<s>
A	O
quorum	B-General_Concept
is	O
the	O
minimum	O
number	O
of	O
votes	O
that	O
a	O
distributed	O
transaction	B-General_Concept
has	O
to	O
obtain	O
in	O
order	O
to	O
be	O
allowed	O
to	O
perform	O
an	O
operation	O
in	O
a	O
distributed	B-Architecture
system	I-Architecture
.	O
</s>
<s>
A	O
quorum-based	O
technique	O
is	O
implemented	O
to	O
enforce	O
consistent	O
operation	O
in	O
a	O
distributed	B-Architecture
system	I-Architecture
.	O
</s>
<s>
Quorum-based	O
voting	O
can	O
be	O
used	O
as	O
a	O
replica	B-Architecture
control	O
method	O
,	O
</s>
<s>
as	O
well	O
as	O
a	O
commit	B-Language
method	O
to	O
ensure	O
transaction	B-General_Concept
atomicity	B-General_Concept
in	O
the	O
presence	O
of	O
network	B-Algorithm
partitioning	I-Algorithm
.	O
</s>
<s>
In	O
a	O
distributed	O
database	O
system	O
,	O
a	O
transaction	B-General_Concept
could	O
execute	O
its	O
operations	O
at	O
multiple	O
sites	O
.	O
</s>
<s>
Since	O
atomicity	B-General_Concept
requires	O
every	O
distributed	O
transaction	B-General_Concept
to	O
be	O
atomic	O
,	O
the	O
transaction	B-General_Concept
must	O
have	O
the	O
same	O
fate	O
(	O
commit	B-Language
or	O
abort	B-Application
)	O
at	O
every	O
site	O
.	O
</s>
<s>
In	O
case	O
of	O
network	B-Algorithm
partitioning	I-Algorithm
,	O
sites	O
are	O
partitioned	O
and	O
the	O
partitions	O
may	O
not	O
be	O
able	O
to	O
communicate	O
with	O
each	O
other	O
.	O
</s>
<s>
This	O
is	O
where	O
a	O
quorum-based	O
technique	O
comes	O
in	O
.	O
</s>
<s>
The	O
fundamental	O
idea	O
is	O
that	O
a	O
transaction	B-General_Concept
is	O
executed	O
if	O
the	O
majority	O
of	O
sites	O
vote	O
to	O
execute	O
it	O
.	O
</s>
<s>
Let	O
us	O
assume	O
that	O
the	O
total	O
number	O
of	O
votes	O
in	O
the	O
system	O
is	O
V	O
and	O
the	O
abort	B-Application
and	O
commit	B-Language
quorums	B-General_Concept
are	O
Va	O
and	O
Vc	O
,	O
respectively	O
.	O
</s>
<s>
Then	O
the	O
following	O
rules	O
must	O
be	O
obeyed	O
in	O
the	O
implementation	O
of	O
the	O
commit	B-Language
protocol	O
:	O
</s>
<s>
Before	O
a	O
transaction	B-General_Concept
commits	O
,	O
it	O
must	O
obtain	O
a	O
commit	B-Language
quorum	B-General_Concept
Vc.The	O
total	O
of	O
at	O
least	O
one	O
site	O
that	O
is	O
prepared	O
to	O
commit	B-Language
and	O
zero	O
or	O
more	O
sites	O
waiting	O
Vc	O
.	O
</s>
<s>
Before	O
a	O
transaction	B-General_Concept
aborts	O
,	O
it	O
must	O
obtain	O
an	O
abort	B-Application
quorum	B-General_Concept
VaThe	O
total	O
of	O
zero	O
or	O
more	O
sites	O
that	O
are	O
prepared	O
to	O
abort	B-Application
or	O
any	O
sites	O
waiting	O
Va	O
.	O
</s>
<s>
The	O
first	O
rule	O
ensures	O
that	O
a	O
transaction	B-General_Concept
cannot	O
be	O
committed	O
and	O
aborted	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
The	O
next	O
two	O
rules	O
indicate	O
the	O
votes	O
that	O
a	O
transaction	B-General_Concept
has	O
to	O
obtain	O
before	O
it	O
can	O
terminate	O
one	O
way	O
or	O
the	O
other	O
.	O
</s>
<s>
To	O
ensure	O
serializability	B-Operating_System
,	O
no	O
two	O
transactions	O
should	O
be	O
allowed	O
to	O
read	O
or	O
write	O
a	O
data	O
item	O
concurrently	O
.	O
</s>
<s>
In	O
case	O
of	O
replicated	O
databases	O
,	O
a	O
quorum-based	O
replica	B-Architecture
control	O
protocol	O
can	O
be	O
used	O
to	O
ensure	O
that	O
no	O
two	O
copies	O
of	O
a	O
data	O
item	O
are	O
read	O
or	O
written	O
by	O
two	O
transactions	O
concurrently	O
.	O
</s>
<s>
The	O
quorum-based	O
voting	O
for	O
replica	B-Architecture
control	O
is	O
due	O
to	O
[	O
Gifford	O
,	O
1979 ]	O
.	O
</s>
<s>
Each	O
operation	O
then	O
has	O
to	O
obtain	O
a	O
read	O
quorum	B-General_Concept
(	O
Vr	O
)	O
or	O
a	O
write	O
quorum	B-General_Concept
(	O
Vw	O
)	O
to	O
read	O
or	O
write	O
a	O
data	O
item	O
,	O
respectively	O
.	O
</s>
<s>
If	O
a	O
given	O
data	O
item	O
has	O
a	O
total	O
of	O
V	O
votes	O
,	O
the	O
quorums	B-General_Concept
have	O
to	O
obey	O
the	O
following	O
rules	O
:	O
</s>
<s>
Additionally	O
,	O
it	O
ensures	O
that	O
a	O
read	O
quorum	B-General_Concept
contains	O
at	O
least	O
one	O
site	O
with	O
the	O
newest	O
version	O
of	O
the	O
data	O
item	O
.	O
</s>
<s>
The	O
two	O
rules	O
ensure	O
that	O
one-copy	B-Operating_System
serializability	I-Operating_System
is	O
maintained	O
.	O
</s>
