<s>
Multi-master	B-Operating_System
replication	I-Operating_System
is	O
a	O
method	O
of	O
database	O
replication	B-Architecture
which	O
allows	O
data	O
to	O
be	O
stored	O
by	O
a	O
group	O
of	O
computers	O
,	O
and	O
updated	O
by	O
any	O
member	O
of	O
the	O
group	O
.	O
</s>
<s>
The	O
multi-master	B-Operating_System
replication	I-Operating_System
system	O
is	O
responsible	O
for	O
propagating	O
the	O
data	O
modifications	O
made	O
by	O
each	O
member	O
to	O
the	O
rest	O
of	O
the	O
group	O
and	O
resolving	O
any	O
conflicts	O
that	O
might	O
arise	O
between	O
concurrent	O
changes	O
made	O
by	O
different	O
members	O
.	O
</s>
<s>
Multi-master	B-Operating_System
replication	I-Operating_System
can	O
be	O
contrasted	O
with	O
primary-replica	B-Operating_System
replication	B-Architecture
,	O
in	O
which	O
a	O
single	O
member	O
of	O
the	O
group	O
is	O
designated	O
as	O
the	O
"	O
master	O
"	O
for	O
a	O
given	O
piece	O
of	O
data	O
and	O
is	O
the	O
only	O
node	O
allowed	O
to	O
modify	O
that	O
data	O
item	O
.	O
</s>
<s>
Allowing	O
only	O
a	O
single	O
master	O
makes	O
it	O
easier	O
to	O
achieve	O
consistency	O
among	O
the	O
members	O
of	O
the	O
group	O
,	O
but	O
is	O
less	O
flexible	O
than	O
multi-master	B-Operating_System
replication	I-Operating_System
.	O
</s>
<s>
Multi-master	B-Operating_System
replication	I-Operating_System
can	O
also	O
be	O
contrasted	O
with	O
failover	B-General_Concept
clustering	I-General_Concept
where	O
passive	O
replica	B-Architecture
servers	O
are	O
replicating	O
the	O
master	O
data	O
in	O
order	O
to	O
prepare	O
for	O
takeover	O
in	O
the	O
event	O
that	O
the	O
master	O
stops	O
functioning	O
.	O
</s>
<s>
Often	O
,	O
communication	O
and	O
replication	B-Architecture
in	O
Multi-master	O
systems	O
are	O
handled	O
via	O
a	O
type	O
of	O
Consensus	B-Operating_System
algorithm	I-Operating_System
,	O
but	O
can	O
also	O
be	O
implemented	O
via	O
custom	O
or	O
proprietary	O
algorithms	O
specific	O
to	O
the	O
software	O
.	O
</s>
<s>
The	O
primary	O
purposes	O
of	O
multi-master	B-Operating_System
replication	I-Operating_System
are	O
increased	O
availability	O
and	O
faster	O
server	O
response	O
time	O
.	O
</s>
<s>
Consistency	O
:	O
Most	O
multi-master	B-Operating_System
replication	I-Operating_System
systems	O
are	O
only	O
loosely	O
consistent	O
,	O
i.e.	O
</s>
<s>
Performance	O
:	O
Eager	O
replication	B-Architecture
systems	O
are	O
complex	O
and	O
increase	O
communication	O
latency	O
.	O
</s>
<s>
Many	O
directory	O
servers	O
are	O
based	O
on	O
Lightweight	B-Protocol
Directory	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
LDAP	B-Protocol
)	O
and	O
implement	O
multi-master	B-Operating_System
replication	I-Operating_System
.	O
</s>
<s>
One	O
of	O
the	O
more	O
prevalent	O
multi-master	B-Operating_System
replication	I-Operating_System
implementations	O
in	O
directory	O
servers	O
is	O
Microsoft	O
's	O
Active	B-Protocol
Directory	I-Protocol
.	O
</s>
<s>
Within	O
Active	B-Protocol
