1 software processes

8/12/2019 1 Software Processes

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Topics covered

concepts of software processes and software

process models;

three generic software process models and when

they might be used;

fundamental process activities of software

requirements engineering, software

development, testing, and evolution; coping with changes in the software

requirements and design;

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The software process

A structured set of activities required to develop asoftware system.

Many different software processes but all involve:

Specificationdefining what the system should do;

Design and implementationdefining the organization of thesystem and implementing the system;

Validationchecking that it does what the customer wants;

Evolutionchanging the system in response to changingcustomer needs.

A software process model is an abstract representation of aprocess. It presents a description of a process from someparticular perspective.

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Software process descriptions

When we describe and discuss processes, weusually talk about the activities in these processessuch as specifying a data model, designing a userinterface, etc. and the ordering of these activities.

Process descriptions may also include: Products, which are the outcomes of a process

activity;

Roles, which reflect the responsibilities of the people

involved in the process; Pre- and post-conditions, which are statements that

are true before and after a process activity has beenenacted or a product produced.

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Software process models

A software process model is an abstract representation of aprocess. It presents a description of a process from some particularperspective.

The waterfall model

Plan-driven model. Separate and distinct phases of specification anddevelopment.

Incremental development

Specification, development and validation are interleaved. May beplan-driven or agile.

Reuse-oriented software engineering

The system is assembled from existing components. May be plan-driven or agile.

In practice, most large systems are developed using a process thatincorporates elements from all of these models.

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The waterfall model

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Waterfall model problems

Inflexible partitioning of the project into distinctstages makes it difficult to respond to changingcustomer requirements.

Therefore, this model is only appropriate when therequirements are well-understood and changes will be

fairly limitedduring the design process.

Few business systems have stable requirements.

The waterfall model is mostly used for large

systems engineering projects where a system isdeveloped at several sites.

In those circumstances, the plan-driven nature of thewaterfall model helps coordinate the work.

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Incremental development

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Incremental development benefits

The cost of accommodating changing customerrequirements is reduced.

The amount of analysis and documentation that has to beredone is much less than is required with the waterfall model.

It is easier to get customer feedback on the development

work that has been done. Customers can comment on demonstrations of the software

and see how much has been implemented.

More rapid delivery and deployment of useful software tothe customer is possible.

Customers are able to use and gain value from the softwareearlier than is possible with a waterfall process.

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Incremental development problems

The process is not visible. Managers need regular deliverables to measure

progress. If systems are developed quickly, it is notcost-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 toimprove the software, regular change tends to corruptits structure. Incorporating further software changesbecomes increasingly difficult and costly.

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Reuse-oriented software engineering

Based on systematic reuse where systems are integratedfrom existing components or COTS (Commercial-off-the-shelf) systems.

Process stages

Requirement specification;

Component analysis;

Requirements modification;

System design with reuse;

Development and integration;

Validation.

Reuse is now the standard approach for building manytypes of business system

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Types of software component

Stand-alone software systems (COTS) that areconfigured for use in a particular environment.

Collections of objects that are developed as a

package to be integrated with a componentframework such as .NET or J2EE.

Web services that are developed according to

service standards and which are available forremote invocation.

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Software design and implementation

The process of converting the systemspecification into an executable system.

Software design

Design a software structure that realises thespecification;

Implementation

Translate this structure into an executable program;

The activities of design and implementation are

closely related and may be inter-leaved.

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A general model of the design process

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Design activities

Architectural design,where you identify the overallstructure of the system, the principal components(sometimes called sub-systems or modules), theirrelationships and how they are distributed.

Interface design,where you define the interfacesbetween system components.

Component design, where you take each systemcomponent and design how it will operate.

Database design, where you design the system datastructures and how these are to be represented in adatabase.

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Software validation

Verification and validation (V & V) is intended toshow that a system conforms to its specificationand meets the requirements of the systemcustomer.

Involves checking and review processes andsystem testing.

System testing involves executing the system with

test cases that are derived from the specificationof the real data to be processed by the system.

Testing is the most commonly used V & V activity.

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Stages of testing

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Testing stages

Development or component testing Individual components are tested independently;

Components may be functions or objects or coherentgroupings of these entities.

System testing

Testing of the system as a whole. Testing of emergentproperties is particularly important.

Acceptance testing

Testing with customer data to check that the systemmeets the customers needs.

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Software evolution

Software is inherently flexible and can change.

As requirements change through changing

business circumstances, the software that

supports the business must also evolve andchange.

Although there has been a demarcation

between development and evolution(maintenance) this is increasingly irrelevant as

fewer and fewer systems are completely new.

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System evolution

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Key points up to now

Software processes are the activities involvedin producing a software system. Software

process models are abstract representations

of these processes.

General process models describe the

organization of software processes. Examples

of these general models include the waterfallmodel, incremental development, and reuse-

oriented development.

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Key points up to now

Requirements engineering is the process of developinga software specification.

Design and implementation processes are concernedwith transforming a requirements specification into an

executable software system. Software validation is the process of checking that the

system conforms to its specification and that it meetsthe real needs of the users of the system.

Software evolution takes place when you changeexisting software systems to meet new requirements.The software must evolve to remain useful.

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Reducing the costs of rework

Change avoidance, where the software process

includes activities that can anticipate possible changesbefore significant rework is required.

For example, a prototype system may be developed toshow some key features of the system to customers.

Change tolerance, where the process is designed sothat changes can be accommodated at relatively lowcost.

This normally involves some form of incrementaldevelopment. Proposed changes may be implemented in

increments that have not yet been developed. If this isimpossible, then only a single increment (a small part ofthe system) may have be altered to incorporate thechange.

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Software prototyping

A prototype is an initial version of a systemused to demonstrate concepts and try out

design options.

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

with requirements elicitation and validation

In design processes to explore options anddevelop a UI design

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Benefits of prototyping

Improved system usability A closer match to users real needs

Improved design quality

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Throw-away prototypes

Prototypes should be discarded afterdevelopment as they are not a good basis for aproduction 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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Incremental delivery

Rather than deliver the system as a singledelivery, the development and delivery is brokendown into increments with each incrementdelivering part of the required functionality.

User requirements are prioritised and the highestpriority requirements are included in earlyincrements.

Once the development of an increment is started,the requirements are frozen though requirementsfor later increments can continue to evolve.

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Incremental delivery

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Incremental delivery advantages

Customer value can be delivered with eachincrement so system functionality is available

earlier.

Early increments act as a prototype to helpelicit requirements for later increments.

Lower risk of overall project failure.

The highest priority system services tend to

receive the most testing.

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S i l d l f th ft


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Spiral model of the software process

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Spiral model usage

Spiral model has been very influential inhelping 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

development.

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Key points


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Key points

Processes should include activities to copewith change. This may involve a prototyping

phase that helps avoid poor decisions on

requirements and design.

Processes may be structured for iterative

development and delivery so that changes

may be made without disrupting the system

as a whole.

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