ITGS

Software Reliability

ITGS

• All IT systems are a combination of:– Hardware– Software– People– Data

• Problems with any of these parts, or a combination of them, can cause an IT system to fail.

ITGS

• All software contains bugs– Errors and mistakes made by the

programmers

• Safety critical systems– Systems where failure could cause injury or

death• Special measures must be taken to keep the

number of failures as low as possible

ITGS• Some infamous software bugs:

– The Millennium Bug (Y2K or Year 2000)• Computer software created before 1990 often stored

dates using 2 digits instead of 4• Purpose of this was to save money - memory was

expensive, and every digit saved helped• As the year 2000 approached, people realized that these

systems were representing the year 2000 as ‘00’ - which meant that it preceded ‘99, and was indistinguishable from the years 1900, 1800, etc.

• Thanks to advance publicity, most problems were fixed, although some credit cards were refused because of expiration date issues

• Reveals a lot about the assumptions humans make when using computers

ITGS• Infamous software bugs, cont’d

– Denver Airport Baggage System• System designed to automatically transfer

passengers’ bags from the check-in desk to the airplane - Feb. 1995 (cost $234 million to develop)

• Problems: bags falling out of carts, getting stuck on conveyor belts, labels getting dirty and not being able to be read

• System abandoned in 2005 • Illustrates how had it is for computers to

work in the real world

ITGS

ITGS

• Infamous software bugs, continued

– Mars Climate Orbiter - 1998• Designed to land on Mars and study its

surface (cost - $327 million)– Destroyed as it attempted to land

because it entered the atmosphere at a higher speed than intended

»Some of the project’s software developers had used metric units for their calculations, while others had used Imperial units (pounds) - this led to the simple miscalculation

ITGS

Case study: Therac-25 (review)– A computer-controlled machine for

administering radiation treatment to cancer patients in 1982• 2 modes:

– Low energy electron mode, designed to focus on a specific part of the body

– High energy x-ray mode, designed to distribute energy over a wider area of the body

» A tungsten shield would move into place during x-ray mode to protect the patient from harm

•ITGS• Therac-25 reused software from two previous

models in which there had been no problems• The designers also removed hardware safety locks

which physically prevented certain erroneous conditions

• Accident 1 (June 1985)– Large overdose given to breast cancer patient– Nurse noticed she was warm after treatment, but hospital

denied any mistake and she received future treatments– Patient lost use of one arm and had to have both breasts

removed

ITGS• Accident 2, July 1985

– Machine gave an error message during treatment: the message ‘No Dose’ prompted the hospital tech to start it again

• He ended up starting again 5 times, not realizing that each time the patient had been given a radiation dose

• Overall, the patient received 13,000 - 17,000 rads – 200 rads is a typical dose, and 1000 rads can be fatal)

• Accident 4, March 1986– Malfunction ‘54” error

• Treatment stopped immediately, but the patient had received an overdose

– She suffered severe neurological damage and died 3 wks later– The nurse was able to recreate the sequence of the error:

» It turns out she entered info too quickly, then altered it

• Finally removed from sserice after 3 people were killed and 3 more seriously injured

ITGS• User interfaces

– Command Line Interface (CLI)• The user interacts with the computer solely by typing

commands– Some of the earliest interfaces– Most modern operating systems have a command prompt (also

called a terminal) built in» PC: type ‘cmd’, press enter» Mac: Applications>Utilities

ITGS

• User interfaces– Menu driven Interface (MDI)

• Let the user make decisions from a series of predetermined options that may be spread across multiple screens or menus

– ATMs, restaurants, etc.

ITGS• Ways to get help

– Read-me files• Included with software files, these detail last minute changes or known problems which have

not been included in the printed documentation.– Web:

• FAQ’s (frequently asked questions)• Tutorials

– Wizards:• Can guide users through a complex task by breaking it into steps and asking a series of

questions