Rapid software development

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 1

Objectives

    To explain how an iterative, incremental development process leads to faster delivery of more useful software To discuss the essence of agile development methods To explain the principles and practices of extreme programming To explain the roles of prototyping in the software process ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 2

Topics covered

    Agile methods Extreme programming Rapid application development Software prototyping ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 3

Rapid software development

   Because of rapidly changing business environments , businesses have to respond to new opportunities and competition.

Almost business operations require software and rapid development and delivery is often the most critical requirement for software systems.

Businesses may be willing to accept lower quality software if rapid delivery of essential functionality is possible.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 4

Requirements

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

Therefore a waterfall model is impractical .

of development An approach to development based on iterative specification and delivery is the only way to deliver useful software quickly.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 5

Characteristics of rapid software development

1.

2.

3.

4.

5.

The processes of specification, design, implementation are concurrent . There is no detailed specification.

Design documentation is minimised.

The system is developed in a series of increments . End users evaluate each increment and make proposals for later increments.

System user interfaces (which are difficult to specify in advance) , are usually developed using an interactive development system (for example, interfaces might be developed by using an

exploratory development

, [see pg. 69 evolutionary development] ).

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 6

A general process model for incremental development process

It is very important!

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 7

Advantages of incremental development

  Accelerated delivery of customer services . Each increment delivers the highest priority functionality to the customer.

User engagement with the system . Users have to be involved in the development which means the system is more likely to meet their requirements and the users are more committed to the system.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 8

Problems with incremental development (1/2)

  Management problems • Progress can be hard to judge there is no documentation and problems hard to find because to demonstrate what has been done.

• Incremental development may require new and unfamiliar technologies to ensure rapid delivery: managers may find it difficult to use existing stuff (these stuff might lack of skills).

Contractual problems • • Usually, normal contracts include a specification documentation ; Without a specification, different forms of contract

but

….. have to be used, 1. Developers may be unhappy to accept a fixed-price ; 2. Customers may be unhappy to pay developers for the time spent in the projects; ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 9

 

Problems with incremental development (2/2)

Validation problems 1.

2.

3.

Without a specification , what is the system being tested against?

If there is a specification a V&V team can early use it parallel with the system implementation; for test in If there is no such a specification independent validation incrementally developed system is difficult.

of Maintenance problems 1.

2.

3.

Continual change tends to corrupt software structure expensive to change and evolve making it to meet new requirements.

more This means that only the original developers may understand the software: • It could be profitable to use refactoring to maintain the system well structured; Moreover, if specialized technology, such as become obsolete. (See next slides for RAD)

RAD

(

Rapid Application Development

) is used to support incremental development and delivery)

rapid development

the

RAD

(based on technology itself may ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 10

•   

Large systems

For some large systems , incremental iterative development and delivery may be impractical; This is especially true: 1.

2.

3.

when multiple teams are working on different sites .

Embedded systems development; where software depends on hardware Critical system: safety, security; But also these systems suffer the problem of uncertain and changing requirements , so: • a hybrid process iteratively.

may be used where a prototype is developed As we will see in the last slides, you can get some of the benefits of an incremental development by a rapid and iterative implementation of a throw-away prototype within a waterfall or plan-based development approach .

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 11

Prototyping

   For some large systems , incremental iterative prototyping may be used where an experimental system is developed as a basis for: 1.

2.

3.

formulating and /or validating requirements; exploring new design solution (e.g., design interface); testing.

A prototype system is not developed to be delivered to the customer ,

but

…..

….. the term

evolutionary prototyping

is sometimes used to as a synonym of

incremental development

(i.e., the prototype is not discarded but evolve to meet the customer’s requirements) ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 12

Incremental development and prototyping

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 13

Incremental development and throw-away prototyping: different objectives   The objective of incremental development to deliver a working system to end-users: • is The development starts with those requirements which are best understood and which have highest priority .

The objective of throw-away prototyping is to validate or derive the system requirements : • The prototyping process starts with those requirements which are poorly understood in order to find out more about them.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 14

Incremental development and throw-away prototyping: different quality of the system   Incremental development.

The intermediate system evolves into the final system, so the former should: 1.

Have the same organisational standard and quality; 2.

3.

Have a robust structure; Be reliable and efficient; Throw-away prototyping.

The intermediate system has short life time, so: 1.

2.

3.

It must be possible to change the system rapidly; Long-term maintainability is not required; High performance and reliability are not required; ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 15

Agile methods

  Dissatisfaction with the overheads involved in design methods led to the creation of agile methods . These methods: • Focus on the code design; rather than the documentation and • Are based on an iterative approach development; to software • Are intended to deliver working software quickly evolve and this quickly to meet changing requirements.

