<s>
Fault	B-General_Concept
tolerance	I-General_Concept
is	O
the	O
property	O
that	O
enables	O
a	O
system	O
to	O
continue	O
operating	O
properly	O
in	O
the	O
event	O
of	O
the	O
failure	O
of	O
one	O
or	O
more	O
faults	O
within	O
some	O
of	O
its	O
components	O
.	O
</s>
<s>
Fault	B-General_Concept
tolerance	I-General_Concept
is	O
particularly	O
sought	O
after	O
in	O
high-availability	B-General_Concept
,	O
mission-critical	B-Application
,	O
or	O
even	O
life-critical	B-Application
systems	I-Application
.	O
</s>
<s>
The	O
ability	O
of	O
maintaining	O
functionality	O
when	O
portions	O
of	O
a	O
system	O
break	O
down	O
is	O
referred	O
to	O
as	O
graceful	B-General_Concept
degradation	I-General_Concept
.	O
</s>
<s>
A	O
fault-tolerant	B-General_Concept
design	I-General_Concept
enables	O
a	O
system	O
to	O
continue	O
its	O
intended	O
operation	O
,	O
possibly	O
at	O
a	O
reduced	O
level	O
,	O
rather	O
than	O
failing	O
completely	O
,	O
when	O
some	O
part	O
of	O
the	O
system	O
fails	O
.	O
</s>
<s>
That	O
is	O
,	O
the	O
system	O
as	O
a	O
whole	O
is	O
not	O
stopped	O
due	O
to	O
problems	O
either	O
in	O
the	O
hardware	B-Architecture
or	O
the	O
software	O
.	O
</s>
<s>
Within	O
the	O
scope	O
of	O
an	O
individual	O
system	O
,	O
fault	B-General_Concept
tolerance	I-General_Concept
can	O
be	O
achieved	O
by	O
anticipating	O
exceptional	O
conditions	O
and	O
building	O
the	O
system	O
to	O
cope	O
with	O
them	O
,	O
and	O
,	O
in	O
general	O
,	O
aiming	O
for	O
self-stabilization	B-Operating_System
so	O
that	O
the	O
system	O
converges	O
towards	O
an	O
error-free	O
state	O
.	O
</s>
<s>
In	O
any	O
case	O
,	O
if	O
the	O
consequence	O
of	O
a	O
system	O
failure	O
is	O
so	O
catastrophic	O
,	O
the	O
system	O
must	O
be	O
able	O
to	O
use	O
reversion	B-General_Concept
to	O
fall	B-General_Concept
back	I-General_Concept
to	O
a	O
safe	O
mode	O
.	O
</s>
<s>
The	O
first	O
known	O
fault-tolerant	B-General_Concept
computer	O
was	O
SAPO	B-Device
,	O
built	O
in	O
1951	O
in	O
Czechoslovakia	O
by	O
Antonín	O
Svoboda	O
.	O
</s>
<s>
Its	O
basic	O
design	O
was	O
magnetic	B-General_Concept
drums	I-General_Concept
connected	O
via	O
relays	O
,	O
with	O
a	O
voting	O
method	O
of	O
memory	B-General_Concept
error	I-General_Concept
detection	O
(	O
triple	B-Error_Name
modular	I-Error_Name
redundancy	I-Error_Name
)	O
.	O
</s>
<s>
Eventually	O
,	O
they	O
separated	O
into	O
three	O
distinct	O
categories	O
:	O
machines	O
that	O
would	O
last	O
a	O
long	O
time	O
without	O
any	O
maintenance	O
,	O
such	O
as	O
the	O
ones	O
used	O
on	O
NASA	O
space	O
probes	O
and	O
satellites	B-Application
;	O
computers	O
that	O
were	O
very	O
dependable	O
but	O
required	O
constant	O
monitoring	O
,	O
such	O
as	O
those	O
used	O
to	O
monitor	O
and	O
control	O
nuclear	O
power	O
plants	O
or	O
supercollider	O
experiments	O
;	O
and	O
finally	O
,	O
computers	O
with	O
a	O
high	O
amount	O
of	O
runtime	O
which	O
would	O
be	O
under	O
heavy	O
use	O
,	O
such	O
as	O
many	O
of	O
the	O
supercomputers	O
used	O
by	O
insurance	O
companies	O
for	O
their	O
probability	O
monitoring	O
.	O
</s>
<s>
This	O
computer	O
had	O
a	O
backup	O
of	O
memory	B-General_Concept
arrays	O
to	O
use	O
memory	B-General_Concept
recovery	O
methods	O
and	O
thus	O
it	O
was	O
called	O
the	O
JPL	O
Self-Testing-And-Repairing	O
computer	O
