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Software Processes. Objectives. To introduce software process models To describe three generic process models and when they may be used To describe outline process models for requirements engineering, software development, testing and evolution To explain the Rational Unified Process model
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Objectives • To introduce software process models • To describe three generic process models and when they may be used • To describe outline process models for requirements engineering, software development, testing and evolution • To explain the Rational Unified Process model • To introduce CASE technology to support software process activities
Topics covered • Software process models • Process iteration • Process activities • The Rational Unified Process • Computer-aided software engineering
The software process • A structured set of activities required to develop a software system • Specification; • Design & implementation; • Validation; • Evolution. • A software process model is an abstractrepresentation of a process. It presents a description of a process from some particular perspective.
Generic software process models • The waterfall model • Separate and distinct phases of specification and development. • Evolutionary development • Specification, development and validation are interleaved. • Component-based software engineering • The system is assembled from existing components.
Waterfall model phases • Requirements analysis and definition: • define the system’s services, constrains and goals. • System and software design: • identify and describe the fundamental software system abstractions and relationships • Implementation and unit testing: • code the system as a set of programs or program units. Unit testing involves verifying that each unit meets its specification.
Waterfall model phases • 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. • Operation and maintenance: • The system is installed. Maintenance involves correcting errors which were not discovered in earlier stages of the life cycle, improving the implementation of system units and enhancing the system’s services as new requirements are discovered. • The main drawback of the waterfall model is the difficulty of accommodating change after the process is underway. One phase has to be complete before moving onto 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.
Evolutionary development • Specification, development and validation activities are interleaved rather than separate, with rapid feedback across activities. • Two types: • Exploratory development • Objective is to work with customers and to evolve a final system from an initial outline specification. Should start with well-understood requirements and add new features as proposed by the customer. • Throw-away prototyping • understand the customer’s requirements and hence develop a better requirements definition for the system.. Should start with poorly understood requirements to clarify what is really needed.
Evolutionary development • Problems • Lack of process visibility; • Systems are often poorly structured; • Special skills (e.g. in languages for rapid prototyping) may be required. • Applicability • For small or medium-size interactive systems; • For parts of large systems (e.g. the user interface); • For short-lifetime systems.
Waterfall VS. Evolutionary • An evolutionary approach to software development is often more effective than the waterfall approach in producing systems that meet the immediate needs of customers
Component-based software engineering • looking for designs or code which is similar to the particular requirements; modify them as needed and incorporate with the system. • Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems.
Processes of Component-based software engineering • Process stages • Component analysis; • Requirements modification; • System design with reuse; • Development and integration. • This approach is becoming increasingly used as component standards have emerged.
Advantages of Component-based software engineering • Reducing the amount of software to be developed • Reducing cost and risks. • Leading to faster delivery of the software
disadvantages of Component-based software engineering • Requirements are inevitable and may lead to a system that does not meet the real needs of users. • In evolution phase, we may lost some control over the system as new versions of the reusable components.
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. • Two (related) approaches • Incremental delivery; • Spiral development.
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.
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.
Incremental development disadvantages • Increments should be relatively small (no more than 20,000 lines of code) because • It is difficult to map the customer’s requirements onto increments of the right size • Hard to identify the errors. • This can be developed by Extreme programming
Extreme programming • An approach to development based on the development and delivery of very small increments of functionality. • Relies on constant code improvement, user involvement in the development team and pairwise programming. • Covered in Chapter 17
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 in the process. • No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required. • Risks are explicitly assessed and resolved throughout the process.
Spiral model sectors • Objective setting • Specific objectives for the phase are identified. • Constrains on the process and product are identify. • Risk assessment and reduction • Risks are assessed and activities put in place to reduce the key risks. • Ex: if there is a risk of inappropriate requirements, develop a prototype system. • Development and validation • A development model for the system is chosen which can be any of the generic models. • The waterfall model may be the most appropriate development model if the main identified risk is sub-system integration. • For example, if user interface risks are dominant, an appropriate development model might be evolutionary prototyping. • Planning • The project is reviewed and the next phase of the spiral is planned.
Spiral model • The main difference between the spiral and others software process models is to recognize the risks • Risks means are we going wrong. • For example, if we are going to use a new programming language, a risk is that the available compilers are unreliable or do not produce sufficiently efficient object code. • Risk result schedule and cost overrun
Computer-aided software engineering • Computer-aided software engineering (CASE) is software to support software development and evolution processes. • software development process: • requirements engineering • design • program development • and testing. • CASE tools therefore include design editors, data dictionaries, compilers, debuggers, system building tools and so on.
CASE technology provides software process support by automating some process activities and by providing information about the software that is being developed. • Activity automation • Graphical editors for system model development; • Data dictionary to manage design entities; • Graphical UI builder for user interface construction; • Debuggers to support program fault finding; • Automated translators to generate new versions of a program.
Case technology • Case technology has led to significant improvements in the software process. However, these are not the order of magnitude improvements that were once predicted;reasons • Software engineering requires creative thought - this is not readily automated; • Software engineering is a team activity and, for large projects, much time is spent in team interactions. CASE technology does not really support these.
CASE classification • Classification helps us understand the different types of CASE tools and their support for process activities. • Functional perspective • Tools are classified according to their specific function. • Process perspective • Tools are classified according to process activities that are supported. • Integration perspective • Tools are classified according to their organisation into integrated units.