Directory	I-Protocol
,	O
objects	O
that	O
are	O
updated	O
on	O
one	O
Domain	B-Protocol
Controller	I-Protocol
are	O
then	O
replicated	O
to	O
other	O
domain	B-Protocol
controllers	I-Protocol
through	O
multi-master	B-Operating_System
replication	I-Operating_System
.	O
</s>
<s>
It	O
is	O
not	O
required	O
for	O
all	O
domain	B-Protocol
controllers	I-Protocol
to	O
replicate	O
with	O
each	O
other	O
as	O
this	O
would	O
cause	O
excessive	O
network	O
traffic	O
in	O
large	O
Active	B-Protocol
Directory	I-Protocol
deployments	O
.	O
</s>
<s>
Instead	O
,	O
domain	B-Protocol
controllers	I-Protocol
have	O
a	O
complex	O
update	O
pattern	O
that	O
ensures	O
that	O
all	O
servers	O
are	O
updated	O
in	O
a	O
timely	O
fashion	O
without	O
excessive	O
replication	B-Architecture
traffic	O
.	O
</s>
<s>
Some	O
Active	B-Protocol
Directory	I-Protocol
needs	O
are	O
however	O
better	O
served	O
by	O
Flexible	B-General_Concept
single	I-General_Concept
master	I-General_Concept
operation	I-General_Concept
.	O
</s>
<s>
CA	O
Directory	O
supports	O
multi-master	B-Operating_System
replication	I-Operating_System
.	O
</s>
<s>
OpenDS	B-Language
(	O
and	O
its	O
successor	O
product	O
OpenDJ	B-Language
)	O
implemented	O
multi-master	O
since	O
version	O
1.0	O
.	O
</s>
<s>
The	O
OpenDS/OpenDJ	O
multi-master	B-Operating_System
replication	I-Operating_System
is	O
asynchronous	O
,	O
it	O
uses	O
a	O
log	O
with	O
a	O
publish-subscribe	O
mechanism	O
that	O
allows	O
scaling	O
to	O
a	O
large	O
number	O
of	O
nodes	O
.	O
</s>
<s>
OpenDS/OpenDJ	O
replication	B-Architecture
does	O
conflict	O
resolution	O
at	O
the	O
entry	O
and	O
attribute	O
level	O
.	O
</s>
<s>
OpenDS/OpenDJ	O
replication	B-Architecture
can	O
be	O
used	O
over	O
a	O
wide	O
area	O
network	O
.	O
</s>
<s>
OpenLDAP	B-Language
,	O
the	O
widely	O
used	O
open-source	B-License
LDAP	B-Protocol
server	O
,	O
implements	O
multi-master	B-Operating_System
replication	I-Operating_System
since	O
version	O
2.4	O
(	O
October	O
2007	O
)	O
.	O
</s>
<s>
Apache	B-Application
CouchDB	I-Application
uses	O
a	O
simple	O
,	O
HTTP-based	O
multi-master	B-Operating_System
replication	I-Operating_System
system	O
built	O
from	O
its	O
use	O
of	O
an	O
append-only	O
data-store	O
and	O
use	O
of	O
Multiversion	B-Operating_System
Concurrency	I-Operating_System
Control	I-Operating_System
(	O
MVCC	B-Operating_System
)	O
.	O
</s>
<s>
Each	O
document	O
contains	O
a	O
revision	O
ID	O
,	O
so	O
every	O
record	O
stores	O
the	O
evolutionary	O
timeline	O
of	O
all	O
previous	O
revision	O
IDs	O
leading	O
up	O
to	O
itself	O
—	O
which	O
provides	O
the	O
foundation	O
of	O
CouchDB	B-Application
's	O
MVCC	B-Operating_System
system	O
.	O
</s>
<s>
"	O
The	O
replication	B-Architecture
process	O
only	O
copies	O
the	O
last	O
revision	O
of	O
a	O
document	O
,	O
so	O
all	O
previous	O
revisions	O
that	O
were	O
only	O
on	O
the	O
source	O
database	O
are	O
not	O
copied	O
to	O
the	O
destination	O
database.	O
"	O
</s>
<s>
The	O
CouchDB	B-Application
