The Software Processes

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

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.

Software processes Description

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.

Process descriptions may also include:

Products, which are the outcomes of a processing activity;

Roles, which reflect the responsibilities of the people involved in the process;

Pre- and post-conditions, are statements that are true before and after a processing activity has been enacted or a product produced.

Plan-driven and agile processes

Plan-driven processes are processes where all of the process activities are 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.

Software Processes Model

The following are some of the software processes models:

1) The waterfall model
  • Plan-driven model. Separate and distinct phases of specification and development.
2) Incremental development
  • Specification, development, and validation are interleaved. May be plan-driven or agile.
3) Integration and configuration
  • The system is assembled from existing configurable 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.

The Waterfall Model

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

The main drawback of this software process model is the difficulty of accommodating change after the process is underway. In principle, a phase has to be complete before moving on to the next phase.

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.

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.
Incremental Development

Incremental Development Benefits

1) 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.

2) 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.

3) More rapid delivery and deployment of useful software to the customer is possible.

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

Incremental Development Problems 

1) The process is not visible.

  • 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.

2) 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.
Integration and Configuration

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

Reused elements may be configured to adapt their behaviour and functionality to a user’s requirements

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

Reuse is covered in more depth in Chapter 15.

Types of reusable software

Stand-alone application systems (sometimes called COTS) are configured for use in a particular environment.

Collections of objects that are developed as a package to be integrated with a component framework such as .NET or J2EE.

Web services that are developed according to service standards and which are available for remote invocation.

Reuse-Oriented Software Engineering

Key processes stages
  • Requirements specification
  • Software discovery and evaluation
  • Requirements refinement
  • Application system configuration
  • Component adaptation and integration

Advantages and Disadvantages

  • Reduced costs and risks as less software are developed from scratch
  • Faster delivery and deployment of the system
  • But requirements compromises are inevitable so the system may not meet the real needs of users
  • Loss of control over the evolution of reused system elements

Processes 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.

For example, in the waterfall model, they are organized in sequence, whereas in incremental development they are interleaved.

The requirement engineering Processes

Software Specification

A) The process of establishing what services are required and the constraints on the system’s operation and development.

B) Requirements engineering process

  • Requirements elicitation and analysis– What do the system stakeholders require or expect from the system?
  • Requirements specification– Defining the requirements in detail
  • Requirements validation– Checking the validity of the requirements
Software design and implementation

The process of converting the system specification into an executable system.

Software design

  • Design a software structure that realizes the specification;

Implementation

  • Translate this structure into an executable program;

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

A general model of the design Processes 

Design activities

Architectural design, where you identify the overall structure of the system, the principal components (subsystems or modules), their relationships, and how they are distributed.

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

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

Component selection and design, where you search for reusable components. If unavailable, you design how it will operate.

System implementation

The software is implemented either by developing a program or programs or by configuring an application system.

Design and implementation are interleaved activities for most types of software systems.

Programming is an individual activity with no standard process.

Debugging is the activity of finding program faults and correcting these faults.

Software validation

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

Involves checking and reviewing 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.

Stages of testing

Testing stages

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.

Customer testing

  • Testing with customer data to check that the system meets the customer’s needs.

Testing phases in a plan-driven software process (V-model)

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 and change.

Although there has been a demarcation between development and evolution (maintenance) this is increasingly irrelevant as fewer and fewer systems are completely new.

System Evolution

Coping with the 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 requires application changes

Change leads to rework so the costs of change include both rework (e.g. re-analyzing requirements) as well as the costs of implementing new functionality

Reducing the costs of the rework

Change anticipation, 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 a 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.
Coping with the changing requirements

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 design decisions. This approach supports change anticipation.

²Incremental delivery, where system increments are delivered to the customer for comment and experimentation. This supports both change avoidance and changes tolerance.

