SE2 - Software Process_2.pdf

Chapter II: Software Process Software Engineering Plan ¤ Software Process Models ¤ Process Activities ¤ Coping with change Plan ¤ Software Process Models ¤ Process Activities ¤ Coping with change Chapter 2 Software Processes The software process ¤ A structured set of activities required t...

Chapter II: Software Process Software Engineering Plan ¤ Software Process Models ¤ Process Activities ¤ Coping with change Plan ¤ Software Process Models ¤ Process Activities ¤ Coping with change Chapter 2 Software Processes The software process ¤ A structured set of activities required to develop a software system. ¤ Many different software processes but all involve: ü Specification – defining what the system should do; ü Design and implementation – defining the organization of the system and implementing the system; ü Validation – checking that it does what the customer wants; ü 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. 4 Chapter 2 Software Processes 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: ü Products, which are the outcomes of a process activity; ü Roles, which reflect the responsibilities of the people involved in the process; ü Pre- and post-conditions, which are statements that are true before and after a process activity has been enacted or a product produced. 5 Chapter 2 Software Processes 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. 6 Chapter 2 Software Processes Software process models ¤ The waterfall model ü Plan-driven model. Separate and distinct phases of specification and development. ¤ Incremental development ü Specification, development and validation are interleaved. May be plan-driven or agile. ¤ Reuse-oriented software engineering ü The system is assembled from existing 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. 7 Chapter 2 Software Processes The waterfall model 8 Chapter 2 Software Processes Waterfall model ¤ Advantages: ü The waterfall model is the classic process model (easy to understand) ü It is simple, widely known, easy to understand and use. ü In some respect, waterfall is the “common sense” approach : define- before-design, design-before-code ü Works well on small project with a short-time development 9 Chapter 2 Software Processes Waterfall model ¤ Disadvantages: ⤬ Hard to follow the sequential step (ex: can’t understand requirement well) no theory to tell what to do with requirement ⤬ Sometimes unrealistic to expect accurate requirements early in a project. ⤬ No inherent risk management and expensive to change decisions, ”swimming upstream”. ⤬ Software is delivered late, delays discovery of serious errors. ⤬ A high risk for the large and long-term development project 10 Chapter 2 Software Processes V model 11 Chapter 2 Software Processes Incremental development Concurrent activities Initial Specification version Outline Intermediate description Development versions Final Validation version 12 Chapter 2 Software Processes Incremental development benefits ¤ The cost of accommodating changing customer requirements is reduced. ü 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. ü 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. ü Customers are able to use and gain value from the software earlier than is possible with a waterfall process. 13 Chapter 2 Software Processes Incremental development problems ¤ The process is not visible. ü 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. ü 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. 14 Chapter 2 Software Processes Reuse-oriented software engineering ¤ Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the- shelf) systems. ¤ Process stages ü Component analysis; ü Requirements modification; ü System design with reuse; ü Development and integration. ¤ Reuse is now the standard approach for building many types of business system 15 Chapter 2 Software Processes Reuse-oriented software engineering ¤ Main characteristics: Ø Makes intensive use of existing reusable components Ø The focus is on integrating the components rather than on creating them from the scratch ¤ Advantages: Ø Reduces considerably the software to be developed “in- house” Ø Allows faster delivery Ø In principle, more reliable systems, due to using previously tested components 16 Chapter 2 Software Processes Reuse-oriented software engineering ¤ Disadvantages: Ø Compromises in requirements are needed Ø Less control over the system’s evolution ¤ Applicability: Ø When there is a pool of existing components that could satisfy the requirements of the new product 17  Chapter 2 Software Processes Reuse-oriented software engineering Requirements Component Requirements System design specification analysis modification with reuse Development System and integration validation 18 Plan ¤ Software Process Models ¤ Process Activities ¤ Coping with change Chapter 2 Software Processes 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. In the waterfall model, they are organized in sequence, whereas in incremental development they are inter- leaved. 20  Chapter 2 Software Processes Software specification ¤ The process of establishing what services are required and the constraints on the system’s operation and development. ¤ Requirements engineering process Ø Feasibility study ü Is it technically and financially feasible to build the system? Ø Requirements elicitation and analysis ü What do the system stakeholders require or expect from the system? Ø Requirements specification ü Defining the requirements in detail Ø Requirements validation ü Checking the validity of the requirements 21 Chapter 2 Software Processes The requirements engineering process Requirements Feasibility elicitation and study analysis Requirements specification Feasibility Requirements report validation System models User and system requirements Requirements document 22 Chapter 2 Software Processes Software design and implementation ¤ The process of converting the system specification into an executable system. ¤ Software design ü Design a software structure that realises the specification; ¤ Implementation ü Translate this structure into an executable program; ¤ The activities of design and implementation are closely related and may be inter-leaved. 23 Chapter 2 Software Processes A general model of the design process Design inputs Platform Requirements Data information specification description Design activities Architectural Interface Component design design design Database design Design outputs System Database Interface Component architecture specification specification specification 24 Chapter 2 Software Processes Design activities ¤ Architectural design, where you identify the overall structure of the system, the principal components (sometimes called sub-systems or modules), their relationships and how they are distributed. ¤ Interface design, where you define the interfaces between system components. ¤ Component design, where you take each system component and design how it will operate. ¤ Database design, where you design the system data structures and how these are to be represented in a database. 25 Chapter 2 Software Processes 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. 