<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
multiple	B-Operating_System
granularity	I-Operating_System
locking	I-Operating_System
(	O
MGL	O
)	O
is	O
a	O
locking	B-Operating_System
method	O
used	O
in	O
database	B-Application
management	I-Application
systems	I-Application
(	O
DBMS	O
)	O
and	O
relational	B-Application
databases	I-Application
.	O
</s>
<s>
In	O
multiple	B-Operating_System
granularity	I-Operating_System
locking	I-Operating_System
,	O
locks	O
are	O
set	O
on	O
objects	O
that	O
contain	O
other	O
objects	O
.	O
</s>
<s>
This	O
can	O
be	O
thought	O
of	O
as	O
a	O
tree	B-Application
of	O
objects	O
,	O
where	O
each	O
node	B-Data_Structure
contains	O
its	O
children	O
.	O
</s>
<s>
A	O
lock	O
on	O
this	O
structure	O
(	O
such	O
as	O
a	O
shared	O
or	O
exclusive	O
lock	O
)	O
locks	O
the	O
targeted	O
node	B-Data_Structure
as	O
well	O
as	O
all	O
of	O
its	O
descendants	O
.	O
</s>
<s>
Multiple	B-Operating_System
granularity	I-Operating_System
locking	I-Operating_System
is	O
usually	O
used	O
with	O
non-strict	B-Operating_System
two-phase	I-Operating_System
locking	I-Operating_System
to	O
guarantee	O
serializability	B-Operating_System
.	O
</s>
<s>
In	O
addition	O
to	O
shared	O
(	O
S	O
)	O
locks	O
and	O
exclusive	O
(	O
X	O
)	O
locks	O
from	O
other	O
locking	B-Operating_System
schemes	O
,	O
like	O
strict	B-Operating_System
two-phase	I-Operating_System
locking	I-Operating_System
,	O
MGL	O
also	O
uses	O
intentional	O
"	O
locks	O
"	O
,	O
which	O
do	O
not	O
directly	O
lock	O
a	O
node	B-Data_Structure
,	O
but	O
instead	O
denote	O
the	O
existence	O
,	O
or	O
intent	O
to	O
add	O
,	O
a	O
lock	O
of	O
the	O
specified	O
type	O
lower	O
in	O
the	O
node	B-Data_Structure
hierarchy	O
.	O
</s>
<s>
To	O
lock	O
a	O
node	B-Data_Structure
in	O
S	O
(	O
or	O
X	O
)	O
,	O
MGL	O
has	O
the	O
transaction	B-General_Concept
lock	O
on	O
all	O
of	O
its	O
ancestors	O
with	O
IS	O
(	O
or	O
IX	O
)	O
,	O
so	O
if	O
a	O
transaction	B-General_Concept
locks	O
a	O
node	B-Data_Structure
in	O
S	O
(	O
or	O
X	O
)	O
,	O
no	O
other	O
transaction	B-General_Concept
can	O
access	O
its	O
ancestors	O
in	O
X	O
(	O
or	O
S	O
and	O
X	O
)	O
.	O
</s>
<s>
Determining	O
what	O
level	O
of	O
granularity	O
to	O
use	O
for	O
locking	B-Operating_System
is	O
done	O
by	O
locking	B-Operating_System
the	O
finest	O
level	O
possible	O
(	O
i.e.	O
,	O
at	O
the	O
lowest	O
leaf	O
)	O
,	O
and	O
then	O
escalating	O
these	O
locks	O
to	O
higher	O
levels	O
in	O
the	O
file	O
hierarchy	O
to	O
cover	O
more	O
records	O
or	O
file	O
elements	O
as	O
needed	O
in	O
a	O
process	O
known	O
as	O
"	O
lock	O
escalation	O
"	O
.	O
</s>
<s>
MGL	O
locking	B-Operating_System
modes	O
are	O
compatible	O
with	O
each	O
other	O
as	O
defined	O
in	O
the	O
following	O
matrix	O
.	O
</s>
<s>
Following	O
the	O
locking	B-Operating_System
protocol	O
and	O
the	O
compatibility	O
matrix	O
,	O
if	O
one	O
transaction	B-General_Concept
holds	O
a	O
node	B-Data_Structure
in	O
S	O
mode	O
,	O
no	O
other	O
transactions	O
can	O
have	O
locked	O
any	O
ancestor	O
in	O
X	O
mode	O
.	O
</s>
