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Software Engineering : Process Models

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software engineering; software development life cycle models

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Software Engineering : Process Models

  1. 1. Software Engineering Principles Ajit K Nayak, Ph.D. ajitnayak@soauniversity.ac.in 9338749992 Lecture Notes – 2 Process Models
  2. 2. Why Study Software Engineering? • To acquire skills to develop large programs. – Exponential growth in complexity and difficulty level with size. – The ad hoc approach breaks down when size of software increases. • Ability to solve complex programming problems: – How to break large projects into smaller and manageable parts? • Learn techniques of: – specification, design, interface development, testing, project management, etc.
  3. 3. Evolution of Design Techniques Object-Oriented Exploratory Style Data flow-based Data structure- based Control flow- based
  4. 4. Modern S/W Development Practices • Use of Life Cycle Models – Software is developed through several well-defined stages: • requirements analysis and specification, • design, • coding, • testing, etc. • Emphasis has shifted from error correction to error prevention. • Modern practices emphasize on detection of errors as close to their point of introduction as possible. • Unlike exploratory style, now coding is considered only a small part of program development effort.
  5. 5. Software Life Cycle • The life cycle of a software represents the series of identifiable stages through which it evolves during its life time • Also called Software Development Life Cycle Model (SDLC)
  6. 6. Classical Waterfall Model • Classical waterfall model divides life cycle into phases: Feasibility Study Req. Analysis & Spec. Design Implementation Testing Maintenance • It is simple and idealistic model • Hard to use for any non- trivial SD project • All other models are extension to this model.
  7. 7. Relative Effort for Phases • Phases between feasibility study and testing known as development phases. • Among all life cycle phases maintenance phase consumes maximum effort. • Among development phases, testing phase consumes the maximum effort. 0 10 20 30 40 50 60 Req.Sp Design Coding Test Maintnce
  8. 8. Feasibility Study • Main aim of feasibility study is to determine whether developing the product – financially worthwhile – technically feasible. • First roughly understand what the customer wants: – different data which would be input to the system, – processing needed on these data, – output data to be produced by the system, – various constraints on the behavior of the system.
  9. 9. Activities during Feasibility Study • Work out an overall understanding of the problem. • Formulate different solution strategies. • Examine alternate solution strategies in terms of: – resources required, – cost of development, and – development time. • Perform a cost/benefit analysis: – To determine which solution is the best. – None of the solutions may be feasible due to: • high cost, • resource constraints, • technical reasons.
  10. 10. Requirements Analysis and Specification • Aim of this phase: – understand the exact requirements of the customer, – document them properly. • Consists of two distinct activities: – requirements gathering and analysis – requirements specification.
  11. 11. Requirements Analysis - I • To Collect all related data from the customer: – analyze the collected data to clearly understand what the customer wants, – find out any inconsistencies and incompleteness in the requirements, – resolve all inconsistencies and incompleteness. • Usually collected from the end-users through interviews and discussions. • Example: business accounting software – interview all the accountants of the organization to find out their requirements
  12. 12. Requirements Analysis - II • The data collected initially from the users: – would usually contain several contradictions and ambiguities: – each user typically has only a partial and incomplete view of the system. • Ambiguities and contradictions must be identified, resolved by discussions with the customers. • Then, requirements are organized into a Software Requirements Specification (SRS) document. • Engineers doing requirements analysis and specification are designated as analysts.
  13. 13. Design • Design phase transforms requirements specification into a form suitable for implementation in some programming language. • In technical terms: – during design phase, software architecture is derived from the SRS document. • Two design approaches: – traditional approach, – object oriented approach.
  14. 14. Object Oriented Design • First identify various objects (real world entities) occurring in the problem: • Example: the objects in a pay-roll software may be: – employees, managers, pay-roll register, Departments, etc. • identify the relationships among the objects. • OOD has several advantages: – lower development effort, – lower development time, – better maintainability.
