Critical Software

Safety Critical Software

Systems where failure could cause death or injury are called safety critical systems. For example, nuclear reactor and flight control systems.

Security Cricital Software

Systems where failure could lead to reavealing classified, confidential business or personal data are called security cirtical systems. For example, payroll systems.

Business Critical Software

Systems where failure could affect important operations are called business critical systems.


Example- Patriot Missiles

Anti-missile patrior missiles failed to intercept an Iraqi SCUD missile on 25/2/91- the SCUD struck a US barracks in Dhahran. Other SCUD's got through to Isreal and Saudi Arabia.

Reason for failure

  • The system measured time in 1/10 sec, truncated from binary representation .0001100110011....
  • As the system was upgraded from anti-aircraft to anti-missile, greater accuracy introduced - but not everywhere in the code
  • Two modules got out of step by 1/3 sec after 100 hours operation. Target not acquired
  • Defect not found in testing as the spec called for 14 hour continuous operation only

    Many critical systems failures are multifactorial: "a reliable system can't fail in a simple way"