<s>
Data	B-General_Concept
synchronization	I-General_Concept
is	O
the	O
process	O
of	O
establishing	O
consistency	B-General_Concept
between	O
source	O
and	O
target	O
data	O
stores	O
,	O
and	O
the	O
continuous	O
harmonization	O
of	O
the	O
data	O
over	O
time	O
.	O
</s>
<s>
It	O
is	O
fundamental	O
to	O
a	O
wide	O
variety	O
of	O
applications	O
,	O
including	O
file	B-Application
synchronization	I-Application
and	O
mobile	O
device	O
synchronization	O
.	O
</s>
<s>
Data	B-General_Concept
synchronization	I-General_Concept
can	O
also	O
be	O
useful	O
in	O
encryption	O
for	O
synchronizing	O
public	O
key	O
servers	O
.	O
</s>
<s>
There	O
are	O
tools	O
available	O
for	O
file	B-Application
synchronization	I-Application
,	O
version	B-Architecture
control	I-Architecture
(	O
CVS	B-Application
,	O
Subversion	B-Application
,	O
etc	O
.	O
</s>
<s>
)	O
,	O
distributed	O
filesystems	O
(	O
Coda	B-Application
,	O
etc	O
.	O
</s>
<s>
)	O
,	O
and	O
mirroring	O
(	O
rsync	B-Application
,	O
etc	O
.	O
</s>
<s>
However	O
,	O
only	O
version	B-Architecture
control	I-Architecture
and	O
file	B-Application
synchronization	I-Application
tools	O
can	O
deal	O
with	O
modifications	O
to	O
more	O
than	O
one	O
copy	O
of	O
the	O
files	O
.	O
</s>
<s>
File	B-Application
synchronization	I-Application
is	O
commonly	O
used	O
for	O
home	O
backups	O
on	O
external	O
hard	B-Device
drives	I-Device
or	O
updating	O
for	O
transport	O
on	O
USB	O
flash	O
drives	O
.	O
</s>
<s>
Version	B-Architecture
control	I-Architecture
tools	O
are	O
intended	O
to	O
deal	O
with	O
situations	O
where	O
more	O
than	O
one	O
user	O
attempts	O
to	O
simultaneously	O
modify	O
the	O
same	O
file	O
,	O
while	O
file	O
synchronizers	O
are	O
optimized	O
for	O
situations	O
where	O
only	O
one	O
copy	O
of	O
the	O
file	O
will	O
be	O
edited	O
at	O
a	O
time	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
although	O
version	B-Architecture
control	I-Architecture
tools	O
can	O
be	O
used	O
for	O
file	B-Application
synchronization	I-Application
,	O
dedicated	O
programs	O
require	O
less	O
overhead	O
.	O
</s>
<s>
This	O
normally	O
requires	O
that	O
the	O
devices	O
storing	O
the	O
files	O
are	O
always	O
connected	O
,	O
but	O
some	O
distributed	O
file	O
systems	O
like	O
Coda	B-Application
allow	O
disconnected	O
operation	O
followed	O
by	O
reconciliation	O
.	O
</s>
<s>
The	O
merging	O
facilities	O
of	O
a	O
distributed	O
file	O
system	O
are	O
typically	O
more	O
limited	O
than	O
those	O
of	O
a	O
version	B-Architecture
control	I-Architecture
system	I-Architecture
because	O
most	O
file	O
systems	O
do	O
not	O
keep	O
a	O
version	O
graph	O
.	O
</s>
<s>
Several	O
theoretical	O
models	O
of	O
data	B-General_Concept
synchronization	I-General_Concept
exist	O
in	O
the	O
research	O
literature	O
,	O
and	O
the	O
problem	O
is	O
also	O
related	O
to	O
the	O
problem	O
of	O
Slepian	B-Error_Name
–	I-Error_Name
Wolf	I-Error_Name
coding	I-Error_Name
in	O
information	O
theory	O
.	O
</s>
<s>
Then	O
data	B-General_Concept
synchronization	I-General_Concept
is	O
the	O
process	O
of	O
reducing	O
edit	O
distance	O
between	O
and	O
,	O
up	O
to	O
the	O
ideal	O
distance	O
of	O
zero	O
.	O
</s>
<s>
It	O
is	O
sometimes	O
possible	O
to	O
transform	O
the	O
problem	O
to	O
one	O
of	O
unordered	O
data	O
through	O
a	O
process	O
known	O
as	O
shingling	B-General_Concept
(	O
splitting	O
the	O
strings	O
into	O
shingles	O
)	O
.	O
</s>
<s>
The	O
first	O
step	O
is	O
usually	O
replication	B-Architecture
,	O
which	O
involves	O
making	O
multiple	O
copies	O
of	O
the	O
data	O
and	O
keeping	O
them	O
all	O
up	O
to	O
date	O
as	O
changes	O
are	O
made	O
.	O
</s>
<s>
Paxos	B-Operating_System
and	O
Raft	B-Operating_System
are	O
more	O
complex	O
protocols	O
that	O
exist	O
to	O
solve	O
problems	O
with	O
transient	O
effects	O
during	O
failover	O
,	O
such	O
as	O
two	O
instances	O
thinking	O
they	O
are	O
the	O
master	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
If	O
corrupt	O
or	O
out-of-date	O
data	O
may	O
be	O
present	O
on	O
some	O
nodes	O
,	O
this	O
approach	O
may	O
also	O
benefit	O
from	O
the	O
use	O
of	O
an	O
error	B-Error_Name
correction	I-Error_Name
code	I-Error_Name
.	O
</s>
<s>
DHTs	B-Operating_System
and	O
Blockchains	B-Application
try	O
to	O
solve	O
the	O
problem	O
of	O
synchronization	O
between	O
many	O
nodes	O
(	O
hundreds	O
to	O
billions	O
)	O
.	O
</s>
