Chapter 2 software processes
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Chapter 2 – Software Processes. Lecture 1. Topics covered. Software process models Process activities Coping with change The B ohem model. Software process descriptions. A software process is a set of related activities that leads to the production of a software product.

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Chapter 2 – Software Processes

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Chapter 2 software processes

Chapter 2 – Software Processes

Lecture 1

Chapter 2 Software Processes


Topics covered

Topics covered

  • Software process models

  • Process activities

  • Coping with change

  • The Bohem model

Chapter 2 Software Processes


Software process descriptions

Software process descriptions

  • A software process is a set of related activities that leads to the production of a software product.

  • When we describe and discuss processes, we usually talk about the activities in these processes such as specifying a data model, designing a user interface, etc. and the ordering of these activities.

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Software process descriptions1

Software process descriptions

  • 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 been enacted or a product produced.

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The software process

Many different software processes but all involve:

Specification – defining what the system should do;

Design and implementation – defining the organization of the system and implementing the system;

Validation – checking that it does what the customer wants;

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

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

The software process

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Process types plan driven and agile processes

Process types: Plan-driven and agile processes

  • Plan-driven processes are processes where all of the process activitiesare planned in advance and progress is measured against this plan.

  • In 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.

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Software process models

The waterfall model

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

Incremental development

Specification, development and validation are interleaved. May be plan-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 that incorporates elements from all of these models.

Software process models

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1 st model the waterfall model

1st model: The waterfall model

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Waterfall model phases

There are separate identified phases in the waterfall model:

Requirements analysis and definition

System and software design

Implementation and unit testing

Integration and system testing

Operation and maintenance

Waterfall model phases

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Waterfall model

Waterfall model

  • In principle:

    • A phase has to be complete before moving onto the next phase.

    • The result of each phase is one or more documents that are approved (‘signed off’).

  • In practice, these stages overlap and feed information to each other

  • Because of the costs of producing and approving documents, iterations can be costly and involve significant rework.

  • The main drawback of the waterfall model is the difficulty of accommodating change after the process is underway.

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Waterfall model benefits

Waterfall model benefits

  • Simple and easy to manage:

    • The waterfall model is mostly used for large systems engineering projects where a system is developed at several sites. In those circumstances, the plan-driven nature of the waterfall model helps coordinate the work.

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Waterfall model problems

Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements.

Therefore, this model is only appropriate when the requirements are well-understood and changes will be fairly limited during the design process.

Few business systems have stable requirements.

Amount and cost of rework is high

High costs of producing and approving documents

Consume more time….(i.e.more cost)

Waterfall model problems

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2 nd model incremental development

2nd model: Incremental development

  • Incremental development is based on the idea of developing an initial implementation exposing this to user comment and evolving it through several versions until an adequate system has been developed

  • Specification, development, and validation activities are interleaved rather than separate, with rapid feedback across activities.

  • Generally, the early increments of the system include the most important or most urgently required functionality. (i.e. the customer can evaluate the system at a relatively early stage in the development)

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Incremental development

Incremental development

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Incremental development1

Incremental development

  • Incremental development in some form is now the most common approach for the development of application systems.

  • This approach can be either plan-driven, agile, or, more usually, a mixture of these approaches.

    • In a plan-driven approach, the system increments are identified in advance;

    • In an agile approach, the early increments are identified but the development of later increments depends on progress and customer priorities.

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Incremental development benefits

The cost of accommodating changing customer requirements is reduced.

The amount of analysis and documentation that has to be redone 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 to the customer is possible.....(time)

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

Incremental development benefits

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Incremental development problems

Incremental development problems

  • The process is not visible.(i.e. difficult to manage)

    • Managers need regular 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.

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3 rd model reuse oriented software engineering

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

Process stages

Component analysis;

Requirements modification;

System design with reuse;

Development and integration.

Reuse is now the standard approach for building many types of business system

3rd model: Reuse-oriented software engineering

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Reuse oriented software engineering

Reuse-oriented software engineering

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Process activities

Process activities

  • Real software processes are inter-leaved sequences 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 and evolution are organized differently in different development processes. In the waterfall model, they are organized in sequence, whereas in incremental development they are inter-leaved.

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Software specification requirements engineering

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

It is critical stage of the software process as errors at this stage inevitably lead to later problems in the system design and implementation.

The requirements engineering process aims to produce an agreed requirements document .

Requirements are usually presented at two levels of detail:

User requirements: which are abstract statements of the system requirements for the customer and end-user of the system. It’s for customers

system requirements which are a more detailed description of the functionality to be provided. It is for system developer.

Software specification (/ Requirements engineering )

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Software specification requirements engineering1

Software specification (/ Requirements engineering )

  • Requirements engineering process

    • Feasibility study

      • Is it technically and financially feasible to build the system?

    • Requirements elicitation and analysis

      • What do the system stakeholders require or expect from the system?

    • Requirements specification

      • Defining the requirements in a document.

    • Requirements validation

      • Checking the validity of the requirements

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The requirements engineering process

The requirements engineering process

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Software design and implementation

Software design

It is a description of the structure of the software to be implemented, the data models and structures used by the system, the interfaces between system components and, sometimes, the algorithms used.

Designers do not arrive at a finished design immediately but develop the design iteratively.

Implementation

The process of converting the system specification into an executable system

The activities of design and implementation are closely related and may be inter-leaved.