26 Chapter 2 Software Processes Stages of testing Component Acceptance System testing testing testing 27 Chapter 2 Software Processes Testing stages ¤ Development or component testing ü Individual components are tested independently; ü Components may be functions or objects or coherent groupings of these entities. ¤ System testing ü Testing of the system as a whole. Testing of emergent properties is particularly important. ¤ Acceptance testing ü Testing with customer data to check that the system meets the customer’s needs. 28  Chapter 2 Software Processes Testing phases in a plan-driven software process Requirements System System Detailed specification specification design design System Sub-system Module and Acceptance unit code integration integration test plan test plan test plan and test Acceptance System Sub-system Service test integration test integration test 29 Chapter 2 Software Processes 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 Chapter 2 Software Processes System evolution Define system Assess existing Propose system Modify requirements systems changes systems Existing New systems system 31 Plan ¤ Software Process Models ¤ Process Activities ¤ Coping with change Chapter 2 Software Processes Coping with change ¤ Change is inevitable in all large software projects. ü Business changes lead to new and changed system requirements ü New technologies open up new possibilities for improving implementations ü Changing platforms require application changes ¤ Change leads to rework so the costs of change include both rework (e.g. re-analysing requirements) as well as the costs of implementing new functionality 33 Chapter 2 Software Processes Reducing the costs of rework ¤ Change avoidance, where the software process includes activities that can anticipate possible changes before significant rework is required. ü For example, a prototype system may be developed to show some key features of the system to customers. ¤ Change tolerance, where the process is designed so that changes can be accommodated at relatively low cost. ü This normally involves some form of incremental development. Proposed changes may be implemented in increments that have not yet been developed. If this is impossible, then only a single increment (a small part of the system) may have be altered to incorporate the change. 34 Chapter 2 Software Processes Software prototyping ¤ A prototype is an initial version of a system used to demonstrate concepts and try out design options. ¤ A prototype can be used in: ü The requirements engineering process to help with requirements elicitation and validation; ü In design processes to explore options and develop a UI design; ü In the testing process to run back-to-back tests. 35 Chapter 2 Software Processes Benefits of prototyping ¤ Improved system usability. ¤ A closer match to users’ real needs. ¤ Improved design quality. ¤ Improved maintainability. ¤ Reduced development effort. 36 Chapter 2 Software Processes The process of prototype development Establish Define prototype prototype Develop Evaluate objectives functionality prototype prototype Prototyping Outline Executable Evaluation plan definition prototype report 37 Chapter 2 Software Processes Prototype development ¤ May be based on rapid prototyping languages or tools ¤ May involve leaving out functionality ü Prototype should focus on areas of the product that are not well-understood; ü Error checking and recovery may not be included in the prototype; ü Focus on functional rather than non-functional requirements such as reliability and security 38 Chapter 2 Software Processes Throw-away prototypes ¤ Prototypes should be discarded after development as they are not a good basis for a production system: ü It may be impossible to tune the system to meet non- functional requirements; ü Prototypes are normally undocumented; ü The prototype structure is usually degraded through rapid change; ü The prototype probably will not meet normal organisational quality standards. 39 Chapter 2 Software Processes 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. 40 Chapter 2 Software Processes Incremental development and delivery ¤ Incremental development ü Develop the system in increments and evaluate each increment before proceeding to the development of the next increment; ü Normal approach used in agile methods; ü Evaluation done by user/customer proxy. ¤ Incremental delivery ü Deploy an increment for use by end-users; ü More realistic evaluation about practical use of software; ü Difficult to implement for replacement systems as increments have less functionality than the system being replaced. 41  Chapter 2 Software Processes Incremental delivery Define outline Assign requirements Design system Develop system requirements to increments architecture increment System incomplete? Validate Integrate Validate Deploy increment increment system increment System complete? Final system 42 Chapter 2 Software Processes Incremental delivery 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. 43  Chapter 2 Software Processes Incremental delivery problems ¤ Most systems require a set of basic facilities that are used by different parts of the system. ü As requirements are not defined in detail until an increment is to be implemented, it can be hard to identify common facilities that are needed by all increments. ¤ The essence of iterative processes is that the specification is developed in conjunction with the software. ü However, this conflicts with the procurement model of many organizations, where the complete system specification is part of the system development contract. 44 Chapter 2 Software Processes Boehm’s spiral model ¤ 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. 45 Chapter 2 Software Processes Boehm’s spiral model of the software process Determine objectives, Evaluate alternatives, alternatives and identify, resolve risks constraints Risk analysis Risk analysis Risk analysis Opera- Prototype 3 tional Prototype 2 protoype Risk REVIEW analysis Proto- type 1 Requirements plan Simulations, models, benchmarks Life-cycle plan Concept of Operation S/W requirements Product design Detailed Requirement design Development plan validation Code Design Unit test Integration and test plan V&V Integration Plan next phase test Acceptance Service test Develop, verify next-level product 46 Chapter 2 Software Processes Spiral model sectors ¤ Objective setting ü Specific objectives for the phase are identified. ¤ Risk assessment and reduction ü Risks are assessed and activities put in place to reduce the key risks. ¤ Development and validation ü A development model for the system is chosen which can be any of the generic models. ¤ Planning ü The project is reviewed and the next phase of the spiral is planned. 47 Chapter 2 Software Processes Spiral model usage ¤ Spiral model has been very influential in helping people think about iteration in software processes and introducing the risk-driven approach to development. ¤ In practice, however, the model is rarely used as published for practical software development. 48 Video Links ¤ https://www.youtube.com/watch?v=Fi3_BjVzpqk ¤ https://www.youtube.com/watch?v=i-QyW8D3ei0 ¤ https://www.youtube.com/watch?v=4w8VhoBVcPw ¤ https://www.youtube.com/watch?v=ZzAX7Z7CLDs ¤ https://www.youtube.com/watch?v=Y_A0E1ToC_I ¤ https://www.youtube.com/watch?v=W2CD_sUHtOo ¤ https://www.youtube.com/watch?v=Dfi4cxCC_I0 The End!