  15. 15. Implementation • Implementation phase is also known as coding and unit testing phase – software design is translated into source code. – each module of the design is coded, – each module is unit tested • tested independently as a stand alone unit, and debugged, – each module is documented. • The purpose of unit testing to test test if individual modules work correctly. • The end product of implementation phase – a set of program modules that have been tested individually.
  16. 16. Integration and System Testing • Different modules are integrated in a planned manner through a number of steps • During each integration step, the partially integrated system is tested. • After all the modules have been successfully integrated and tested: – system testing is carried out. • Goal of system testing: – ensure that the developed system functions according to its requirements as specified in the SRS document.
  17. 17. Maintenance • Maintenance of any software product requires much more effort than the effort to develop the product itself. • development effort to maintenance effort is typically 40:60. • Corrective maintenance: – Correct errors which were not discovered during the product development phases. • Perfective maintenance: – Improve implementation of the system – enhance functionalities of the system. • Adaptive maintenance: – Port software to a new environment, – e.g. to a new computer or to a new operating system.
  18. 18. Limitations • Classical waterfall model is idealistic – assumes that no defect is introduced during any development activity. – in practice defects do get introduced in almost every phase of the life cycle. • Defects usually get detected much later in the life cycle – For example, a design defect might go unnoticed till the coding or testing phase. • Once a defect is detected: – we need to go back to the phase where it was introduced – redo some of the work done during that and all subsequent phases.
  19. 19. Iterative Waterfall Model - I Feasibility Study Req. Analysis & Specification Design Coding & Unit test Integration & System Testing Maintenance There are feedback paths in the Iterative waterfall model
  20. 20. Iterative Waterfall Model - II • Errors should be detected – in the same phase in which they are introduced. • For example: – if a design problem is detected in the design phase itself, then the problem can be taken care of much more easily. – if it is identified at the end of the integration and system testing phase, then it would be difficult, • because rework must be carried out not only to the design but also to code and test phases
  21. 21. Iterative Waterfall Model - III • The principle of detecting errors as close to its point of introduction as possible is known as phase containment of errors. • Iterative waterfall model is by far the most widely used model. • Almost every other model is derived from the waterfall model. • Irrespective of the life cycle model actually followed the documents should reflect a classical waterfall model of development.
  22. 22. Use Waterfall models if… • Project is large, expensive. • Project has clear objectives and solution. • Pressure does not exist for immediate implementation. • Project requirements can be stated unambiguously and comprehensively. • Project requirements are stable or unchanging during the system development life cycle. • User community is fully knowledgeable in the business and application. • Team members may be inexperienced.
  23. 23. Don’t Use these models when … • Large projects where the requirements are not well understood or are changing for any reasons such as external changes, changing expectations, budget changes or rapidly changing technology. • Web Information Systems (WIS) – primarily due to the pressure of implementing a WIS project quickly; – the continual evolution of the project requirements; – the need for experienced, flexible team members drawn from multiple disciplines; and – the inability to make assumptions regarding the users’ knowledge level. • Real-time systems. • Event-driven systems. • Leading-edge applications
  24. 24. V Model • It is an extension to iterative waterfall model • Used in systems requiring high reliability Req. Analysis & Specification High-level Design System Testing Integration Testing Unit Testing Coding Unit test design System test design Unit test design • Two Phases • Development • Validation
  25. 25. V Model • It is a variant of the Waterfall model – emphasizes verification and validation – Both V&V activities are spread over the entire life cycle. • In every phase of development: – Testing activities are planned in parallel with development. • Strengths – Starting from early stages , it emphasize planning for verification and validation of the software – Each deliverable is made testable, easy to use • Weaknesses – Does not support overlapping of phases, does not handle iterations or phases, does not easily handle later changes in requirements
  26. 26. Prototyping Model - I • Before starting actual development, – a working prototype of the system should first be built. Requirements Gathering Quick Design Refine Requirements Build Prototype Customer Evaluation of Prototype Design Implement Test Maintain Customer satisfied • A prototype is a toy implementation of a system: – limited functional capabilities, – low reliability, – inefficient performance.