Agile methods are probably best suited to small/medium-sized business systems or PC products.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 16

Principles of agile methods

Principle Customer involvement Incremental delivery People not process Embrace change Maintain simplicity Description The customer s hould be closely involved throughout the development process. Their role is provide and prioritise new system requirements and to evaluate the iterations of the system.

The software is developed in increments with the customer specifying the requireme nts to be included in each increment.

The skills of the development team should be recognised and exploited. T he team should be left to develop their own ways of working without prescriptive processes.

Expect the system requirements to change and design the system so that it can acc ommodate these changes.

Focus on simplicity in both the software being developed and in the development process used. Wherever possible, actively work to eliminate complexity from the system.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 17

Problems with agile methods (1/2)

  Customer involvement is a good idea ,

BUT:

1. it can be difficult to keep the interest of customers who are involved in the process.

2. it could be difficult to find customer that can represent all system stakeholders.

Team members involvement may be unsuited to the intense that characterises agile methods: 1.

They may not interact well with other team members. ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 18

Problems with agile methods (2/2)

  Prioritising changes can be difficult where there are multiple stakeholders: 1.

Typically different stakeholders gives different priorities to different changes .

Maintaining simplicity requires extra work : 1.

Under pressure from delivery scheduling , team members may not have time to simplify the system in order to keep it maintainable.  Contracts may be a problem iterative development: as with other approaches to 1.

As discussed in chapter 6, requirements document may be part of the contract ……. If there is not such a documentation!

To summarize the discussion above: all method to software development have some problems.

• Agile method are suitable for small and medium-sized systems. ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 19

Extreme programming

  Perhaps the best-known and most widely used agile method: • Good practice: 1.

2.

iterative development.

customer involvement to “EXTREME” levels.

• • • • Extreme Programming (XP) takes an “EXTREME” approach to iterative development.

• All requirements are expressed as stories).

scenarios (called user Scenarios are implemented as series of task .

New versions may be built several times per day ; Increments are delivered to customers every 2 weeks ; All tests must be run for every build and the build is only accepted if tests run successfully .

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 20

The XP release cycle

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 21

Extreme programming practices 1

Incr emental plann ing Small Releases Sim ple Design Test fir st deve lopment Refactoring Requi rements are recorded on Story Cards and the Stories to be included in a release are determi ned by the tim e avail able and their relative priorit y. The deve lopers break these Stories into deve lopment ŌTasksÕ.

The mi nim al useful set of func tiona lit y that provides busines s value is deve loped fir st. Releases of the system are fr equen t and incrementall y add func tiona lit y to the fi rst release.

Enough de sign is carried out to meet the cu rrent requir ements and no more.

An automated un it t est framewo rk is used to write tests for a new piece of func tiona lit y be fore that fun ction alit y it self is im plemented.

All developers are expe cted to refactor the code con tinuous ly as soon as possible code im prove ments are found . This keeps the code sim ple and maintainable.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 22

Extreme programming practices 2

Pair Progra mmi ng Coll ective Owne rship Continuou s Integr ation Sustainab le pace On-sit e Customer Deve lopers work in pair s, che cking ea ch otherÕs work and providing the suppo rt to always do a good job.

The pa ir s of deve lopers work on all areas of the system, so that no islands of expe rtise deve lop and all the deve lopers own a ll t he code. Anyon e can chang e any thing.

As soon as work on a task is complete it is integrated into the who le system. After any such integration, all the un it tests in the system must pass.

Large amounts of ove r-time are not considered acceptable as the net effect is often to reduc e code qua lit y and medium term produc tivity A representative of the end -user of the system (t he Customer) shou ld be ava il able full tim e for the use of the XP team. In an extreme programmi ng p rocess, the cus tomer is a member of the deve lopment team and is responsible for bringing system requirements to the team for im plementation.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 23

XP and agile principles

     Incremental development is supported through small, frequent system releases.

Customer involvement means full-time customer engagement with the team.

People not process through pair programming, collective ownership and a process that avoids long working hours.

Change supported through regular system releases.

Maintaining simplicity of code.

through constant

refactoring

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 24

Requirements scenarios

     In XP, user requirements are expressed as scenarios or user stories.

These are written on cards and the development team break them down into implementation tasks .

Each task represent a discrete feature .

The tasks are the basis of schedule estimates.

and cost The customer chooses the stories for inclusion in the next release based on their priorities and the schedule estimates.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 25

Story card for document downloading

Downloading an d printing an article

First, you select the article that you want from a disp layed list. then have to tell the system how you will pay for it - this can either be through a subscription, through a company account or by credit card.

