1
Q
attributes of dependability
A
• Safety: absence of harm to people and environment • Availability: the readiness for correct service • Integrity: absence of improper system alterations • Reliability: continuity of correct service • Maintainability: ability to undergo modifications and repairs
2
Q
metric Reliability
A
MTTF
3
Q
Fault, Error & Failures
A
• Fault: a defect within the system or a situation that can lead to failure • Error: manifestation (symptom) of the fault - an unexpected behaviour • Failure: system not performing its intended function
Effects in time:
Transient/ Intermittent / Permanent
4
Q
Dependability techniques
A
- Fault prevention
- Fault removal
- Fault tolerance
- Fault forecasting
5
Q
Goal of system verification and
validation
A
is to remove faults
6
Q
Goal of hazard/risk analysis
A
is to focus
on more important faults
7
Q
Goal of fault tolerance
A
is to reduce
effects of errors if they appear -
eliminate or delay failures
8
Q
Fault model
A
describes the foreseen
faults in fault tolerance
9
Q
Node failures
A
– Crash
– Omission
– Timing
– Byzantine
10
Q
Channel failures
A
in Distributed systems: – Crash (and potential partitions) – Message loss – Message delay – Erroneous/arbitrary messages
11
Q
On-line error management
A
• Detection: By program or its environment • Mitigation: – Fault containment by architectural choices – Fault tolerance using redundancy • in software (redundancy in space or time) • in hardware • in data
12
Q
Static Redundancy
A
Used all the time (whether an error has appeared or not), just in case… – SW: N-version programming – HW: Voting systems – Data: Parity bits, checksums
13
Q
Dynamic Redundancy
A
Used when error appears and specifically aids the treatment – SW: • Space: Exceptions, Rollback recovery • Time: Re-computing a result – HW: Switching to back-up module – Data: Self-correcting codes
14
Q
Byzantine agreement protocol
A
a
Real Time Systems Exam
flashcards
Decks in class (3)
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