Ch17 Rapid SW Dev

8/10/2019 Ch17 Rapid SW Dev

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Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 17 Slide 1

Rapid software development


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Objectives

To explain how an iterative, incrementaldevelopment process leads to faster deliveryof more useful software

To discuss the essence of agile developmentmethodsTo explain the principles and practices ofextreme programming

To explain the roles of prototyping in thesoftware process


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Topics covered

Agile methodsExtreme programmingRapid application developmentSoftware prototyping


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Rapid software development

Because of rapidly changing businessenvironments, businesses have to respondto new opportunities and competition.

This requires software and rapiddevelopment and delivery often to be themost critical requirement for softwaresystems.

Businesses may be willing to accept lowerquality software if rapid delivery of essentialfunctionality is possible.


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Requirements

Because of the changing environment, it isoften impossible to arrive at a stable,consistent set of system requirements.

Therefore a waterfall model of developmentis impractical and an approach todevelopment based on iterative specificationand delivery is the only way to deliversoftware quickly.


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Characteristics of RAD processes

The processes of specification, design andimplementation are concurrent. There is no detailedspecification and design documentation isminimised.The system is developed in a series of increments.End users evaluate each increment and makeproposals for later increments.System user interfaces are usually developed usingan interactive development system.


7/46Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 17 Slide 7

An iterative development process



Advantages of incremental development

Accelerated delivery of customer services .Each increment delivers the highest priorityfunctionality to the customer.

User engagement with the system . Usershave to be involved in the developmentwhich means the system is more likely tomeet their requirements and the users aremore committed to the system.


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Problems with incrementaldevelopment (cont.)

Validation problems Without a specification, what is the system

being tested against?

Maintenance problems Continual change tends to corrupt software

structure making it more expensive to changeand evolve to meet new requirements.



Prototyping

For some large systems, incrementaliterative development and delivery may beimpractical; this is especially true when

multiple teams are working on different sites.Prototyping, where an experimental systemis developed as a basis for formulating therequirements may be used. This system isthrown away when the system specificationhas been agreed.



Incremental development andprototyping



Conflicting objectives

The objective of incremental development isto deliver a working system to end-users.The development starts with those

requirements which are best understood.The objective of throw-away prototyping is tovalidate or derive the system requirements.The prototyping process starts with thoserequirements which are poorly understood.



Agile methods

Dissatisfaction with the overheads involved in designmethods has led to the creation of agile methods.These methods:

Focus on the code rather than the design; Are based on an iterative approach to software

development; Are intended to deliver working software quickly and

evolve this quickly to meet changing requirements.

Agile methods are probably best suited tosmall/medium-sized business systems or PCproducts.


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Principles of agile methods


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Problems with agile methods

It can be difficult to keep the interest of customerswho are involved in the process. Pressures from job;represent all stateholders.

Team members may be unsuited to the intenseinvolvement that characterises agile methods.Prioritising changes can be difficult where there aremultiple stakeholders.

Maintaining simplicity requires extra work.Contracts may be a problem as with otherapproaches to iterative development.


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Extreme programming

Perhaps the best-known and most widelyused agile method.Extreme Programming (XP) takes an

extreme approach to iterative development. New versions may be built several times perday;

Increments are delivered to customers every 2weeks;

All tests must be run for every build and thebuild is only accepted if tests run successfully.


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Extreme programming practices 1


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Extreme programming practices 2


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Requirements scenarios

In XP, user requirements are expressed asscenarios or user stories.These are written on cards and thedevelopment team break them down intoimplementation tasks. These tasks are thebasis of schedule and cost estimates.

The customer chooses the stories forinclusion in the next release based on theirpriorities and the schedule estimates.


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Story card for document downloading


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XP and change

Conventional wisdom in softwareengineering is to design for change. It isworth spending time and effort anticipatingchanges as this reduces costs later in the lifecycle.XP, however, maintains that this is notworthwhile as changes cannot be reliably

anticipated.Rather, it proposes constant codeimprovement (refactoring) to make changeseasier when they have to be implemented.


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Testing in XP

Test-first development.Incremental test development fromscenarios.User involvement in test development andvalidation.

