<s>
In	O
theoretical	O
computer	O
science	O
,	O
the	O
PACELC	B-Operating_System
theorem	I-Operating_System
is	O
an	O
extension	O
to	O
the	O
CAP	B-Operating_System
theorem	I-Operating_System
.	O
</s>
<s>
It	O
states	O
that	O
in	O
case	O
of	O
network	O
partitioning	O
(	O
P	O
)	O
in	O
a	O
distributed	O
computer	O
system	O
,	O
one	O
has	O
to	O
choose	O
between	O
availability	O
(	O
A	O
)	O
and	O
consistency	O
(	O
C	O
)	O
(	O
as	O
per	O
the	O
CAP	B-Operating_System
theorem	I-Operating_System
)	O
,	O
but	O
else	O
(	O
E	O
)	O
,	O
even	O
when	O
the	O
system	O
is	O
running	O
normally	O
in	O
the	O
absence	O
of	O
partitions	O
,	O
one	O
has	O
to	O
choose	O
between	O
latency	O
(	O
L	O
)	O
and	O
consistency	O
(	O
C	O
)	O
.	O
</s>
<s>
PACELC	B-Operating_System
builds	O
on	O
the	O
CAP	B-Operating_System
theorem	I-Operating_System
.	O
</s>
<s>
PACELC	B-Operating_System
goes	O
further	O
and	O
states	O
that	O
an	O
additional	O
trade-off	O
exists	O
:	O
between	O
latency	O
and	O
consistency	O
,	O
even	O
in	O
absence	O
of	O
partitions	O
,	O
thus	O
providing	O
a	O
more	O
complete	O
portrayal	O
of	O
the	O
potential	O
consistency	O
trade-offs	O
for	O
distributed	O
systems	O
.	O
</s>
<s>
The	O
PACELC	B-Operating_System
theorem	I-Operating_System
was	O
first	O
described	O
by	O
Daniel	O
J	O
.	O
Abadi	O
from	O
Yale	O
University	O
in	O
2010	O
in	O
a	O
blog	O
post	O
,	O
which	O
he	O
later	O
clarified	O
in	O
a	O
paper	O
in	O
2012	O
.	O
</s>
<s>
The	O
purpose	O
of	O
PACELC	B-Operating_System
is	O
to	O
address	O
his	O
thesis	O
that	O
"	O
Ignoring	O
the	O
consistency/latency	O
trade-off	O
of	O
replicated	O
systems	O
is	O
a	O
major	O
oversight	O
[	O
in	O
CAP ]	O
,	O
as	O
it	O
is	O
present	O
at	O
all	O
times	O
during	O
system	O
operation	O
,	O
whereas	O
CAP	O
is	O
only	O
relevant	O
in	O
the	O
arguably	O
rare	O
case	O
of	O
a	O
network	O
partition.	O
"	O
</s>
<s>
The	O
PACELC	B-Operating_System
theorem	I-Operating_System
was	O
proved	O
formally	O
in	O
2018	O
in	O
a	O
SIGACT	O
News	O
article	O
.	O
</s>
<s>
Original	O
database	O
PACELC	B-Operating_System
ratings	O
are	O
from	O
.	O
</s>
<s>
The	O
default	O
versions	O
of	O
,	O
Cassandra	B-Application
,	O
Riak	B-General_Concept
,	O
and	O
Cosmos	B-Application
DB	I-Application
are	O
PA/EL	O
systems	O
:	O
if	O
a	O
partition	O
occurs	O
,	O
they	O
give	O
up	O
consistency	O
for	O
availability	O
,	O
and	O
under	O
normal	O
operation	O
they	O
give	O
up	O
consistency	O
for	O
lower	O
latency	O
.	O
</s>
<s>
Fully	O
ACID	O
systems	O
such	O
as	O
VoltDB/H	O
-Store	O
,	O
Megastore	O
,	O
MySQL	B-Operating_System
Cluster	I-Operating_System
,	O
and	O
PostgreSQL	B-Application
are	O
PC/EC	O
:	O
they	O
refuse	O
to	O
give	O
up	O
consistency	O
,	O
and	O
will	O
pay	O
the	O
availability	O
and	O
latency	O
costs	O
to	O
achieve	O
it	O
.	O
</s>
<s>
Bigtable	B-Application
and	O
related	O
systems	O
such	O
as	O
HBase	B-Language
are	O
also	O
PC/EC	O
.	O
</s>
<s>
Amazon	O
DynamoDB	O
(	O
launched	O
January	O
2012	O
)	O
is	O
quite	O
different	O
from	O
the	O
which	O
was	O
considered	O
for	O
the	O
PACELC	B-Operating_System
paper	O
.	O
</s>
<s>
Cosmos	B-Application
DB	I-Application
supports	O
five	O
tunable	O
consistency	O
levels	O
that	O
allow	O
for	O
tradeoffs	O
between	O
C/A	O
during	O
P	O
,	O
and	O
L/C	O
during	O
E	O
.	O
Cosmos	B-Application
DB	I-Application
never	O
violates	O
the	O
specified	O
consistency	O
level	O
,	O
so	O
it	O
's	O
formally	O
CP	O
.	O
</s>
<s>
MongoDB	B-Application
can	O
be	O
classified	O
as	O
a	O
PA/EC	O
system	O
.	O
</s>
<s>
FaunaDB	O
implements	O
Calvin	O
,	O
a	O
transaction	O
protocol	O
created	O
by	O
Dr.	O
Daniel	O
Abadi	O
,	O
the	O
author	O
of	O
the	O
PACELC	B-Operating_System
theorem	I-Operating_System
,	O
and	O
offers	O
users	O
adjustable	O
controls	O
for	O
LC	O
tradeoff	O
.	O
</s>