  27. 27. Why Prototyping Model? • It is also a derivative of waterfall model • To illustrate to the customer – input data formats, messages, reports, or interactive dialogs. • Examine technical issues associated with product development: – Often major design decisions depend on issues like: – response time of a hardware controller, – efficiency of a sorting algorithm, etc. • It is impossible to ``get it right'' the first time – we must plan to throw away the first product if we want to develop a good product.
  28. 28. Prototyping Model - II • Start with approximate requirements. • Carry out a quick design. • Prototype model is built using several short-cuts: – Short-cuts might involve using inefficient, inaccurate, or dummy functions. – A function may use a table look-up rather than performing the actual computations. • The developed prototype is submitted to the customer for his evaluation: – Based on the user feedback, requirements are refined. – This cycle continues until the user approves the prototype. – The actual system is developed using the classical waterfall approach.
  29. 29. Prototyping Model - III • Requirements analysis and specification phase becomes redundant: – final working prototype (with all user feedbacks incorporated) serves as an animated requirements specification. • Design and code for the prototype is usually thrown away – However, the experience gathered from developing the prototype helps a great deal while developing the actual product. • Even though construction of a working prototype model involves additional cost --- overall development cost might be lower for: – systems with unclear user requirements, – systems with unresolved technical issues. • Many user requirements get properly defined and technical issues get resolved: – these would have appeared later as change requests and resulted in incurring massive redesign costs.
  30. 30. Major difficulties of Waterfall-Based Models • Difficulty in accommodating change requests during development. – 40% of requirements change during development • High cost incurred in developing custom applications. • Heavy weight processes.
  31. 31. Incremental Model • Idea – take advantage of what was being learned during the development of earlier, incremental, deliverable versions of the system. – Learning comes from both the development and use of the system… – Start with a simple implementation of a subset of the software requirements and iteratively enhance the evolving sequence of versions. – At each version design modifications are made along with adding new functional capabilities.
  32. 32. Evolutionary Model - I • In Evolutionary model (aka successive versions or incremental model) – The system is broken down into several modules which can be incrementally implemented and delivered. • First develop the core modules of the system. • The initial product skeleton is refined into increasing levels of capability: – by adding new functionalities in successive versions. A B C A A B
  33. 33. Advantages / Disadvantages • Users get a chance to experiment with a partially developed system: – much before the full working version is released, • Helps finding exact user requirements: – much before fully working system is developed. • Core modules get tested thoroughly: – reduces chances of errors in final product. • Often, difficult to subdivide problems into functional units: – which can be incrementally implemented and delivered. • Evolutionary model is useful for very large problems, – where it is easier to find modules for incremental implementation.
  34. 34. Use • Most Appropriate – Large projects where requirements are not well understood or are changing due to – external changes, changing expectations, budget changes or rapidly changing technology. – Web Information Systems (WIS ) and event-driven systems. – Leading-edge applications. • Least Appropriate – Very small projects of very short duration. – Integration and architectural risks are very low. – Highly interactive applications where the data for the project already exists (completely or in part), and the project largely comprises analysis or reporting of the data.
  35. 35. Rapid Application Development (RAD) • Also known as Rapid Prototyping Model • Clients do not know what they exactly wanted until they saw a working system. – In waterfall models, customer can’t see the S/W until the development is complete in all respects. • Working – Functional modules are developed in parallel as prototypes. – Integrated to make the complete product for faster product delivery. – Follow iterative and incremental model. – Prototypes developed are reusable. • Reduces the communication gap between customers and developers
  36. 36. RAD Model
  37. 37. Usability • Suitable – Customized product developed for one or two customers only – Performance and reliability are not critical. – The system can be split into several independent modules. • Unsuitable – Few plug-in components are available – High performance or reliability required – No precedence for similar products exists – The system cannot be modularized.
