<s>
A	O
wait-for	B-Operating_System
graph	I-Operating_System
in	O
computer	B-General_Concept
science	I-General_Concept
is	O
a	O
directed	O
graph	O
used	O
for	O
deadlock	B-Operating_System
detection	O
in	O
operating	B-General_Concept
systems	I-General_Concept
and	O
relational	B-Application
database	I-Application
systems	O
.	O
</s>
<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
system	O
that	O
allows	O
concurrent	O
operation	O
of	O
multiple	O
processes	O
and	O
locking	O
of	O
resources	O
and	O
which	O
does	O
not	O
provide	O
mechanisms	O
to	O
avoid	O
or	O
prevent	O
deadlock	B-Operating_System
must	O
support	O
a	O
mechanism	O
to	O
detect	O
deadlocks	B-Operating_System
and	O
an	O
algorithm	O
for	O
recovering	O
from	O
them	O
.	O
</s>
<s>
One	O
such	O
deadlock	B-Operating_System
detection	O
algorithm	O
makes	O
use	O
of	O
a	O
wait-for	B-Operating_System
graph	I-Operating_System
to	O
track	O
which	O
other	O
processes	O
a	O
process	O
is	O
currently	O
blocking	O
on	O
.	O
</s>
<s>
In	O
a	O
wait-for	B-Operating_System
graph	I-Operating_System
,	O
processes	O
are	O
represented	O
as	O
nodes	O
,	O
and	O
an	O
edge	O
from	O
process	O
to	O
implies	O
is	O
holding	O
a	O
resource	O
that	O
needs	O
and	O
thus	O
is	O
waiting	O
for	O
to	O
release	O
its	O
lock	O
on	O
that	O
resource	O
.	O
</s>
<s>
The	O
possibility	O
of	O
a	O
deadlock	B-Operating_System
is	O
implied	O
by	O
graph	O
cycles	O
in	O
the	O
conjunctive	O
case	O
,	O
and	O
by	O
knots	O
in	O
the	O
disjunctive	O
case	O
.	O
</s>
<s>
There	O
is	O
no	O
simple	O
algorithm	O
for	O
detecting	O
the	O
possibility	O
of	O
deadlock	B-Operating_System
in	O
the	O
final	O
case	O
.	O
</s>
