Software Reliability

1
Software Reliability

• The most important dynamic characteristic of
  most software systems.. Sommerville (5th ed.)
  p365

2
Sommerville on Reliability

• Sommerville deals with reliability in several
  places
• 16.2 pp 359 -364
• 17.1 pp 373 - 364
• 21.2 pp 470 - 476
• this lecture attempts to address all these
  aspects of reliability

3
Definition

• The probability of failure-free operation within
  a specific time when used in a specific
  environment for a specific purpose
• But,
• use of system
• perception of reliability

4
Faults and Failures

• faults in every system
• faults lead to failures only if
• those parts of the system are used
• failures are perceived as such
• reliability is
• about failures not faults
• related to use
• difficult to apply to unacceptable failures

5
Sources of Faults

• specification
• design
• implementation
• maintenance

6
Estimation of Reliability

• depends on
• what is expected of the system
• does it operate continuously?
• transaction systems, operating systems
• is it invoked when required
• desktop systems
• application domain
• potentially dangerous?
• potential high financial cost?

7
Reliability Specification

• failure class
• description
• appropriate measure for the system
• value for that measure

8
Statistical Testing

• define operation profile
• generate appropriate test cases
• apply tests
• record time (in correct units) between failures
• for AVAIL record down time
• calculate measure - ensure it is significant
• result - reliability at a point in time

9
Reliability Modelling

• purpose
• give an estimate of when (or if) the system will
  meet its reliability specification
• method
• decide on a model
• measure time to each failure
• fit to model, predict time to meet specification
• cost

10
Reliability Models

• linear improvement
• random improvement (possibly -ve.)
• curve fitting

11
Summary

• reliability depends on use and perception
• use quantitative reliability specifications
• measure depends on the system
• use of statistical testing
• use of reliability models