.	O
</s>
<s>
It	O
could	O
detect	O
its	O
own	O
errors	B-General_Concept
and	O
fix	O
them	O
or	O
bring	O
up	O
redundant	O
modules	O
as	O
needed	O
.	O
</s>
<s>
These	O
needed	O
computers	O
with	O
massive	O
amounts	O
of	O
uptime	B-Device
that	O
would	O
fail	O
gracefully	O
enough	O
with	O
a	O
fault	O
to	O
allow	O
continued	O
operation	O
while	O
relying	O
on	O
the	O
fact	O
that	O
the	O
computer	O
output	O
would	O
be	O
constantly	O
monitored	O
by	O
humans	O
to	O
detect	O
faults	O
.	O
</s>
<s>
For	O
instance	O
,	O
F14	B-Device
CADC	I-Device
had	O
built-in	O
self-test	O
and	O
redundancy	B-Application
.	O
</s>
<s>
In	O
general	O
,	O
the	O
early	O
efforts	O
at	O
fault-tolerant	B-General_Concept
designs	I-General_Concept
were	O
focused	O
mainly	O
on	O
internal	O
diagnosis	O
,	O
where	O
a	O
fault	O
would	O
indicate	O
something	O
was	O
failing	O
and	O
a	O
worker	O
could	O
replace	O
it	O
.	O
</s>
<s>
SAPO	B-Device
,	O
for	O
instance	O
,	O
had	O
a	O
method	O
by	O
which	O
faulty	O
memory	B-General_Concept
drums	I-General_Concept
would	O
emit	O
a	O
noise	O
before	O
failure	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
N-model	O
redundancy	B-Application
,	O
where	O
faults	O
cause	O
automatic	O
fail-safes	B-General_Concept
and	O
a	O
warning	O
to	O
the	O
operator	O
,	O
and	O
it	O
is	O
still	O
the	O
most	O
common	O
form	O
of	O
level	O
one	O
fault-tolerant	B-General_Concept
design	I-General_Concept
in	O
use	O
today	O
.	O
</s>
<s>
Tandem	O
Computers	O
,	O
in	O
1976	O
and	O
Stratus	B-General_Concept
were	O
among	O
the	O
first	O
companies	O
specializing	O
in	O
the	O
design	O
of	O
fault-tolerant	B-General_Concept
computer	I-General_Concept
systems	I-General_Concept
for	O
online	B-General_Concept
transaction	I-General_Concept
processing	I-General_Concept
.	O
</s>
<s>
Hardware	B-Architecture
fault	B-General_Concept
tolerance	I-General_Concept
sometimes	O
requires	O
that	O
broken	O
parts	O
be	O
taken	O
out	O
and	O
replaced	O
with	O
new	O
parts	O
while	O
the	O
system	O
is	O
still	O
operational	O
(	O
in	O
computing	O
known	O
as	O
hot	B-Device
swapping	I-Device
)	O
.	O
</s>
<s>
Such	O
a	O
system	O
implemented	O
with	O
a	O
single	O
backup	O
is	O
known	O
as	O
single	O
point	O
tolerant	O
and	O
represents	O
the	O
vast	O
majority	O
of	O
fault-tolerant	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
It	O
is	O
helpful	O
if	O
the	O
time	O
between	O
failures	O
is	O
as	O
long	O
as	O
possible	O
,	O
but	O
this	O
is	O
not	O
specifically	O
required	O
in	O
a	O
fault-tolerant	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Fault	B-General_Concept
tolerance	I-General_Concept
is	O
notably	O
successful	O
in	O
computer	O
applications	O
.	O
</s>
<s>
Tandem	O
Computers	O
built	O
their	O
entire	O
business	O
on	O
such	O
machines	O
,	O
which	O
used	O
single-point	O
tolerance	O
to	O
create	O
their	O
NonStop	B-Architecture
systems	O
with	O
uptimes	B-Device
measured	O
in	O
years	O
.	O
</s>
<s>
Fail-safe	B-General_Concept
architectures	O
may	O
encompass	O
also	O
the	O
computer	O
software	O
,	O
for	O
example	O
by	O
process	O
replication	B-Architecture
.	O
</s>
<s>
Data	O
formats	O
may	O
also	O
be	O
designed	O
to	O
degrade	B-General_Concept
gracefully	I-General_Concept
.	O
</s>
<s>
HTML	B-Language
for	O
example	O
,	O
is	O
designed	O