replicator	O
acts	O
as	O
a	O
simple	O
HTTP	O
client	O
acting	O
on	O
both	O
a	O
and	O
database	O
.	O
</s>
<s>
Bi-directional	O
replication	B-Architecture
is	O
the	O
result	O
of	O
merely	O
doing	O
another	O
replication	B-Architecture
with	O
the	O
and	O
values	O
swapped	O
.	O
</s>
<s>
ArangoDB	B-Operating_System
is	O
a	O
native	O
multi-model	O
database	B-Application
system	I-Application
using	O
multi-master	B-Operating_System
replication	I-Operating_System
.	O
</s>
<s>
Clusters	B-Architecture
in	O
ArangoDB	B-Operating_System
use	O
the	O
CP	O
master/master	O
model	O
with	O
no	O
single	O
point	O
of	O
failure	O
.	O
</s>
<s>
When	O
a	O
cluster	O
encounters	O
a	O
network	O
partition	O
,	O
ArangoDB	B-Operating_System
prefers	O
to	O
maintain	O
its	O
internal	O
consistency	O
over	O
availability	O
.	O
</s>
<s>
Cloudant	B-General_Concept
,	O
a	O
distributed	O
database	B-Application
system	I-Application
,	O
uses	O
largely	O
the	O
same	O
HTTP	O
API	O
as	O
Apache	B-Application
CouchDB	I-Application
,	O
and	O
exposes	O
the	O
same	O
ability	O
to	O
replicate	O
using	O
Multiversion	B-Operating_System
Concurrency	I-Operating_System
Control	I-Operating_System
(	O
MVCC	B-Operating_System
)	O
.	O
</s>
<s>
Cloudant	B-General_Concept
databases	B-Application
can	O
replicate	O
between	O
each	O
other	O
,	O
but	O
internally	O
,	O
nodes	O
within	O
Cloudant	B-General_Concept
clusters	B-Architecture
use	O
multi-master	B-Operating_System
replication	I-Operating_System
to	O
stay	O
in	O
sync	O
with	O
each	O
other	O
and	O
provide	O
high	O
availability	O
to	O
API	O
consumers	O
.	O
</s>
<s>
eXtremeDB	B-Application
Cluster	O
is	O
the	O
clustering	O
sub-system	O
for	O
McObject	O
's	O
eXtremeDB	B-Application
embedded	O
database	O
product	O
family	O
.	O
</s>
<s>
It	O
maintains	O
database	O
consistency	O
across	O
multiple	O
hardware	O
nodes	O
by	O
replicating	O
transactions	O
in	O
a	O
synchronous	O
manner	O
(	O
two-phase	B-General_Concept
commit	I-General_Concept
)	O
.	O
</s>
<s>
An	O
important	O
characteristic	O
of	O
eXtremeDB	B-Application
Cluster	O
is	O
transaction	B-General_Concept
replication	B-Architecture
,	O
in	O
contrast	O
to	O
log	O
file-based	O
,	O
SQL	O
statement-based	O
,	O
or	O
other	O
replication	B-Architecture
schemes	O
that	O
may	O
or	O
may	O
not	O
guarantee	O
the	O
success	O
or	O
failure	O
of	O
entire	O
transactions	O
.	O
</s>
<s>
Accordingly	O
,	O
eXtremeDB	B-Application
Cluster	O
is	O
an	O
ACID	O
compliant	O
system	O
(	O
not	O
BASE	O
or	O
eventual	B-General_Concept
consistency	I-General_Concept
)	O
;	O
a	O
query	O
executed	O
on	O
any	O
cluster	O
node	O
will	O
return	O
the	O
same	O
result	O
as	O
if	O
executed	O
on	O
any	O
other	O
cluster	O
node	O
.	O
</s>
<s>
Database	B-General_Concept
clusters	I-General_Concept
implement	O
multi-master	B-Operating_System
replication	I-Operating_System
using	O
one	O
of	O
two	O
methods	O
.	O
</s>
<s>
Asynchronous	O
multi-master	B-Operating_System
replication	I-Operating_System
