Transcript Data Independence - Dr Chaamwe

MIS
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CA – 40%
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Assignment
Test
Exam – 60%
Lecture slides
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www.chaamwe.weebly.com
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Taught Courses
Dr N Chaamwe
Dr N Chaamwe
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Examples of DBSs:
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Purchases from the supermarket
Purchases using your credit card
Booking a holiday at the travel agents
Using the local library
Taking out insurance
Using the Internet
Studying at university
Dr N Chaamwe
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File-based Systems
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Collection of application programs that
perform
services for the end users (e.g. reports).
Each program defines and manages its
own data.
Dr N Chaamwe
Dr N Chaamwe
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Separation and isolation of data
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Each program maintains its own set of data.
 Users of one program may be unaware of
potentially useful data held by other programs.
Duplication of data
 Same data is held by different programs.
 Wasted space and potentially different values
and/or different formats for the same item
Dr N Chaamwe
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Data dependence
 File structure is defined in the program code.
Incompatible file formats
 Programs are written in different languages, and
so cannot easily access each others files.
Fixed Queries/Proliferation of application programs
 Programs are written to satisfy particular
functions.
 Any new requirement needs a new program.
Dr N Chaamwe
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Arose because:
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Definition of data was embedded in
application programs, rather than being
stored separately and independently.
No control over access and manipulation of
data beyond that imposed by application
programs.
Result
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the database and Database Management
System (DBMS).Dr N Chaamwe
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DB:
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Shared collection of logically related data (and a
description of this data), designed to meet the
information needs of an organization.
System catalog (metadata) provides description of
data to enable program–data independence.
Logically related data comprises entities,
attributes, and relationships of an organization's
information.
Dr N Chaamwe
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DBMS:
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A software system that enables users to
define, create, and maintain the database
and which provides controlled access to
this database.
Dr N Chaamwe
Dr N Chaamwe
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Data definition language (DDL).
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Permits specification of data types,
structures and any data constraints. All
specifications are stored in the database.
Provides basic data manipulation
operations on data held in the database.
Dr N Chaamwe
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Data manipulation language (DML).
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General enquiry facility (query language) of
the data.
Procedural DML
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allows user to tell system exactly how to
manipulate data.
Non-Procedural DML
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allows user to state what data is needed rather
than how it is to be retrieved.
Dr N Chaamwe
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Data Control language (DCL).
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General administration & security facility
(same syntax as query language) used
mainly by the DBA.
Dr N Chaamwe
Views
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A view mechanism.
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Provides users with only the data they want or
need to use.
Allows each user to have his or
her own
view of the database.
A view is essentially some subset of the
database.
Dr N Chaamwe
Views Benefits include:
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Reduce complexity;
Provide a level of security;
Provide a mechanism to customize the
appearance of the database;
Present a consistent, unchanging picture of
the structure of the database, even if the
underlying database is changed
Dr N Chaamwe
Three-level schema
architecture
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ANSI-SPARC architecture is named
after the committee that proposed
it, the American National Standard
Institute, Standards Planning And
Requirements Committee.
Dr N Chaamwe
Objectives of Three-Level
Architecture
• All users should be able to access same data.
• A user's view is immune to changes made in
other views.
• Users should not need to know physical
database storage details.
Dr N Chaamwe
Objectives of Three-Level
Architecture
DBA should be able to change database
storage structures without affecting the users'
views.
• Internal structure of database should be
unaffected by changes to physical aspects of
storage.
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Dr N Chaamwe
Objectives of Three-Level
Architecture
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DBA should be able to change
conceptual structure of database
without affecting all users.
Dr N Chaamwe
Dr N Chaamwe
The External level
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The external level (also known as the user
logical level) is the one closest to the
users.
It is the level concerned with the way the
data is seen by individual users.
Dr N Chaamwe
The External level
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The external schema is used to
describe the external level.
It contains a description of a
portion of the database that is of
concern to the specific user.
Dr N Chaamwe
The External level
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The external view is described in
terms of external records, which
may be different form the actual
stored records.
The view may have a different
representation of the same data
Dr N Chaamwe
The Conceptual level
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The level represents the community
view of the database as seen by the
database administrator.
It is also known as the community
logical level or even sometimes just the
logical level.
Dr N Chaamwe
The Conceptual level
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The conceptual schema is used to
describe what data is stored in the
database and the relationships
among the data.
The description includes the
structure and constraints for the
whole database
Dr N Chaamwe
The Conceptual level
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the conceptual schema defines the
logical structure of all data in the
database.
The conceptual schema is defined by a
Data Definition Language (DDL).
There is only one conceptual schema
for the database.
Dr N Chaamwe
The Internal level
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It is the description of the
implementation of the conceptual
schema by means of physical
storage structures.
It summarizes how the data are
stored on secondary storage
devices such as disks and tapes.
Dr N Chaamwe
The Internal level
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However, it is still one level above the
actual physical storage, which is usually
managed by the operating system.
The internal level does not deal with
the physical records but it deals with
the internal records (stored records).
Dr N Chaamwe
The Internal level
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Details of how the address space is
mapped to physical storage are
highly system-specific (e.g. a block
or a page)
Dr N Chaamwe
Dr N Chaamwe
Dr N Chaamwe
Logical Data
Independence
Refers to immunity of external
schemas to changes in conceptual
schema.