to	O
be	O
forward	B-General_Concept
compatible	I-General_Concept
,	O
allowing	O
Web	B-Application
browsers	I-Application
to	O
ignore	O
new	O
and	O
unsupported	O
HTML	B-Language
entities	O
without	O
causing	O
the	O
document	O
to	O
be	O
unusable	O
.	O
</s>
<s>
Additionally	O
,	O
some	O
sites	O
,	O
including	O
popular	O
platforms	O
such	O
as	O
Twitter	O
(	O
until	O
December	O
2020	O
)	O
,	O
provide	O
an	O
optional	O
lightweight	O
front	O
end	O
that	O
does	O
not	O
rely	O
on	O
JavaScript	B-Language
and	O
has	O
a	O
minimal	B-Device
layout	O
,	O
to	O
ensure	O
wide	O
accessibility	O
and	O
outreach	O
,	O
such	O
as	O
on	O
game	O
consoles	O
with	O
limited	O
web	O
browsing	O
capabilities	O
.	O
</s>
<s>
A	O
highly	O
fault-tolerant	B-General_Concept
system	I-General_Concept
might	O
continue	O
at	O
the	O
same	O
level	O
of	O
performance	O
even	O
though	O
one	O
or	O
more	O
components	O
have	O
failed	O
.	O
</s>
<s>
A	O
system	O
that	O
is	O
designed	O
to	O
fail	B-General_Concept
safe	I-General_Concept
,	O
or	O
fail-secure	B-General_Concept
,	O
or	O
fail	O
gracefully	O
,	O
whether	O
it	O
functions	O
at	O
a	O
reduced	O
level	O
or	O
fails	O
completely	O
,	O
does	O
so	O
in	O
a	O
way	O
that	O
protects	O
people	O
,	O
property	O
,	O
or	O
data	O
from	O
injury	O
,	O
damage	O
,	O
intrusion	O
,	O
or	O
disclosure	O
.	O
</s>
<s>
In	O
computers	O
,	O
a	O
program	O
might	O
fail-safe	B-General_Concept
by	O
executing	O
a	O
graceful	O
exit	O
(	O
as	O
opposed	O
to	O
an	O
uncontrolled	O
crash	O
)	O
in	O
order	O
to	O
prevent	O
data	O
corruption	O
after	O
experiencing	O
an	O
error	O
.	O
</s>
<s>
A	O
system	O
that	O
is	O
designed	O
to	O
experience	O
graceful	B-General_Concept
degradation	I-General_Concept
,	O
or	O
to	O
fail	O
soft	O
(	O
used	O
in	O
computing	O
,	O
similar	O
to	O
"	O
fail	B-General_Concept
safe	I-General_Concept
"	O
<	O
ref>Stallings	O
,	O
W	O
(	O
2009	O
)	O
:	O
Operating	O
Systems	O
.	O
</s>
<s>
In	O
computing	O
an	O
example	O
of	O
graceful	B-General_Concept
degradation	I-General_Concept
is	O
that	O
if	O
insufficient	O
network	O
bandwidth	O
is	O
available	O
to	O
stream	O
an	O
online	O
video	O
,	O
a	O
lower-resolution	O
version	O
might	O
be	O
streamed	O
in	O
place	O
of	O
the	O
high-resolution	O
version	O
.	O
</s>
<s>
Progressive	O
enhancement	O
is	O
an	O
example	O
in	O
computing	O
,	O
where	O
web	O
pages	O
are	O
available	O
in	O
a	O
basic	O
functional	O
format	O
for	O
older	O
,	O
small-screen	O
,	O
or	O
limited-capability	O
web	B-Application
browsers	I-Application
,	O
but	O
in	O
an	O
enhanced	O
version	O
for	O
browsers	B-Application
capable	O
of	O
handling	O
additional	O
technologies	O
or	O
that	O
have	O
a	O
larger	O
display	O
available	O
.	O
</s>
<s>
In	O
fault-tolerant	B-General_Concept
computer	I-General_Concept
systems	I-General_Concept
,	O
programs	O
that	O
are	O
considered	O
robust	B-Application
are	O
designed	O
to	O
continue	O
operation	O
despite	O
an	O
error	O
,	O
exception	O
,	O
or	O
invalid	O
input	O
,	O
instead	O
of	O
crashing	O
completely	O
.	O
</s>
<s>
Software	O
brittleness	O
is	O
the	O
opposite	O
of	O
robustness	B-Application
.	O
</s>
<s>
Resilient	B-General_Concept
networks	I-General_Concept
continue	O
to	O
transmit	O
data	O
despite	O
the	O
failure	O
of	O
some	O
links	O
or	O
nodes	O
;	O
resilient	O
