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Software Chapter Processes
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Objectives
To introduce software process and software process models. To describe three generic process models and when they may be used. To outline process models for requirements engineering, software development, testing and evolution. To explain the Rational Unied Process model. To introduce CASE technology to support software process activities.
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Topics covered
Software process models Process iteration Process activities The Rational Unied Process Computer-aided software engineering
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The software process
A software process is a coherent set of activities for specifying, designing, implementing and testing software systems. It is a structured set of activities required to develop a software system. Fundamental activities: Specication Design & Implementation Validation Evolution
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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Generic software process models
1. The waterfall model Separate and distinct phases of specication and development. 2. Evolutionary development Specication and development are interleaved. 3. Reuse-based development The system is assembled from existing components. There are many variants of these models e.g. formal development where a waterfall-like process is used but the specication is a formal specication that is rened through several stages to an implementable design.
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Waterfall model
Based on more general system engineering processes model Classic approach to the systems development life cycle Describes a development method that is linear and sequential Distinct goals for each phase of development One phase has to be completed before moving onto the next phase and there is no turning back
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Waterfall model
The waterfall model has the following phases: 1. Requirements analysis and denition 2. System and software design 3. Implementation and unit testing 4. Integration and system testing 5. Operation and maintenance
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Waterfall model
Requirements definition System and software design
Implementation and unit testing
Integration and system testing
Operation and maintenance
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Waterfall model advantages
Each phase of development proceeds in strict order, without any overlapping or iterative steps. It allows for departmentalization and managerial control. A schedule can be set with deadlines for each phase. The output of each phase is one or more documents that are approved (signed o). Testing is inherent to every phase.
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Waterfall model problems
The main drawback of the waterfall model is the diculty of accommodating change after the process is underway. Inexible partitioning of the project into distinct stages makes it dicult 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. Alternatives to the waterfall model include joint application development (JAD), rapid application development (RAD), synch and stabilize, build and x, and the spiral model.
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Waterfall model: Requirements denition
The systems services, constraints, and goals are established by consulting with system users. They are then dened in detail and serve as a system specification.
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Waterfall model: System and software design
Partition the requirements to either software or hardware systems. Establish an overall system architecture. Identify and describe the fundamental software system abstractions and their relationships.
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Waterfall model: Implementation and unit testing
The software design is realized as a set of programs or program units. Then verify that each unit meets its specication.
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Waterfall model: Integration and system testing
The individual program units or programs are integrated and tested as a complete system to ensure that the software requirements have been met. Then, the software sustem is delivered to the customer.
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Waterfall model: Operation and maintenance
Usually, the longest life-cycle phase. The system is installed and put to work. Maintenance involves correcting errors which were not discovered in earlier stages improving the implementation of system units enhancing the systems services as new requirements are discovered.
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Generic software process models
1. The waterfall model Separate and distinct phases of specication and development. 2. Evolutionary development Specication and development are interleaved. 3. Reuse-based development The system is assembled from existing components.
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Evolutionary development
It is based on the idea of exploratory development. Objective is to work with customers and to evolve a nal system from an initial outline specication. Should start with well-understood requirements. Development and validation activities are carried out concurrently. There are two types of evolutionary development: 1. Exploratory development 2. Throw-away prototyping
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Evolutionary development
Specification
Initial version Intermediate versions Final version
Outline description
Development
Validation Concurrent activities
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1. Exploratory development
Starts with the parts of the system which are understood and the system evolves by adding new features. The system evolves by adding new features as they are proposed by the customer.
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2. Throw-away prototyping
Objective is to understand the system requirements. Should start with poorly understood requirements to clarify what is really needed. The prototype concentrates on experimenting with those parts which are poorly understood.
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Problems
Lack of process visibility Systems are often poorly structured Special skills (e.g. in languages for rapid prototyping) may be required
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Applicability
For small or medium-size interactive systems For parts of large systems (e.g. the user interface) For short-lifetime systems
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Generic software process models
1. The waterfall model Separate and distinct phases of specication and development. 2. Evolutionary development Specication and development are interleaved. 3. Reuse-based development The system is assembled from existing components.
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Component-based software engineering
Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-o-the-shelf) systems. The process stages are: 1. Component analysis 2. Requirements modication 3. System design with reuse 4. Development and integration This approach is becoming more important but still limited experience with it.
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Component-based software engineering
Requirements specification Component analysis
System design with reuse
Requirements modification
Development and integration
System validation
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Process iteration
System requirements ALWAYS evolve in the course of a project so process iteration where earlier stages are reworked is always part of the process for large systems. Iteration can be applied to any of the generic process models. There are two (related) approaches: Incremental delivery Spiral development
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1. Incremental development
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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1. Incremental development
Define outline requirements Validate system
Final system
Assign requirements to increments
Integrate increment
Design system architecture
Validate increment
Develop system increment System incomplete
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Incremental development 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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Extreme programming
A new approach to development based on the development and delivery of very small increments of functionality. It relies on constant code improvement, user involvement in the development team and pairwise programming. See Chapter 17.