commits	O
data	O
changes	O
to	O
a	O
deferred	O
transaction	B-General_Concept
queue	O
which	O
is	O
periodically	O
processed	O
on	O
all	O
databases	B-Application
in	O
the	O
cluster	O
.	O
</s>
<s>
Synchronous	O
multi-master	B-Operating_System
replication	I-Operating_System
uses	O
Oracle	B-General_Concept
's	O
two-phase	B-General_Concept
commit	I-General_Concept
functionality	O
to	O
ensure	O
that	O
all	O
databases	B-Application
with	O
the	O
cluster	O
have	O
a	O
consistent	O
dataset	B-General_Concept
.	O
</s>
<s>
Microsoft	B-Application
SQL	I-Application
provides	O
multi-master	B-Operating_System
replication	I-Operating_System
through	O
peer-to-peer	O
replication	B-Architecture
.	O
</s>
<s>
It	O
provides	O
a	O
scale-out	O
and	O
high-availability	B-General_Concept
solution	O
by	O
maintaining	O
copies	O
of	O
data	O
across	O
multiple	O
nodes	O
.	O
</s>
<s>
Built	O
on	O
the	O
foundation	O
of	O
transactional	O
replication	B-Architecture
,	O
peer-to-peer	O
replication	B-Architecture
propagates	O
transactionally	O
consistent	O
changes	O
in	O
near	O
real-time	O
.	O
</s>
<s>
At	O
a	O
basic	O
level	O
,	O
it	O
is	O
possible	O
to	O
achieve	O
a	O
multi-master	B-Operating_System
replication	I-Operating_System
scheme	O
beginning	O
with	O
MySQL	B-Application
version	O
3.23	O
with	O
circular	O
replication	B-Architecture
.	O
</s>
<s>
Departing	O
from	O
that	O
,	O
MariaDB	B-Application
and	O
MySQL	B-Application
ship	O
with	O
some	O
replication	B-Architecture
support	O
,	O
each	O
of	O
them	O
with	O
different	O
nuances	O
.	O
</s>
<s>
MariaDB	B-Application
:	O
natively	O
supports	O
multi-master	B-Operating_System
replication	I-Operating_System
since	O
version	O
10.0	O
,	O
but	O
conflict	O
resolution	O
is	O
not	O
supported	O
,	O
so	O
each	O
master	O
must	O
contain	O
different	O
databases	B-Application
.	O
</s>
<s>
On	O
MySQL	B-Application
,	O
this	O
is	O
named	O
multi-source	O
available	O
since	O
version	O
.	O
</s>
<s>
MySQL	B-Application
:	O
MySQL	B-Application
Group	O
Replication	B-Architecture
,	O
a	O
plugin	O
for	O
virtual	O
synchronous	O
multi-master	O
with	O
conflict	O
handling	O
and	O
distributed	O
recovery	O
was	O
released	O
with	O
.	O
</s>
<s>
MySQL	B-Operating_System
Cluster	I-Operating_System
supports	O
conflict	O
detection	O
and	O
resolution	O
between	O
multiple	O
masters	O
since	O
version	O
6.3	O
for	O
true	O
multi-master	O
capability	O
for	O
the	O
MySQL	B-Application
Server	I-Application
.	O
</s>
<s>
There	O
is	O
also	O
an	O
external	O
project	O
,	O
Galera	O
Cluster	O
created	O
by	O
,	O
that	O
provides	O
true	O
multi-master	O
capability	O
,	O
based	O
on	O
a	O
fork	O
of	O
the	O
InnoDB	O
storage	O
engine	O
and	O
custom	O
replication	B-Architecture
plug-ins	O
.	O
</s>
<s>
Replication	B-Architecture
is	O
synchronous	O
,	O
so	O
no	O
conflict	O
is	O
possible	O
.	O
</s>
<s>
Percona	B-Language
XtraDB	O
Cluster	O
also	O
is	O
a	O
combination	O
of	O
Galera	O
replication	B-Architecture
