This document discusses availability and reliability in systems. Availability is defined as the probability that a system will be operational to deliver requested services, while reliability is the probability of failure-free operation over time. Both can be expressed as percentages. Availability takes into account repair times, whereas reliability does not. Faults can lead to errors and failures if not addressed through techniques like fault avoidance, detection, and tolerance.

1. Availability and Reliability
Availability and reliability, 2013
Slide 1

2. Principal dependability
properties
Availability and reliability, 2013
Slide 2

3. • Reliability
– The probability of failure-free system
operation over a specified time in a
given environment for a given purpose
Availability and reliability, 2013
Slide 3

4. • Availability
– The probability that a system, at a point in
time, will be operational and able to deliver
the requested services
Availability and reliability, 2013
Slide 4

5. Availability specification
• Both reliability and availability attributes
can be expressed as numbers:
– Availability of 0.999 means that the system
is up and running for 99.9% of the time;
Availability and reliability, 2013
Slide 5

6. Reliability specification
• Probability of failure on demand
(POFOD) of 0.0001 means that on
average 1 in 10, 000 demands for
service from a system will fail in some
way
Availability and reliability, 2013
Slide 6

7. Availability and reliability
• Availability and reliability are closely
related
– Obviously if a system is unavailable it is not
delivering the specified system services.
Availability and reliability, 2013
Slide 7

8. • However, it is possible to have systems
with low reliability that must be
available.
– So long as system failures can be repaired
quickly and does not damage data, some
system failures may not be a problem.
Availability and reliability, 2013
Slide 8

9. • Availability is therefore best considered
as a separate attribute reflecting
whether or not the system can deliver its
services.
• Availability takes repair time into
account, if the system has to be taken
out of service to repair faults.
Availability and reliability, 2013
Slide 9

10. Availability perception
• Availability is usually expressed as a
percentage of the time that the system
is available to deliver services e.g.
99.9%.
Availability and reliability, 2013
Slide 10

11. Availability and reliability, 2013
Slide 11

12. Subjective availability
• The number of users affected by the
service outage.
– Loss of service in the middle of the night is
less important for many systems than loss
of service during peak usage periods.
Availability and reliability, 2013
Slide 12

13. • The length of the outage.
– The longer the outage, the more the
disruption. Several short outages are less
likely to be disruptive than 1 long outage.
Long repair times are a particular problem.
Availability and reliability, 2013
Slide 13

14. Reliability metrics
• Probability of failure on demand
(POFOD)
– Probability that a system will not deliver a
service correctly when requested
– Used for systems where demands are
infrequent and intermittent
Availability and reliability, 2013
Slide 14

15. • Rate of occurrence of failure (ROCOF)
– Number of system failures in a given time
period
– Used for transaction processing systems
with frequent and regular transactions
Availability and reliability, 2013
Slide 15

16. •
Fault
–
•
Error
–
•
A characteristic of a software system that can lead to a
system error.
An erroneous system state that can lead to system behavior
that is unexpected by system users.
Failure
–
An event that occurs at some point in time when the system
does not deliver a service as expected by its users.
Availability and reliability, 2013
Slide 16

17. Faults-errors-failures
Fault
Error
Failure
Availability and reliability, 2013
Slide 17

18. Faults and failures
• Failures are a usually a result of system
errors.
• The incorrect state causes undesirable
system behaviour
• Incorrect state is a consequence of
executing faulty code
Availability and reliability, 2013
Slide 18

19. • However, faults do not necessarily
result in system errors
– The erroneous system state resulting from
the fault may be transient and ‘corrected’
before an error arises.
– The faulty code may never be executed.
Availability and reliability, 2013
Slide 19

20. • Errors do not necessarily lead to system
failures
– The error can be corrected by built-in error
detection and recovery
– The failure can be protected against by
built-in protection facilities. These may, for
example, protect system resources from
system errors
Availability and reliability, 2013
Slide 20

21. Reliability achievement
• Fault avoidance
– Development technique are used that
either minimise the possibility of
mistakes or trap mistakes before they
result in the introduction of system
faults.
Availability and reliability, 2013
Slide 21

22. • Fault detection and removal
– Verification and validation techniques
that increase the probability of
detecting and correcting errors before
the system goes into service are
used.
Availability and reliability, 2013
Slide 22

23. • Fault tolerance
– Run-time techniques are used to
ensure that system faults do not result
in system errors and/or that system
errors do not lead to system failures.
Availability and reliability, 2013
Slide 23

24. Summary
• Availability is the probability that a
system will be available when a service
request is made
• Reliability is the probablity that a system
will deliver a service as expected by
users
Availability and reliability, 2013
Slide 24

25. Summary
• Software faults lead to state errors lead
to operational failures
• Fault avoidance, detection and
tolerance are strategies for achieving
reliability
Availability and reliability, 2013
Slide 25