Ch2 SW Processes Focused(2).pdf

Fundamentals of Software Engineering Typical problems to solve 1 Example 1 Typical problems to solve include : Build a system which allows all students to electronically record their attendance for each timetabled class. 2 Example 2 Typical problems to solve include : Build a system which char...

Fundamentals of Software Engineering Typical problems to solve 1 Example 1 Typical problems to solve include : Build a system which allows all students to electronically record their attendance for each timetabled class. 2 Example 2 Typical problems to solve include : Build a system which charges motorists to travel into a city. 3 Example 3 Typical problems to solve include Build a system to control aircraft travelling through British Airspace. 4 Example 4 Typical problems to solve include Build a system to control the operation a power station. 5 Question 1 How do you progress from a vague idea for an application to a working solution? 6 Answer Use Software Processes ! 7 Question 2 "Do we need different approaches for different types of application/system" 8 Answer 1 Yes, it is likely that different type of problems will require different approaches and hence different software processes 9 Software Processes 10 Question Why Use Software Processes ? 11 Answer 2 Why Use Software Processes? Think programming in the large 12 Topics covered Software process models  Process activities  Coping with change  13 The software process A structured set of activities required to develop a software system.  A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective.  14 The software process cont.  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. 15 Software process models 16 List 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 ◦ 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. 17 Test Question 1 You are developing a student record system for a University. You have developed this type of system before. Do you use : 1. 2. 3. Integrated and Configuration Approach A Plan driven approach Incremental approach 18 Test Question 2 You are developing a pumping system for a hydro electric dam. This type of system is governed by national legislation. Do you use : 1. 2. 3. Integrated and Configuration Approach A Plan driven approach Incremental approach 19 Test Question 3 You are developing an online booking system for a motor vehicle repair garage. The owners of the garage want to make better use of information technology to help their business operate but do not have a clear view of how to do this. Do you use : 1. 2. 3. Integrated and Configuration Approach A Plan driven approach Incremental approach 20 The waterfall model 21 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. 22 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. 23 Incremental development 24 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. 25 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. 26 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. 27 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.  28 Key process stages Requirements specification  Software discovery and evaluation  Requirements refinement  Application system configuration  Component adaptation and integration  29 Reuse-oriented software engineering 30 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  31 Process Model activities 32 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. 33 The requirements engineering process 34 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 35 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 inter-leaved. 36 A general model of the design process 37 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.  38 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.  39 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.  40 Stages of testing 41 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. 42 Testing phases in a plan-driven software process (V-model) 43 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.  44 System evolution 45 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. 46 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.  47