<s>
Optimistic	O
replication	B-Architecture
,	O
also	O
known	O
as	O
lazy	B-General_Concept
replication	I-General_Concept
,	O
is	O
a	O
strategy	O
for	O
replication	B-Architecture
,	O
in	O
which	O
replicas	B-Architecture
are	O
allowed	O
to	O
diverge	O
.	O
</s>
<s>
Traditional	O
pessimistic	O
replication	B-Architecture
systems	O
try	O
to	O
guarantee	O
from	O
the	O
beginning	O
that	O
all	O
of	O
the	O
replicas	B-Architecture
are	O
identical	O
to	O
each	O
other	O
,	O
as	O
if	O
there	O
was	O
only	O
a	O
single	O
copy	O
of	O
the	O
data	O
all	O
along	O
.	O
</s>
<s>
Optimistic	O
replication	B-Architecture
does	O
away	O
with	O
this	O
in	O
favor	O
of	O
eventual	B-General_Concept
consistency	I-General_Concept
,	O
meaning	O
that	O
replicas	B-Architecture
are	O
guaranteed	O
to	O
converge	O
only	O
when	O
the	O
system	O
has	O
been	O
quiesced	O
for	O
a	O
period	O
of	O
time	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
there	O
is	O
no	O
longer	O
a	O
need	O
to	O
wait	O
for	O
all	O
of	O
the	O
copies	O
to	O
be	O
synchronized	O
when	O
updating	O
data	O
,	O
which	O
helps	O
concurrency	B-Operating_System
and	O
parallelism	B-Operating_System
.	O
</s>
<s>
The	O
trade-off	O
is	O
that	O
different	O
replicas	B-Architecture
may	O
require	O
explicit	O
reconciliation	O
later	O
on	O
,	O
which	O
might	O
then	O
prove	O
difficult	O
or	O
even	O
insoluble	O
.	O
</s>
<s>
An	O
optimistic	O
replication	B-Architecture
algorithm	O
consists	O
of	O
five	O
elements	O
:	O
</s>
<s>
One	O
well-known	O
example	O
of	O
a	O
system	O
based	O
on	O
optimistic	O
replication	B-Architecture
is	O
the	O
CVS	O
version	O
control	O
system	O
,	O
or	O
any	O
other	O
version	O
control	O
system	O
which	O
uses	O
the	O
paradigm	O
.	O
</s>
<s>
A	O
special	O
case	O
of	O
replication	B-Architecture
is	O
synchronization	O
,	O
where	O
there	O
are	O
only	O
two	O
replicas	B-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
personal	O
digital	O
assistants	O
(	O
PDAs	O
)	O
allow	O
users	O
to	O
edit	O
data	O
either	O
on	O
the	O
PDA	O
or	O
a	O
computer	O
,	O
and	O
then	O
to	O
merge	B-General_Concept
these	O
two	O
datasets	O
together	O
.	O
</s>
<s>
Note	O
,	O
however	O
,	O
that	O
replication	B-Architecture
is	O
a	O
broader	O
problem	O
than	O
synchronization	O
,	O
since	O
there	O
may	O
be	O
more	O
than	O
two	O
replicas	B-Architecture
.	O
</s>
<s>
The	O
replication	B-Architecture
behaviour	O
of	O
such	O
an	O
installation	O
may	O
differ	O
from	O
a	O
live	O
environment	O
in	O
ways	O
that	O
mean	O
that	O
replication	B-Architecture
lag	O
is	O
unlikely	O
to	O
be	O
observed	O
in	O
testing	O
,	O
masking	O
replication-sensitive	O
bugs	O
.	O
</s>
