Designing Information Systems

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					 CP2236: Information Systems Design


Information Systems Development (ISD)

   Systems Development Life Cycle
     Overview of Analysis Phase
      Overview of Design Phase

                                      1
  Information Systems Development
               (ISD)
A logical series of development phases
followed in an organisation to conduct all
the steps necessary to analyse, design,
implement & maintain information systems.

These logical series of development phases
- known as System Development Life Cycle
(SDLC).                                      2
Systems Development Life Cycle (SDLC)
         Scope & objectives
                   Feasibility
                       Analysis            Time
•Waterfall Model        Design
– Proven approach         Implement
                               Maintain
– Lacks flexibility
                                  Review
– Time lag
– Ensures points of review
                                                  3
          Alternative Approaches (1)
• Waterfall SDLC with iterations:
Scope & objectives

           Feasibility

                Analysis
                         Design
                            Implement
                                   Maintain
                                              Review   4
         Alternative Approaches (2)
• Beer mat and hack.

• Prototyping / Rapid Application Development.
  –   Uses a different SDLC
  –   System developed in smaller units
  –   Iterative                     Analysis
  –   Less time lag
  –   User participation    Review           Design

                                    Code
                                                      5
         Alternative Approaches (4)
Spiral model includes best features of both the classic
Waterfall SDLC and the Prototyping approach.


    Planning
                                            Risk Analysis




  Evaluation                              Development


                                                            6
         Alternative Approaches (5)
Each spiral consists of four main activities:

• Planning: setting project objectives; defining
  alternatives; further planning on the next spiral; etc.

• Risk Analysis: analysis of alternatives & the
  identification & solution of risks.

• Development: designing, coding and testing etc. in
  increments.

• Evaluation: user evaluation of each spiral and then the
  final product.
                                                            7
         SDLC Stages and Deliverables (1)
Scope & Objectives     Terms of Reference

                       Feasibility Report
Feasibility

                       Requirement specification,
Analysis               Physical & Logical models

                       Logical models of required
Design                 system, physical design and
                       detailed design documentation
                                                    8
      SDLC Stages and Deliverables (2)
                    Code - program
Implementation:     documentation
Code, Test &        Test - Test plans from unit to
Install             system & acceptance testing
                    Install - system changeover,
                    installation of hw/sw, user &
                    system documentation and
                    training
Maintain            Maintenance procedures

Review              Evaluation report
                                                     9
        Analysis - Overview (1)
Scope & Objectives         PRL      RC
    Feasibility            current system’s
                            DFDs
       Analysis             LDS
          Design            I/Os
Time        Implement       Process desc.
                Maintain
                  Review      Requirements
                              Specification
                              of req.System10
          Analysis - Overview (2)

PHYSICAL ANALYSIS
                                              Current
                                              Physical
      Problem                                  DFDs
 Requirements List
  (Requirements
    Catalogue)                   Logicalisation


      Current                                Current
      System         Data Store Entity       Logical
       LDS            Cross Reference         DFDs


LOGICAL ANALYSIS


                                                         11
         Analysis - Overview (3)
• Analysis - gathering & documentation of information
  about the current system & the required system.

• To achieve this - a number of techniques are used.
   – Problem Requirement List feeds into Requirements
     Catalogue: contain information about current
     problems & future requirements
   (logical: functional & physical: non-functional)

   – Data Flow Diagrams (Processing): contain
    information about the processing, inputs and
    outputs and data stores to be accessed.             12
          Analysis - Overview (4)
– Logical Data Structures (Data): contain
  information about the groups of data (entities) and
  their relationships.
– Input\Output Descriptions: contain information
  about data that flows into and out of the system.
– Process Descriptions: contain information about
  how the processes actually work & these can be
  specified using various techniques
  (e.g. Structured English, Decision Tables &
  Decision Trees).
                                                        13
      Analysis - Physical & Logical
• Physical analysis includes all physical aspects
  – e.g. Filing cabinets, Locations, (Who, When,
    Where, etc).


• Physical analysis become logical when all the
  physical aspects are removed by logicalisation.
  – Therefore there are 2 versions of the DFDs current
    physical and current logical.


                                                         14
     Analysis - Data & Processing
• DFDs model information about the processing
  in the current system.

• LDS models information about the data in the
  current system.

• Data & Processing are two separate but
  essential components of an Information System.

                                                 15
         Analysis - Techniques
• The techniques used to perform analysis are
  taken from Structured Systems Analysis and
  Design Methodology (SSADM).

• There are other techniques and ways of
  combining them in order to perform analysis.




                                                 16
          Design - Overview (1)
Scope & Objectives
       Feasibility
           Analysis
                 Design
   Time              Implement
                        Maintain
                            Review   17
         Design - Overview (2)
PRL         RC        Requirements
current system’s      Specification
 DFDs                 of req. System
 LDS
 I/Os
 Process desc.         Design

                          System
                      Specification
                      of req. System   18
   Design - Overview (3)

        Design


Required          Required
 Logical          Physical
 Design            Design    19
         Design - Overview (4)

Logical Design: gives a detailed logical
definition of the required system.

Physical Design: takes the products of the
logical design and produces specifications
for the target hardware and software.


                                           20
           Design - Overview (5)
Logical Design:
•No reference made to any technical
 environment.
•Capable of implementation on a variety of
 platforms.
•Act as a model of how the system will
 satisfy the user requirements.
•Reflect underlying business rules &
 activities rather than physical constraints.
                                                21
          Design - Overview (6)
Logical Design - blueprint is the delivered
product of the required system.
It includes the following:
 • Requirements Catalogue
 • Required system DFDs
 • Required system LDS
 • Relational Data Analysis (normalised set
    of tables and rationalised set of tables)

                                                22
          Design - Overview (7)
Physical Design:
•Logical design translated into a physical
 design based on the technical environment
 selected, e.g. platform, programming
 language, database etc.
•Produces program specifications, physical
 files or database definitions and user
 interfaces for a selected target hardware &
 software.
                                               23
           Design - Overview (8)
Physical design includes contents of the
following components;

  • Data design - (requirements for data to be
    stored in a database)
  • Outputs design - (reports, displays)
  • Inputs design - (forms, screens, dialogues)
  • Programs design - (structure of programs to
    collect, transform & output data)
                                              24
         Design Overview (9)

                    Requirements Catalogue

                                    I/O
                 Data Design
DESIGN                           Descriptions

                Interface      Program Design
                Design

                System
                               Documentation:
                changeover
                               System & User
                strategies
                                                25
   Physical and Logical Models
Analysis                  Design
 Current               Required
 Logical               Logical



Current                Required
Physical               Physical
                                   26

				
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