chapter 2 software processes slide 1 chapter 2 software processes

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 (and Verification)• 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 process/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.


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)Requirements

definition

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

Validation Finalversion

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription

Concurrentactivities


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.

Primary Reuse-Affected Process stages:

• Component analysis (What’s available?)• Requirements modification (Can we live without some things?)• 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)

(Can we live withoutsome things?)


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

Requirementsdocument


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 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 (human-based) checking processes (e.g., inspections/reviews, formal verification) and (machine-based) program testing.

Program 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 existingsystems

Define systemrequirements

Propose systemchanges

Modifysystems

Newsystem

Existingsystems

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 system is developed incrementally, accommodating

changing customer requirements during development may be less costly.

• Information hiding techniques (isolating potentially changeable design decisions) may be employed in product design to reduce costs after development.


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. ( change AVOIDANCE)• Experimentation stimulates anticipation of how a

system could/may actually be used.• Attempting to use functions together to

accomplish some (higher-level) 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 designs3. “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

Outlinerequirements

Developprototype

Evaluateprototype

Specifysystem

Developsoftware

Validatesystem

Deliveredsoftwaresystem

Reusablecomponents

Elicit/validate REQMTS

Fidelity

Number of features few many

low

highVertical prototype

Horizontal prototypepoints of comparable effort


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.”







“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 (typeless, interpretive)

• Database programming (RAD / 4GLs)

• 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 the “incremental development”

generic model)


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)


Incremental Delivery

Valida teincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Valida tesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem

(cont'd)


incremental development versus Incremental Delivery

incremental development• Incrementally develop the system in versions that are made

available for customer evaluation & feedback (but not necessarily for actual work in the customer’s own environment) before proceeding to the development of the next version;

• 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)

Many 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 2Prototype 3

Opera-tionalprotoype

Concept ofOperation

Simulations, models, benchmarks

S/Wrequirements

Requirementvalidation

DesignV&V

Productdesign Detailed

design

CodeUnit 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

chapter 2 software processes slide 1 chapter 2 software processes

Documents

software system

different software processes

generic software process

wrong software processes

practical processes

change software prototyping

process models generic

rational unified process