Chapter 2 - University of Florida · Actual software processes are interleaved or se- ... fun cti on ali ty Develo p p ro to ty pe Ev alu at e p ro to ty pe Pro to ty pi ng p lan

Chapter 2 Software Processes Slide 1

Chapter 2

Software Processes


Topics covered

Software processes and process models

Generic models:

• Waterfall

• Incremental development

• Reuse-oriented software engineering

Basic process activities:

• Specification

• Development

• Validation

• Evolution

(cont'd)


Topics covered (cont’d)

Coping with change

Software prototyping:

• Uses of prototypes

• Classifying prototypes

• User Interface prototyping

(Mills’) Incremental Delivery (cf. Incremental

development)

Boehm’s spiral model

The Rational Unified Process (an example of a

modern software development method)


Software processes and

process models


The software process

A process is a structured set of activities required to develop a software system.

Many different software processes, but all involve:

• Specification – defining what the system should do;

• Design & implementation;

• Validation – checking that it does what the customer wants (and

Verification – checking that it does what is specified);

• Evolution – changing the system in response to changing customer needs.

A process MODEL is an abstract representation of a process. It presents a description of a process from some particular perspective

Models should be as simple as possible, but no simpler. – A. Einstein

[struhk-cherd] –adjective; having a

clearly defined structure or organization.


The software process

A process is a structured set of activities required to develop a software system.

Many different software processes, but all involve:

• Specification – defining what the system should do;

• Design & implementation;

• Validation – checking that it does what the customer wants (and

Verification – checking that it does what is specified);

• Evolution – changing the system in response to changing customer needs.

A process MODEL is an abstract representation of a process. It presents a description of a process from some particular perspective

Models should be as simple as possible, but no simpler. – A. Einstein

[struhk-cherd] –adjective; having a

clearly defined structure or organization.


Plan-driven and agile processes

Plan-driven processes: all of the process activities

are planned in advance and progress is measured

against this plan.

Agile processes: planning is incremental and it is

easier to change the process to reflect changing

customer requirements.

In practice, most practical processes include

elements of both plan-driven and agile approaches.

There are no right or wrong software processes!


Generic models


Generic software process models

The Waterfall Model – plan-driven model; separate and

distinct phases of specification and development. not iterative

Incremental Development – specification, development,

& validation are interleaved; may be plan-driven or agile.

Reuse-Based Development – e.g., “component-

based SE”: the system is assembled from existing

components; may be plan-driven or agile.

-----------------------------

In practice, most large systems are developed using a pro-

cess that incorporates elements from all of these models.

(And, at no additional cost: Boehm’s Spiral Model.)


Waterfall model (W. Royce)

Requirementsdefinition

System andsoftware design

Implementationand unit testing

Integration andsystem testing

Operation andmaintenance


Waterfall model (cont’d)

There are separate phases in the waterfall model:

• Requirements analysis and definition

• System and software design

• Implementation and unit testing

• Integration and system testing

• Operation and maintenance

Model is mostly used for large systems engineering

projects where a system is developed at several sites.

In such circumstances, its plan-driven nature helps in

coordinating the work.


Waterfall model problems

Inflexible partitioning of the project into distinct stages

makes it difficult to respond to changing customer

requirements.

• Thus, more appropriate when requirements are well-

understood to begin with.

• This applies to relatively few systems.

---------------------------------------------

In general, the drawback of the waterfall model is the

difficulty of accommodating change after the pro-

cess is underway.


Incremental development

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outline

description

Concurrent

activities


Incremental development benefits

Reduces cost of accommodating changing customer

requirements - amount of analysis and documentation that

has to be re-done is much less than is required with the waterfall

model.

Easier to get customer feedback - customers can

comment on demonstrations of the software and see how much

has been implemented.

More rapid delivery & deployment of useful software

to the customer is possible - customers may be able to

use and gain value from the software earlier than is possible

with a waterfall process.


Incremental development problems

Lack of process visibility - lack of document deliverables

to measure progress. If systems are developed quickly, it

is not cost-effective to produce documents that reflect

every version of the system.

System structure tends to degrade as new

increments are added - unless time and money is spent

on refactoring to improve the software, regular change

tends to corrupt its structure. Incorporating further

software changes becomes increasingly difficult and

costly.


Reuse-oriented software engineering

Based on systematic (as opposed to serendipitous)

reuse - systems are integrated from existing components or COTS

(Commercial-Off-The-Shelf) systems.

Process stages

• Component analysis (Assessing what’s available.)

• Requirements modification (Why is this needed?)

• System design with reuse

• Development and integration

“Reuse” is now the standard approach for building many

types of business systems.

