Critical systems are critical ;p and it is important they are dependable.
Four principal dimensions of dependability:
a) availability - ability to deliver services when required
b) reliability - ability to deliver services as specified
c) safety - ability to operate without catastrophic failure
d) security - ability to protect itself from intrusion
we do not necessarily need all these 4 (in particular system)
System properties connected to dependability:
a) repairability - the organisation using software can repair it
b) maintainability - can be adapted easily to changing conditions
c) survivability - ability to deliver services even if being under attack or while part of the system is disabled; recovery from attack
d) error tolerance - the extent to which the system avoids and tolerate user input errors
failure - event when software does not deliver what expected
/\
||
error - state which leads to unexpected behaviour
/\
||
fault - property of software which can lead to an error
