IB Computer ScienceSection 1: Systems life cycle and software development

The systems life cycle

Analysis

Design

Implementation

Operation & Maintenance

StagesThe guide gives these five as the stages of the software life cycle

Analysis: Data collection, interviews, etc, user requirement, feasibility report

Design: Data structures, algorithms, files. Hardware requirements. Data flow and object model.

Implementation: Also called installation. Direct changeover

Phased introduction

Parallel running

Operation: Detailed planning using GANTT and PERT charts

Maintenance: Bug fixing

But I have also seen these on past exam papers

Validation

Documentation

Analysis: Data collection methods

Analysis: Requirements Specification

Defining what the client wants Inputs: what data/information will

the system require Outputs: what information is

expected from the system Human resources requirements Schedule

Critical success factors. Key objectives.

GANTT and PERT charts

GANTT

List of activities

Order in which they are to be done

Total time required

PERT

Module diagram

Dependencies between modules

Analysis: IPO

Input Processing Output

What data or

information will the system need?

What will the system do to

the data? How will

information be stored and represented?

What information should the

system give its users?

Three stages of programming:

Analysis: Feasibility Report

Feasibility means “Can it be done?”

Brief description of the proposed system

Estimated costs Financial, technical, legal

responsibility Estimated completion date

Analysis: Systems Flowcharting

Tape Storag

e

Disk Storag

e

Document

Process

Input/ Output

Manual Input

Documents

Analysis: Pay System Example

Master FileTransaction

File

Payslips

Calculate Pay

Hours worked

Updated Master

File

(Inputs)

(Processing)

(Outputs)

Analysis: Systems FlowchartPast Paper Question

Analysis: Systems FlowchartPast Paper Answer

Customer orders are collected on paper, keyed in, and stored in a customer orders file. A stock master file is searched to determine whether sufficient stock is available, and a report produced.

The mark scheme awarded one mark for each of the boxes, up to a maximum of five boxes. In my opinion you could have left out the keyboard input, or you could have specified the master file as tape drive (sequential access storage), and you would still get full marks.

Analysis: Review

Why is data collection important? What are the methods of data collection?

What is a requirements specification? What does it contain and what purpose does it serve?

Outline the features of a feasibility report.

Annotate the systems flowchart you created in the past paper exercise. Try to think of another computerised process you could model with a systems flowchart. Discuss with a colleague and prepare the flowchart.

Resources: Computer Science Java Enabled, IB Computing website, Richard Jones’ site (Int. Sch. Toulouse).

Design Stage

Forms (data capture) How the data will get in to the system

Classes, Data structures and Input Files How the data will be represented and stored

Algorithms How the data will be processed

Hardware The components required

Reports, Lists, Output Files What will the output of the system be?

Systems Flowchart Showing the whole system

Other concepts

Modularity: Breaking down the software to make it easier to understand. Can be done in several ways. Classes, input-related, output-related, processing-related.

Prototyping: Quickly building a partially-functioning version of the system with a view to getting constructive feedback from the user to help clarify the requirements.

CASE tools and IDEs: Computer Aided Software Engineering and Integrated Development Environment. Provide useful tools such as debugging, code-highlighting, entity-relationship diagrams, automatic instance variable encapsulation, etc.

Testing

Imagine you have a textbox that should only accept values from 0-100.

Normal data: Data that the system should expect, ie that is well within the normal range. Eg 23, 56, 89, etc

Extreme data: Data at the boundaries of what is acceptable, eg -1, 0, 1, 99, 100, 101

Abnormal data: Data that is outside the normal range of expected data and which perhaps should produce an error, eg -34, 155.

Also be familiar with tracing algorithms, debugging

White-box testing: Testing done by the programmer, focusing on an understanding of how the program should function

Black-box testing: Testing done by the user, focusing on an understanding of what the program should achieve, but not how it should achieve it.

Implementation Parallel running:

Keep the old system and the new system running at the same time. Adv: No disruption to business because even if new system doesn’t work, old system is still available. Disadv: Twice as much work required to keep both systems running.

Phased introduction: Bring the new system in gradually, replacing the old system function by function.

Adv: Can be the best of both worlds between Parallell Running and Direct Changeover. Disadv: Not often possible to replace a system bit by bit.

Direct changeover (big bang): “Flicking the switch” between old system and new system. Adv: Avoids extra

overhead of having two systems running in parallel. Disadv: Disruption to the business if the new system doesn’t work properly.

Things to consider: Bugs in the new system

Cost of running two systems in parallel

Training new users

Interruption to business

Operation and Maintenance

Constant review

Performance evaluation

Bug-fixing

Feeds back into the Analysis stage to create a cycle

Documentation Two types:

System documentation: Intended for programmers so they can maintain the system. Lists and descriptions of modules, classes, variables, data structures, hardware requirements, etc.

User documentation: Intended for users of the system so they can operate it usefully. Illustrated instructions, how to install, how to operate, etc.

Master File vs Transaction File

Master File Permanent

Complete set of records

Transaction File Temporary

Contains only those records that have been changed recently

Used to update the master file

Batch vs Online vs Real-TimeBatch Processing

A large amount of input happens over time and then then whole set of input is processed in one go

Examples: Any monthly billing, eg internet billing (you access the internet lots of times over the course of a month, then at the end of the month your ISP totals your usage and gives you the bill)

Online Processing (also known as interactive processing)

Input is processed (almost) immediately

Example: Flight booking system. As soon as you book the seat it is yours. Can you explain why flight bookings could not be processed in batches?

Real-Time Processing

Input is processed immediately and continuously

There is generally no user

Input comes from sensors

Examples: Auto-pilot. Large volume of data harvested from multiple sensors continuously. System reacts in real time.

Validation vs Verification

Validation

A validation check just checks if input is possible, appropriate or reasonable

Keppler accidentally enters 81 for his age, instead of 18

This is valid because he could be 81

Validation check does not check if it's actually true

Examples are range check (eg is aged entered between 0 and 100) and type check (eg has the user entered a number rather than a string)

Verification

The process of checking if data is true, factually correct

Two ways of doing it:

Visual checking, ie proofreading

Double-entry of data, ie inputting the whole data set again and checking if both sets are the same

Social Effects of Computer Systems

Positive More leisure time for people because computers do their work

Online commerce reduces the need for travel

Communication more immediate

More accessibility for elderly or disabled people

Computers can do dangerous/repetitive jobs

Labour-saving devices eg washing machines, security systems, etc

Negative Training required

Purchase of a computer sometimes required (cost)

Unemployment in areas where computers do work, eg factories

Longer hours worked by people always contactable eg email, mobile phone

Some health issues, eg eye-strain, back problems, RSI (repetitive strain injury)

SL P2 N 2009

SL P1 M 2009