CSI 101 Elements of ComputingSpring 2009

Lecture #2 Development Life Cycle of a Computer Application

Monday January 26th, 2009

What is a Life Cycle?

Process from idea to execution

It's all the steps required to create a computer program from a general problem statement

Why a Life Cycle?

Systematic approach

Everybody can follow

Repeatable success

Checkpoints● Checkpoints are steps along the way, prior to

completion, where you can stop and check your progress

● Allows you to know if you are on schedule or behind● Can check for major errors that would be time-

consuming to fix later

Development Life CycleRequirements Gathering

Design

High level

Low level

Create Data Dictionary

Program

Functions

Interfaces

Testing

Documentation

Step 1 – Requirements Gathering

Most important step

Why?● The better you understand the task, the better your

focus and the better product you create● Eliminates trial and error of constantly requiring user

feedback

Lists the functionality the user wants

What data changes and how

Diagrams how user wants to see it

Interface – how user interacts with program

Step 2 - Design

Many large companies have two levels

High level design is for interfaces and libraries

How outside elements are needed or provided by each component

Identifies functional boundaries

Very general

Low level design is for key functionality

Outline process performed by each component

Step 3 – Data Dictionary

Create reusable body of data elements

Provides structure and template to other component parts of a large application

Can even be reused in other programs

Can be included in object libraries

Object libraries are repositories of reusable data structures

Reinforces customer view of data while planning how machine will implement it

Evolution of Data DictionaryIn procedural languages, it was optional

Mostly used by computers with frequent reviews (like IBM) or database applications

Not considered necessary, as functions were most important

Object-Oriented Programming changed all that

Now transactions and transformation of data focus

We'll discuss Object-Oriented Programming later in this course

Importance of Data DictionaryWasn't crucial until Object-Oriented Programming

Object-Oriented Programming focuses on taking input data and creating output data

Hides complex functionality inside enclosed “modules”

Programs become series of functions that use these “modules”

Most modern programs are written in Object-Oriented Programming languages

Allows identification of crucial objects that might be used throughout program

Step 4 - ProgramNot a patchwork anymore

Systematic approach

Inside out

Take care of internal functions

Expand outwards

Look for recursion and reuse opportunities

Internal subroutine blocks

Take advantage of DLLs

So... What's a DLL?Dynamic Link Library

2 stages to decomposing code to machine level

Compilation : translating programming language into machine-understandable code

Linking : putting together machine-language code pieces into a single executable program

Dynamic link libraries contain precompiled pieces of code that can be linked with program

Advantages of DLLs

Precompiled pieces contain templates or parameters, so the precompiled code can be adjusted

This means code can be changed without having to recompile

Can make conditional changes that are based on data defined or calculated when the program runs

Hence name “dynamic” - changeable

Step 5 - Testing

Here's where companies got crazy

Thought the more you tested, the better the result

“Work the bugs out”

Can never find every problem

Why not?

Can never anticipate every possible move

The fallacy of “idiot proofing”

Auto-Builds

Code builds from automated computerized design packages

Thought – instead of spending time removing errors, why not prevent inserting them?

If requirements gathering was good, errors inserted by humans

Worked great, but not widely used

Programmers still like control

Did require tweaking

Test levelsUnit test – test individual component

Function test – test individual user function as it processes

System Test – test full package as it interacts with components

Regression Test – ensure upgrades don't harm existing applications

This is the step Microsoft forgets

Useability Test – ease of use

Reliability Test – stability

Sometimes called Stress Test

Unit Test

Usually performed by programmer

Test each line of code

Often done in a Debug environment of the Development environment package where the code is written

Involves stepping through the code and checking important data values

Test all condition results and loop pathsWe’ll discuss conditions and loops when we get into Visual Basic programming

Function TestOften done by Project Leader

Tests an entire path based upon different initial data values

Ensures function returns expected values without failing

Often tests expectable “incorrect” data as well to ensure program won’t fail

System Test

Usually handled by a separate team

Create a test environment that simulates customer’s environment

Might require multiple tests to simulate multiple environments

Specific tests are performed, based upon environment and customer need

Tests are created beforehand, while program is being coded

Regression Test

Frequently done along with System Test

Uses applications obtained from the customer

Ensures new version will work with additional applications created under the old one

Useability Test

Performed by customers

Often call “Beta Testing”

A small group of customers are shown Users Guide and allowed to try new program

They determine if the program is easy to understand and use

Reliability TestAlso called “Stress Testing”

Often done by System Test team

Designed to see how program responds to busy system

Run lots of applications at same time as program

Competing for processor time

Rarely done anymore

Processor power is greater, so not much of a concern anymore

Documentation

Not truly a “step”

Best to document as you go along

What to document?

Anything that someone else would need to know to modify the program

Data structures and what they represent

Complex formulas or functions

Changes from earlier versions

Library Documentation

Used for descriptions of DLL packages

Describes function

Describes expected input and form

Describes form of output

Describes what and how to customize

Also used for Class Libraries in Object-Oriented Programming languages

Design Documentation

Used to maintain accurate overview of project

Original created after Requirements

Used to distribute work among team

Ensures completeness

Update if design changes or additions occur

“Living document”

Checkpoint Documentation

Created as part of project schedule

Shows functions, data, or components complete at specific time

Allows for prioritization

Helps see slowdowns before they cause major project delays

User Documentation

Documents provided to user

Users Guide – how to use tool

Detailed, often with screen shots

Step by step instructions

Covers common functions

Implementation Guide – how to customize

Developers Guide – libraries and interfaces for expanding

WIKI Documentation

Many documents are now links to Internet files

Updateable by any customer

Called “wiki”s or “living documents”

Anyone with permission can provide an update

Often checked by someone who “owns” the document