  38. 38. Spiral Model  Each loop of the spiral represents a phase of the software process:  the innermost loop might be concerned with system feasibility,  the next loop with system requirements definition,  the next one with system design, and so on. (1) Determine Objectives (2) Identify & Resolve Risks (3) Develop Next Level of Product (4) Customer Evaluation of Prototype
  39. 39. 4 Quadrants - I • Objective Setting (First Quadrant) • Identify objectives of the phase – Examine the risks associated with these objectives. • Risk: – any adverse circumstance that might hamper successful completion of a software project. • Find alternate solutions possible. • Risk Assessment and Reduction (Second Quadrant) • For each identified project risk – a detailed analysis is carried out. – Steps are taken to reduce the risk. • Example: if there is a risk that the requirements are inappropriate: – a prototype system may be developed
  40. 40. 4 Quadrants - II • Development and Validation (Third quadrant) – develop and validate the next level of the product. • Review and Planning (Fourth quadrant) – review the results achieved so far with the customer and plan the next iteration around the spiral. • With each iteration around the spiral – progressively more complete version of the software gets built.
  41. 41. Spiral Model as a meta model • Subsumes all discussed models – a single loop spiral represents waterfall model. – iterations through the spiral are evolutionary levels. – enables understanding and reacting to risks during each iteration along the spiral. – Uses prototyping as a risk reduction mechanism – retains the step-wise approach of the waterfall model.
  42. 42. Agile Development Models I • Used to overcome the shortcomings of the waterfall model of development. – Proposed in mid-1990s • The agile model was primarily designed to help a project to adapt to change requests • In this model, the requirements are decomposed into many small incremental parts that can be developed over, one to four weeks each.
  43. 43. Agile Development Models II • Characteristics – Quick project completion – Makes the process light weight • Removing activities not necessary to specific project. • Removing anything that wastes time and effort. – Emphasize face-to-face communication with the client, with small team size may be working on-site. – Emphasizes incremental release of working software. – Deployment of pair-programming. • Popular SDLC Models – Extreme Programming (XP) – Scrum – Unified process – Crystal – DSDM – Lean – . . .
  44. 44. Agile Model: Principal Techniques • User stories: – Simpler than use cases. • Metaphors: – Based on user stories, developers propose a common vision of what is required. • Spike: – A very simple program to explore potential solutions should be considered. • Refactor: – Restructure code without affecting behavior, improve efficiency, structure, etc.
  45. 45. Methodology • Face-to-face communication favoured over written documents. • To facilitate face-to-face communication, – development team to share a single office space. – Team size is deliberately kept small (5-9 people) – This makes the agile model most suited to the development of small projects. • At a time, only one increment is planned, developed, deployed at the customer site. – No long-term plans are made. • An iteration may not add significant functionality, – but still a new release is invariably made at the end of each iteration – Delivered to the customer for regular use.
  46. 46. Comparisons - I • Waterfall Model – steps through in a planned sequence over – Progress is measured in terms of delivered artifacts like Requirement specifications, design documents, test plans, code reviews, etc. • Agile model sequences delivery of working versions of a product in several increments. • Agile teams use the waterfall model on a small scale.
  47. 47. Comparisons - II • RAD Model Vs Agile model – RAD is based on designing quick-and-dirty prototypes, which are then refined into production quality code. – Agile model does not recommend developing prototypes – Agile projects logically break down the solution into a number of features: • These are incrementally developed and delivered – Systematic development of each incremental feature is emphasized.
  48. 48. Comparisons - III • Exploratory programming vs Agile Model • Similarity – Frequent re-evaluation of plans, – Emphasis on face-to-face communication, – Relatively sparse use of documents. • Contrast – Agile teams, however, do follow defined and disciplined processes and carry out rigorous designs
  49. 49. Thank You Acknowledgements Dr. Rajib Mall, IIT, KGP

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