buildings	O
and	O
infrastructure	O
are	O
likewise	O
expected	O
to	O
prevent	O
complete	O
failure	O
in	O
situations	O
like	O
earthquakes	O
,	O
floods	O
,	O
or	O
collisions	O
.	O
</s>
<s>
A	O
system	O
with	O
high	O
failure	B-Operating_System
transparency	I-Operating_System
will	O
alert	O
users	O
that	O
a	O
component	O
failure	O
has	O
occurred	O
,	O
even	O
if	O
it	O
continues	O
to	O
operate	O
with	O
full	O
performance	O
,	O
so	O
that	O
failure	O
can	O
be	O
repaired	O
or	O
imminent	O
complete	O
failure	O
anticipated	O
.	O
</s>
<s>
A	O
single	O
fault	O
condition	O
is	O
a	O
situation	O
where	O
one	O
means	O
for	O
protection	O
against	O
a	O
hazard	B-General_Concept
is	O
defective	O
.	O
</s>
<s>
Providing	O
fault-tolerant	B-General_Concept
design	I-General_Concept
for	O
every	O
component	O
is	O
normally	O
not	O
an	O
option	O
.	O
</s>
<s>
Associated	O
redundancy	B-Application
brings	O
a	O
number	O
of	O
penalties	O
:	O
increase	O
in	O
weight	O
,	O
size	O
,	O
power	O
consumption	O
,	O
cost	O
,	O
as	O
well	O
as	O
time	O
to	O
design	O
,	O
verify	O
,	O
and	O
test	O
.	O
</s>
<s>
"	O
Fault-Tolerant	B-General_Concept
Design	I-General_Concept
"	O
,	O
Springer	O
,	O
2013	O
,	O
</s>
<s>
In	O
a	O
car	O
,	O
the	O
radio	O
is	O
not	O
critical	O
,	O
so	O
this	O
component	O
has	O
less	O
need	O
for	O
fault	B-General_Concept
tolerance	I-General_Concept
.	O
</s>
<s>
Some	O
components	O
,	O
like	O
the	O
drive	O
shaft	O
in	O
a	O
car	O
,	O
are	O
not	O
likely	O
to	O
fail	O
,	O
so	O
no	O
fault	B-General_Concept
tolerance	I-General_Concept
is	O
needed	O
.	O
</s>
<s>
How	O
expensive	O
is	O
it	O
to	O
make	O
the	O
component	O
fault	B-General_Concept
tolerant	I-General_Concept
?	O
</s>
<s>
However	O
,	O
the	O
similarly	O
critical	O
systems	O
for	O
actuating	O
the	O
brakes	O
under	O
driver	O
control	O
are	O
inherently	O
less	O
robust	B-Application
,	O
generally	O
using	O
a	O
cable	O
(	O
can	O
rust	O
,	O
stretch	O
,	O
jam	O
,	O
snap	O
)	O
or	O
hydraulic	O
fluid	O
(	O
can	O
leak	O
,	O
boil	O
and	O
develop	O
bubbles	O
,	O
absorb	O
water	O
and	O
thus	O
lose	O
effectiveness	O
)	O
.	O
</s>
<s>
Thus	O
in	O
most	O
modern	O
cars	O
the	O
footbrake	O
hydraulic	O
brake	O
circuit	O
is	O
diagonally	O
divided	O
to	O
give	O
two	O
smaller	O
points	O
of	O
failure	O
,	O
the	O
loss	O
of	O
either	O
only	O
reducing	O
brake	O
power	O
by	O
50%	O
and	O
not	O
causing	O
as	O
much	O
dangerous	O
brakeforce	O
imbalance	O
as	O
a	O
straight	O
front-back	O
or	O
left-right	O
split	O
,	O
and	O
should	O
the	O
hydraulic	O
circuit	O
fail	O
completely	O
(	O
a	O
relatively	O
very	O
rare	O
occurrence	O
)	O
,	O
there	O
is	O
a	O
failsafe	B-General_Concept
in	O
the	O
form	O
of	O
the	O
cable-actuated	O
parking	O
brake	O
that	O
operates	O
the	O
otherwise	O
relatively	O
weak	O
rear	O
brakes	O
,	O
but	O
can	O
still	O
bring	O
the	O
vehicle	O
to	O
a	O
safe	O
halt	O
in	O
conjunction	O
with	O
transmission/engine	O
braking	O
so	O
long	O
as	O
the	O
demands	O
on	O
it	O
are	O
in	O
line	O
with	O
normal	O
traffic	O
flow	O
.	O
</s>
<s>
Therefore	O
,	O
no	O
redundancy	B-Application
is	O
built	O
into	O
it	O
per	O
se	O
(	O
and	O
it	O
typically	O
uses	O
a	O
cheaper	O
,	O
lighter	O
,	O
but	O
less	O
hardwearing	O
cable	O
actuation	O
system	O
)	O
,	O
and	O
it	O
can	O
suffice	O
,	O
if	O
this	O
