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