<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
Lease	B-Operating_System
is	O
a	O
contract	O
that	O
gives	O
its	O
holder	O
specified	O
rights	O
to	O
some	O
resource	O
for	O
a	O
limited	O
period	O
.	O
</s>
<s>
Because	O
it	O
is	O
time-limited	O
,	O
a	O
lease	B-Operating_System
is	O
an	O
alternative	O
to	O
a	O
lock	B-Operating_System
for	O
resource	O
serialization	B-Application
.	O
</s>
<s>
A	O
traditional	O
resource	O
lock	B-Operating_System
is	O
granted	O
until	O
it	O
is	O
explicitly	O
released	O
by	O
the	O
locking	B-Operating_System
client	O
process	O
.	O
</s>
<s>
Reasons	O
why	O
a	O
lock	B-Operating_System
might	O
not	O
be	O
released	O
include	O
:	O
</s>
<s>
By	O
contract	O
,	O
a	O
lease	B-Operating_System
is	O
valid	O
for	O
a	O
limited	O
period	O
,	O
after	O
which	O
it	O
automatically	O
expires	O
,	O
making	O
the	O
resource	O
available	O
for	O
reallocation	O
by	O
a	O
new	O
client	O
.	O
</s>
<s>
The	O
term	O
'	O
lease	B-Operating_System
 '	O
was	O
applied	O
to	O
this	O
concept	O
in	O
a	O
1989	O
paper	O
by	O
Cary	O
G	O
.	O
Gray	O
and	O
David	O
R	O
.	O
Cheriton	O
,	O
but	O
similar	O
concepts	O
(	O
expiring	O
tokens	O
and	O
breakable	O
locks	O
with	O
timeouts	O
)	O
had	O
been	O
used	O
in	O
prior	O
systems	O
.	O
</s>
<s>
Leases	B-Operating_System
are	O
commonly	O
used	O
in	O
distributed	O
systems	O
for	O
applications	O
ranging	O
from	O
DHCP	B-Protocol
address	I-Protocol
allocation	I-Protocol
to	O
file	B-Application
locking	I-Application
,	O
but	O
they	O
are	O
not	O
(	O
by	O
themselves	O
)	O
a	O
complete	O
solution	O
:	O
</s>
<s>
There	O
must	O
be	O
some	O
means	O
of	O
notifying	O
the	O
lease	B-Operating_System
holder	O
of	O
the	O
expiration	O
and	O
preventing	O
that	O
agent	O
from	O
continuing	O
to	O
rely	O
on	O
the	O
resource	O
.	O
</s>
<s>
Often	O
,	O
this	O
is	O
done	O
by	O
requiring	O
all	O
requests	O
to	O
be	O
accompanied	O
by	O
an	O
access	O
token	O
,	O
which	O
is	O
invalidated	O
if	O
the	O
associated	O
lease	B-Operating_System
has	O
expired	O
.	O
</s>
<s>
If	O
a	O
lease	B-Operating_System
is	O
revoked	O
after	O
the	O
lease	B-Operating_System
holder	O
has	O
started	O
operating	O
on	O
the	O
resource	O
,	O
revocation	O
may	O
leave	O
the	O
resource	O
in	O
a	O
compromised	O
state	O
.	O
</s>
<s>
In	O
such	O
situations	O
,	O
it	O
is	O
common	O
to	O
use	O
Atomic	B-General_Concept
transactions	I-General_Concept
to	O
ensure	O
that	O
updates	O
that	O
do	O
not	O
complete	O
have	O
no	O
effect	O
.	O
</s>
