Software engineering 23 software reliability

May 8, 20201 like777 views

Software Reliability is the probability of failure-free software operation for a specified period of time in a specified environment. Software Reliability is also an important factor affecting system reliability. ... The high complexity of software is the major contributing factor of Software Reliability problems.

Software Engineering: 23
Software Reliability
Prof Neeraj Bhargava
Vaibhav Khanna
Department of Computer Science
School of Engineering and Systems Sciences
Maharshi Dayanand Saraswati University Ajmer

Notion of Reliability
• Aims at fault-free performance of software
systems
• Software reliability goes hand-in-hand with
software verification
– Input: collection of software test results
– Goal: assess the validity of the software system
• Targets safety-critical software

Role of Reliability in Software
Engineering

Error, Fault and Failure
• Error: human action that results in software
containing a fault
• Fault: a cause for an internal error (failure)
• Failure: any observable divergence of software
behavior in execution from user needs
• Failure intensity: the number of failures per time
unit

More Basic Notions
• Failure: any observable divergence of software
behavior in execution from user needs
• Failure intensity: the number of failures per natural
or time unit. Failure intensity is a way of expressing
reliability.
• Availability: The probability that at a given time that
a system or a capability of a system functions
satisfactorily in a specified environment.
• If you are given an average downtime per failure, then
availability implies a kind of reliability.

Classical Definition of Reliability
• Software Reliability is the probability that a system
will operate without failure under given
environmental conditions for a specified period of
time.
• We express reliability on a scale from 0 to 1:
– highly reliable system will have a reliability measure close
to 1, and
– unreliable system will have a measure close to 0.
• Reliability is measured over execution time so that
it more accurately reflects system usage.
• GOAL: reliability must be quantified so that we can compare
software systems

Characters of Software Reliability
• Failures are primarily due to design faults.
– Repairs are made by modifying the design to make it robust
against conditions that can trigger a failure.
• There is no wear-out phenomena.
– Software errors occur without warning.
– “Old” code can exhibit an increasing failure rate as a function of
errors induced while making upgrades.
– External environment conditions do not affect software reliability.
– Internal environmental conditions, such as insufficient memory or
inappropriate clock speeds do affect software reliability.
• Reliability is not time dependent.
– Failures occur when the logic path that contains an error is
executed.
– Reliability growth is observed as errors are detected and
corrected.

Software Reliability Modeling
• A software reliability
model specifies the
general form of the
dependence of the failure
process on the principal
factors that affect it:
- Time,
- fault introduction,
- fault removal,
- operational
environment

Assignment
• Explain the concept of software reliability and
its importance.
• Thank You

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