OBJECTS

Object Oriented ???????

OBJECTS

Does the relational model meet the needs of the 21st Century?Can the world be modeled better through objects?What are objects?Where does object oriented ??? fit into the IS world?Should I get rid of all my tables and convert?

Object-Oriented OO convenient label for a collection of

interconnected ideas OO approach views computer system

as a collection of interacting objects Objects viewed as things

– things interact– people interact with things

Need to know what an object does Component approach

Object-oriented Approach Object-oriented analysis

– defining all of the types of objects that are part of a users work environment

Object-oriented design– define all the types of objects in computer system

and how they interact– UML

Object-oriented programming– programmers write statements that define types of

objects– C++, JAVA, Small Talk

Object-oriented Approach con’t

Object databases - ultimate management system for multimedia, documents and other info.

Object brokers (Agents) - in distributed systems, allow objects to dynamically discover each other and interact across machines and through the web

Object frameworks - provide flexible, customizable, prefabricated software subsystems.

OO?? Object Orientation - a set of design and development

principles based on the idea of conceptually autonomous structures. Each autonomous structure represents a real world user's entity with the ability to interact with itself and other objects.

Object - an abstract representation of a real world entity that has a unique identity, embedded properties, and the ability to interact with other objects and itself.

Class - collection of similar objects with shared attributes and methods.

Core Model

Class– Object belongs to only one class– is an instance of the class– provides the basis for queries

Class Hierarchy and Inheritance– single inheritance -hierarchy– multiple inheritance- rooted and connected

directed acyclic graph (DAG)

Core Model

Object Identifier– unique– system-wide

Attributes– value of attribute also object– domain is a class of attribute values

Encapsulation and Message Passing

Object-oriented concepts Abstraction, encapsulation,

information hiding. Objects and attributes. Object identity. Methods and messages. Classes, subclasses, superclasses,

and inheritance. Overloading. Polymorphism and dynamic binding.

OODBMS

Object OrientedFeatures

ConventionalDBMS Features

OODBMS

Comparison between Rel DB Obj DB

relations which are sets of tuples

Tuple components primitive types

Persistence Backup & Recovery Transactions Concurrency Security & Integrity Administration

classes which are sets of objects

Object components complex types

Inheritance hierarchy Methods Methods integral part

of class and inherited DML and host

language can be the same

Weaknesses of RDBMSs

Poor Representation of "Real World" Entities– Normalization leads to relations that do

not correspond to entities in "real world".

Semantic Overloading– Relational model has only one construct

for representing data and data relationships: the relation.

– Relational model is semantically overloaded.

Weaknesses of RDBMSs

Poor Support for Integrity and Enterprise Constraints

Homogeneous Data Structure– Relational model assumes both

horizontal and vertical homogeneity. – Many RDBMSs now allow Binary Large

Objects (BLOBs).

Weaknesses of RDBMSs

Limited Operations– RDBMs only have a fixed set of

operations which cannot be extended.

Difficulty Handling Recursive Queries– Extremely difficult to produce

recursive queries.– Extension proposed to relational

algebra to handle this type of query is unary transitive (recursive) closure, operation.

Weaknesses of RDBMSs

Limited Operations– RDBMs only have a fixed set of

operations which cannot be extended.

Difficulty Handling Recursive Queries– Extremely difficult to produce recursive

queries.– Extension proposed to relational algebra

to handle this type of query is unary transitive (recursive) closure, operation.

Weaknesses of RDBMSs

Other Problems with RDBMSs– Transactions are generally short-lived

and concurrency control protocols not suited for long-lived transactions.

– Schema changes are difficult. – RDBMSs are poor at navigational

access.

BENEFITS OF OODBMS More semantic informatiom Multimedia, CAD, CAM User defined data types Versioning Class Reusability Faster development (inheritance,

reusability) Portability

DRAWBACKS TO OODBMS Still developing Lack of standards Is there a sufficient theoretical foundation? Is this a throwback to hierarchical systems No ad hoc query language No set of tools Steep learning curve

CURRENT DATABASE FEATURES Language

– DDL– DML– DCL

Concurrency Control Recovery Utilities

13 Rules for an OODBMSfrom the OODBS Manifesto

System must support complex objects Object identifier must be supported Objects must be encapsulated Systems must support types or classes System must support inheritance System must avoid premature binding System must be computationally complete

LAST 6 RULES System must be extensible System must be able to remember data

locations System must be able to handle very large

databases System must support concurrent users System must be able to recover from

hardware and software Data query must be simple

Key Tensions Two data models needed

– object model (data & programs) vs. relational model (data & constraints)

– Must one model dominate? Are they built simultaneously?

– relational model needs some object features Impedance mismatch

– object-at-a-time navigation vs. set-at-a-time queries– programmatic vs. declarative integrity enforcement

Concurrency discordance– single image vs. shared database mindset

Interoperability– what other applications must use database?

Commercial OODBMSs GemStone from Gemstone Systems Inc., Itasca from Ibex Knowledge Systems SA, Objectivity/DB from Objectivity Inc., ObjectStore from eXcelon Corp., Ontos from Ontos Inc., Poet from Poet Software Corp., Jasmine from Computer

Associates/Fujitsu, Versant from Versant Object Technology.

NEXT GENERATION DB SYSTEMS NEED THE ABILITY TO:

represent and manipulate complex nested objects store and retrieve long data define and manipulate arbitrary data types represent and manage changes over time

(versions) represent semantic modeling concepts specify rules and extended constraints manage long-duration cooperative transactions

Relationships

Relationships represented using reference attributes, typically implemented using OIDs.

Consider how to represent following binary relationships according to their cardinality:– 1:1– 1:*– *:*.

1:1 Relationship Between Objects A and B

Add reference attribute to A and, to maintain referential integrity, reference attribute to B.

1:* Relationship Between Objects A and B

Add reference attribute to B and attribute containing set of references to A.

*:* Relationship Between Objects A and B

Add attribute containing set of references to each object.

For relational database design, would decompose *:N into two 1:* relationships linked by intermediate entity. Can also represent this model in an ODBMS.

*:* Relationships

Alternative Design for *:* Relationships