Ch11 - Software Engineering 9th – Sommerville
questions with verified answers
4 Principal Properties of Dependability - SARS Ans✓✓✓-1. Security
Systems should protect themselves and their data from external interference.
2. Availability
Systems should be available to deliver service and perform as expected.
3. Reliability
Systems should consistently deliver services as specified.
4. Safety
Systems should not behave in an unsafe way.
System dependability Ans✓✓✓-For many computer-based systems, the most
important system property is the dependability of the system.
The dependability of a system reflects the user's degree of trust in that system. It
reflects the extent of the user's confidence that it will operate as users expect and
that it will not 'fail' in normal use.
Dependability covers the related systems attributes of reliability, availability and
security. These are all inter-dependent.
Importance of dependability Ans✓✓✓-System failures may have widespread
effects with large numbers of people affected by the failure.
Systems that are not dependable and are unreliable, unsafe or insecure may be
rejected by their users.
The costs of system failure may be very high if the failure leads to economic losses
or physical damage.
, Undependable systems may cause information loss with a high consequent
recovery cost.
3 Causes of failure Ans✓✓✓-1. Hardware failure
Hardware fails because of design and manufacturing errors or because
components have reached the end of their natural life.
2. Software failure
Software fails due to errors in its specification, design or implementation.
3. Operational failure
Human operators make mistakes. Now perhaps the largest single cause of system
failures in socio-technical systems.
SARS Ans✓✓✓-1. Security
A judgment of how likely it is that the system can resist accidental or deliberate
intrusions.
2. Availability
The probability that the system will be up and running and able to deliver useful
services to users.
3. Reliability
The probability that the system will correctly deliver services as expected by
users.
4. Safety
A judgment of how likely it is that the system will cause damage to people or its
environment.
4 Other dependability properties - SERM
questions with verified answers
4 Principal Properties of Dependability - SARS Ans✓✓✓-1. Security
Systems should protect themselves and their data from external interference.
2. Availability
Systems should be available to deliver service and perform as expected.
3. Reliability
Systems should consistently deliver services as specified.
4. Safety
Systems should not behave in an unsafe way.
System dependability Ans✓✓✓-For many computer-based systems, the most
important system property is the dependability of the system.
The dependability of a system reflects the user's degree of trust in that system. It
reflects the extent of the user's confidence that it will operate as users expect and
that it will not 'fail' in normal use.
Dependability covers the related systems attributes of reliability, availability and
security. These are all inter-dependent.
Importance of dependability Ans✓✓✓-System failures may have widespread
effects with large numbers of people affected by the failure.
Systems that are not dependable and are unreliable, unsafe or insecure may be
rejected by their users.
The costs of system failure may be very high if the failure leads to economic losses
or physical damage.
, Undependable systems may cause information loss with a high consequent
recovery cost.
3 Causes of failure Ans✓✓✓-1. Hardware failure
Hardware fails because of design and manufacturing errors or because
components have reached the end of their natural life.
2. Software failure
Software fails due to errors in its specification, design or implementation.
3. Operational failure
Human operators make mistakes. Now perhaps the largest single cause of system
failures in socio-technical systems.
SARS Ans✓✓✓-1. Security
A judgment of how likely it is that the system can resist accidental or deliberate
intrusions.
2. Availability
The probability that the system will be up and running and able to deliver useful
services to users.
3. Reliability
The probability that the system will correctly deliver services as expected by
users.
4. Safety
A judgment of how likely it is that the system will cause damage to people or its
environment.
4 Other dependability properties - SERM
