<s>
Split-brain	B-General_Concept
is	O
a	O
computer	O
term	O
,	O
based	O
on	O
an	O
analogy	O
with	O
the	O
medical	O
Split-brain	B-General_Concept
syndrome	O
.	O
</s>
<s>
It	O
indicates	O
data	O
or	O
availability	O
inconsistencies	O
originating	O
from	O
the	O
maintenance	O
of	O
two	O
separate	O
data	O
sets	O
with	O
overlap	O
in	O
scope	O
,	O
either	O
because	O
of	O
servers	O
in	O
a	O
network	B-Architecture
design	I-Architecture
,	O
or	O
a	O
failure	O
condition	O
based	O
on	O
servers	O
not	O
communicating	O
and	O
synchronizing	O
their	O
data	O
to	O
each	O
other	O
.	O
</s>
<s>
This	O
last	O
case	O
is	O
also	O
commonly	O
referred	O
to	O
as	O
a	O
network	B-Algorithm
partition	I-Algorithm
.	O
</s>
<s>
Although	O
the	O
term	O
split-brain	B-General_Concept
typically	O
refers	O
to	O
an	O
error	O
state	O
,	O
split-brain	B-General_Concept
DNS	B-Protocol
(	O
or	O
split-horizon	B-Protocol
DNS	I-Protocol
)	O
is	O
sometimes	O
used	O
to	O
describe	O
a	O
deliberate	O
situation	O
where	O
internal	O
and	O
external	O
DNS	B-Protocol
services	I-Protocol
for	O
a	O
corporate	O
network	O
are	O
not	O
communicating	O
,	O
so	O
that	O
separate	O
DNS	B-Protocol
name	I-Protocol
spaces	O
are	O
to	O
be	O
administered	O
for	O
external	O
computers	O
and	O
for	O
internal	O
ones	O
.	O
</s>
<s>
This	O
requires	O
a	O
double	O
administration	O
,	O
and	O
if	O
there	O
is	O
domain	O
overlap	O
in	O
the	O
computer	O
names	O
,	O
there	O
is	O
a	O
risk	O
that	O
the	O
same	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
(	O
FQDN	B-Protocol
)	O
,	O
may	O
ambiguously	O
occur	O
in	O
both	O
name	O
spaces	O
referring	O
to	O
different	O
computer	O
IP	O
addresses	O
.	O
</s>
<s>
High-availability	B-General_Concept
clusters	I-General_Concept
usually	O
use	O
a	O
heartbeat	O
private	O
network	O
connection	O
which	O
is	O
used	O
to	O
monitor	O
the	O
health	O
and	O
status	O
of	O
each	O
node	O
in	O
the	O
cluster	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
split-brain	B-General_Concept
syndrome	O
may	O
occur	O
when	O
all	O
of	O
the	O
private	O
links	O
go	O
down	O
simultaneously	O
,	O
but	O
the	O
cluster	O
nodes	O
are	O
still	O
running	O
,	O
each	O
one	O
believing	O
they	O
are	O
the	O
only	O
one	O
running	O
.	O
</s>
<s>
One	O
current	O
implementation	O
for	O
this	O
approach	O
is	O
Hazelcast	B-Application
,	O
which	O
does	O
automatic	O
reconciliation	O
of	O
its	O
key-value	O
store	O
.	O
</s>
<s>
Once	O
a	O
network	B-Algorithm
partitioning	I-Algorithm
has	O
been	O
detected	O
,	O
access	O
to	O
the	O
sub-partitions	O
is	O
limited	O
in	O
order	O
to	O
guarantee	O
consistency	O
.	O
</s>
<s>
A	O
typical	O
approach	O
,	O
as	O
described	O
by	O
Coulouris	O
et	O
al.	O
,	O
is	O
to	O
use	O
a	O
quorum-consensus	O
approach	O
.	O
</s>
<s>
This	O
allows	O
the	O
sub-partition	O
with	O
a	O
majority	O
of	O
the	O
votes	O
to	O
remain	O
available	O
,	O
while	O
the	O
remaining	O
sub-partitions	O
should	O
fall	O
down	O
to	O
an	O
auto-fencing	O
mode	O
.	O
</s>
<s>
One	O
current	O
implementation	O
for	O
this	O
approach	O
is	O
the	O
one	O
used	O
by	O
MongoDB	B-Application
replica	O
sets	O
.	O
</s>
<s>
And	O
another	O
such	O
implementation	O
is	O
Galera	O
replication	O
for	O
MariaDB	B-Application
and	O
MySQL	B-Application
.	O
</s>
<s>
Modern	O
commercial	O
general-purpose	O
HA	B-General_Concept
clusters	I-General_Concept
typically	O
use	O
a	O
combination	O
of	O
heartbeat	O
network	O
connections	O
between	O
cluster	O
hosts	O
,	O
and	O
quorum	B-General_Concept
witness	O
storage	O
.	O
</s>
<s>
In	O
such	O
clusters	O
(	O
without	O
quorum	B-General_Concept
)	O
,	O
if	O
a	O
member	O
fails	O
,	O
even	O
if	O
the	O
members	O
normally	O
assign	O
primary	O
and	O
secondary	O
statuses	O
to	O
the	O
hosts	O
,	O
there	O
is	O
at	O
least	O
a	O
50%	O
probability	O
that	O
a	O
2-node	O
HA	B-General_Concept
cluster	I-General_Concept
will	O
totally	O
fail	O
until	O
human	O
intervention	O
is	O
provided	O
,	O
to	O
prevent	O
multiple	O
members	O
becoming	O
active	O
independently	O
and	O
either	O
directly	O
conflicting	O
or	O
corrupting	O
data	O
.	O
</s>
