1. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 1 Rapid software development

2. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 2 Objectives l To explain how an iterative, incremental development process leads to faster delivery of more useful software l To discuss the essence of agile development methods l To explain the principles and practices of extreme programming l To explain the roles of prototyping in the software process

3. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 3 Topics covered l Agile methods l Extreme programming l Software prototyping

4. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 4 Rapid software development l Because of rapidly changing business environments, businesses have to respond to new opportunities and competition. l This requires software and rapid development and delivery is not often the most critical requirement for software systems. l Businesses may be willing to accept lower quality software if rapid delivery of essential functionality is possible.

5. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 5 Unstable requirements l Because of the changing environment, it is often impossible to arrive at a stable, consistent set of system requirements. l Therefore a waterfall model of development is impractical and an approach to development based on iterative specification and delivery is the only way to deliver software quickly.

6. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 6 Characteristics of rapid software development processes l The processes of specification, design and implementation are concurrent. There is no detailed specification and design documentation is minimised. l The system is developed in a series of increments. End users evaluate each increment and make proposals for later increments. l System user interfaces are usually developed using an interactive development system.

7. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 7 An iterative development process

8. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 8 Advantages of incremental development l Accelerated delivery of customer services. Each increment delivers the highest priority functionality to the customer. l User engagement with the system. Users have to be involved in the development which means the system is more likely to meet their requirements and the users are more committed to the system.

9. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 9 Agile methods l Dissatisfaction with the overheads involved in design methods led to the creation of agile methods. These methods: Focus on the code rather than the design; Are based on an iterative approach to software development; Are intended to deliver working software quickly and evolve this quickly to meet changing requirements. l Agile methods are probably best suited to small/medium-sized business systems or PC products.

10. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 10 Principles of agile methods

11. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 11 Example Agile Methods* l Agile Modeling l Adaptive Software Development l Crystal l Feature Driven Development l Dynamic Systems Development Method l Lean Software Development l Scrum l Test-Driven Design l Xbreed l XP – Extreme Programming *List taken from http://www.agilealliance.org

12. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 12 Problems with agile methods l It can be difficult to keep the interest of customers who are involved in the process. l Team members may be unsuited to the intense involvement that characterises agile methods. l Prioritising changes can be difficult where there are multiple stakeholders. l Maintaining simplicity requires extra work. l Contracts may be a problem as with other approaches to iterative development.

13. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 13 Topics covered l Agile methods l Extreme programming l Software prototyping

14. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 14 Extreme programming l Perhaps the best-known and most widely used agile method. l Extreme Programming (XP) takes an ‘extreme’ approach to iterative development. New versions may be built several times per day; Increments are delivered to customers every 2 weeks; All tests must be run for every build and the build is only accepted if tests run successfully.

15. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 15 The XP release cycle

16. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 16 Extreme programming practices 1

17. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 17 Extreme programming practices 2

18. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 18 XP and agile principles l Incremental development is supported through small, frequent system releases. l Customer involvement means full-time customer engagement with the team. l People not process through pair programming, collective ownership and a process that avoids long working hours. l Change supported through regular system releases. l Maintaining simplicity through constant refactoring of code.

19. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 19 Requirements scenarios l In XP, user requirements are expressed as scenarios or user stories. l These are written on cards and the development team break them down into implementation tasks. These tasks are the basis of schedule and cost estimates. l The customer chooses the stories for inclusion in the next release based on their priorities and the schedule estimates.

20. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 20 Story card for document downloading

21. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 21 Task cards for document downloading

22. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 22 XP and change l Conventional wisdom in software engineering is to design for change. It is worth spending time and effort anticipating changes as this reduces costs later in the life cycle. l XP, however, maintains that this is not worthwhile as changes cannot be reliably anticipated. YAGNI principle (“You aren't gonna need it.”) l Rather, it proposes constant code improvement (refactoring) to make changes easier when they have to be implemented.

23. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 23 Refactoring l Extreme programming depends on refactoring to address the problem of poor structure due to evolutionary development l Refactoring - redesigning or rewriting code while preserving its behavior. l Candidates: duplicate code, long methods, etc. l Isn’t it risky to keep rewriting code, especially code that has previously worked? l Solution: Test-first development

24. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 24 Testing in XP l Test-first development. l Incremental test development from scenarios. l User involvement in test development and validation. l Automated test harnesses are used to run all component tests each time that a new release is built.

25. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 25 Test case description Test 4:Test credit card validity Input: A string representing the credit card number and two integers representing the month and year when the card expires Tests: Check that all bytes in the string are digits Check that the month lies between 1 and 12 and the year is greater than or equal to the current year. Using the first 4 digits of the credit card number, check that the card issuer is valid by looking up the card issuer table. Check credit card validity by submitting the card number and expiry date information to the card issuer Output: OK or error message indicating that the card is invalid

26. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 26 Test-first development l Writing tests before code clarifies the requirements to be implemented. l Tests are written as programs rather than data so that they can be executed automatically. The test includes a check that it has executed correctly. l All previous and new tests are automatically run when new functionality is added. Thus checking that the new functionality has not introduced errors.

27. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 27 Pair programming l In XP, programmers work in pairs, sitting together to develop code. l This helps develop common ownership of code and spreads knowledge across the team. l It serves as an informal review process as each line of code is looked at by more than 1 person. l It encourages refactoring as the whole team can benefit from this. l Measurements suggest that development productivity with pair programming is similar to that of two people working independently.

28. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 28 Problems with extreme programming l Pair programming is not for everyone. Works best when both programmers are able to communicate, are highly disciplined, and have above- average skills Skill set: refactoring experience, knowledgeable on design patterns, have testing discipline l Difficult to scale to large projects. Refactoring effort increases with the number of requirements Cannot maintain tightly coordinated teamwork when managing a large team of pair programmers Combinatorial explosion of test cases Reliance on developers’ tacit knowledge may backfire

29. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 29 Agile Modeling l Agile software development does not necessarily exclude design activities altogether. l Agile modeling applies the principles of agile software development to the creation and maintenance of models and documentation. l Can work with many other software process models like XP or RUP. From http://www.agilemodeling.com

30. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 30 Some Agile Modeling concepts l Multiple models Not restricted to a particular set (like UML) Use the right modeling tool for the right purpose l No separate design phase Start with an initial high level architecture Detailed design is evolved throughout the implementation Conduct “model storming sessions” followed by coding

31. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 31 Topics covered l Agile methods l Extreme programming l Software prototyping

32. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 32 Prototyping l For some large systems, incremental iterative development and delivery may be impractical; this is especially true when multiple teams are working on different sites. l Prototyping, where an experimental system is developed as a basis for formulating the requirements may be used. This system is thrown away when the system specification has been agreed.

33. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 33 Software prototyping l A prototype is an initial version of a system used to demonstrate concepts and try out design options. l A prototype can be used in: The requirements engineering process to help with requirements elicitation and validation; In design processes to explore options and develop a UI design; In the testing process to run back-to-back tests. l Incremental development recommends that developers start with the best-understood requirements. l Prototyping allows developers to start with the least understood requirements.

34. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 34 Benefits of prototyping l Improved system usability. l A closer match to users’ real needs. l Improved design quality. l Improved maintainability. l Reduced development effort.

35. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 35 Back to back testing

36. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 36 The prototyping process

37. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 37 Throw-away prototypes l Prototypes should be discarded after development as they are not a good basis for a production system: It may be impossible to tune the system to meet non-functional requirements; Prototypes are normally undocumented; The prototype structure is usually degraded through rapid change; The prototype probably will not meet normal organisational quality standards.

38. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 38 Incremental development and prototyping Incremental development Throw-away prototyping Delivered system Executable prototype + System specification Outline requirements

39. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 39 Conflicting objectives l The objective of incremental development is to deliver a working system to end-users. The development starts with those requirements which are best understood. l The objective of throw-away prototyping is to validate or derive the system requirements. The prototyping process starts with those requirements which are poorly understood.

40. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 17 Slide 40 Key points l An iterative approach to software development leads to faster delivery of software. There are two approaches: incremental development and rapid prototyping. l Agile methods like Extreme Programming are incremental development methods that aim to reduce development overhead and so produce software faster. l When implementing a throw-away prototype, start with the requirements you least understand; in incremental development, start with the best- understood requirements.