After this, you get a copy right form from the system to fill in and, when you have submitted this, the article you want is downloaded onto your computer.

You then choose a printer and a cop y of the article is printed. You tell the system if printing has been successful.

If the article is a print-only article, you canÕt keep the PDF version so it is automatically deleted from y our computer.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 26

  

XP and change

Conventional wisdom in software engineering is to design for change process (i.e., for traditional software it is worth spending time and effort anticipating changes life cycle.

as this reduces costs later in the XP, however, has discarded this principle . XP maintains that this is not worthwhile as changes cannot be reliably anticipated: • Often changes anticipated never materialise and completely different changes are actually made .

Rather, it proposes constant code improvement (refactoring) to make changes easier when they have to be implemented by maintaining a good structure .

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 27

Testing in XP (1/3)

Testing practice is different from iterative development and plan-based development:   In XP, there is no a full system specification used by an external team that to develop system test.

can be As already said, this can lead to very informal testing and validation process.

 XP avoid some of the problems by reducing the likelihood of testing and validation that producing new system increments will produce error.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 28

Testing in XP (2/3)

 

Test-first development:

1. In this way it is possible to define both an interface and behavior.

2.

Problems with requirements and interface misunderstanding are reduced.

In general, this can be achieved when requirements and implementation are strictly related.

3. In XP 2. is possible because cards are broken down into implementing tasks which are principal unit of implementation .

Incremental test development from scenarios:

1. Each task generates one or more unit test .

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 29

Testing in XP (3/3)

 

User involvement in test development and validation:

1. Users help developers by means of acceptance tests.

Automated test harnesses are used to run all tests each time that a new release is built:

1.

Ad-hoc testing components are written to execute the tests automatically: automatic submission of inputs and checking of related outputs.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 30

Task cards for document downloading

Task 1: Imple me nt principal workflow Task 2: Imple me nt article catalog and se lection Task 3: Imple me nt payme nt collection

Payment may be made in 3 dif ferent ways. The user selects which way they wish to pay. If the user has a library subscription, then they can input the subscriber key which should be checked by the system. Alternatively, they can input an organisational account number. If this is valid, a debit of the cost of the article is p osted to this account. Finally, they may input a 16 digit credit card number and exp iry date. This should be checked for validity and, if valid a debit is posted to that credit card account.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 31

Test case description

Te st 4: Te st cre dit card validity Input:

A string representing the credit card number and two integers representing the month and year when the card expires

Te sts:

Check that all by tes in the string are digits Check that the month lies between 1 and 12 and the year is greater than or equal to the current y ear.

Using the first 4 digits of the credit card number, check that the card issuer is valid by looking up the card issuer table. Check credit card validity by submitting the card number and expiry date information to the card issuer

O utpu t:

OK or error message indicating that the card is invalid ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 32

Test-first development (1/2)

   

TO SUMMARIZE:

Writing tests before code clarifies the requirements implemented.

to be Tests are written as programs rather than data so that they can be executed automatically. The test includes a check that it has executed correctly.

All previous and new tests are automatically run when new functionality is added (regression testing) . Thus checking that the new functionality has not introduced errors.

 P.S.: test-first development avoid problems of • In fact, usually so it is a developers work at a faster pace tendency to skip tests .

“test-lag” : than the tester and ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 33

Test-first development (2/2)

 However, test-first development does not always work: 1. Programmers often prefer programming to testing .

2. Programmers write sometimes incomplete tests: 1. For example, it could be difficult to write test for user interface .

3. Customers may feel that providing the requirements is enough and can be reluctant to get involved in the testing process too.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 34



Pair programming

 In XP, programmers work in pairs , sitting together to develop code (the pairs are changed) .

ADVANTAGES and DISADVANTAGES:

1.

2.

This helps develop common ownership of code and spreads knowledge across the team (“egoless” programming).

It serves as an informal review process more than 1 person: • as each line of code is looked at by obviously this is a less formal and cheaper process with respect to formal program inspection and hence it probably does not find so many error .

3.

It encourages refactoring • and the whole team can benefit from this: if you work alone with yourself and if you practice refactoring you may be judged less efficient …. but if you work in pair ….. and pair are changed … Measurements suggest that development productivity with pair programming is similar to that of two people working independently.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 35

Rapid Application Development

   Agile methods (as iterative development approaches) a lot of attention

but

other approaches to R have received apid A pplication D evelopment have been used for many years.

These are mainly designed to develop data-intensive business applications and rely on programming and presenting information from a database.

These techniques are usually organized as a set of tool that allow data: 1.

2.

3.

4.

5.

