<s>
Eventual	B-General_Concept
consistency	I-General_Concept
is	O
a	O
consistency	B-General_Concept
model	I-General_Concept
used	O
in	O
distributed	B-Architecture
computing	I-Architecture
to	O
achieve	O
high	O
availability	O
that	O
informally	O
guarantees	O
that	O
,	O
if	O
no	O
new	O
updates	O
are	O
made	O
to	O
a	O
given	O
data	O
item	O
,	O
eventually	O
all	O
accesses	O
to	O
that	O
item	O
will	O
return	O
the	O
last	O
updated	O
value	O
.	O
</s>
<s>
Eventual	B-General_Concept
consistency	I-General_Concept
,	O
also	O
called	O
optimistic	B-General_Concept
replication	I-General_Concept
,	O
is	O
widely	O
deployed	O
in	O
distributed	B-Architecture
systems	I-Architecture
and	O
has	O
origins	O
in	O
early	O
mobile	O
computing	O
projects	O
.	O
</s>
<s>
A	O
system	O
that	O
has	O
achieved	O
eventual	B-General_Concept
consistency	I-General_Concept
is	O
often	O
said	O
to	O
have	O
converged	O
,	O
or	O
achieved	O
replica	O
convergence	O
.	O
</s>
<s>
Eventual	B-General_Concept
consistency	I-General_Concept
is	O
a	O
weak	O
guarantee	O
–	O
most	O
stronger	O
models	O
,	O
like	O
linearizability	B-General_Concept
,	O
are	O
trivially	O
eventually	B-General_Concept
consistent	I-General_Concept
.	O
</s>
<s>
Eventually-consistent	O
services	O
are	O
often	O
classified	O
as	O
providing	O
BASE	O
semantics	O
(	O
basically-available	O
,	O
soft-state	O
,	O
eventual	B-General_Concept
consistency	I-General_Concept
)	O
,	O
in	O
contrast	O
to	O
traditional	O
ACID	O
(	O
atomicity	B-General_Concept
,	O
consistency	O
,	O
isolation	O
,	O
durability	O
)	O
.	O
</s>
<s>
Eventual	B-General_Concept
consistency	I-General_Concept
is	O
sometimes	O
criticized	O
as	O
increasing	O
the	O
complexity	O
of	O
distributed	B-Architecture
software	I-Architecture
applications	O
.	O
</s>
<s>
This	O
is	O
partly	O
because	O
eventual	B-General_Concept
consistency	I-General_Concept
is	O
purely	O
a	O
liveness	B-Application
guarantee	O
(	O
reads	O
eventually	O
return	O
the	O
same	O
value	O
)	O
and	O
does	O
not	O
guarantee	O
safety	B-Application
:	O
an	O
eventually	B-General_Concept
consistent	I-General_Concept
system	O
can	O
return	O
any	O
value	O
before	O
it	O
converges	O
.	O
</s>
<s>
Timestamps	B-Operating_System
and	O
vector	B-Operating_System
clocks	I-Operating_System
are	O
often	O
used	O
to	O
detect	O
concurrency	O
between	O
updates	O
.	O
</s>
<s>
Whereas	O
eventual	B-General_Concept
consistency	I-General_Concept
is	O
only	O
a	O
liveness	B-Application
guarantee	O
(	O
updates	O
will	O
be	O
observed	O
eventually	O
)	O
,	O
strong	O
eventual	B-General_Concept
consistency	I-General_Concept
(	O
SEC	O
)	O
adds	O
the	O
safety	B-Application
guarantee	O
that	O
any	O
two	O
nodes	O
that	O
have	O
received	O
the	O
same	O
(	O
unordered	O
)	O
set	O
of	O
updates	O
will	O
be	O
in	O
the	O
same	O
state	O
.	O
</s>
<s>
A	O
common	O
approach	O
to	O
ensure	O
SEC	O
is	O
conflict-free	B-Operating_System
replicated	I-Operating_System
data	I-Operating_System
types	I-Operating_System
.	O
</s>
