<s>
Multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
(	O
MCC	O
or	O
MVCC	O
)	O
,	O
is	O
a	O
concurrency	B-Operating_System
control	I-Operating_System
method	O
commonly	O
used	O
by	O
database	B-Application
management	I-Application
systems	I-Application
to	O
provide	O
concurrent	B-Operating_System
access	I-Operating_System
to	O
the	O
database	O
and	O
in	O
programming	O
languages	O
to	O
implement	O
transactional	B-Operating_System
memory	I-Operating_System
.	O
</s>
<s>
Without	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
if	O
someone	O
is	O
reading	O
from	O
a	O
database	O
at	O
the	O
same	O
time	O
as	O
someone	O
else	O
is	O
writing	O
to	O
it	O
,	O
it	O
is	O
possible	O
that	O
the	O
reader	O
will	O
see	O
a	O
half-written	O
or	O
inconsistent	B-General_Concept
piece	O
of	O
data	O
.	O
</s>
<s>
Isolation	O
is	O
implemented	O
by	O
means	O
of	O
a	O
concurrency	B-Operating_System
control	I-Operating_System
protocol	O
.	O
</s>
<s>
Locks	O
are	O
known	O
to	O
create	O
contention	B-General_Concept
especially	O
between	O
long	O
read	O
transactions	O
and	O
update	O
transactions	O
.	O
</s>
<s>
Any	O
changes	O
made	O
by	O
a	O
writer	O
will	O
not	O
be	O
seen	O
by	O
other	O
users	O
of	O
the	O
database	O
until	O
the	O
changes	O
have	O
been	O
completed	O
(	O
or	O
,	O
in	O
database	O
terms	O
:	O
until	O
the	O
transaction	B-General_Concept
has	O
been	O
committed	O
.	O
)	O
</s>
<s>
The	O
version	O
that	O
each	O
transaction	B-General_Concept
sees	O
depends	O
on	O
the	O
isolation	B-General_Concept
level	I-General_Concept
implemented	O
.	O
</s>
<s>
The	O
most	O
common	O
isolation	B-General_Concept
level	I-General_Concept
implemented	O
with	O
MVCC	O
is	O
snapshot	B-Operating_System
isolation	I-Operating_System
.	O
</s>
<s>
With	O
snapshot	B-Operating_System
isolation	I-Operating_System
,	O
a	O
transaction	B-General_Concept
observes	O
a	O
state	O
of	O
the	O
data	O
as	O
of	O
when	O
the	O
transaction	B-General_Concept
started	O
.	O
</s>
<s>
Read	O
transactions	O
under	O
MVCC	O
typically	O
use	O
a	O
timestamp	O
or	O
transaction	B-General_Concept
ID	O
to	O
determine	O
what	O
state	O
of	O
the	O
DB	O
to	O
read	O
,	O
and	O
read	O
these	O
versions	O
of	O
the	O
data	O
.	O
</s>
<s>
Read	O
and	O
write	O
transactions	O
are	O
thus	O
isolated	B-General_Concept
from	O
each	O
other	O
without	O
any	O
need	O
for	O
locking	O
.	O
</s>
<s>
However	O
,	O
despite	O
locks	O
being	O
unnecessary	O
,	O
they	O
are	O
used	O
by	O
some	O
MVCC	O
databases	B-Application
such	O
as	O
Oracle	O
.	O
</s>
<s>
PostgreSQL	B-Application
can	O
use	O
this	O
approach	O
with	O
its	O
process	O
.	O
</s>
<s>
Other	O
databases	B-Application
split	O
the	O
storage	O
blocks	O
into	O
two	O
parts	O
:	O
the	O
data	O
part	O
and	O
an	O
undo	O
log	O
.	O
</s>
<s>
For	O
a	O
document-oriented	B-General_Concept
database	I-General_Concept
it	O
also	O
allows	O
the	O
system	O
to	O
optimize	O
documents	O
by	O
writing	O
entire	O
documents	O
onto	O
contiguous	O
sections	O
of	O
disk	O
—	O
when	O
updated	O
,	O
the	O
entire	O
document	O
can	O
be	O
re-written	O
rather	O
than	O
bits	O
and	O
pieces	O
cut	O
out	O
or	O
maintained	O
in	O
a	O
linked	O
,	O
non-contiguous	O
database	O
structure	O
.	O
</s>
<s>
MVCC	O
uses	O
