2. Software Engineering I Preface Lecturer: Paul Dunne Course web site:http://gmitweb.gmit.ie/pdunne

3. Course material  The course based on: oIan Sommerville’s “Software Engineering 6 th Ed.” A number of copies (of the 5 th ed) are held in the library (005.1) See also http://www.software-engin.com/http://www.software-engin.com/ oAlso recommended is Robert Pressman’s “Software Engineering – A practitioners approach 5 th Ed.” A number of copies in the library (005.1)

4. Course layout  Class lectures: o2 hrs per week  Labs o2hrs per week oHTML and XML oProgramming oAssignments must be finished on time  Code must have comments  Course work/assignments

5. Recent Disasters  ‘Those who ignore the lessons of the past are doomed to repeat them’ (Napoleon) oSome prominent disasters  London Ambulance  Ariane 5 oDon’t forget that most are hidden

6. London Ambulance Dispatching System The full introduction of a computer system effectively did away with oThe radio as a mechanism for staff to dispatch ambulances.  The computer dispatched crews to calls oHaving people answer the phones  The computer queued calls from emergency callers Problems  During the morning rush it became obvious, to crews and control room staff, that calls were going missing in the system; ambulances were arriving late or doubling up on calls.  Distraught emergency callers were also held in a queuing system which failed to put them through for up to 30 minutes. Outcome It was said that it was hard to verify how many people might have died because of the delays but it could have been as many as 20. From http://128.240.150.127/Risks/13.88.html#subj1.1

7. Ariane 5 June 4 th 1996: It took the European Space Agency 10 years and $7 billion to produce Ariane5, a giant rocket capable of propelling six tons of payload into orbit. The rocket exploded (39 seconds into its maiden voyage scattering fiery rubble across the mangrove swamps of French Guiana) due to small computer program trying to stuff a 64-bit number into a 16-bit space. Ariane 5 (along with its payload of four expensive and uninsured scientific satellites valued at $500 million) was destroyed. This problem had begun an instant before, when the spacecraft swerved off course under the pressure of the three powerful nozzles in its boosters and main engine. The rocket was making an abrupt course correction that was not needed, compensating for a wrong turn that had not taken place. http://www.around.com/ariane.html http://www.ima.umn.edu/~arnold/disasters/ariane.html

8. Ariane 5 (2) Steering was controlled by the on-board computer, (which mistakenly thought the rocket needed a course change because of the numbers and which were really an error message coming from the inertial guidance system). The guidance system had in fact shut down 36.7 seconds after launch, when the guidance system's own computer tried to convert one piece of data -- the sideways velocity of the rocket -- from a 64-bit format to a 16-bit format = overflow error. When the guidance system shut down, it passed control to an identical, redundant unit, which was there to provide backup in case of just such a failure. The second unit (having the same software) failed too. In an earlier design decision, the programmers had decided that this particular velocity figure would never be large enough to cause trouble. After all, it never had been before. But Ariane 5 was a faster rocket than Ariane 4. One extra absurdity: the calculation containing the bug actually served no purpose once the rocket was in the air. Its only function was to align the system before launch. So it should have been turned off. But engineers chose to leave this function running for the first 40 seconds of flight -- a "special feature" meant to make it easy to restart the system in the event of a brief hold in the countdown.

9. Understanding the Problems  Considering the problems in software development and the goals that software development seeks to achieve. These are: oMeeting the user’s needs oLow cost of production oHigh performance oPortability oLow cost of maintenance oHigh reliability; oDelivery on time  Lets look at these in turn….

10. Understanding the Problems  Meeting the User’s Needs oA piece of software must do what its users want of it (Obvious) oHowever,  software not always used quite as intended  Using Microsoft Word to do drawings!  Different people have different (conflicting) requirements  Full featured drawing package required – but only by some  Communicating Requirements is fraught with difficulties oDeduce from this that a large proportion of systems do not meet the needs of their users. oMain problem of software development seems to lie in requirements analysis and design “When a passenger of foot heave in sight, tootle the horn. Trumpet him melodiously at first, but if he still obstacles your passage tootle him with vigor.” From a brochure at a Tokyo car rental firm

11. Understanding the Problems  The Cost of Software Production : oProgrammer Productivity = 10 to 20 lines of code a day? oPredicting Software Costs  how long it will take to write a piece of software?  common for time scale to be underestimated by up to 50% oHardware v Software Costs  In 1955 software cost typically only about 10% of a project, now it is the hardware that is only 10% (or much less) oThe Impact of the Personal Computer  Cheap, Available oSoftware Packages  Off the shelf solutions. oThe IT Revolution  Software increasing in complexity.

12.  An experiment was conducted to compare programmer productivity and defect rates for Java and C++. A modified version of the Personal Software Process (PSP) was used to gather defect rate*, bug rate**, and productivity data on C++ and Java during two real world development projects. oA typical C++ program had two to three times as many bugs per line of code as a typical Java program. oC++ also generated between 15% and 50% more defects, and perhaps took six times as long to debug. oJava was between 30% and 200% more productive, in terms of lines of code per minute. oWhen defects were measured against development time, Java and C++ show no difference, but C++ had 2 to 3 times as many bugs per hour.  The experiment shows that it is possible to experimentally measure the fitness of a programming language. Comparing Observed Bug and Productivity Rates for Java and C++ Geoffrey Phipps Spirus - 2002 http://www.spirus.com.au/papersAndTalks/javaVsC++.pdf *A defect is either a bug, or an error detected during compile time **A bug is defined to be a problem detected during testing or deployment.