happens	O
on	O
a	O
hill	O
,	O
to	O
use	O
the	O
footbrake	O
to	O
momentarily	O
hold	O
the	O
vehicle	O
still	O
,	O
before	O
driving	O
off	O
to	O
find	O
a	O
flat	O
piece	O
of	O
road	O
on	O
which	O
to	O
stop	O
.	O
</s>
<s>
On	O
cheaper	O
,	O
slower	O
utility-class	O
machines	O
,	O
even	O
if	O
the	O
front	O
wheel	O
should	O
use	O
a	O
hydraulic	O
disc	O
for	O
extra	O
brake	O
force	O
and	O
easier	O
packaging	O
,	O
the	O
rear	O
will	O
usually	O
be	O
a	O
primitive	O
,	O
somewhat	O
inefficient	O
,	O
but	O
exceptionally	O
robust	B-Application
rod-actuated	O
drum	O
,	O
thanks	O
to	O
the	O
ease	O
of	O
connecting	O
the	O
footpedal	O
to	O
the	O
wheel	O
in	O
this	O
way	O
and	O
,	O
more	O
importantly	O
,	O
the	O
near	O
impossibility	O
of	O
catastrophic	O
failure	O
even	O
if	O
the	O
rest	O
of	O
the	O
machine	O
,	O
like	O
a	O
lot	O
of	O
low-priced	O
bikes	O
after	O
their	O
first	O
few	O
years	O
of	O
use	O
,	O
is	O
on	O
the	O
point	O
of	O
collapse	O
from	O
neglected	O
maintenance	O
.	O
</s>
<s>
The	O
basic	O
characteristics	O
of	O
fault	B-General_Concept
tolerance	I-General_Concept
require	O
:	O
</s>
<s>
No	B-Architecture
single	I-Architecture
point	I-Architecture
of	I-Architecture
failure	I-Architecture
–	O
If	O
a	O
system	O
experiences	O
a	O
failure	O
,	O
it	O
must	O
continue	O
to	O
operate	O
without	O
interruption	O
during	O
the	O
repair	O
process	O
.	O
</s>
<s>
Firewalls	B-Application
or	O
other	O
mechanisms	O
that	O
isolate	O
a	O
rogue	O
transmitter	O
or	O
failing	O
component	O
to	O
protect	O
the	O
system	O
are	O
required	O
.	O
</s>
<s>
In	O
addition	O
,	O
fault-tolerant	B-General_Concept
systems	I-General_Concept
are	O
characterized	O
in	O
terms	O
of	O
both	O
planned	O
service	O
outages	O
and	O
unplanned	O
service	O
outages	O
.	O
</s>
<s>
These	O
are	O
usually	O
measured	O
at	O
the	O
application	O
level	O
and	O
not	O
just	O
at	O
a	O
hardware	B-Architecture
level	O
.	O
</s>
<s>
Fault-tolerant	B-General_Concept
systems	I-General_Concept
are	O
typically	O
based	O
on	O
the	O
concept	O
of	O
redundancy	B-Application
.	O
</s>
<s>
Considering	O
the	O
importance	O
of	O
high-value	O
systems	O
in	O
transport	O
,	O
public	O
utilities	O
and	O
the	O
military	O
,	O
the	O
field	O
of	O
topics	O
that	O
touch	O
on	O
research	O
is	O
very	O
wide	O
:	O
it	O
can	O
include	O
such	O
obvious	O
subjects	O
as	O
software	B-Language
modeling	I-Language
and	O
reliability	O
,	O
or	O
hardware	B-General_Concept
design	I-General_Concept
,	O
to	O
arcane	O
elements	O
such	O
as	O
stochastic	O
models	O
,	O
graph	O
theory	O
,	O
formal	O
or	O
exclusionary	O
logic	O
,	O
parallel	B-Operating_System
processing	I-Operating_System
,	O
remote	O
data	O
transmission	O
,	O
and	O
more	O
.	O
</s>
<s>
Spare	O
components	O
address	O
the	O
first	O
fundamental	O
characteristic	O
of	O
fault	B-General_Concept
tolerance	I-General_Concept
in	O
three	O
ways	O
:	O
</s>
<s>
Replication	B-Architecture
:	O
Providing	O
multiple	O
identical	O
instances	O
of	O
the	O
same	O
system	O
or	O
subsystem	O
,	O
directing	O
tasks	O
or	O
requests	O
to	O
all	O
of	O
them	O
in	O
parallel	B-Operating_System
,	O
and	O
choosing	O
the	O
correct	O
result	O
on	O
the	O
basis	O
of	O
a	O
quorum	B-General_Concept
;	O
</s>
<s>
Redundancy	B-Application
:	O
Providing	O
multiple	O
identical	O
