Reliability and Availability
Availability and ReliabilityAvailability and reliability,
2013Slide #Principal dependability properties
Availability and reliability, 2013Slide #ReliabilityThe
probability of failure-free system operation over a specified time
in a given environment for a given purposeAvailability and
reliability, 2013Slide #AvailabilityThe probability that a system,
at a point in time, will be operational and able to deliver the
requested services
Availability and reliability, 2013Slide #Availability
specificationBoth 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, 2013Slide #Reliability
specificationProbability 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, 2013Slide #Availability and
reliabilityAvailability and reliability are closely
relatedObviously if a system is unavailable it is not delivering
the specified system services.
Availability and reliability, 2013Slide #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, 2013Slide #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, 2013Slide #Availability
perceptionAvailability is usually expressed as a percentage of the
time that the system is available to deliver services e.g.
99.9%.Availability and reliability, 2013Slide #
Availability and reliability, 2013Slide #Subjective
availabilityThe 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, 2013Slide #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, 2013Slide #Reliability
metricsProbability of failure on demand (POFOD)Probability that a
system will not deliver a service correctly when requestedUsed for
systems where demands are infrequent and intermittentAvailability
and reliability, 2013Slide #Rate of occurrence of failure
(ROCOF)Number of system failures in a given time periodUsed for
transaction processing systems with frequent and regular
transactions
Availability and reliability, 2013Slide #FaultA characteristic
of a software system that can lead to a system error. ErrorAn
erroneous system state that can lead to system behavior that is
unexpected by system users. FailureAn event that occurs at some
point in time when the system does not deliver a service as
expected by its users.
Availability and reliability, 2013Slide
#Faults-errors-failuresFaultErrorFailureAvailability and
reliability, 2013Slide #Faults and failuresFailures are a usually a
result of system errors.The incorrect state causes undesirable
system behaviourIncorrect state is a consequence of executing
faulty codeAvailability and reliability, 2013Slide #However, faults
do not necessarily result in system errorsThe 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, 2013Slide #Errors do not
necessarily lead to system failuresThe 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, 2013Slide #Reliability
achievementFault avoidanceDevelopment 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, 2013Slide #Fault detection and removalVerification and
validation techniques that increase the probability of detecting
and correcting errors before the system goes into service are
used.
Availability and reliability, 2013Slide #Fault toleranceRun-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, 2013Slide #SummaryAvailability is
the probability that a system will be available when a service
request is madeReliability is the probablity that a system will
deliver a service as expected by usersAvailability and reliability,
2013Slide #SummarySoftware faults lead to state errors lead to
operational failuresFault avoidance, detection and tolerance are
strategies for achieving reliability
Availability and reliability, 2013Slide #