13. Understanding the Problems  The Cost of Software Production :  How is the Cost Made Up?  Helpful to know where errors arise Detected on compilation Detected in testing Possibly Detected after installation

14. Understanding the Problems  Meeting Deadlines : oThis has always been a major problem in software production. oSurveys have shown that this is the worst problem faced by IT managers oThe problem is related to the difficulty of predicting how long it will take to develop something. oIf we don’t know how long its going to take, we cannot hope to meet any deadline.  Software Performance : oOften we must make sure that:  In interactive system responds within a reasonably short time;  A control signal is output to a plant in sufficient time;  batch job is not taking 12 hours when it should take 1.

15. Understanding the Problems  Portability : oIdeally, software should be easy to port oHigh-level languages (Java, C++) and international standards (POSIX,.NET) assist prospects. oHowever, market forces have dominated the situation. oProgrammers can be tied to particular suppliers. oRecent years have seen the emergence of Java technologies oDoes this change the portability issue? A Definition An application is portable across a class of environments to the degree that the effort required to transport and adapt it to a new environment in the class is less than the effort of redevelopment. http://www.csee.wvu.edu/~jdm/research/portability/home.html For a portability tutorial see Jim Mooney’s http://www.csee.wvu.edu/~jdm/research/portability/tutorial/index.html

16. Understanding the Problems  Maintenance : oThere are two types of maintenance:  remedial maintenance is the time spent fixing bugs;  adaptive maintenance is altering software to meet changing user oneeds or to accommodate new hardware and OS constraints. oManagers regard maintenance as a nuisance - because they need to ensure sufficient people to do it. oProgrammers regard it as the least interesting work.  Reliability : oYou can never assume that a piece of software is ever bug free. oTherefore, every large piece of software contains errors

17. Understanding the Problems  A Software Crisis ? : oWe have covered various perceived problems with software oIt fails to do what the users want it to do oIt is expensive oIt is not always fast enough oIts not always easily ported oMaintenance is expensive oIts is unreliable oIt is often late  Meeting user’s needs, reducing costs, improving reliability, and delivery on time are the most important problems

18. A Solution - Software Engineering:  Greater emphasis on systematic development.  Computer assistance for software development (CASE)  A concentration on finding out the user’s requirements  Formal/Semi Formal specification of the requirements of a system  Demonstration of early version of a system (prototyping)  Greater emphases on trying to ensure error free code

19. Software Engineering  Definitions: o Simple Definition: Designing, building and maintaining large software systems o Others:  ‘Software engineering is concerned with the theories, methods and tools for developing, managing and evolving software products ’ I. Sommerville  ‘The practical application of scientific knowledge in the design and construction of computer programs and the associated documentation required to develop, operate and maintain them’ B.W.Boehm  ‘The establishment and use of sound engineering principles in order to obtain economically software that is reliable and works efficiently on real machines’ F.L.Bauer

20. Software Engineering cont.  Definitions cont. o ‘The application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software’ IEEE Standard 610.12 o ‘The technological and managerial discipline concerned with systematic production and maintenance of software products that are developed and modified on time and within cost constraints’ R. Fairley

21. Learning Outcomes oAppreciate the problems associated with developing software oUnderstand the need for a managed approach to Software Development oBe able to define the term ‘Software Engineering’

22. Questions (1) 1. Here are some things that could go wrong with a Computing project (i.e. risks)  a) Changing requirements leading to an over-budget, over-schedule system  b) Incomplete Requirements Specification leading to incomplete/ unusable system  c) Lack of full testing leading to a system that doesn’t work  d) Failure to fully specify and estimate a system  e) The project contains parts that are technically infeasible, meaning that the system cannot be built For each of the following infamous software projects identify the main problem that occurred from the list above: London Ambulance Service dispatch system, Ariane 5 Guidance software

23. Questions (2) Look at the following list of goals for software  a) Meeting the user’s needs  b) Low cost of production  c) High performance  d) Portability  e) Low cost of maintenance  f) High reliability;  g) Delivery on time. For each of the following systems choose the three most important goals to fulfil. (i) A system to manage students fees – to track whether they have been paid, calculate the correct amount, issue invoices, send out reminders etc. (ii) A system to manage the temperature of a nuclear reactor core. (iii) An experimental program to test the efficiency of an algorithm for some mathematical theory. (iv) A new system for the online return of UK self assessment forms for 2002 (v) A program that allows the viewing of football scores on mobile phones.

24. Questions (3) 1. Typically how many lines of code does the average programmer produce per day 2. Which is cheaper – fixing an error at Requirements stage or at Testing stage? 3. Which stage of software development is typically the most expensive? 4. Where are the most errors introduced in software projects 5. What do project managers say is the worst problem (consequence) in Software projects? 6. Differentiate between remedial and adaptive maintenance. 7. ‘All Microsoft software is bug free’ (Bill Gates) – Is he correct? 8. Define Software Engineering