<s>
In	O
theoretical	O
computer	O
science	O
,	O
the	O
CAP	B-Operating_System
theorem	I-Operating_System
,	O
also	O
named	O
Brewer	B-Operating_System
's	I-Operating_System
theorem	I-Operating_System
after	O
computer	O
scientist	O
Eric	O
Brewer	O
,	O
states	O
that	O
any	O
distributed	O
data	O
store	O
can	O
provide	O
only	O
two	O
of	O
the	O
following	O
three	O
guarantees	O
:	O
</s>
<s>
Consistency	B-General_Concept
Every	O
read	O
receives	O
the	O
most	O
recent	O
write	O
or	O
an	O
error	O
.	O
</s>
<s>
Partition	B-Algorithm
tolerance	I-Algorithm
The	O
system	O
continues	O
to	O
operate	O
despite	O
an	O
arbitrary	O
number	O
of	O
messages	O
being	O
dropped	O
(	O
or	O
delayed	O
)	O
by	O
the	O
network	O
between	O
nodes	O
.	O
</s>
<s>
When	O
a	O
network	B-Algorithm
partition	I-Algorithm
failure	O
happens	O
,	O
it	O
must	O
be	O
decided	O
whether	O
to	O
do	O
one	O
of	O
the	O
following	O
:	O
</s>
<s>
Thus	O
,	O
if	O
there	O
is	O
a	O
network	B-Algorithm
partition	I-Algorithm
,	O
one	O
has	O
to	O
choose	O
between	O
consistency	B-General_Concept
or	O
availability	O
.	O
</s>
<s>
Note	O
that	O
consistency	B-General_Concept
as	O
defined	O
in	O
the	O
CAP	B-Operating_System
theorem	I-Operating_System
is	O
quite	O
different	O
from	O
the	O
consistency	B-General_Concept
guaranteed	O
in	O
ACID	O
database	B-General_Concept
transactions	I-General_Concept
.	O
</s>
<s>
Eric	O
Brewer	O
argues	O
that	O
the	O
often-used	O
"	O
two	O
out	O
of	O
three	O
"	O
concept	O
can	O
be	O
somewhat	O
misleading	O
because	O
system	O
designers	O
need	O
only	O
to	O
sacrifice	O
consistency	B-General_Concept
or	O
availability	O
in	O
the	O
presence	O
of	O
partitions	O
,	O
but	O
that	O
in	O
many	O
systems	O
partitions	O
are	O
rare	O
.	O
</s>
<s>
No	O
distributed	O
system	O
is	O
safe	O
from	O
network	O
failures	O
,	O
thus	O
network	B-Algorithm
partitioning	I-Algorithm
generally	O
has	O
to	O
be	O
tolerated	O
.	O
</s>
<s>
In	O
the	O
presence	O
of	O
a	O
partition	O
,	O
one	O
is	O
then	O
left	O
with	O
two	O
options	O
:	O
consistency	B-General_Concept
or	O
availability	O
.	O
</s>
<s>
When	O
choosing	O
consistency	B-General_Concept
over	O
availability	O
,	O
the	O
system	O
will	O
return	O
an	O
error	O
or	O
a	O
time	O
out	O
if	O
particular	O
information	O
cannot	O
be	O
guaranteed	O
to	O
be	O
up	O
to	O
date	O
due	O
to	O
network	B-Algorithm
partitioning	I-Algorithm
.	O
</s>
<s>
When	O
choosing	O
availability	O
over	O
consistency	B-General_Concept
,	O
the	O
system	O
will	O
always	O
process	O
the	O
query	O
and	O
try	O
to	O
return	O
the	O
most	O
recent	O
available	O
version	O
of	O
the	O
information	O
,	O
even	O
if	O
it	O
cannot	O
guarantee	O
it	O
is	O
up	O
to	O
date	O
due	O
to	O
network	B-Algorithm
partitioning	I-Algorithm