(cont'd)


Reuse-oriented software engineering

Requirementsspecification

Componentanalysis

Developmentand integration

System designwith reuse

Requirementsmodification

Systemvalidation

(what’s available?)

(cont'd)


Types of reusable software components

Web services that are developed according to

service standards and which are available for

remote invocation.

Collections of objects that are developed as a

package to be integrated within a component

framework such as .NET or J2EE (Java EE 6).

Stand-alone software systems (COTS) that

are configured for use in a particular environ-

ment.


Basic process activities


Process activities

Actual software processes are interleaved or se-

quences of technical, collaborative and managerial

activities with the overall goal of specifying,

designing, implementing and testing a software

system.

The four basic process activities of specification,

development, validation (& verification), and

evolution are organized differently in different

development processes. In the waterfall model, they

are organized in sequence; in incremental

development they are interleaved.


Software specification / RE

The process of establishing what services are required

and the constraints on the system’s operation and

development.

Requirements Engineering (RE) process:

• Feasibility (technical and otherwise) study

• Requirements elicitation and analysis

• Requirements specification (documentation)

• Requirements validation

(cont'd)


The requirements engineering

process (waterfall perspective)

Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirements

document


Software design and implementation

The process of producing an executable system

based on the specification (waterfall perspective)

• Software design – design a software structure that realizes

the specification.

• Implementation – translate this structure into an executable

program.

Note: the activities of specification, design, and

implementation are closely related and may be

interleaved (as with incremental development).


Design activities

Architectural design: identify overall structure of

system, principal components (sometimes called

sub-systems or modules), their relationships, and

how they are distributed.

Interface design: define the interfaces between

system components.

Component design: design how each system

component will operate.

Database (data structure) design: design the system

data structures and how these are to be represented

in a database.


Software verification & validation

Verification and validation (V&V) determines whether or

not a system (1) conforms to its specification and (2)

meets the requirements of the customer.

Involves checking processes (e.g., inspections/reviews,

formal verification) and program (machine-based)

testing.

Testing is the most commonly used V&V activity

and involves executing program elements with test

cases that are derived from analyzing specifications

and/or program logic.


Testing stages

Development or component testing

• Individual components are tested independently;

• “Components” may be functions or objects or coherent

groupings of these entities.

System testing

• Testing of the system as a whole. Testing of emergent

properties is particularly important.

Acceptance testing

• Testing with customer data to check that the system

meets the customer’s needs/desires.


Software evolution (“maintenance”)

Software is inherently flexible and subject to change.

As requirements change through changing

business/environmental circumstances, the software

must also evolve and change.

The distinction between development and evolution

has become increasingly irrelevant as fewer and

fewer systems are completely new.

(cont'd)


Software evolution (“maintenance”)

Assess existing

systems

Define systemrequirements

Propose systemchanges

Modify

systems

Newsystem

Existing

systems

e.g., change

requests


Coping with change


Inevitability of change

Change is inevitable in all large software projects.

• Business/environmental changes lead to new and

changed system requirements.

• New technologies open up new possibilities for improving

implementations.

• Changing platforms require application changes.

Change costs include both re-work (redoing already

completed work -- e.g., re-analyzing requirements)

and the cost of implementing new functionality.


Reducing the costs of re-work

Change avoidance: includes process activities that can

stimulate user/stakeholder anticipation of needs/require-

ments before development begins.

• E.g., a prototype may allow users/stakeholders to better envi-

sion how the system would actually be used.

Change tolerance: the development process and/or

product may be designed to reduce change costs.

• If the systems is delivered in increments (e.g., via Mills’ Incre-

mental Delivery), re-work may only be required in those

increments that have already been developed.

• Information hiding techniques (isolating potentially changeable

design decisions) may be employed in product design.


Software prototyping


What is prototyping?

An iterative process emphasizing:

• Rapid development

• Concreteness and evaluative use (a “real system” is developed and presented to real users for hands-on evaluation)

• Consideration of alternatives

• Feedback

• Modification


General Prototyping process

Establishprototypeobjectives

Defineprototype

functionality

Developprototype

Evaluateprototype

Prototypingplan

Outlinedefinition

Executableprototype

Evaluationreport

What to include & what

NOT to include.


Uses of prototypes

Principal use is to help customers and

developers better understand system

requirements.

• Experimentation stimulates anticipation of how

a system could/may actually be used.

• Attempting to use functions together to

accomplish some task often reveals subtle

requirements problems.

(cont’d)


Uses of prototypes (cont’d)

Other potential uses:

1. Evaluating proposed solutions for feasibility

(=“Experimental Prototyping”)

2. Develop and evaluate User Interface designs

3. “Back-to-back testing”

4. Training users before system delivery

Prototyping is most often undertaken as a

risk reduction activity.