timestamps	O
(	O
TS	O
)	O
,	O
and	O
incrementing	O
transaction	B-General_Concept
IDs	O
,	O
to	O
achieve	O
transactional	O
consistency	B-General_Concept
.	O
</s>
<s>
MVCC	O
ensures	O
a	O
transaction	B-General_Concept
(	O
T	O
)	O
never	O
has	O
to	O
wait	O
to	O
Read	O
a	O
database	O
object	O
(	O
P	O
)	O
by	O
maintaining	O
several	O
versions	O
of	O
the	O
object	O
.	O
</s>
<s>
Each	O
version	O
of	O
object	O
P	O
has	O
both	O
a	O
Read	O
Timestamp	O
(	O
RTS	O
)	O
and	O
a	O
Write	O
Timestamp	O
(	O
WTS	O
)	O
which	O
lets	O
a	O
particular	O
transaction	B-General_Concept
Ti	O
read	O
the	O
most	O
recent	O
version	O
of	O
the	O
object	O
which	O
precedes	O
the	O
transaction	B-General_Concept
's	O
Read	O
Timestamp	O
RTS(Ti )	O
.	O
</s>
<s>
Like	O
standing	O
in	O
line	O
at	O
the	O
store	O
,	O
you	O
cannot	O
complete	O
your	O
checkout	O
transaction	B-General_Concept
until	O
those	O
in	O
front	O
of	O
you	O
have	O
completed	O
theirs	O
.	O
</s>
<s>
To	O
restate	O
;	O
every	O
object	O
(	O
P	O
)	O
has	O
a	O
Timestamp	O
(	O
TS	O
)	O
,	O
however	O
if	O
transaction	B-General_Concept
Ti	O
wants	O
to	O
Write	O
to	O
an	O
object	O
,	O
and	O
the	O
transaction	B-General_Concept
has	O
a	O
Timestamp	O
(	O
TS	O
)	O
that	O
is	O
earlier	O
than	O
the	O
object	O
's	O
current	O
Read	O
Timestamp	O
,	O
TS(Ti )	O
<	O
RTS(P )	O
,	O
then	O
the	O
transaction	B-General_Concept
is	O
aborted	O
and	O
restarted	O
.	O
</s>
<s>
(	O
This	O
is	O
because	O
a	O
later	O
transaction	B-General_Concept
already	O
depends	O
on	O
the	O
old	O
value	O
.	O
)	O
</s>
<s>
Otherwise	O
,	O
Ti	O
creates	O
a	O
new	O
version	O
of	O
object	O
P	O
and	O
sets	O
the	O
read/write	O
timestamp	O
TS	O
of	O
the	O
new	O
version	O
to	O
the	O
timestamp	O
of	O
the	O
transaction	B-General_Concept
TS	O
←	O
TS(Ti )	O
.	O
</s>
<s>
MVCC	O
is	O
particularly	O
adept	O
at	O
implementing	O
true	O
snapshot	B-Operating_System
isolation	I-Operating_System
,	O
something	O
which	O
other	O
methods	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
frequently	O
do	O
either	O
incompletely	O
or	O
with	O
high	O
performance	O
costs	O
.	O
</s>
<s>
If	O
a	O
long	O
running	O
transaction	B-General_Concept
T2	O
starts	O
a	O
read	O
operation	O
of	O
Object	O
2	O
and	O
Object	O
1	O
after	O
T1	O
committed	O
and	O
there	O
is	O
a	O
concurrent	O
update	O
transaction	B-General_Concept
T3	O
which	O
deletes	O
Object	O
2	O
and	O
adds	O
Object	O
3	O
=	O
"	O
Foo-Bar	O
"	O
,	O
the	O
database	O
state	O
will	O
look	O
like	O
this	O
at	O
time	O
2	O
:	O
</s>
<s>
This	O
is	O
how	O
multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
allows	O
snapshot	B-Operating_System
isolation	I-Operating_System
reads	O
without	O
any	O
locks	O
.	O
</s>
<s>
Multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
is	O
described	O
in	O
some	O
detail	O
in	O
the	O
1981	O
paper	O
"	O
Concurrency	B-Operating_System
Control	I-Operating_System
in	O
Distributed	O
Database	B-Application
Systems	I-Application
"	O
by	O
Phil	O
Bernstein	O
and	O
Nathan	O
Goodman	O
,	O
then	O
employed	O
by	O
the	O
Computer	O
Corporation	O
of	O
America	O
.	O
</s>
<s>
Starkey	O
went	O
on	O
to	O
create	O
the	O
second	O
commercially	O
successful	O
MVCC	O
database	O
-	O
InterBase	B-Application
.	O
</s>
