Software reliability (week 11)

Question 1

How is software reliability defined?

Answer 1

The probability of failure-free software operation for a specified period of time in a specified evnironment

Question 2

Describe Probability of Failure on Demand (POFOD) measure

Answer 2

• Likelihood a system will fail when a request for service is made
• A POFOD of 0.0001 means 1 in 10000 requests may result in a failure
• Very relevant for safety critical systems

Question 3

Describe Rate of Occurrence of Failure (ROCOF) measure

Answer 3

• Frequency of occurrence of failures
• ROCOF of 0.005 means 5 failures are likely in each 1000 time
units
• Relevant for banking/financial systems

Question 4

Describe Mean Time Between Failures (MTBF) measure

Answer 4

• Measure of time between recoverable failures over the lifetime of
the product
• A MTBF of 100, failures may occur between every 100 time units
• Relevant for ‘long duration transaction’ systems such as database
operations, word processor systems, etc
• MTBF should be longer than the typical transaction length

Question 5

Describe Availability (AVAIL) measure

Answer 5

• Measure of how long a system is available for use
• An availability of 0.995 means a system will likely be available for
995 out of 1000 time units
• Relevant for continuous systems
• Such as communication/broadband systems

Question 6

Describe Mean Time to Recover (MTTR)

Answer 6

• Average time taken to recover from a software failure
• Eg: if Amazon MTBF was three years, and the MTTR was one day,
then customers would notice about the long recovery time.
• If the MTBF was, say, twice per day, but the MTTR was less
than a second, then customers would not notice..!
• Backup systems/system redundancy attempts to reduce MTTR to
very low levels

Question 7

What does a reliability specification consist of?

Answer 7

• Need a quantitative statement of the reliability requirement
• A description of the environment where the equipment is to be
used, stored, etc.
• Clear definition of what constitutes a failure
• What tests are used to demonstrate reliability

Question 8

Give an example of reliability specification

Answer 8

For example a radar system which uses high and low
power searching and tracking
• Specify MTBF required for each case
• Condition of use for the system: • Temp, vibration, light, pressure, operator ability
• Successful operation – is it measurable?
• Failure performance specification
• How, who, when, where of the tests to determine
reliability

Question 9

Describe reliability Engineering?

Answer 9

Focuses upon costs of system downtime, spares, personnel, cost of
repair
• Safety Engineering focuses upon accident reduction and risk
reduction (minimising life threatening situations)
• A highly reliable system results not only from good reliability
engineering
• Predicting failure rates, costs, etc
• But also on good development engineering, rigorous methodology
applied etc.

Question 10

How is AVAIL calculated?

Answer 10

MTBF/(MTBF+MTTR)