To be created; To be searched; To be displayed; To presented as report; etc. ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 36

RAD environment tools

    Database programming language that includes basic manipulation operation on data: • Commands (e.g., queries) may be input directly or by means of forms filled by end-user.

Interface generator: • Used to create forms for data input and output . Links to office applications: • • Such spreadsheet for numerical manipulation.

Such as word processor for creation of report template.

Report generators • Used to define and create reports from information in the data base.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 37

A RAD environment

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 38

Interface generation

   Many applications are based around a lot complex forms and developing these forms manually is a time-consuming activity so ….. ….. RAD environments include support for screen generation including: • • Interactive form definition using drag and drop techniques; Form linking where the sequence of forms to be presented is specified; • Form verification where allowed ranges in form fields is defined.

Nowadays RAD environment also support the development of database interfaces on web browsers: • Data can be accessed from anywhere.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 39

Visual programming

   Most RAD systems also include visual programming languages.

Scripting languages such as Visual Basic support visual programming where the prototype is developed by creating a user interface from standard items and associating “components” with these items: • Items such as buttons, screens, text fields and menus have associated script that may be calls to reusable component, special-purpose code or mixture of both. A large library of components exists development.

to support this type of ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 40

Visual programming with reuse

Dat e co m po nent Range checki ng script Draw canv as com po nen t Fi le Edit Views Layo ut 12t h January 2 00 0 3. 876 Opt i on s Help General Index Men u co m po nent User pro m pt com po nen t + script Tree di splay com po nen t

Software Engineering, 7th edition. Chapter 17

©Ian Sommerville 2004 Slide 41

Problems with visual development

 Difficult to coordinate team-based development.

 No explicit and well structured system architecture .

 Complex dependencies between parts of the program can cause maintainability problems.

 Since scripting languages are limited to a specific set of interaction objects, it can be difficult to implement non standard user interface . ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 42

COTS reuse

   Visual development is an approach to RAD integrating fine-grained component.

that relies on An alternative effective approach to rapid development is to configure and link existing off the shelf systems (COTS) that could be complete applications For example, a • A database

requirements management system

(discussed in Chapter 7) could be built by using: to store requirements: • To store requirements.

• A word processor to capture requirements and format reports (e.g., Microsoft Word is a COTS): • To format report of requirements. • A spreadsheet for traceability management (e.g., Microsoft Excel is a COTS): • To manage traceability tables.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 43

Compound documents: a container.

     For some applications functionalities can be developed by using macros (e.g., Words, Excel macros).

This can done by creating a compound document acts as a container .

that This is a document with active elements (such as a spreadsheet) that allow user computations.

Each active element has an associated application is invoked when that element is selected.

which In this way, the document itself is the integrator different applications: • for the different objects in the document are linked and typed accessing an object results in the associated application being initiated. so that ©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 44

Application linking

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 45

Software prototyping

   As already mentioned, there are situations where, for practical (e.g., large and/or critical and/or critical systems) and contractual reasons , an incremental software development cannot be used …..

But

.... you can get some of the benefits development approach .

of an incremental development by a rapid and iterative implementation of a throw-away prototype (i.e., it is not delivered to the customer) within a waterfall or plan-based Generally, a prototype is an initial version of a system used to find out more about the problem and its possible solution.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 46

Software prototyping

 A prototype can be used in: 1.

2.

3.

The requirements engineering process to help with requirements elicitation and validation; The design processes to explore options and develop a UI design; The testing process to run back-to-back tests.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 47

Benefits of prototyping

     Improved system usability.

A closer match to users’ real needs.

Improved design quality.

Improved maintainability.

Reduced development effort.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 48

Back to back testing

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 49

The prototyping process

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 50

Throw-away prototypes

 Prototypes should be discarded after development as they are not a good basis for a production system: 1. It may be impossible to tune the system to meet non-functional requirements (the prototype was not born to meet such requirements) ; 2. Prototypes are normally undocumented (just code) ; 3. The prototype structure is usually degraded rapid change (so it results difficult to maintain) ; through 4. The prototype probably will not meet normal organisational quality standards (they are usually relaxed during prototype development) .

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 51

Key points

    An iterative approach to software development leads to faster delivery of software.

Agile methods are iterative development methods that aim to reduce development overhead and so produce software faster.

Extreme programming includes practices such as systematic testing, continuous improvement and customer involvement.

The approach to testing in XP is a particular strength where executable tests are developed before the code is written.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 52

Key points

   Rapid application development environments include database programming languages, form generation tools and links to office applications.

A throw-away prototype is used to explore requirements and design options.

When implementing a throw-away prototype, start with the requirements you least understand; in incremental development, start with the best-understood requirements.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 17

Slide 53