<s>
Software	B-Architecture
fault	I-Architecture
tolerance	I-Architecture
is	O
the	O
ability	O
of	O
computer	B-Application
software	I-Application
to	O
continue	O
its	O
normal	O
operation	O
despite	O
the	O
presence	O
of	O
system	O
or	O
hardware	B-Architecture
faults	O
.	O
</s>
<s>
Fault-tolerant	B-Architecture
software	I-Architecture
has	O
the	O
ability	O
to	O
satisfy	O
requirements	O
despite	O
failures	O
.	O
</s>
<s>
This	O
is	O
certainly	O
more	O
true	O
of	O
software	O
systems	O
than	O
almost	O
any	O
phenomenon	O
,	O
not	O
all	O
software	O
change	O
in	O
the	O
same	O
way	O
so	O
software	B-Architecture
fault	I-Architecture
tolerance	I-Architecture
methods	O
are	O
designed	O
to	O
overcome	O
execution	O
errors	O
by	O
modifying	O
variable	O
values	O
to	O
create	O
an	O
acceptable	O
program	O
state	O
.	O
</s>
<s>
Fault	B-General_Concept
tolerance	I-General_Concept
must	O
be	O
a	O
key	O
consideration	O
in	O
the	O
early	O
stage	O
of	O
software	O
development	O
.	O
</s>
<s>
There	O
exist	O
different	O
mechanisms	O
for	O
software	B-Architecture
fault	I-Architecture
tolerance	I-Architecture
,	O
among	O
which	O
:	O
</s>
<s>
Computer	O
applications	O
make	O
a	O
call	O
using	O
the	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
to	O
access	O
shared	O
resources	O
,	O
like	O
the	O
keyboard	O
,	O
mouse	O
,	O
screen	O
,	O
disk	O
drive	O
,	O
network	O
,	O
and	O
printer	O
.	O
</s>
<s>
Intensive	O
calculations	O
cause	O
lengthy	O
delays	O
with	O
the	O
same	O
effect	O
as	O
a	O
blocked	O
API	B-Application
call	O
.	O
</s>
<s>
There	O
are	O
two	O
methods	O
used	O
to	O
handle	O
blocking	B-Operating_System
.	O
</s>
<s>
Threading	O
allows	O
a	O
separate	O
sequence	O
of	O
execution	O
for	O
each	O
API	B-Application
call	O
that	O
can	O
block	O
.	O
</s>
<s>
This	O
has	O
the	O
benefit	O
that	O
none	O
of	O
the	O
information	O
about	O
the	O
state	O
of	O
the	O
API	B-Application
call	O
is	O
lost	O
while	O
other	O
activities	O
take	O
place	O
.	O
</s>
<s>
Interrupts	O
typically	O
destroy	O
any	O
information	O
related	O
to	O
the	O
state	O
of	O
a	O
blocked	O
API	B-Application
call	O
or	O
intensive	O
calculation	O
,	O
so	O
the	O
programmer	O
must	O
keep	O
track	O
of	O
this	O
information	O
separately	O
.	O
</s>
<s>
Fault	O
are	O
induced	O
by	O
signals	B-Operating_System
in	O
POSIX	O
compliant	O
systems	O
,	O
and	O
these	O
signals	B-Operating_System
originate	O
from	O
API	B-Application
calls	O
,	O
from	O
the	O
operating	O
system	O
,	O
and	O
from	O
other	O
applications	O
.	O
</s>
<s>
This	O
is	O
called	O
exception	B-General_Concept
handling	I-General_Concept
.	O
</s>
<s>
All	O
other	O
signals	B-Operating_System
can	O
be	O
directed	O
to	O
a	O
handler	O
function	O
.	O
</s>
<s>
Initialized	O
handler	O
functions	O
are	O
paired	O
with	O
each	O
signal	O
when	O
the	B-Application
software	I-Application
starts	O
.	O
</s>
<s>
Hardware	B-Architecture
fault	B-General_Concept
tolerance	I-General_Concept
for	O
software	O
requires	O
the	O
following	O
.	O
</s>
<s>
Backup	B-Protocol
maintains	O
information	O
in	O
the	O
event	O
that	O
hardware	B-Architecture
must	O
be	O
replaced	O
.	O
</s>
<s>
Backup	B-Protocol
requires	O
an	O
information-restore	O
strategy	O
to	O
make	O
backup	B-Protocol
information	O
available	O
on	O
a	O
replacement	O
system	O
.	O
</s>
<s>
This	O
can	O
be	O
achieved	O
using	O
continuous	O
backup	B-Protocol
to	O
a	O
live	O
system	O
that	O
remains	O
inactive	O
until	O
needed	O
(	O
synchronized	O
backup	B-Protocol
)	O
.	O
</s>