Software design and implementation

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A general model of the design process

A general model of the design process

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Design activities

Design activities

  • Architectural design, where you identify the overall structure of the system, the principal components (sometimes called sub-systems or modules), their relationships and how they are distributed.

  • Interface design, where you define the interfaces between system components.

  • Component design, where you take each system component and design how it will operate.

  • Database design, where you design the system data structures and how these are to be represented in a database.

  • NOTE: The activities in the design process vary, depending on the type of systembeing developed.

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Implementation programming coding

Implementation (/programming / coding)

  • Programming is a personal activity and there is no general process that is usually followed.

  • Normally, programmers carry out some testing of the code they have developed. This often reveals program defects that must be removed from the program.

  • Defect testing and debugging are different processes:

    • Testingestablishes the existence of defects.

    • Debugging is concerned with locating and correcting these defects.

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Software validation

Verification and validation (V & V) is intended to show that a system conforms to its specification and meets the requirements of the system customer.

Involves checking and review processes and system testing.

System testing involves executing the system with test cases that are derived from the specification of the real data to be processed by the system.

Testing is the most commonly used V & V activity.

Software validation

A set of inputs, execution preconditions, and expected outcomes developed to verify compliance with a specific requirement

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Stages of testing

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 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.

Testing stages

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Software evolution software maintenance

software evolution (/software maintenance)

  • The flexibility of software systems is essential issue

  • It is very expensive to make changes to the hardware design. changes can be made to software at any time during or after the system development.

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Coping with change

Coping with change

  • Change is inevitable in all large software projects.

    • Business changes lead to new and changed system requirements

    • New technologies open up new possibilities for improving implementations

    • Changing platforms require application changes

  • Change leads to rework so the costs of change include both rework (e.g. re-analysing requirements) & the costs of implementing new functionality

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Reducing the costs of rework approaches

Reducing the costs of rework approaches

  • Change avoidance, where the software process includes activities that can anticipate possible changes before significant rework is required.

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

  • Change tolerance, where the process is designed so that changes can be accommodated at relatively low cost.

    • This normally involves some form of incremental development. Proposed changes may be implemented in increments that have not yet been developed.

    • If this is impossible, then only a single increment (a small part of the system) may have be altered to incorporate the change.

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Coping with change methods

Coping with change methods

  • System prototyping,

    • where a version of the system or part of the system is developed quickly to check the customer’s requirements and the feasibility of some design decisions.

    • This supports change avoidance as it allows users to experiment with the system before delivery and so refine their requirements.

    • The number of requirements change proposals made after delivery is therefore likely to be reduced.

  • Incremental delivery,

    • where system increments are delivered to the customer for comment and experimentation.

    • This supports both change avoidance and change tolerance. It avoids the premature commitment to requirements for the whole system and allows changes to be incorporated into later increments at relatively low cost.

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Software prototyping

A prototype is an initial version of a system used to demonstrate concepts and try out design options.

A prototype can be used to help anticipate changes that may be required: :

In the requirements engineering process, a prototype can help with the elicitation and validation of system requirements.

In the system design process, a prototype can be used to explore particular software solutions and to support user interface design.

Software prototyping

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The process of prototype development

The process of prototype development

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Boehm s spiral model

A risk-driven software process framework (the spiral model) was proposed by Boehm (1988).

Process is represented as a spiral rather than as a sequence of activities with backtracking.

Each loop in the spiral represents a phase in the process (i.e. the innermost loop might be concerned with system feasibility, the next loop with requirements definition, the next loop with system design, and so on)

Boehm’s spiral model

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Boehm s spiral model1

Boehm’s spiral model

  • The spiral model combines change avoidance with change tolerance

  • The main difference between the spiral model and other software process models is its explicit recognition of risk.

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Boehm s spiral model2

Boehm’s spiral model

  • Each loop in the spiral is split into four sectors:

  • 1. Objective setting:

    • Specific objectives for that phase of the project are defined.

    • Constraints on the process and the product are identified and a detailed management plan is drawn up.

    • Project risks are identified. Alternative strategies may be planned.

  • 2. Risk assessment and reduction

    • For each risk, a detailed analysis is carried out. Steps are taken to reduce the risk. (e.g. if there is a risk that the requirements are inappropriate, a prototype system may be developed)

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Boehm s spiral model3

Boehm’s spiral model

  • 3.Development and validation

    • After risk evaluation, a development model for the system is chosen.

    • For example, throwaway prototyping may be the best development approach if user interface risks are dominant. If the main identified risk is sub-system integration, the waterfall model may be the best development model to use.

  • 4. Planning

    • The project is reviewed and a decision made whether to continue with a further loop of the spiral.

    • If it is decided to continue, plans are drawn up for the next phase of the project.

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Boehm s spiral model of the software process

Boehm’s spiral model of the software process

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Spiral model usage

Spiral model usage

  • Spiral model has been very influential in helping people think about iteration in software processes and introducing the risk-driven approach to development.

    • Iteration:means the act of repeating a process with the aim of approaching a desired goal, target or result.

    • Each repetition of the process is also called an "iteration," and the results of one iteration are used as the starting point for the next iteration

  • In practice, however, the model is rarely used as published for practical software development.

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Risk examples

Risk examples

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Risk examples1

Risk examples

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Risk examples2

Risk examples

Chapter 2 Software Processes


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