Chapter10 Maintainence Reliability Quality

8/6/2019 Chapter10 Maintainence Reliability Quality

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Maintenance, Reliability and QualityMaintenance, Reliability and Quality

y Maintenance Types

y Lehmans laws

y Maintenance Process Models

y Reliability Hardware and Software

y Reliability Metrics

y Quality

y ISO -9001, SEI CMM, Six Sigma


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MaintenanceMaintenance

y This phase starts after the software goes

live

y One of the most important phases

y Every development needs maintenance

y All good software continue to evolve after

its development through maintenance

efforts


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Type of maintenanceType of maintenance

y Corrective All the bugs in the system discovered after release is

resolved in this phase

y Adaptive Maintenance is done when platforms changed, OS

upgraded or new interfaces added

y Perfective

This involves adding new features and changingexisting functionalities


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Characteristics of s/w evolutionCharacteristics of s/w evolution

y Lehmans Laws

First Law Software product must change

continuously otherwise they will become

progressively less useful

Second Law The structure of the program

tends to degrade as more and more

maintenance is carried out

Third Law The rate at which code is written

or modified is approximately the same during

development and maintenance


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Software Maintenance Process ModelsSoftware Maintenance Process Models

y Method 1

Small requirement changes

Requirements are not complex in nature

y Method 2

y Requirements are complex

y

Existing applications do not have properdocumentation and design

y Use of reverse engineering and forwardengineering


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Method 1Method 1

Gather change requirements

Analyze change requirements

Devise code change strategies

Apply code changes to old code

Update documents Integrate and test


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Method 2Method 2

Code

Module specs

Design

Requirement Specs

Code

Module specs

Design

New Requirement Specs

Reverse Engineering

Change Requirement


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ReliabilityReliability

y Reliability of a software product:

a concern for most users especially industry users.

An important attribute determining the quality of theproduct.

Users not only want highly reliable products but wantquantitative estimation of reliability before makingbuying decision.

y

Accurate measurement of software reliability: a very difficult problem

Several factors contribute to making measurement ofsoftware reliability difficult


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Hardware Vs Software ReliabilityHardware Vs Software Reliability

y Hardware failures: inherently different from software failures.

y Most hardware failures are due to component

wear and tear: some component no longer functions as specified.

y To fix hardware faults- replace or repair the failed part

y Software faults are latent:

system will continue to fail unless changes are made tothe software design and code.


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Reliability MetricsReliability Metrics

y There are many method by which reliability ofa software product can be measured.

y Reliability can also be a part of the non-

functional requirement in SRS

y Some reliability metrics

Rate of Occurrence Of Failure (ROCOF)

Mean Time to Failure (MTTF)

Mean Type to Repair (MTTR) Mean Time between Failure (MTBF)

Probability of failure on demand(POFOD)

Availability


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Rate of Occurrence Of Failure

(ROCOF)

y ROCOF measures:

The frequency of occurrence offailures.

Observe the behaviour of a softwareproduct in operation:

x over a specified time intervalx calculate the total number of failuresduring the interval.


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Mean Time To Failure (MTTF)Mean Time To Failure (MTTF)

y Average time between two successive

failures:

observed over a large number of failures.

y MTTF is not as appropriate for software as for

hardware:

Hardware fails due to a components wear andtear indicating how frequently the component fails

When a software error is detected and repairedthe same error never appears.


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Mean Type to Repair (MTTR)

y Once the failure occurs, it takes time to

fix it and addition time is lost

y MTTR measures the average time it takes

to fix faults


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Probability of failure on demand(POFOD)

y Unlike other metrics

This metric does not explicitly involve time.

y Measures the likelihood of the system

failing:

when a service request is made.

POFOD of 0.001 means:

x 1 out of 1000 service requests may result in a failure.


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AvailabilityAvailability

y This metric is important for systemslike: telecommunication systems,

operating systems, etc. which are supposedto be never down

where repair and restart time aresignificant and loss of service during thattime is important.

Availability is also important for webapplications


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QualityQuality

y Several quality factors are associated

with a software product :

Correctness

Reliability

Portability

Usability

Reusability

Maintainability


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Software Quality ManagementSoftware Quality Management

SystemSystemy Quality management system (or quality system):

Principal methodology used by organizations toensure that the products have desired quality.

y Quality System Activities Auditing of projects

Development of standards, procedures, andguidelines.

Production of reports for the top management: Summarizing the effectiveness of the quality system in

the organization.

Review of the quality system itself


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CertificationsCertifications

y International Standards Organization

ISO 9000

y SEI CMM (Capability Mature Model)

y Six Sigma


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AssignmentAssignment

y What is the difference between ISO 9001,

9002 and 9003?

y Compare between ISO 9001 and SEI

CMM?

y What are the metrics of six sigma, 5 sigma

and 4 sigma?

Documents

Chapter10 Maintainence Reliability Quality