Types of Faults
- Transient
- Occurs only once and then goes away
- Intermittent
- Irregular, comes and goes
- Permanent
- Occurs only once and then goes away. Continues to exist (until repair)
Propagation
- Fault
- the cause of an error
- Error
- part of a system’s state that may lead to a failure
- Failure
- a system does not operate according to its specifications
- Faults lead to errors, errors lead to failures.
Types of Failures
- Crash failure
- A server halts, but is working correctly until it halts
- Omission failure
- A server fails to respond to incoming requests, fails to receive incoming messages or fails to send messages
- Timing failure
- A server’s response lies outside the specified time interval
- Response failure
- The server’s response is incorrect, value of the response is wrong, or server deviates from the correct flow of control
- Arbitrary (Byzantine) Failure
- A server may produce arbitrary responses at arbitrary times
(k)-Fault-Tolerance
Fail-Silent
[ n=k+1 ] Replica produces either correct responses or no response at all.
Fail-Consistent
[ n=2k+1 ] Replica produces either only correct or only faulty responses
Byzantine
[ n=3k+1 ] Replica inconsistently produces correct or faulty responses