System dependability, 2013 Slide 1

System dependability

System dependability, 2013 Slide 2

System dependability

• For many software-intensive systems, the most important system property is the dependability of the system.

System dependability, 2013 Slide 3

• The dependability of a system reflects the extent of the user’s confidence that it will operate as users expect and that it will not corrupt data or other systems and will not ‘fail’ in normal use.

• Users trust a system that is dependable

System dependability, 2013 Slide 4

Importance of dependability• 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.

System dependability, 2013 Slide 5

• 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.

System dependability, 2013 Slide 6

System dependability, 2013 Slide 7

Principal properties• Availability

– The probability that the system will be up and running and able to deliver useful services to users.

• Reliability– The probability that the system will correctly

deliver services as expected by users.

System dependability, 2013 Slide 8

• Safety

– A judgment of how likely it is that the system will not damage people or the system’s environment.

• Security

– A judgment of how likely it is that the system can resist accidental or deliberate attacks and intrusions.

System dependability, 2013 Slide 9

Dependability attribute dependencies

• Safe system operation depends on the system being available and operating reliably.

• A system may be unreliable because its data has been corrupted by an external attack.

System dependability, 2013 Slide 10

• Denial of service attacks on a system are intended to make it unavailable.

• If a system is infected with a virus, you cannot be confident in its reliability or safety.

System dependability, 2013 Slide 11

Dependability is subjective

• The dependability of a system is not absolute but depends on the judgment and expectations of a system stakeholder.

• What seems to be a system failure to one stakeholder is normal behaviour to another.

System dependability, 2013 Slide 12

User expectations• A banking system does not update user

accounts in real time but queues the transactions and updates accounts twice a day

• A user that expects real time updates may consider this undependable as they cannot be sure whether or not the account balance is accurate

System dependability, 2013 Slide 13

• Users trust a system if they think it is dependable and this is based on previous experience

• Some people trust a system so don’t do backups

• Others, backup obsessively

System dependability, 2013 Slide 14

Dependability and specifications

• Dependability can only be defined formally (objectively) by relating behaviour to the system specification

• A failure can be thought of as a deviation from a specification

System dependability, 2013 Slide 15

• However, many specifications do not reflect the real requirements of the system users.

• A system that conforms to its specification may ‘fail’ from the perspective of some users.

System dependability, 2013 Slide 16

• Furthermore, users don’t read specifications so don’t know how the system is supposed to behave.

• Therefore, a formal definition of dependability may not reflect user experience with the system

System dependability, 2013 Slide 17

Summary• Dependability is a composite system

property that reflects the degree of trust users have in a system

• It includes availability, reliability, safety and security

• Dependability is subjective and depends on the expectations and judgment of system users