<s>
Optimistic	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
(	O
OCC	O
)	O
,	O
also	O
known	O
as	O
optimistic	B-Operating_System
locking	I-Operating_System
,	O
is	O
a	O
concurrency	B-Operating_System
control	I-Operating_System
method	O
applied	O
to	O
transactional	O
systems	O
such	O
as	O
relational	B-Application
database	I-Application
management	I-Application
systems	I-Application
and	O
software	B-Operating_System
transactional	I-Operating_System
memory	I-Operating_System
.	O
</s>
<s>
Optimistic	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
was	O
first	O
proposed	O
by	O
H	O
.	O
T	O
.	O
Kung	O
and	O
John	O
T	O
.	O
Robinson	O
.	O
</s>
<s>
OCC	O
is	O
generally	O
used	O
in	O
environments	O
with	O
low	O
data	B-General_Concept
contention	I-General_Concept
.	O
</s>
<s>
When	O
conflicts	O
are	O
rare	O
,	O
transactions	O
can	O
complete	O
without	O
the	O
expense	O
of	O
managing	O
locks	O
and	O
without	O
having	O
transactions	O
wait	O
for	O
other	O
transactions	O
 '	O
locks	O
to	O
clear	O
,	O
leading	O
to	O
higher	O
throughput	O
than	O
other	O
concurrency	B-Operating_System
control	I-Operating_System
methods	O
.	O
</s>
<s>
However	O
,	O
if	O
contention	O
for	O
data	O
resources	O
is	O
frequent	O
,	O
the	O
cost	O
of	O
repeatedly	O
restarting	O
transactions	O
hurts	O
performance	O
significantly	O
,	O
in	O
which	O
case	O
other	O
concurrency	B-Operating_System
control	I-Operating_System
methods	O
may	O
be	O
better	O
suited	O
.	O
</s>
<s>
Optimistic	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
transactions	O
involve	O
these	O
phases	O
:	O
</s>
<s>
Care	O
must	O
be	O
taken	O
to	O
avoid	O
a	O
time-of-check	B-Error_Name
to	I-Error_Name
time-of-use	I-Error_Name
bug	O
,	O
particularly	O
if	O
this	O
phase	O
and	O
the	O
previous	O
one	O
are	O
not	O
performed	O
as	O
a	O
single	O
atomic	B-General_Concept
operation	I-General_Concept
.	O
</s>
<s>
The	O
stateless	B-Protocol
nature	O
of	O
HTTP	B-Protocol
makes	O
locking	O
infeasible	O
for	O
web	O
user	O
interfaces	O
.	O
</s>
<s>
HTTP	B-Protocol
does	O
provide	O
a	O
form	B-General_Concept
of	O
built-in	O
OCC	O
.	O
</s>
<s>
The	O
response	O
to	O
an	O
initial	O
GET	O
request	O
can	O
include	O
an	O
ETag	B-Protocol
for	O
subsequent	O
PUT	O
requests	O
to	O
use	O
in	O
the	O
If-Match	O
header	O
.	O
</s>
<s>
Any	O
PUT	O
requests	O
with	O
an	O
out-of-date	O
ETag	B-Protocol
in	O
the	O
If-Match	O
header	O
can	O
then	O
be	O
rejected	O
.	O
</s>
<s>
Some	O
database	B-Application
management	I-Application
systems	I-Application
offer	O
OCC	O
natively	O
,	O
without	O
requiring	O
special	O
application	O
code	O
.	O
</s>
<s>
In	O
such	O
cases	O
,	O
the	O
form	B-General_Concept
may	O
include	O
a	O
hidden	O
field	O
with	O
the	O
record	O
's	O
original	O
content	O
,	O
a	O
timestamp	O
,	O
a	O
sequence	O
number	O
,	O
or	O
an	O
opaque	O
token	O
.	O
</s>
<s>
MediaWiki	B-Application
's	O
edit	O
pages	O
use	O
OCC	O
.	O
</s>
<s>
Bugzilla	B-Application
uses	O
OCC	O
;	O
edit	B-Operating_System
conflicts	I-Operating_System
are	O
called	O
"	O
mid-air	O
collisions	O
"	O
.	O
</s>
<s>
The	O
Ruby	B-Application
on	I-Application
Rails	I-Application
framework	O
has	O
an	O
API	O
for	O
OCC	O
.	O
</s>
<s>
The	O
Grails	B-Language
framework	O
uses	O
OCC	O
in	O
its	O
default	O
conventions	O
.	O
</s>
<s>
The	O
GT.M	B-Application
database	O
engine	O
uses	O
OCC	O
for	O
managing	O
transactions	O
(	O
even	O
single	O
updates	O
are	O
treated	O
as	O
mini-transactions	O
)	O
.	O
</s>
<s>
Microsoft	O
's	O
Entity	B-Application
Framework	I-Application
(	O
including	O
Code-First	O
)	O
has	O
built-in	O
support	O
for	O
OCC	O
based	O
on	O
a	O
binary	O
timestamp	O
value	O
.	O
</s>
<s>
Mimer	B-Application
SQL	I-Application
is	O
a	O
DBMS	B-General_Concept
that	O
only	O
implements	O
optimistic	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
.	O
</s>
<s>
Google	B-Application
App	I-Application
Engine	I-Application
data	O
store	O
uses	O
OCC	O
.	O
</s>
<s>
The	O
Apache	B-Language
Solr	I-Language
search	O
engine	O
supports	O
OCC	O
via	O
the	O
_version_	O
field	O
.	O
</s>
<s>
The	O
Elasticsearch	B-Application
search	O
engine	O
supports	O
OCC	O
via	O
the	O
version	O
attribute	O
.	O
</s>
<s>
CouchDB	B-Application
implements	O
OCC	O
through	O
document	O
revisions	O
.	O
</s>
<s>
The	O
MonetDB	B-Application
column-oriented	B-General_Concept
database	I-General_Concept
management	O
system	O
's	O
transaction	O
management	O
scheme	O
is	O
based	O
on	O
OCC	O
.	O
</s>
<s>
Most	O
implementations	O
of	O
software	B-Operating_System
transactional	I-Operating_System
memory	I-Operating_System
use	O
OCC	O
.	O
</s>
<s>
Redis	B-Operating_System
provides	O
OCC	O
through	O
WATCH	O
command	O
.	O
</s>
<s>
Firebird	B-Application
uses	O
Multi-generational	B-Operating_System
architecture	I-Operating_System
as	O
an	O
implementation	O
of	O
OCC	O
for	O
data	O
management	O
.	O
</s>
<s>
DynamoDB	B-Application
uses	O
conditional	O
update	O
as	O
an	O
implementation	O
of	O
OCC	O
.	O
</s>
<s>
Kubernetes	B-General_Concept
uses	O
OCC	O
when	O
updating	O
resources	O
.	O
</s>
<s>
YugabyteDB	B-Operating_System
is	O
a	O
cloud-native	O
database	O
that	O
primarily	O
uses	O
OCC	O
.	O
</s>