– Conceptual schema changes (e.g.
addition/removal of entities).
– Should not require changes to
external schema or rewrites of
application programs.
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Dr N Chaamwe
Physical Data
Independence
– Refers to immunity of conceptual
schema to changes in the internal
schema.
– Internal schema changes (e.g.
using different file organizations,
storage structures/devices).
– Should not require change to
conceptual or external schemas.
Dr N Chaamwe
Data Independence
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Describes the immunity of the
upper levels from the changes in
the lower levels
Dr N Chaamwe
Mappings
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The process of transforming requests
and results between levels are called
mappings.
The three-level architecture involves
certain mappings— one
conceptual/internal mapping and
several external/conceptual mappings.
Dr N Chaamwe
The conceptual/internal
mapping
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defines the correspondence
between the conceptual view and
the stored databases
If a change is made to the storage
definition, then the
conceptual/internal mapping must
be changed accordingly, so that the
conceptual schema can remain
Dr N Chaamwe
The external/conceptual
mapping
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defines the correspondence
between a particular external view
and the conceptual view.
If a change is made to the
conceptual schema definition, then
the External/conceptual mapping
must be changed accordingly, so
that the External Schema can
Dr N Chaamwe
Functions of a DBMS
• Data Storage, Retrieval, and
Update.
• A User-Accessible Catalog.
• Transaction Support.
• Concurrency Control Services.
• Recovery Services.
Dr N Chaamwe
Functions of a DBMS
• Authorization Services.
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Support for Data Communication.
Integrity Services.
Services to Promote Data Independence.
Utility Services.
Dr N Chaamwe
Roles in the Database
Environment
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Data Administrator (DA)
Database Administrator (DBA)
Database Designers (Logical and Physical)
Application Programmers End Users (naive
and sophisticated)
Dr N Chaamwe
Advantages of DBMS
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Control of data redundancy
Data consistency
More information from the same
amount of data.
Sharing of data
Dr N Chaamwe
Advantages of DBMS
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Improved data integrity
Improved security
Enforcement of standards
Economy of scale
Improved data accessibility and
responsiveness
Dr N Chaamwe
Advantages of DBMS
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Balanced conflicting requirements
Increased productivity
Improved maintenance through data
independence
Increased concurrency
Improved backup and recovery services
Dr N Chaamwe
Disadvantages of DBMS
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Complexity
Size
Cost of DBMS
Additional hardware costs
Cost of conversion
Performance
Higher impact of a failure
Dr N Chaamwe
When a DBMS may be
unnecessary
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If the database and applications are
simple, well defined, and not expected
to change
If there are stringent real-time
requirements that may not be met
because of DBMS overhead.
If access to data by multiple users is
not required.
Dr N Chaamwe
Information Resource
Dictionary System (IRDS)
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Also called as a Data Dictionary or
System Catalog
A repository of information (meta-data)
describing the data in the database.
Dr N Chaamwe
Information Resource
Dictionary System (IRDS)
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It is a tool which enables one to control
and manage the information about the
data in the design, implementation and
expansion phases of a DB
Dr N Chaamwe
Information Resource
Dictionary System (IRDS)
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Typically stores:
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Names of authorized users.
It is used by modules such as:
Authorization Control.
Integrity Checker.
Dr N Chaamwe
Information Resource
Dictionary System (IRDS)
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Who benefits from the use of DDS?
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DB administrators
Data administrators
Application personnel
Operating staff
End users
Data auditors.
Dr N Chaamwe
Lifecycle of Database system
development
Lifecycle of Database system
development
Dr N Chaamwe
Lifecycle of Database system
development
Dr N Chaamwe
Lifecycle of Database system
development
Dr N Chaamwe
Planning phase
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Prior to the first phase of our
requirement’s elicitation, a plan
must be put forward showing each
stage of the life cycle and how it
can be performed efficiently and
effectively.
Dr N Chaamwe
System definition phase,
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This is where the scope and
boundaries of the database
application, which includes the
major application areas and user
groups, are defined.
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Dr N Chaamwe
Requirement collection and
analysis Phase
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It is concerned with collecting the
data and analyzing the user
requirements in order to build the
database application.
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Conceptual model.
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Create a conceptual schema (HighLevel), e..g. ERD
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Logical Design
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Map conceptual database schema
to logical database schema
(Representational), it is also called
an Implementation model e.g.
Relational
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Physical Design
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Internal storage structures and file
organisations are specified
(Physical), e.g. B-tree
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The prototype
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This is a working model of the
database application, which allows
the designers or users to visualize
and evaluate how the final system
will look and function.
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Implementation
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The phase involves the creation of
the external, conceptual and
internal
definitions
and
the
application programs.
Dr N Chaamwe
Data Conversion and Loading
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If we have a legacy system at this
stage, then we need to transfer
any existing data into the new
database and converting any
existing application.
Dr N Chaamwe
The testing phase
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Validating
user
requirements
against the application programs
then tests the system. The testing
phase aims at executing the
application programs with intention
to find errors
Dr N Chaamwe
Operational maintenance
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This process involves monitoring &
maintaining the system.
Dr N Chaamwe