Ch2 SW Processes and models.pptx

Chapter 2 – Software Processes 30/10/2014 Chapter 2 Software Processes 1 Topics covered  Software process models  Process activities 30/10/2014 Chapter 2 Software Processes 2 The software process  A structured set of activities required to develop a software system...

Chapter 2 – Software Processes 30/10/2014 Chapter 2 Software Processes 1 Topics covered  Software process models  Process activities 30/10/2014 Chapter 2 Software Processes 2 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. 30/10/2014 Chapter 2 Software Processes 3 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. 30/10/2014 Chapter 2 Software Processes 4 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. 30/10/2014 Chapter 2 Software Processes 5  Software process models 30/10/2014 Chapter 2 Software Processes 6 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. 30/10/2014 Chapter 2 Software Processes 7 The waterfall model 30/10/2014 Chapter 2 Software Processes 8 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. 30/10/2014 Chapter 2 Software Processes 9 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. 30/10/2014 Chapter 2 Software Processes 10 Incremental development 30/10/2014 Chapter 2 Software Processes 11 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. 30/10/2014 Chapter 2 Software Processes 12 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. 30/10/2014 Chapter 2 Software Processes 13 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. 30/10/2014 Chapter 2 Software Processes 14 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. 30/10/2014 Chapter 2 Software Processes 15 Reuse-oriented software engineering 30/10/2014 Chapter 2 Software Processes 16 Key process stages  Requirements specification  Software discovery and evaluation  Requirements refinement  Application system configuration  Component adaptation and integration 30/10/2014 Chapter 2 Software Processes 17 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 30/10/2014 Chapter 2 Software Processes 18  Process activities 30/10/2014 Chapter 2 Software Processes 19 Process 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. 30/10/2014 Chapter 2 Software Processes 20 The requirements engineering process 30/10/2014 Chapter 2 Software Processes 21 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 30/10/2014 Chapter 2 Software Processes 22 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. 30/10/2014 Chapter 2 Software Processes 23 A general model of the design process 30/10/2014 Chapter 2 Software Processes 24 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. 30/10/2014 Chapter 2 Software Processes 25 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. 30/10/2014 Chapter 2 Software Processes 26 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. 30/10/2014 Chapter 2 Software Processes 27 Stages of testing 30/10/2014 Chapter 2 Software Processes 28 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. 30/10/2014 Chapter 2 Software Processes 29 Testing phases in a plan-driven software process (V-model) 30/10/2014 Chapter 2 Software Processes 30 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. 30/10/2014 Chapter 2 Software Processes 31 System evolution 30/10/2014 Chapter 2 Software Processes 32 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. 30/10/2014 Chapter 2 Software Processes 33 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. 30/10/2014 Chapter 2 Software Processes 34