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Software Failures

Software Failures

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Software Failures

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  1. Software Failures Ron Gilmore, CMC Edmonton April 2006

  2. Software Failures • Santayana • The software sector • Observations • Case Study: Therac 25 • Lessons • Engineering Comparisons • Challenges

  3. Santayana (1863 - 1952) • Philosopher, essayist, poet, novelist • The Life of Reason (1905) • "Those who cannot remember the past are condemned to repeat it“ • Lots of other great quotes • Egypt, March 2006

  4. Software Sector • Young – less than a century • Amateurs • Change, churn and failures • Compare to roads, houses, bridges • Professions evolving • Standards evolving • Best practices evolving • Societal awareness evolving

  5. Case Study: Therac 25 • Radiation therapy machines • Atomic Energy of Canada • 1985 to 1987 • Six known “incidents” • Massive radiation overdoses to patients • Order of tens of thousands of rads • At least five deaths!

  6. Therac 25 Root Causes • Institutional causes: • No independent code review • Software not included in reliability design • Documentation “lean” on error codes • AECL did not initially believe complaints

  7. Therac 25 Root Causes • Design Issues: • No preventative hardware interlocks • AECL re-used software from older models which had hardware interlocks • No way for software to verify sensors were working • Arithmetic overflow - safety checks bypassed • Software written in assembly language

  8. Therac 25 Lessons? • Professions? • Standards? • Best practices? • Societal awareness?

  9. Engineering Comparisons • More mature sector • Certification, legislation, compliance • Curriculum: Tacoma Narrows Bridge • Still: London Pedestrian bridge • Still: Confusion re mandate, coverage • Still: budget & schedule - oilsands

  10. Challenges • Education – technical, business • Sensitivity – bad software can kill! • Lots more examples: • Chinook helicopter • Missile detection systems

  11. Constructive Notions • Awareness efforts • Consequences • Core competencies • Systems classifications: • A = Life threatening • B = Business threatening • C = Other