instances	O
of	O
the	O
same	O
system	O
and	O
switching	O
to	O
one	O
of	O
the	O
remaining	O
instances	O
in	O
case	O
of	O
a	O
failure	O
(	O
failover	B-General_Concept
)	O
;	O
</s>
<s>
Diversity	O
:	O
Providing	O
multiple	O
different	O
implementations	O
of	O
the	O
same	O
specification	O
,	O
and	O
using	O
them	O
like	O
replicated	O
systems	O
to	O
cope	O
with	O
errors	B-General_Concept
in	O
a	O
specific	O
implementation	O
.	O
</s>
<s>
All	O
implementations	O
of	O
RAID	B-Architecture
,	O
redundant	B-Architecture
array	I-Architecture
of	I-Architecture
independent	I-Architecture
disks	I-Architecture
,	O
except	O
RAID	B-Architecture
0	O
,	O
are	O
examples	O
of	O
a	O
fault-tolerant	B-General_Concept
storage	B-General_Concept
device	I-General_Concept
that	O
uses	O
data	B-Application
redundancy	I-Application
.	O
</s>
<s>
A	O
lockstep	B-General_Concept
fault-tolerant	B-General_Concept
machine	O
uses	O
replicated	O
elements	O
operating	O
in	O
parallel	B-Operating_System
.	O
</s>
<s>
At	O
any	O
time	O
,	O
all	O
the	O
replications	B-Architecture
of	O
each	O
element	O
should	O
be	O
in	O
the	O
same	O
state	O
.	O
</s>
<s>
The	O
same	O
inputs	O
are	O
provided	O
to	O
each	O
replication	B-Architecture
,	O
and	O
the	O
same	O
outputs	O
are	O
expected	O
.	O
</s>
<s>
The	O
outputs	O
of	O
the	O
replications	B-Architecture
are	O
compared	O
using	O
a	O
voting	O
circuit	O
.	O
</s>
<s>
A	O
machine	O
with	O
two	O
replications	B-Architecture
of	O
each	O
element	O
is	O
termed	O
dual	B-Error_Name
modular	I-Error_Name
redundant	I-Error_Name
(	O
DMR	O
)	O
.	O
</s>
<s>
A	O
machine	O
with	O
three	O
replications	B-Architecture
of	O
each	O
element	O
is	O
termed	O
triple	B-Error_Name
modular	I-Error_Name
redundant	I-Error_Name
(	O
TMR	O
)	O
.	O
</s>
<s>
The	O
voting	O
circuit	O
can	O
determine	O
which	O
replication	B-Architecture
is	O
in	O
error	O
when	O
a	O
two-to-one	O
vote	O
is	O
observed	O
.	O
</s>
<s>
After	O
this	O
,	O
the	O
internal	O
state	O
of	O
the	O
erroneous	O
replication	B-Architecture
is	O
assumed	O
to	O
be	O
different	O
from	O
that	O
of	O
the	O
other	O
two	O
,	O
and	O
the	O
voting	O
circuit	O
can	O
switch	O
to	O
a	O
DMR	O
mode	O
.	O
</s>
<s>
This	O
model	O
can	O
be	O
applied	O
to	O
any	O
larger	O
number	O
of	O
replications	B-Architecture
.	O
</s>
<s>
Lockstep	B-General_Concept
fault-tolerant	B-General_Concept
machines	O
are	O
most	O
easily	O
made	O
fully	O
synchronous	O
,	O
with	O
each	O
gate	O
of	O
each	O
replication	B-Architecture
making	O
the	O
same	O
state	O
transition	O
on	O
the	O
same	O
edge	O
of	O
the	O
clock	O
,	O
and	O
the	O
clocks	O
to	O
the	O
replications	B-Architecture
being	O
exactly	O
in	O
phase	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
possible	O
to	O
build	O
lockstep	B-General_Concept
systems	O
without	O
this	O
requirement	O
.	O
</s>
<s>
Bringing	O
the	O
replications	B-Architecture
into	O
synchrony	O
requires	O
making	O
their	O
internal	O
stored	O
states	O
the	O
same	O
.	O
</s>
<s>
Alternatively	O
,	O
the	O
internal	O
state	O
of	O
one	O
replica	B-Architecture
can	O
be	O
copied	O
to	O
another	O
replica	B-Architecture
.	O
</s>
<s>
Two	O
replicated	O
elements	O
operate	O
in	O
lockstep	B-General_Concept
as	O
a	O
pair	O
,	O
with	O
a	O
voting	O
circuit	O
that	O
detects	O
any	O
mismatch	O
between	O
