Software Processes Chapter 3 Ian Sommerville 2000 Software

Software Processes (Chapter 3) ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 1

Objectives l l To introduce the software process model concept To describe different process models and their pros and cons To outline process models for requirements engineering, software development, testing, and evolution To introduce CASE technology to support software process activities ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 2

The Software Process l A structured set of activities required to develop a software system. These activities include: • • • l Specification Design Implementation Validation Evolution A software process model is an abstract representation of a process. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 3

Generic Software Process Models l The waterfall model • l Evolutionary development • l Separate and distinct phases of specification and development Specification and development are interleaved Reuse-based development • The system is assembled from some (most likely) or all existing components ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 4

Waterfall Model ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 5

Waterfall Model Pros and Cons Pros Cons ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 6

Evolutionary Development Two general types: l Exploratory development • l Objective is to work with the customers to evolve a final system from an initial outline specification. Process starts with the well -understood requirements. Prototyping (throw-away? ) • Objective is to understand the system requirements. Process starts with the poorly understood requirements. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 7

Evolutionary Development ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 8

Evolutionary Development Pros and Cons Pros Cons ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 9

Throw-away Prototyping Pros and Cons Pros Cons ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 10

Reuse-oriented Development l l Based on systematic reuse where systems are integrated from existing components or COTS (commercial-off-the-shelf) systems This approach is becoming more important, but there is still limited experience with it. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 11

Reuse-oriented Development ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 12

Reuse-oriented Development Pros and Cons Pros Cons ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 13

Process Iteration l l l System requirements ALWAYS evolve in the course of a project. So, process iteration where earlier stages are reworked is always part of the process, especially for large systems. Iteration can be applied to any of the generic process models. Examples of two iterative approaches: • • Incremental development Spiral development ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 14

Incremental Development l l l 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. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 15

Incremental Development ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 16

Incremental Development Advantages l l Customers do not have to wait until the entire system is delivered until they can gain value from it. 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. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 17

Spiral Development l l 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. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 18

Spiral model of the software process ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 19

Spiral Model Sectors l Objective setting • l Risk assessment and reduction • l Risks are assessed and activities put in place to reduce the key risks Development and validation • l Specific objectives for the phase are identified A development model for the system is chosen which can be any of the generic models Planning • The project is reviewed and the next phase of the spiral is planned ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 20

Requirements Engineering The process of establishing l what services are required of the system l the constraints on the system’s operation and development ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 21

The Requirements Engineering Process ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 22

Software Design and Implementation The process of converting the system specification into an executable system. l Software design • l Implementation • l Design a software structure that realizes the specification Translate this structure into an executable program The activities of design and implementation are closely related and may be inter-leaved. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 23

The Software Design Process ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 24

Design Methods Systematic approaches to developing a software design l The design is usually documented as a set of graphical models. l Possible models • • Data-flow model Entity-relation-attribute model Structural model Object models ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 25

Programming and Debugging l l l 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. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 26

Software Validation l l l Verification and validation 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. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 27

The Testing Process ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 28

Testing Stages l Unit testing • l Modules are integrated into sub-systems and tested. The focus here should be on interface testing System testing • l Related collections of dependent components are tested Sub-system testing • l Individual components are tested Testing of the system as a whole. Testing of emergent properties Acceptance testing • Testing with customer data to check that it is acceptable ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 29

Testing Phases ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 30

Software Evolution l l l Software is inherently flexible and can change (as opposed to hardware). In the past, there has been a demarcation between development and evolution (maintenance). This is increasingly irrelevant as fewer and fewer systems are completely new. Software engineering should be thought of as an evolutionary process where software is continually changed over its lifetime in response to customer needs. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 31

System Evolution ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 32

Automated Process Support (CASE) l l Computer-aided software engineering (CASE) is software used to support software development and evolution processes. 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 ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 33

Case Technology l Case technology has led to significant improvements in the software process, though not the order of magnitude improvements that were once predicted. • • Software engineering requires creative thought - this is not readily automatable. Software engineering is a team activity and, for large projects, much time is spent in team interactions. CASE technology does not really support these. ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 34

CASE Classification helps us understand the different types of CASE tools and their support for process activities. l Functional perspective • l Process perspective • l Tools are classified according to their specific function Tools are classified according to process activities that are supported Integration perspective • Tools are classified according to their organization into integrated units ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 35

Functional Tool Classification ©Ian Sommerville 2000 Software Engineering, 6 th edition. Chapter 1 Slide 36

Activity-based classification