postgrad extends Student
• (extent postgrads)
• attribute string thesis_title

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Object Interchange Format

• Used to dump/load current state of ODBMS to/from
  a set of files
• e.g. Sarah PersonName Sarah,
• PersonAddressStreet Willow Lane,
• City Durham,
• Phone CountryCode 44,
• AreaCode 191,
• PersonCode 1234

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Object Query Language

• Similar to SQL92
• extensions complex objects, object identity,
  path expressions, polymorphism, operation
  invocation, late binding
• e.g. select distinct x.age
• from Persons x
• where x.name Pat
• Return literal of type setltstructgt
• select distinct struct(a x.age, s x.sex)
• from Persons x
• where x.name Pat

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OQL Examples

• Path Expressions
• select c.address
• from Persons p, p.children c
• where p.address.street Main Street
• and count(p.children) gt 2
• and c.address.city ! p.address.city
• Methods
• select max(select c.age from p.children c)
• from Persons p
• where p.name Paul

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OQL Polymorphism Examples

• Late Binding
• select p.activities
• from Persons p
• Class Indication
• select ((Student)p).grade
• from Persons p
• where course of study in p.activities

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Java Data Objects

• ODMG disbanded in 2001
• ODMG Java Data Binding superceded by JDO
• JDO
•   Provides transparent persistence
•   Scales from embedded to enterprise
•   Integrates with EJB and J2EE
•   Is being widely adopted in the database
  industry
• More information at www.odmg.org

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P/FDM

• An implementation of Shipmans functional data
  model and query language DAPLEX
• Built on top of the Prolog programming language
• Supports object-oriented extensions
• e.g. inheritance, methods, complex objects
• We will be using it as an object-oriented DBMS in
  the module

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Comparison

• class Student
• (extent students)
• attribute short id
• attribute string name
• attribute string address
• attribute date dob
• attribute Lecturer tutor
• relationship setltModulegt takes
• inverse Module takenby
• short age()
• select s.address, t.tutor_name
• from students s, s.tutor t
• where s.name Fred

• declare student -gt entity
• declare id(student) -gt integer
• declare name(student) -gt string
• declare address(student) -gt string
• declare dob(student) -gt string
• declare tutor(student) -gt lecturer
• declare takes(student) -gtgt module
• define age(student) -gt
• for each s in student
• such that name(s) Fred
• print(address(s), tutor_name(tutor(s)))

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Summary

• Object-oriented databases used for niche
  applications in the market place
• They are an object-oriented extension of semantic
  databases
• Atkinson et al produced a manifesto to try to
  state what concepts are necessary for an OODBMS
• ODMG devised a standard but in some ways it was
  too late because most OODBMS had been implemented
  by then

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References

• Shipman, The Functional Data Model and the Data
  Language DAPLEX, ACM TODS 6(1) 1981
• Connolly Begg, chapters 21 and 22
• Date 7th ed., chapter on Object-Oriented
  databases
• Atkinson et al, Object-Oriented Database System
  Manifesto, Proc. 1st Intl Conference on DOOD,
  Japan, 1989.
• Cattell, et. al., The Object Data Standard
  ODMG3.0