<s>
In	O
database	B-Application
systems	I-Application
,	O
consistency	B-General_Concept
(	O
or	O
correctness	O
)	O
refers	O
to	O
the	O
requirement	O
that	O
any	O
given	O
database	B-General_Concept
transaction	I-General_Concept
must	O
change	O
affected	O
data	O
only	O
in	O
allowed	O
ways	O
.	O
</s>
<s>
Any	O
data	O
written	O
to	O
the	O
database	O
must	O
be	O
valid	O
according	O
to	O
all	O
defined	O
rules	O
,	O
including	O
constraints	O
,	O
cascades	O
,	O
triggers	B-Language
,	O
and	O
any	O
combination	O
thereof	O
.	O
</s>
<s>
Consistency	B-General_Concept
can	O
also	O
be	O
understood	O
as	O
after	O
a	O
successful	O
write	O
,	O
update	O
or	O
delete	O
of	O
a	O
Record	O
,	O
any	O
read	O
request	O
immediately	O
receives	O
the	O
latest	O
value	O
of	O
the	O
Record	O
.	O
</s>
<s>
Consistency	B-General_Concept
is	O
one	O
of	O
the	O
four	O
guarantees	O
that	O
define	O
ACID	O
transactions	B-General_Concept
;	O
however	O
,	O
significant	O
ambiguity	O
exists	O
about	O
the	O
nature	O
of	O
this	O
guarantee	O
.	O
</s>
<s>
The	O
guarantee	O
that	O
any	O
transactions	B-General_Concept
started	O
in	O
the	O
future	O
necessarily	O
see	O
the	O
effects	O
of	O
other	O
transactions	B-General_Concept
committed	O
in	O
the	O
past	O
.	O
</s>
<s>
As	O
these	O
various	O
definitions	O
are	O
not	O
mutually	O
exclusive	O
,	O
it	O
is	O
possible	O
to	O
design	O
a	O
system	O
that	O
guarantees	O
"	O
consistency	B-General_Concept
"	O
in	O
every	O
sense	O
of	O
the	O
word	O
,	O
as	O
most	O
relational	B-Application
database	I-Application
management	I-Application
systems	I-Application
in	O
common	O
use	O
today	O
arguably	O
do	O
.	O
</s>
<s>
The	O
CAP	B-Operating_System
theorem	I-Operating_System
is	O
based	O
on	O
three	O
trade-offs	O
,	O
one	O
of	O
which	O
is	O
"	O
atomic	O
consistency	B-General_Concept
"	O
(	O
shortened	O
to	O
"	O
consistency	B-General_Concept
"	O
for	O
the	O
acronym	O
)	O
,	O
about	O
which	O
the	O
authors	O
note	O
,	O
"	O
Discussing	O
atomic	O
consistency	B-General_Concept
is	O
somewhat	O
different	O
than	O
talking	O
about	O
an	O
ACID	O
database	O
,	O
as	O
database	B-General_Concept
consistency	I-General_Concept
refers	O
to	O
transactions	B-General_Concept
,	O
while	O
atomic	O
consistency	B-General_Concept
refers	O
only	O
to	O
a	O
property	O
of	O
a	O
single	O
request/response	O
operation	O
sequence	O
.	O
</s>
<s>
In	O
the	O
CAP	B-Operating_System
theorem	I-Operating_System
,	O
you	O
can	O
only	O
have	O
two	O
of	O
the	O
following	O
three	O
properties	O
:	O
consistency	B-General_Concept
,	O
availability	O
,	O
or	O
partition	O
tolerance	O
.	O
</s>
<s>
Therefore	O
,	O
consistency	B-General_Concept
may	O
have	O
to	O
be	O
traded	O
off	O
in	O
some	O
database	B-Application
systems	I-Application
.	O
</s>