their	O
operations	O
and	O
outputs	O
a	O
signal	O
indicating	O
that	O
there	O
is	O
an	O
error	O
.	O
</s>
<s>
Pair-and-spare	O
requires	O
four	O
replicas	B-Architecture
rather	O
than	O
the	O
three	O
of	O
TMR	O
,	O
but	O
has	O
been	O
used	O
commercially	O
.	O
</s>
<s>
Failure-oblivious	O
computingFailure-oblivious	O
computing	O
is	O
a	O
technique	O
that	O
enables	O
computer	B-Application
programs	I-Application
to	O
continue	O
executing	O
despite	O
errors	B-General_Concept
.	O
</s>
<s>
It	O
can	O
handle	O
invalid	O
memory	B-General_Concept
reads	O
by	O
returning	O
a	O
manufactured	O
value	O
to	O
the	O
program	O
,	O
which	O
in	O
turn	O
,	O
makes	O
use	O
of	O
the	O
manufactured	O
value	O
and	O
ignores	O
the	O
former	O
memory	B-General_Concept
value	O
it	O
tried	O
to	O
access	O
,	O
this	O
is	O
a	O
great	O
contrast	O
to	O
typical	B-General_Concept
memory	I-General_Concept
checkers	I-General_Concept
,	O
which	O
inform	O
the	O
program	O
of	O
the	O
error	O
or	O
abort	O
the	O
program	O
.	O
</s>
<s>
Recovery	O
shepherding	O
is	O
a	O
lightweight	O
technique	O
to	O
enable	O
software	B-Application
programs	I-Application
to	O
recover	O
from	O
otherwise	O
fatal	O
errors	B-General_Concept
such	O
as	O
null	O
pointer	O
dereference	O
and	O
divide	O
by	O
zero	O
.	O
</s>
<s>
It	O
uses	O
the	O
just-in-time	O
binary	B-Application
instrumentation	I-Application
framework	O
Pin	O
.	O
</s>
<s>
For	O
17	O
of	O
18	O
systematically	O
collected	O
real	O
world	O
null-dereference	O
and	O
divide-by-zero	O
errors	B-General_Concept
,	O
a	O
prototype	O
implementation	O
enables	O
the	O
application	O
to	O
continue	O
to	O
execute	O
to	O
provide	O
acceptable	O
output	O
and	O
service	O
to	O
its	O
users	O
on	O
the	O
error-triggering	O
inputs	O
.	O
</s>
<s>
The	O
circuit	B-Language
breaker	I-Language
design	I-Language
pattern	I-Language
is	O
a	O
technique	O
to	O
avoid	O
catastrophic	O
failures	O
in	O
distributed	O
systems	O
.	O
</s>
<s>
Redundancy	B-Application
is	O
the	O
provision	O
of	O
functional	O
capabilities	O
that	O
would	O
be	O
unnecessary	O
in	O
a	O
fault-free	O
environment.Laprie	O
,	O
J	O
.	O
C	O
.	O
(	O
1985	O
)	O
.	O
</s>
<s>
"	O
Dependable	O
Computing	O
and	O
Fault	B-General_Concept
Tolerance	I-General_Concept
:	O
Concepts	O
and	O
Terminology	O
"	O
,	O
Proceedings	O
of	O
15th	O
International	O
Symposium	O
on	O
Fault-Tolerant	B-General_Concept
Computing	I-General_Concept
(	O
FTSC-15	O
)	O
,	O
pp	O
.	O
</s>
<s>
The	O
idea	O
of	O
incorporating	O
redundancy	B-Application
in	O
order	O
to	O
improve	O
the	O
reliability	O
of	O
a	O
system	O
was	O
pioneered	O
by	O
John	O
von	O
Neumann	O
in	O
the	O
1950s.von	O
Neumann	O
,	O
J	O
.	O
</s>
<s>
Two	O
kinds	O
of	O
redundancy	B-Application
are	O
possible:Avizienis	O
,	O
A	O
.	O
</s>
<s>
"	O
Fault-Tolerant	B-General_Concept
Systems	I-General_Concept
"	O
,	O
IEEE	O
Transactions	O
on	O
Computers	O
,	O
vol	O
.	O
</s>
<s>
1304	O
–	O
1312	O
space	O
redundancy	B-Application
and	O
time	O
redundancy	B-Application
.	O
</s>
<s>
Space	O
redundancy	B-Application
provides	O
additional	O
components	O
,	O
functions	O
,	O
or	O
data	O
items	O
that	O
are	O
unnecessary	O
for	O
fault-free	O
operation	O
.	O
</s>
<s>
Space	O
redundancy	B-Application
is	O
further	O
classified	O
into	O
hardware	B-Architecture
,	O
