System dependability is a composite system property that reflects the degree of trust users have in a system. It is determined by availability, reliability, safety, and security. Dependability is subjective as it depends on user expectations - a system deemed dependable by one user may be seen as unreliable by another if it does not meet their expectations. Formal specifications of dependability do not always capture real user experiences.

1. System dependability
System dependability, 2013
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2. System dependability
• For many software-intensive
systems, the most important
system property is the
dependability of the system.
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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
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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.
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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.
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6. System dependability, 2013
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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.
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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.
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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.
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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.
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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.
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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
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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
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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
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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.
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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
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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
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