Chapter 2 – Software Processes

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Topics covered

• Software process models
• Process activities
• Coping with change
• Process improvement

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The software process

• 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 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.

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Software process descriptions

• 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 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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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.

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Software process models

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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.
• Integration and configuration/Reuse-oriented SWE
• 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.

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

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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.
• 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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Incremental development �

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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.
• Customers are able to use and gain value from the software earlier 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 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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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 covered in more depth in Chapter 15.

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Types of reusable software

• Stand-alone application systems (sometimes called COTS) that 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.

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Reuse-oriented software engineering

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Key process stages

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

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Advantages and disadvantages

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

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Process activities

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Process activities

• Real software processes are interleaved 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.

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The requirements engineering process�

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Software specification

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

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Software design and implementation

• The process of converting the system specification into an executable system.
• Software design
• Design a software structure that realises the specification;
• Implementation
• Translate this structure into an executable program;
• The activities of design and implementation are closely related and may be interleaved.

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A general model of the design process �

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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.

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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 system.
• Programming is an individual activity with no standard process.
• Debugging is the activity of finding program faults and correcting these faults.

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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.

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Stages of testing�

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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.

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

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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 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.

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System evolution

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Coping with change

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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) as well as the costs of implementing new functionality

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Reducing the costs of 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 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 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.
• Incremental delivery, where system increments are delivered to the customer for comment and experimentation. This supports both change avoidance and change tolerance.

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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 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 to run back-to-back tests.

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Benefits of prototyping

• Improved system usability.
• A closer match to users’ real needs.
• Improved design quality.
• Improved maintainability.
• Reduced development effort.

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The process of prototype development�

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Prototype development

• May be based on rapid prototyping languages or tools
• May involve leaving out functionality
• 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

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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 organisational quality standards.

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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 prioritised 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.

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Incremental development and delivery

• Incremental development
• Develop the system in increments and evaluate each increment before proceeding to the development of the next increment;
• Normal approach used in agile methods;
• Evaluation done by user/customer proxy.
• Incremental delivery
• Deploy an increment for use by end-users;
• More realistic evaluation about practical use of software;
• Difficult to implement for replacement systems as increments have less functionality than the system being replaced.

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Incremental delivery

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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.

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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.

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Process improvement

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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.

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Approaches to improvement

• The process maturity approach, which focuses on improving process and project management and introducing good software engineering practice.
• 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, which 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.

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The process improvement cycle

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Process improvement activities

• 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.
• 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.
• 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.

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Process measurement

• Wherever possible, quantitative process data �should be collected
• However, where organisations 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.

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Process metrics

• Time 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.

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Capability maturity levels

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The SEI capability maturity model

• Initial
• Essentially uncontrolled
• Repeatable
• Product management procedures defined and used
• Defined
• Process management procedures and strategies defined �and used
• Managed
• Quality management strategies defined and used
• Optimising
• Process improvement strategies defined and used

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Key points

• 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.

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Key points

• 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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Key points

• Processes may be structured for iterative development and delivery so that changes may be made without disrupting the system as a whole.
• The principal approaches to process improvement are agile approaches, geared to reducing process overheads, and maturity-based approaches based on better process management and the use of good software engineering practice.
• The SEI process maturity framework identifies maturity levels that essentially correspond to the use of good software engineering practice.

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30/10/2014