software	O
and	O
information	O
redundancy	B-Application
,	O
depending	O
on	O
the	O
type	O
of	O
redundant	O
resources	O
added	O
to	O
the	O
system	O
.	O
</s>
<s>
In	O
time	O
redundancy	B-Application
the	O
computation	O
or	O
data	O
transmission	O
is	O
repeated	O
and	O
the	O
result	O
is	O
compared	O
to	O
a	O
stored	O
copy	O
of	O
the	O
previous	O
result	O
.	O
</s>
<s>
The	O
current	O
terminology	O
for	O
this	O
kind	O
of	O
testing	O
is	O
referred	O
to	O
as	O
'	O
In	O
Service	O
Fault	B-General_Concept
Tolerance	I-General_Concept
Testing	O
or	O
ISFTT	O
for	O
short	O
.	O
</s>
<s>
Fault-tolerant	B-General_Concept
design	I-General_Concept
's	O
advantages	O
are	O
obvious	O
,	O
while	O
many	O
of	O
its	O
disadvantages	O
are	O
not	O
:	O
</s>
<s>
To	O
continue	O
the	O
above	O
passenger	O
vehicle	O
example	O
,	O
with	O
either	O
of	O
the	O
fault-tolerant	B-General_Concept
systems	I-General_Concept
it	O
may	O
not	O
be	O
obvious	O
to	O
the	O
driver	O
when	O
a	O
tire	O
has	O
been	O
punctured	O
.	O
</s>
<s>
Another	O
variation	O
of	O
this	O
problem	O
is	O
when	O
fault	B-General_Concept
tolerance	I-General_Concept
in	O
one	O
component	O
prevents	O
fault	O
detection	O
in	O
a	O
different	O
component	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
component	O
B	O
performs	O
some	O
operation	O
based	O
on	O
the	O
output	O
from	O
component	O
A	O
,	O
then	O
fault	B-General_Concept
tolerance	I-General_Concept
in	O
B	O
can	O
hide	O
a	O
problem	O
with	O
A	O
.	O
</s>
<s>
If	O
component	O
B	O
is	O
later	O
changed	O
(	O
to	O
a	O
less	O
fault-tolerant	B-General_Concept
design	I-General_Concept
)	O
the	O
system	O
may	O
fail	O
suddenly	O
,	O
making	O
it	O
appear	O
that	O
the	O
new	O
component	O
B	O
is	O
the	O
problem	O
.	O
</s>
<s>
Even	O
if	O
the	O
operator	O
is	O
aware	O
of	O
the	O
fault	O
,	O
having	O
a	O
fault-tolerant	B-General_Concept
system	I-General_Concept
is	O
likely	O
to	O
reduce	O
the	O
importance	O
of	O
repairing	O
the	O
fault	O
.	O
</s>
<s>
If	O
the	O
faults	O
are	O
not	O
corrected	O
,	O
this	O
will	O
eventually	O
lead	O
to	O
system	O
failure	O
,	O
when	O
the	O
fault-tolerant	B-General_Concept
component	O
fails	O
completely	O
or	O
when	O
all	O
redundant	O
components	O
have	O
also	O
failed	O
.	O
</s>
<s>
For	O
certain	O
critical	O
fault-tolerant	B-General_Concept
systems	I-General_Concept
,	O
such	O
as	O
a	O
nuclear	O
reactor	O
,	O
there	O
is	O
no	O
easy	O
way	O
to	O
verify	O
that	O
the	O
backup	O
components	O
are	O
functional	O
.	O
</s>
<s>
Both	O
fault-tolerant	B-General_Concept
components	O
and	O
redundant	O
components	O
tend	O
to	O
increase	O
cost	O
.	O
</s>
<s>
Manned	O
spaceships	O
,	O
for	O
example	O
,	O
have	O
so	O
many	O
redundant	O
and	O
fault-tolerant	B-General_Concept
components	O
that	O
their	O
weight	O
is	O
increased	O
dramatically	O
over	O
unmanned	O
systems	O
,	O
which	O
do	O
n't	O
require	O
the	O
same	O
level	O
of	O
safety	O
.	O
</s>
<s>
A	O
fault-tolerant	B-General_Concept
design	I-General_Concept
may	O
allow	O
for	O
the	O
use	O
of	O
inferior	O
components	O
,	O
which	O
would	O
have	O
otherwise	O
made	O
the	O
system	O
inoperable	O
.	O
</s>
<s>
There	O
is	O
a	O
difference	O
between	O
fault	B-General_Concept
tolerance	I-General_Concept
and	O
systems	O
that	O
rarely	O
have	O
problems	O
.	O
</s>