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2. Spiral development
Process is represented as a spiral rather than as a sequence of activities with backtracking. Each loop in the spiral represents a phase of the software process: the innermost loop might be concerned with system feasability the next loop with system requirements denition the next loop with system design and so on No xed phases such as specication or design - loops in the spiral are chosen depending on what is required. Risks are explicitly assessed and resolved throughout the process. Each loop in the spiral is split into four model sectors:
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2. Spiral development
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Spiral model sectors
1. Objective setting. Specic objectives for the phase are identied. 2. Risk assessment and reduction. Risks are assessed and activities put in place to reduce the key risks. 3. Development and validation. A development model for the system is chosen which can be any of the generic models. 4. Planning. The project is reviewed and the next phase of the spiral is planned.
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Process activities
1. Software specication 2. Software design and implementation 3. Software validation 4. Software evolution Dierent processes organise them dierently waterfall model: they are organised in sequence evolutionary development: they are interleaved There is no wright or wrong way to organise them
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Software specication
The process of establishing what are services required and the constraints on the systems operation and development. Also called the requirements engineering process. There are four phases: Feasibility study; Requirements elicitation and analysis; Requirements specication; Requirements validation.
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Software specication
Feasability study
Requirements elicitation and analysis Requirements specification Requirements validation
Feasability report System models User and System requirements Requirements document
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Software design and implementation
The process of converting the system specication into an executable system. Involves: 1. Software design. Design a software structure that realises the specication. 2. 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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Design process activities
1. Architectural design 2. Abstract specication 3. Interface design 4. Component design 5. Data structure design 6. Algorithm design
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Design process activities
Architectural design System architecture
Abstract specification
Software specification
Interface Requirements specification Component design design
Interface specification
Component specification
Data structure design
Data structure specification
Algorithm design
Algorithm specification
Design activities
Design products
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Design process activities
1. Architectural design. Identify and document the sub-systems making up the system and their relationships. 2. Abstract specication. For each sub-system, produce an abstract specication of its services and the constraints under which it must operate. 3. Interface design. For each sub-system, design and document its interface with other sub-systems. 4. Component design. Allocate services to dierent components; design the interfaces of these components. 5. Data structure design. Specify and design the data structures used in the system implementation. 6. Algorithm design. Specify and design the algorithms used to provide services.
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Structured design methods
Systematic approaches to developing a software design. The design is usually documented as a set of graphical models. Possible models: 1. Data-ow model 2. Sequence model 3. State transition model 4. Structural model 5. Object models
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Programming and debugging
Translating a design into a program and removing errors from that program. Programming is a personal activity - there is no generic programming process. Programmers carry out some program testing to discover faults in the program and remove these faults in the debugging process.
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Programming and debugging
The debugging process: 1. Locate error 2. Design error repair 3. Repair error 4. Re-test program
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Programming and debugging
Locate error
Design error repair
Repair error
Retest program
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Software validation
Verication and validation is intended to show that a system conforms to its specication and meets the requirements of the system customer. It involves checking and review processes and system testing System testing involves executing the system with test cases that are derived from the specication of the real data to be processed by the system.
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The testing process
Component testing
System testing
Acceptace testing
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Stages in the testing process
1. Component (Unit) testing. Individual components are tested independently. Components may be functions or objects or coherent groupings of these entities. 2. System testing. Testing of the system as a whole. Testing of emergent properties. 3. Acceptance testing. Testing with customer data to check that it is acceptable and meets the customers needs.
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Testing phases
How test plans are the links between testing and development activities:
Requirements specification System specification System design Detailed design
Acceptance test plan
System integration test plan
Subsystem integration test plan
Module and unit code and test
Service
Acceptance test
System integration test
Subsystem integration test
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Software evolution
Software is inherently exible 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
Define system requirements
Assess existing systems
Propose system changes
Modify systems
Existing system
New system
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The Rational Unied Process
A modern process model derived from the work on the UML and associated process. Good example of a hybrid process model. 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 practice.
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Dynamic RUP: phase model
Phase
iteration
Inception
Elaboration
Construction
Transition
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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.
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RUP good practice
Develop software iteratively Manage requirements Use component-based architectures Visually model software Verify software quality Control changes to software
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Static RUP: workows
The activities in the development process are called workows. There are six core process workows: Business modelling Requirements Analysis and design Implementation Testing Deployment There are three supporting workows: Conguration and change management Project management Environment
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Critics of RUP
Phases are dynamic and have goals Workows are static and are technical activities The advantages: separation of phases and workows reco...