2. Fault Tolerance
Fault-tolerant computing is the art and science of
building computing systems that
continue to operate satisfactorily in the presence of
faults. A fault-tolerant system may be
able to tolerate one or more fault-types including –
i) transient(cause by external disturbance),
intermittent(cause by marginal designed error) or
permanent hardware faults,
ii) software and hardware design errors,
iii) operator errors, or
iv) externally induced upsets or physical damage.
5. Availability & Reliability
Availability: A measurement of whether
a system is ready to be used immediately
◦ System is available at any given moment
Reliability: A measurement of whether
a system can run continuously without
failure
◦ System continues to function for a long
period of time
6. Safety & Maintainability
Safety: A measurement of how safe failures
are
◦ System fails, nothing serious happens
◦ For instance, high degree of safety is required for
systems controlling nuclear power plants
Maintainability: A measurement of how
easy it is to repair a system
◦ A highly maintainable system may also show a
high degree of availability
◦ Failures can be detected and repaired
automatically. Self-healing systems.
7. What is Fault?
A system fails when it cannot meet its promises
(specifications)
An error is part of a system state that may lead to
a failure
A fault is the cause of the error
Fault-Tolerance: the system can provide services
even in the presence of faults
Faults can be:
◦ Transient (appear once and disappear)
◦ Intermittent (appear-disappear-reappear behavior)
A loose contact on a connector intermittent fault
◦ Permanent (appear and persist until repaired)
8. Failure Model
Type of Failure Description
Crash failure A server halts, but is working correctly until it halts
Omission failure
Receive omission
Send omission
A server fails to respond to incoming requests
A server fails to receive incoming messages
A server fails to send messages
Timing failure A server's response lies outside the specified time
interval
Response failure
Value failure
State transition
failure
The server's response is incorrect
The value of the response is wrong
The server deviates from the correct flow of control
Arbitrary failure
(Byzantine failure)
A server may produce arbitrary responses at
arbitrary times
9. Error Detection
Error detection is a detection of errors
caused by noise or other impairments during
transmission from the transmitter to the
receiver.
There are many schemes of error
detection:-
1. Repetition codes.
2. Parity bits.
3. Checksums.
4. Cyclic redundancy checks.
5. Cryptography hash functions.
10. System Recovery
We have talk a lot about fault tolerance
but not talk about what happen after fault
has occurred.
A process that exhibits a failure has to be
able to recover to a correct state
There are two type of recovery:
1. Backward Recovery.
2. Forward Recovery.
11. Backward Recovery
The goal of backward recovery is to bring
the system from an erroneous state back
to a prior correct state
The state of the system must be recorded
- checkpointed - from time to time, and
then restored when things go wrong
Examples
◦ Reliable communication through packet
retransmission
12. Forward Recovery
The goal of forward recovery is to bring a
system from an erroneous state to a
correct new state (not a previous state)
Examples:
◦ Reliable communication via erasure(a
correction made by erasing) correction, such
as an (n, k) block erasure code.
13. Fault Masking
Fault Masking is a structural redundancy
technique that completely masks faults
within a set of redundant modules.
Redundancy is key technique for hiding
failures.
Redundancy, however, can have an
adverse impact on the performance of a
system. For example, it can increase the
length of transmitted data or increase
the resource consumption.
14. Reconfiguration
Reconfiguration is the “process of
eliminating a faulty entity from a system
and restoring the system to some
operational condition or state”.
When we use Reconfiguration process
designer must be concerned with fault
detection, fault location, fault containment,
and fault recovery.
15. Redundancy
In engineering redundancy is the
duplication of critical components or
function of a system with the intention of
increasing reliability of the system.
Redundancy are four types:-
1. Hardware(such as DMR & TMR)
2. Software(N-version programming)
3. Time(transient fault detection such as
Alternate logic)
4. Information(error detection or
correction)
16. Conclusion
Fault-tolerance is achieved by applying a set of
analysis and design techniques to create systems
with dramatically improved dependability.As new
technologies are developed and new applications
arise, new fault-tolerance approaches are also
needed. Now chips contain complex, highly-
integrated functions, and hardware and software
must be crafted to meet a variety of standards to
be economically viable.Thus a great deal of
current research focuses on implementing fault
tolerance using COTS (Commercial-Off-The-
Shelf) technology.