library	O
and	O
MySQL	B-Application
supporting	O
multi-master	O
.	O
</s>
<s>
Various	O
options	O
for	O
synchronous	O
multi-master	B-Operating_System
replication	I-Operating_System
exist	O
.	O
</s>
<s>
Postgres-XL	B-Language
which	O
is	O
available	O
under	O
the	O
Mozilla	O
Public	O
License	O
,	O
and	O
(	O
now	O
known	O
as	O
)	O
which	O
is	O
available	O
under	O
the	O
same	O
license	O
as	O
PostgreSQL	B-Application
itself	O
are	O
examples	O
.	O
</s>
<s>
The	O
replication	B-Architecture
documentation	O
for	O
PostgreSQL	B-Application
categorises	O
the	O
different	O
types	O
of	O
replication	B-Architecture
available	O
.	O
</s>
<s>
BDR	O
is	O
aimed	O
at	O
eventual	O
inclusion	O
in	O
PostgreSQL	B-Application
core	O
and	O
has	O
been	O
benchmarked	O
as	O
demonstrating	O
significantly	O
enhanced	O
performance	O
over	O
earlier	O
options	O
.	O
</s>
<s>
BDR	O
includes	O
replication	B-Architecture
of	O
data	O
writes	O
(	O
DML	O
)	O
,	O
as	O
well	O
as	O
changes	O
to	O
data	O
definition	O
(	O
DDL	O
)	O
and	O
global	O
sequences	O
.	O
</s>
<s>
The	O
latest	O
version	O
BDR	O
3.6	O
provides	O
column-level	O
conflict	O
detection	O
,	O
CRDTs	O
,	O
eager	O
replication	B-Architecture
,	O
multi-node	O
query	O
consistency	O
,	O
and	O
many	O
other	O
features	O
.	O
</s>
<s>
Within	O
Ingres	B-Application
Replicator	O
,	O
objects	O
that	O
are	O
updated	O
on	O
one	O
Ingres	B-Application
server	O
can	O
then	O
be	O
replicated	O
to	O
other	O
servers	O
whether	O
local	O
or	O
remote	O
through	O
multi-master	B-Operating_System
replication	I-Operating_System
.	O
</s>
<s>
It	O
is	O
not	O
required	O
for	O
all	O
Ingres	B-Application
servers	O
in	O
an	O
environment	O
to	O
replicate	O
with	O
each	O
other	O
as	O
this	O
could	O
cause	O
excessive	O
network	O
traffic	O
in	O
large	O
implementations	O
.	O
</s>
<s>
Instead	O
,	O
Ingres	B-Application
Replicator	O
allows	O
the	O
appropriate	O
data	O
to	O
be	O
replicated	O
to	O
the	O
appropriate	O
servers	O
without	O
excessive	O
replication	B-Architecture
traffic	O
.	O
</s>
<s>
This	O
means	O
that	O
some	O
servers	O
in	O
the	O
environment	O
can	O
serve	O
as	O
failover	O
candidates	O
while	O
other	O
servers	O
can	O
meet	O
other	O
requirements	O
such	O
as	O
managing	O
a	O
subset	O
of	O
columns	O
or	O
tables	O
for	O
a	O
departmental	O
solution	O
,	O
a	O
subset	O
of	O
rows	O
for	O
a	O
geographical	O
region	O
or	O
one-way	O
replication	B-Architecture
for	O
a	O
reporting	O
server	O
.	O
</s>
<s>
In	O
the	O
event	O
of	O
a	O
source	O
,	O
target	O
,	O
or	O
network	O
failure	O
,	O
data	O
integrity	O
is	O
enforced	O
through	O
this	O
two-phase	B-General_Concept
commit	I-General_Concept
protocol	I-General_Concept
by	O
ensuring	O
that	O
either	O
the	O
whole	O
transaction	B-General_Concept
is	O
replicated	O
,	O
or	O
none	O
of	O
it	O
is	O
.	O
</s>
<s>
In	O
addition	O
,	O
Ingres	B-Application
Replicator	O
can	O
operate	O
over	O
RDBMS	O
’s	O
from	O
multiple	O
vendors	O
to	O
connect	O
them	O
.	O
</s>