Classifying prototypes

By purpose:

• Throw-away prototyping – to elicit and validate

requirements

• Experimental prototyping – to evaluate proposed

solutions for feasibility, performance, etc.

horizontal vs. vertical (breadth vs. depth)

mockups vs. breadboards (form vs. function)

“Wizard of Oz” prototyping (Turing test reversed)


Throw-away prototyping

Outline

requirements

Develop

prototype

Evaluate

prototype

Specify

system

Developsoftware

Validatesystem

Deliveredsoftwaresystem

Reusablecomponents

Elicit/validate REQMTS



By purpose:


requirements








By purpose:


requirements






F

i

d

e

l

i

t

y

Number of features few many

low

high Vertical prototype

Horizontal prototype points of

comparable effort



By purpose:


requirements







Quin Tech “Self-service check-in and

baggage drop-off design”

“The design was tested through a full-scale mock-up.”


Electronic circuit on a bread-

board (REUK.co.uk)

“There is no need to solder anything, and the components can be moved

around and the circuit modified thousands of times without damaging parts.”



By purpose:


requirements





“Wizard of Oz” prototyping (Turing test reversed?)


The Turing Test (Alan Turing, 1950)


The Wizard of Oz exposed…

“The truth is the Wizard was an illusion created by a man hidden behind a curtain.”


Prototyping versus simulation

What’s the difference between prototyping and

simulation?


Throw-away prototype delivery

Developers may be “pressurized” to deliver a throw-

away prototype as the final system.

This is problematic...

• It may be impossible to meet non-functional

requirements with the prototype.

• The prototype is almost certainly undocumented.

• The system (prototype) may be poorly structured

and therefore difficult to maintain.

• Normal organizational quality standards may not

have been applied.

?

Air Tank

Developer

User Mgmt

No, no, no! I won’t

deliver the prototype to

you!

“Pressurizing” the

Developer


Implementation techniques

Various techniques may be used to implement

prototypes:

• Dynamic, high-level languages

• Database programming (RAD)

• Component and application assembly

These are NOT mutually exclusive – they are

often used together.

Visual programming is also an inherent part of

most prototype development systems.


User interface prototyping

User interface development consumes an

increasing part of overall system develop-

ment costs.

It is usually impossible to pre-specify the look

and feel of a complex user interface in an

effective way. Thus, prototyping is essential.

(cont’d)


User interface prototyping (cont’d)

Aim is to allow users to gain direct experi-

ence with the interface.

Without this, it is almost impossible to judge

usability.

Often a two-stage process:

• paper prototypes are developed initially,

• followed by a series of increasingly sophis-

ticated automated prototypes.


Paper prototyping

Step through scenarios using sketches of the

interface.

Use storyboards to present a series of interactions

with the system.

Paper prototyping is a cost-effective way of obtaining

user reactions to an interface design proposal.


(Mills’) Incremental Delivery

(not to be confused with

“incremental development”)


Incremental Delivery

Rather than deliver the system as a single unit, the

development and delivery is broken down into

increments, each of which incorporates part of the

required functionality.

User requirements are prioritized and the highest

priority requirements are included in early

increments.

Once the development of an increment is started, its

requirements are “frozen” while requirements for

later increments can continue to evolve. (Compromise between Waterfall & Incremental Development)

(cont’d)



Valida te

increment

Develop systemincrement

Design systemarchitecture

Integrateincrement

Valida te

system

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem

(cont'd)


Incremental development versus


Incremental development

• Develop the system in increments that are made available for

customer evaluation & feedback (but not usually for actual

work in the customer’s own environment) before proceeding to

the development of the next increment;

• Normal approach used in agile methods (but may also be used

in plan-driven development);

• Evaluation undertaken by user/customer (or a proxy).

(Mills’) Incremental Delivery

• Deploy increments for actual work-place use by end-users;

• Permits more realistic evaluation of practical usefulness;

• Difficult to carry out for replacement systems as increments

provide less functionality than the system being replaced.


Advantages of Incremental Delivery

Useful functionality is delivered with each increment,

so customers derive value early.

Early increments assist in eliciting requirements for

later increments.

Lower risk of overall project failure.

The highest priority system services tend to receive

the most testing. (They're subject to more “validation” steps.)

(cont'd)


Potential problems with Incremental

Delivery

Requirements may NOT be partitionable into usable,

stand-alone increments. (e.g., consider a compiler)

Most systems require a set of basic facilities that are used by

different parts of the system. But since requirements are not

defined in detail until an increment is to be implemented, it can

be hard to identify common facilities that are needed by all

increments to be useful.