Automated test harnesses are used to run all

component tests each time that a newrelease is built.


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Task cards for document downloading

Task 1: Implement principal workf low

Task 2: Implement article catalog and selection

Task 3: Implement payment collection

Payment may be made in 3 different ways. The user selects which way they wish to pay. If the user has a library subscription, then they can input thesubscriber key which should be checked by thesystem. Alternatively, they can input an organisationalaccount number . If this is valid, a debit of the costof the article is posted to this account. Finally, theymay input a 16 digit credit card number and expirydate. This should be checked for validity and, if valid a debit i s posted to that credit card account.


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Test case description


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Test-first development

Writing tests before code clarifies therequirements to be implemented.Tests are written as programs rather than

data so that they can be executedautomatically. The test includes a check thatit has executed correctly.

All previous and new tests are automatically

run when new functionality is added. Thuschecking that the new functionality has notintroduced errors.


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Pair programming

In XP, programmers work in pairs, sitting together todevelop code.This helps develop common ownership of code andspreads knowledge across the team.It serves as an informal review process as each lineof code is looked at by more than 1 person.It encourages refactoring as the whole team canbenefit from this.

Measurements suggest that developmentproductivity with pair programming is similar to thatof two people working independently.


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Rapid application development

Agile methods have received a lot ofattention but other approaches to rapidapplication development have been used for

many years.These are designed to develop data-intensive business applications and rely onprogramming and presenting informationfrom a database.


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RAD environment tools

Database programming languageInterface generatorLinks to office applicationsReport generators


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A RAD environment


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Interface generation

Many applications are based around complex formsand developing these forms manually is a time-consuming activity.RAD environments include support for screengeneration including:

Interactive form definition using drag and droptechniques;

Form linking where the sequence of forms to be

presented is specified; Form verification where allowed ranges in form fields is

defined.


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Visual programming

Scripting languages such as Visual Basicsupport visual programming where theprototype is developed by creating a user

interface from standard items andassociating components with these items

A large library of components exists tosupport this type of developmentThese may be tailored to suit the specificapplication requirements


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Visual programming with reuse


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Problems with visual development

Difficult to coordinate team-baseddevelopment.No explicit system architecture.Complex dependencies between parts of theprogram can cause maintainability problems.


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COTS reuse

An effective approach to rapid developmentis to configure and link existing off the shelfsystems.

For example, a requirements managementsystem could be built by using: A database to store requirements; A word processor to capture requirements and

format reports; A spreadsheet for traceability management;


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Compound documents

For some applications, a prototype can be createdby developing a compound document.This is a document with active elements (such as aspreadsheet) that allow user computations.Each active element has an associated applicationwhich is invoked when that element is selected.The document itself is the integrator for the differentapplications.


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Application linking


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Software prototyping

A prototype is an initial version of a systemused to demonstrate concepts and try outdesign options.

A prototype can be used in: The requirements engineering process to help

with requirements elicitation and validation; In design processes to explore options and

develop a UI design; In the testing process to run back-to-back tests.


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Benefits of prototyping

Improved system usability. A closer match to users real needs. Improved design quality.Improved maintainability.Reduced development effort.


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Back to back testing


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The prototyping process


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Throw-away prototypes

Prototypes should be discarded afterdevelopment as they are not a good basisfor a production system: It may be impossible to tune the system to meet

non-functional requirements; Prototypes are normally undocumented; The prototype structure is usually degraded

through rapid change; The prototype probably will not meet normal

organisational quality standards.


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Key points

An iterative approach to software development leadsto faster delivery of software.

Agile methods are iterative development methodsthat aim to reduce development overhead and so

produce software faster.Extreme programming includes practices such assystematic testing, continuous improvement andcustomer involvement.The approach to testing in XP is a particular strengthwhere executable tests are developed before thecode is written.


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Key points

Rapid application development environmentsinclude database programming languages,form generation tools and links to officeapplications.

A throw-away prototype is used to explorerequirements and design options.When implementing a throw-away prototype,

start with the requirements you leastunderstand; in incremental development,start with the best-understood requirements.

Ch17 Rapid SW Dev

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