.	O
</s>
<s>
In	O
the	O
absence	O
of	O
a	O
partition	O
,	O
both	O
availability	O
and	O
consistency	B-General_Concept
can	O
be	O
satisfied	O
.	O
</s>
<s>
Database	O
systems	O
designed	O
with	O
traditional	O
ACID	O
guarantees	O
in	O
mind	O
such	O
as	O
RDBMS	B-Application
choose	O
consistency	B-General_Concept
over	O
availability	O
,	O
whereas	O
systems	O
designed	O
around	O
the	O
BASE	B-General_Concept
philosophy	O
,	O
common	O
in	O
the	O
NoSQL	B-General_Concept
movement	O
for	O
example	O
,	O
choose	O
availability	O
over	O
consistency	B-General_Concept
.	O
</s>
<s>
In	O
2002	O
,	O
Seth	O
Gilbert	O
and	O
Nancy	O
Lynch	O
of	O
MIT	O
published	O
a	O
formal	O
proof	O
of	O
Brewer	B-Operating_System
's	I-Operating_System
conjecture	I-Operating_System
,	O
rendering	O
it	O
a	O
theorem	O
.	O
</s>
<s>
In	O
2012	O
,	O
Brewer	O
clarified	O
some	O
of	O
his	O
positions	O
,	O
including	O
why	O
the	O
often-used	O
"	O
two	O
out	O
of	O
three	O
"	O
concept	O
can	O
be	O
somewhat	O
misleading	O
because	O
system	O
designers	O
only	O
need	O
to	O
sacrifice	O
consistency	B-General_Concept
or	O
availability	O
in	O
the	O
presence	O
of	O
partitions	O
;	O
partition	O
management	O
and	O
recovery	O
techniques	O
exist	O
.	O
</s>
<s>
Brewer	O
also	O
noted	O
the	O
different	O
definition	O
of	O
consistency	B-General_Concept
used	O
in	O
the	O
CAP	B-Operating_System
theorem	I-Operating_System
relative	O
to	O
the	O
definition	O
used	O
in	O
ACID	O
.	O
</s>
<s>
A	O
similar	O
theorem	O
stating	O
the	O
trade-off	O
between	O
consistency	B-General_Concept
and	O
availability	O
in	O
distributed	O
systems	O
was	O
published	O
by	O
Birman	O
and	O
Friedman	O
in	O
1996	O
.	O
</s>
<s>
The	O
PACELC	B-Operating_System
theorem	I-Operating_System
,	O
introduced	O
in	O
2010	O
,	O
builds	O
on	O
CAP	O
by	O
stating	O
that	O
even	O
in	O
the	O
absence	O
of	O
partitioning	O
,	O
there	O
is	O
another	O
trade-off	O
between	O
latency	O
and	O
consistency	B-General_Concept
.	O
</s>
<s>
PACELC	B-Operating_System
means	O
,	O
if	O
partition	O
(	O
P	O
)	O
happens	O
,	O
the	O
trade-off	O
is	O
between	O
availability	O
(	O
A	O
)	O
and	O
consistency	B-General_Concept
(	O
C	O
)	O
;	O
Else	O
(	O
E	O
)	O
,	O
the	O
trade-off	O
is	O
between	O
latency	O
(	O
L	O
)	O
and	O
consistency	B-General_Concept
(	O
C	O
)	O
.	O
</s>
<s>
Blockchain	B-Application
technology	I-Application
often	O
sacrifices	O
immediate	O
consistency	B-General_Concept
for	O
availability	O
and	O
partition	B-Algorithm
tolerance	I-Algorithm
.	O
</s>
<s>
By	O
requiring	O
a	O
specific	O
number	O
of	O
"	O
confirmations	O
"	O
,	O
Blockchain	B-Application
consensus	O
algorithms	O
are	O
basically	O
reduced	O
to	O
eventual	B-General_Concept
consistency	I-General_Concept
.	O
</s>
