<s>
A	O
precedence	B-General_Concept
graph	I-General_Concept
,	O
also	O
named	O
conflict	O
graph	O
and	O
serializability	B-Operating_System
graph	O
,	O
is	O
used	O
in	O
the	O
context	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
in	O
databases	O
.	O
</s>
<s>
The	O
precedence	B-General_Concept
graph	I-General_Concept
for	O
a	O
schedule	B-General_Concept
S	O
contains	O
:	O
</s>
<s>
A	O
precedence	B-General_Concept
graph	I-General_Concept
of	O
the	O
schedule	B-General_Concept
D	O
,	O
with	O
3	O
transactions	O
.	O
</s>
<s>
As	O
there	O
is	O
a	O
cycle	O
(	O
of	O
length	O
2	O
;	O
with	O
two	O
edges	O
)	O
through	O
the	O
committed	O
transactions	O
T1	O
and	O
T2	O
,	O
this	O
schedule	B-General_Concept
(	O
history	O
)	O
is	O
not	O
Conflict	O
serializable	B-Operating_System
.	O
</s>
<s>
Notice	O
,	O
that	O
the	O
commit	O
of	O
Transaction	O
2	O
does	O
not	O
have	O
any	O
meaning	O
regarding	O
the	O
creation	O
of	O
a	O
precedence	B-General_Concept
graph	I-General_Concept
.	O
</s>
<s>
Algorithm	O
to	O
test	O
Conflict	O
Serializability	B-Operating_System
of	O
a	O
Schedule	B-General_Concept
S	O
along	O
with	O
an	O
example	O
schedule	B-General_Concept
.	O
</s>
<s>
For	O
each	O
transaction	O
Tx	O
participating	O
in	O
schedule	B-General_Concept
S	O
,	O
create	O
a	O
node	O
labeled	O
Ti	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
.	O
</s>
<s>
Thus	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
contains	O
T1	O
,	O
T2	O
,	O
T3	O
.	O
</s>
<s>
For	O
each	O
case	O
in	O
S	O
where	O
Tj	O
executes	O
a	O
read_item(X )	O
after	O
Ti	O
executes	O
a	O
write_item(X )	O
,	O
create	O
an	O
edge	O
(	O
Ti	O
→	O
Tj	O
)	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
.	O
</s>
<s>
For	O
each	O
case	O
in	O
S	O
where	O
Tj	O
executes	O
a	O
write_item(X )	O
after	O
Ti	O
executes	O
a	O
read_item(X )	O
,	O
create	O
an	O
edge	O
(	O
Ti	O
→	O
Tj	O
)	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
.	O
</s>
<s>
For	O
each	O
case	O
in	O
S	O
where	O
Tj	O
executes	O
a	O
write_item(X )	O
after	O
Ti	O
executes	O
a	O
write_item(X )	O
,	O
create	O
an	O
edge	O
(	O
Ti	O
→	O
Tj	O
)	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
.	O
</s>
<s>
The	O
schedule	B-General_Concept
S	O
is	O
serializable	B-Operating_System
if	O
and	O
only	O
if	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
has	O
no	O
cycles	O
.	O
</s>
<s>
As	O
T1	O
and	O
T2	O
constitute	O
a	O
cycle	O
,	O
the	O
above	O
example	O
is	O
not	O
(	O
conflict	O
)	O
serializable	B-Operating_System
.	O
</s>
