<s>
Failover	B-General_Concept
is	O
switching	O
to	O
a	O
redundant	O
or	O
standby	O
computer	B-Application
server	I-Application
,	O
system	O
,	O
hardware	O
component	O
or	O
network	O
upon	O
the	O
failure	O
or	O
abnormal	B-Application
termination	I-Application
of	O
the	O
previously	O
active	O
application	B-Application
,	O
server	B-Application
,	O
system	O
,	O
hardware	O
component	O
,	O
or	O
network	O
in	O
a	O
computer	B-Architecture
network	I-Architecture
.	O
</s>
<s>
Failover	B-General_Concept
and	O
switchover	B-General_Concept
are	O
essentially	O
the	O
same	O
operation	O
,	O
except	O
that	O
failover	B-General_Concept
is	O
automatic	O
and	O
usually	O
operates	O
without	O
warning	O
,	O
while	O
switchover	B-General_Concept
requires	O
human	O
intervention	O
.	O
</s>
<s>
Systems	O
designers	O
usually	O
provide	O
failover	B-General_Concept
capability	O
in	O
servers	O
,	O
systems	O
or	O
networks	O
requiring	O
near-continuous	O
availability	O
and	O
a	O
high	O
degree	O
of	O
reliability	O
.	O
</s>
<s>
At	O
the	O
server	B-Application
level	O
,	O
failover	B-General_Concept
automation	O
usually	O
uses	O
a	O
"	O
heartbeat	B-Operating_System
"	O
system	O
that	O
connects	O
two	O
servers	O
,	O
either	O
through	O
using	O
a	O
separate	O
cable	O
(	O
for	O
example	O
,	O
RS-232	O
serial	O
ports/cable	O
)	O
or	O
a	O
network	O
connection	O
.	O
</s>
<s>
In	O
the	O
most	O
common	O
design	O
,	O
as	O
long	O
as	O
a	O
regular	O
"	O
pulse	O
"	O
or	O
"	O
heartbeat	B-Operating_System
"	O
continues	O
between	O
the	O
main	O
server	B-Application
and	O
the	O
second	O
server	B-Application
,	O
the	O
second	O
server	B-Application
will	O
not	O
bring	O
its	O
systems	O
online	O
;	O
however	O
a	O
few	O
systems	O
actively	O
use	O
all	O
servers	O
and	O
can	O
failover	B-General_Concept
their	O
work	O
to	O
remaining	O
servers	O
after	O
a	O
failure	O
.	O
</s>
<s>
There	O
may	O
also	O
be	O
a	O
third	O
"	O
spare	O
parts	O
"	O
server	B-Application
that	O
has	O
running	O
spare	O
components	O
for	O
"	O
hot	O
"	O
switching	O
to	O
prevent	O
downtime	O
.	O
</s>
<s>
The	O
second	O
server	B-Application
takes	O
over	O
the	O
work	O
of	O
the	O
first	O
as	O
soon	O
as	O
it	O
detects	O
an	O
alteration	O
in	O
the	O
"	O
heartbeat	B-Operating_System
"	O
of	O
the	O
first	O
machine	O
.	O
</s>
<s>
Some	O
systems	O
have	O
the	O
ability	O
to	O
send	O
a	O
notification	O
of	O
failover	B-General_Concept
.	O
</s>
<s>
Certain	O
systems	O
,	O
intentionally	O
,	O
do	O
not	O
failover	B-General_Concept
entirely	O
automatically	O
,	O
but	O
require	O
human	O
intervention	O
.	O
</s>
<s>
This	O
"	O
automated	O
with	O
manual	O
approval	O
"	O
configuration	O
runs	O
automatically	O
once	O
a	O
human	O
has	O
approved	O
the	O
failover	B-General_Concept
.	O
</s>
<s>
Failback	B-General_Concept
is	O
the	O
process	O
of	O
restoring	O
a	O
system	O
,	O
component	O
,	O
or	O
service	O
previously	O
in	O
a	O
state	O
of	O
failure	O
back	O
to	O
its	O
original	O
,	O
working	O
state	O
,	O
and	O
having	O
the	O
standby	O
system	O
go	O
from	O
functioning	O
back	O
to	O
standby	O
.	O
</s>
<s>
The	O
use	O
of	O
virtualization	B-General_Concept
software	O
has	O
allowed	O
failover	B-General_Concept
practices	O
to	O
become	O
less	O
reliant	O
on	O
physical	O
hardware	O
through	O
the	O
process	O
referred	O
to	O
as	O
migration	B-General_Concept
in	O
which	O
a	O
running	O
virtual	O
machine	O
is	O
moved	O
from	O
one	O
physical	O
host	O
to	O
another	O
,	O
with	O
little	O
or	O
no	O
disruption	O
in	O
service	O
.	O
</s>
<s>
The	O
term	O
"	O
failover	B-General_Concept
"	O
,	O
although	O
probably	O
in	O
use	O
by	O
engineers	O
much	O
earlier	O
,	O
can	O
be	O
found	O
in	O
a	O
1962	O
declassified	O
NASA	O
report	O
.	O
</s>
<s>
The	O
term	O
"	O
switchover	B-General_Concept
"	O
can	O
be	O
found	O
in	O
the	O
1950s	O
when	O
describing	O
'	O
 "	O
Hot	O
"	O
and	O
"	O
Cold	O
"	O
Standby	O
Systems	O
 '	O
,	O
with	O
the	O
current	O
meaning	O
of	O
immediate	O
switchover	B-General_Concept
to	O
a	O
running	O
system	O
(	O
hot	O
)	O
and	O
delayed	O
switchover	B-General_Concept
to	O
a	O
system	O
that	O
needs	O
starting	O
(	O
cold	O
)	O
.	O
</s>
<s>
A	O
conference	O
proceedings	O
from	O
1957	O
describes	O
computer	O
systems	O
with	O
both	O
Emergency	O
Switchover	B-General_Concept
(	O
i.e.	O
</s>
<s>
failover	B-General_Concept
)	O
and	O
Scheduled	O
Failover	B-General_Concept
(	O
for	O
maintenance	O
)	O
.	O
</s>