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 in:

  • The requirements engineering process to help with requirements elicitation and validation;
  • In design processes to explore options and develop a UI design;
  • In the testing process run back-to-back tests.
Benefits of prototyping
  • Improved system usability.
  • A closer match to users’ real needs.
  • Improved design quality.
  • Improved maintainability.
  • Reduced development effort.
The process of prototype development

Prototype development

It May be based on rapid prototyping languages or tools

It May involve leaving out functionality

  • The prototype should focus on areas of the product that are not well-understood;
  • Error checking and recovery may not be included in the prototype;
  • Focus on functional rather than non-functional requirements such as reliability and security
Throw-away prototypes

Prototypes should be discarded after development as they are not a good basis for a production 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 organizational quality standards.
Incremental delivery

Rather than deliver the system as a single delivery, the development and delivery is broken down into increments with each increment delivering 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, the requirements are frozen though requirements for later increments can continue to evolve.

Incremental development and delivery

A) Incremental development

  • Develop the system in increments and evaluate each increment before proceeding to the development of the next increment;
  • The normal approach used in agile methods;
  • Evaluation is done by user/customer proxy.

B) Incremental delivery

  • Deploy an increment for use by end-users;
  • More realistic evaluation of practical use of software;
  • Difficult to implement replacement systems as increments have less functionality than the system being replaced.
Incremental delivery
Incremental delivery advantages
  • Customer value can be delivered with each increment so system functionality is available earlier.
  • Early increments act as a prototype to help elicit requirements for later increments.
  • Lower risk of overall project failure.
  • The highest priority system services tend to receive the most testing.
Incremental Delivery problems

Most systems require a set of basic facilities that are used by different parts of the system.

  • As 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.

The essence of iterative processes is that the specification is developed in conjunction with the software.

  • However, this conflicts with the procurement model of many organizations, where the complete system specification is part of the system development contract.

Process Improvement

Many software companies have turned to software process improvement as a way of enhancing the quality of their software, reducing costs, or accelerating their development processes.

Process improvement means understanding existing processes and changing these processes to increase product quality and/or reduce costs and development time.

Approaches to improvement

The process maturity approach focuses on improving process and project management and introducing good software engineering practices.

  • The level of process maturity reflects the extent to which good technical and management practice has been adopted in organizational software development processes.

The agile approach focuses on iterative development and the reduction of overheads in the software process.

  • The primary characteristics of agile methods are rapid delivery of functionality and responsiveness to changing customer requirements.
The Process improvement cycle

Process improvement activities

A) Process measurement

  • You measure one or more attributes of the software process or product. These measurements forms a baseline that helps you decide if process improvements have been effective.

B) Process analysis

  • The current process is assessed, and process weaknesses and bottlenecks are identified. Process models (sometimes called process maps) that describe the process may be developed.

C) Process change

  • Process changes are proposed to address some of the identified process weaknesses. These are introduced and the cycle resumes to collect data about the effectiveness of the changes.
Process measurement

Wherever possible, quantitative process data should be collected

  • However, where organizations do not have clearly defined process standards this is very difficult as you don’t know what to measure. A process may have to be defined before any measurement is possible.

Process measurements should be used to assess process improvements

  • But this does not mean that measurements should drive the improvements. The improvement driver should be the organizational objectives.
Process metrics

Time is taken for process activities to be completed

  • E.g. Calendar time or effort to complete an activity or process.

Resources required for processes or activities

  • E.g. Total effort in person-days.

Number of occurrences of a particular event

  • E.g. Number of defects discovered.
Capability maturity levels

The SEI capability maturity model

A) Initial

  • Essentially uncontrolled

B) Repeatable

  • Product management procedures defined and used

C) Defined

  • Process management procedures and strategies defined
    and used

D) Managed

  • Quality management strategies defined and used

E Optimising

  • Process improvement strategies defined and used

KEY POINTS TO REMEMBER

  • Software processes are the activities involved in 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 ‘waterfall’ model, incremental development, and reuse-oriented development.
  • Requirements engineering is the process of developing a software specification.
  • Design and implementation processes are concerned with 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 meets the real needs of the users of the system.
  • Software evolution takes place when you change existing software systems to meet new requirements. The software must evolve to remain useful.
  • Processes should include activities such as prototyping and incremental delivery to cope with change.

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