The essence of an iterative process is that the specification is

developed in conjunction with the software. But this conflicts

with the procurement model of many organizations, where

the complete specification is part of the system development

contract (also a problem with incremental development).


Spiral development model


Boehm’s spiral development model

Process is represented as a spiral rather than a

sequence of activities.

Each loop in the spiral represents a phase in the

process.

No fixed phases such as specification or design

– loops in the spiral are chosen depending on

what is required.

Explicitly incorporates risk assessment and

resolution throughout the process.

(cont'd)





Riskanalysis

Riskanalysis

Riskanalysis

Riskanalysis Proto-

type 1

Prototype 2

Prototype 3Opera-tionalprotoype

Concept ofOperation

Simulations, models, benchmarks

S/Wrequirements

Requirementvalidation

DesignV&V

Productdesign Detailed

design

Code

Unit test

IntegrationtestAcceptance

testService Develop, verifynext-level product

Evaluate alternativesidentify, resolve risks

Determine objectivesalternatives and

constraints

Plan next phase

Integrationand test plan

Developmentplan

Requirements planLife-cycle plan

REVIEW


Spiral model quadrants

Objective Setting – specific objectives for the phase are

identified. Project risks and alternative strategies are identified.

Risk Assessment and Reduction – risks are assessed

and activities put in place to reduce the key risks.

Development and Validation – a development model

for the system is chosen which can be any of the generic

models.

Planning – the project is reviewed and the next phase of the

spiral is planned.


Spiral model usage

The model has been very influential in

helping people think about iteration in

software processes and introducing the risk-

driven approach to development.

In practice, however, the model is rarely used

as published for practical software develop-

ment.


RUP


The Rational Unified Process

A modern process model derived from the work on

the UML and its associated process.

A hybrid process model that brings together aspects

of the generic process models discussed previ-

ously…it represents a new generation of generic

processes.

Normally described from 3 perspectives

• A dynamic perspective that shows phases over time;

• A static perspective that shows process activities;

• A practice perspective that suggests good practices.


RUP phase model

Phase iteration

Inception Elaboration Construction Transition

cf. Waterfall Model: RUP phases are more closely

related to business rather than technical concerns.

“in-phase iteration” “cross-phase iteration”


RUP phases

Inception

• Establish the business case for the system.

Elaboration

• Develop an understanding of the problem domain and the

system architecture.

Construction

• System design, programming and testing.

Transition

• Deploy the system in its operating environment.


Static workflows (process activities) in RUP

Workflow Description

Business modelling The business processes are modelled using business

use cases.

Requirements Actors who interact with the system are identified and

use cases are developed to model the system

requirements.

Analysis and design A design model is created and documented using

architectural models, component models, object

models and sequence models.

Implementation The components in the system are implemented and

structured into implementation sub-systems.

Automatic code generation from design models helps

accelerate this process.


RUP “good practices”

Develop software iteratively: plan increments

based on customer priorities and deliver highest

priority increments first.

Manage requirements: explicitly document and

keep track of changes.

Use component-based architectures: organize

system architecture as a set of reusable

components.

(cont'd)


RUP “good practices” (cont’d)

Visually model software: using graphical UML

models to present static and dynamic views.

Verify software quality: ensure software meets

organizational quality standards.

Control changes to software: using change

management system and configuration management

tools.


Key points

Software processes are the activities involved

in producing and evolving a software system.

They are represented abstractly by software

process models.

Generic models are very general and represent

different approaches to development.

Examples are the waterfall model, incremental

development, and reuse-oriented development.

(cont'd)


Key points (cont’d)

Requirements engineering is the process of establishing what services are required and the constraints on the system’s operation and development.

Design and implementation processes produce an executable system based on the specification.

V&V involves checking that the system meets its specification and satisfies user needs.

Evolution is concerned with modifying the system after it is placed in use. Software must evolve to remain useful.

(cont'd)



Processes should include activities to cope with

change. This may involve a prototyping phase

that helps avoid poor decisions on requirements

and design.

Throw-away prototyping is used to explore

requirements and design options.

Rapid development of prototypes is essential.

This usually requires leaving out functionality or

relaxing non-functional constraints.

(cont'd)



Prototyping may be essential for parts of the

system such as the user interface which cannot

be effectively pre-specified.

Users must be involved in prototype evaluation.

The Rational Unified Process is a modern

generic process model that is organized into

phases (inception, elaboration, construction and

transition) but separates activities (requirements,

analysis and design, etc.) from these phases.


Chapter 2

Software Processes

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Chapter 2 - University of Florida · Actual software processes are interleaved or se- ... fun cti on ali ty Develo p p ro to ty pe Ev alu at e p ro to ty pe Pro to ty pi ng p lan