Lecture 8 - 2010-4-8

Lecture 8 - 2010-4-8 - 4/8/2010 What We Will Cover Failures...

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4/8/2010 1 What We Will Cover Failures and Errors in Computer Systems Case Study: The Therac-25 Increasing Reliability and Safety Dependence, Risk, and Progress Failures and Errors in Computer Systems Most computer applications are so complex it is virtually impossible to produce programs with no errors The cause of failure is often more than one factor Computer professionals must study failures to learn how to avoid them Computer professionals must study failures to understand the impacts of poor work Failures and Errors in Computer Systems (cont.) Individual Problems: Billing errors Inaccurate and misinterpreted data in databases Large population where people may share names Automated processing may not be able to recognize special cases Overconfidence in the accuracy of data Errors in data entry Lack of accountability for errors
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4/8/2010 2 Failures and Errors in Computer Systems (cont.) System Failures: Businesses have gone bankrupt after spending huge amounts on computer systems that failed Voting system in 2000 presidential election Failures and Errors in Computer Systems (cont.) Denver Airport: Baggage system failed due to real world problems, problems in other systems and software errors Main causes: Time allowed for development was insufficient Denver made significant changes in specifications after the project began Failures and Errors in Computer Systems (cont.) High-level Causes of Computer-System Failures: Lack of clear, well thought out goals and specifications Poor management and poor communication among customers, designers, programmers, etc. Pressures that encourage unrealistically low bids, low budget requests, and underestimates of time requirements Use of very new technology, with unknown reliability and problems Refusal to recognize or admit a project is in trouble
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4/8/2010 3 Failures and Errors in Computer Systems (cont.) Safety-Critical Applications: A-320: "fly-by-the-wire" airplanes (many systems are controlled by computers and not directly by the pilots) Between 1988-1992 four planes crashed Air traffic control is extremely complex, and includes computers on the ground at airports, devices in thousands of airplanes, radar, databases, communications, and so on - all of which must work in real time, tracking airplanes that move very fast In spite of problems, computers and other technologies have made air travel safer Case Study: The Therac-25
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Lecture 8 - 2010-4-8 - 4/8/2010 What We Will Cover Failures...

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