SE 131 CHAPTER 12 - RELIABILITY ENGINEERING

In general, software customers expect all software to be dependable. However, for non-critical applications, they may be willing to accept some system failures.

True

False

Some applications (critical systems) have very high reliability requirements. Choose all that have a very high reliability requirement.

Medical systems

Aerospace systems

Telecommunications and power systems

Business System

Which best defines human error or mistake?

Human behavior that results in the introduction of faults into a system

Human behavior that results in the user misusing the product

The programmer making poor design choices

A hacker trying to create system faults into a product

Which best defines a system fault?

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.

An event that occurs at some point in time when the system does not deliver a service as expected by its users

Human behavior that results in the introduction of faults into a system.

Which best defines a system 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.

An event that occurs at some point in time when the system does not deliver a service as expected by its users

Human behavior that results in the introduction of faults into a system.

Which best defines a system failure?

A characteristic of a software system that can lead to a system error.

Human behavior that results in the introduction of faults into a system

An erroneous system state that can lead to system behavior that is unexpected by system users.

An event that occurs at some point in time when the system does not deliver a service as expected by its users

Failures are a usually a result of system errors that are derived from faults in the system. However, faults do not necessarily result in system errors.

True

False

Choose all that explain why 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

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

Choose all that explain why errors do not necessarily lead to system failures.

The erroneous system state resulting from the fault may be transient and ‘corrected’ before an error arises.

The faulty code may never be executed.

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

Which best explains fault avoidance?

The system is developed in such a way that human error is avoided and thus system faults are minimised.

Verification and validation techniques are used to discover and remove faults in a system before it is deployed.

The system is designed so that faults in the delivered software do not result in system failure.

Which best explains fault detection?

The system is developed in such a way that human error is avoided and thus system faults are minimised.

Verification and validation techniques are used to discover and remove faults in a system before it is deployed.

The system is designed so that faults in the delivered software do not result in system failure.

The development process is organised so that faults in the system are detected and repaired before delivery to the customer.

Which best explains fault tolerance?

The system is designed so that faults in the delivered software do not result in system failure.

Verification and validation techniques are used to discover and remove faults in a system before it is deployed.

The system is developed in such a way that human error is avoided and thus system faults are minimised.

The development process is organised so that faults in the system are detected and repaired before delivery to the customer.

One explains reliability while the other availability. Choose the one that describes reliability

The probability of failure-free system operation over a specified time in a given environment for a given purpose

The probability that a system, at a point in time, will be operational and able to deliver the requested services

Reliability can only be defined formally with respect to a system specification. However, many specifications are incomplete or incorrect – hence, a system that conforms to its specification may ‘fail’ from the perspective of system users.

True

False

Availability is usually expressed as a percentage of the time that the system is available to deliver services e.g. 99.95%.

True

False

Choose all that are TRUE statements for reliability in use.

Removing X% of the faults in a system will not necessarily improve the reliability by X%.

Program defects may be in rarely executed sections of the code so may never be encountered by users. Removing these does not affect the perceived reliability.

Users adapt their behaviour to avoid system features that may fail for them.

A program with known faults may therefore still be perceived as reliable by its users.

SE 131 CHAPTER 12 - RELIABILITY ENGINEERING

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