<s>
In	O
concurrency	B-Operating_System
control	I-Operating_System
of	O
databases	O
,	O
transaction	B-General_Concept
processing	I-General_Concept
(	O
transaction	O
management	O
)	O
,	O
and	O
various	O
transactional	B-General_Concept
applications	O
(	O
e.g.	O
,	O
transactional	B-Operating_System
memory	I-Operating_System
and	O
software	B-Operating_System
transactional	I-Operating_System
memory	I-Operating_System
)	O
,	O
both	O
centralized	O
and	O
distributed	B-Architecture
,	O
a	O
transaction	O
schedule	B-General_Concept
is	O
serializable	B-Operating_System
if	O
its	O
outcome	O
(	O
e.g.	O
,	O
the	O
resulting	O
database	O
state	O
)	O
is	O
equal	O
to	O
the	O
outcome	O
of	O
its	O
transactions	B-General_Concept
executed	O
serially	O
,	O
i.e.	O
</s>
<s>
Transactions	B-General_Concept
are	O
normally	O
executed	O
concurrently	O
(	O
they	O
overlap	O
)	O
,	O
since	O
this	O
is	O
the	O
most	O
efficient	O
way	O
.	O
</s>
<s>
Serializability	B-Operating_System
is	O
the	O
major	O
correctness	O
criterion	O
for	O
concurrent	O
transactions	B-General_Concept
 '	O
executions	O
.	O
</s>
<s>
It	O
is	O
considered	O
the	O
highest	O
level	O
of	O
isolation	B-General_Concept
between	O
transactions	B-General_Concept
,	O
and	O
plays	O
an	O
essential	O
role	O
in	O
concurrency	B-Operating_System
control	I-Operating_System
.	O
</s>
<s>
Strong	B-Operating_System
strict	I-Operating_System
two-phase	I-Operating_System
locking	I-Operating_System
(	O
SS2PL	O
)	O
is	O
a	O
popular	O
serializability	B-Operating_System
mechanism	O
utilized	O
in	O
most	O
of	O
the	O
database	O
systems	O
(	O
in	O
various	O
variants	O
)	O
since	O
their	O
early	O
days	O
in	O
the	O
1970s	O
.	O
</s>
<s>
Serializability	B-Operating_System
theory	I-Operating_System
provides	O
the	O
formal	O
framework	O
to	O
reason	O
about	O
and	O
analyze	O
serializability	B-Operating_System
and	O
its	O
techniques	O
.	O
</s>
<s>
Serializability	B-Operating_System
is	O
used	O
to	O
keep	O
the	O
data	O
in	O
the	O
data	O
item	O
in	O
a	O
consistent	O
state	O
.	O
</s>
<s>
Serializability	B-Operating_System
is	O
a	O
property	O
of	O
a	O
transaction	O
schedule	B-General_Concept
(	O
history	O
)	O
.	O
</s>
<s>
It	O
relates	O
to	O
the	O
isolation	B-General_Concept
property	O
of	O
a	O
database	B-General_Concept
transaction	I-General_Concept
.	O
</s>
<s>
Serializability	B-Operating_System
of	O
a	O
schedule	B-General_Concept
means	O
equivalence	O
(	O
in	O
the	O
outcome	O
,	O
the	O
database	O
state	O
,	O
data	O
values	O
)	O
to	O
a	O
serial	O
schedule	B-General_Concept
(	O
i.e.	O
,	O
sequential	O
with	O
no	O
transaction	O
overlap	O
in	O
time	O
)	O
with	O
the	O
same	O
transactions	B-General_Concept
.	O
</s>
<s>
It	O
is	O
the	O
major	O
criterion	O
for	O
the	O
correctness	O
of	O
concurrent	O
transactions	B-General_Concept
 '	O
schedule	B-General_Concept
,	O
and	O
thus	O
supported	O
in	O
all	O
general	O
purpose	O
database	O
systems	O
.	O
</s>
<s>
The	O
rationale	O
behind	O
serializability	B-Operating_System
is	O
the	O
following	O
:	O
</s>
<s>
If	O
each	O
transaction	O
is	O
correct	O
by	O
itself	O
,	O
i.e.	O
,	O
meets	O
certain	O
integrity	O
conditions	O
,	O
then	O
a	O
schedule	B-General_Concept
that	O
comprises	O
any	O
serial	O
execution	O
of	O
these	O
transactions	B-General_Concept
is	O
correct	O
(	O
its	O
transactions	B-General_Concept
still	O
meet	O
their	O
conditions	O
)	O
:	O
"	O
Serial	O
"	O
means	O
that	O
transactions	B-General_Concept
do	O
not	O
overlap	O
in	O
time	O
and	O
cannot	O
interfere	O
with	O
each	O
other	O
,	O
i.e	O
,	O
complete	O
isolation	B-General_Concept
between	O
each	O
other	O
exists	O
.	O
</s>
<s>
Any	O
order	O
of	O
the	O
transactions	B-General_Concept
is	O
legitimate	O
,	O
if	O
no	O
dependencies	O
among	O
them	O
exist	O
,	O
which	O
is	O
assumed	O
(	O
see	O
comment	O
below	O
)	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
a	O
schedule	B-General_Concept
that	O
comprises	O
any	O
execution	O
(	O
not	O
necessarily	O
serial	O
)	O
that	O
is	O
equivalent	O
(	O
in	O
its	O
outcome	O
)	O
to	O
any	O
serial	O
execution	O
of	O
these	O
transactions	B-General_Concept
,	O
is	O
correct	O
.	O
</s>
<s>
Schedules	O
that	O
are	O
not	O
serializable	B-Operating_System
are	O
likely	O
to	O
generate	O
erroneous	O
outcomes	O
.	O
</s>
<s>
Well	O
known	O
examples	O
are	O
with	O
transactions	B-General_Concept
that	O
debit	O
and	O
credit	O
accounts	O
with	O
money	O
:	O
If	O
the	O
related	O
schedules	O
are	O
not	O
serializable	B-Operating_System
,	O
then	O
the	O
total	O
sum	O
of	O
money	O
may	O
not	O
be	O
preserved	O
.	O
</s>
<s>
This	O
and	O
violations	O
of	O
possibly	O
needed	O
other	O
invariant	B-Application
preservations	O
are	O
caused	O
by	O
one	O
transaction	O
writing	O
,	O
and	O
"	O
stepping	O
on	O
"	O
and	O
erasing	O
what	O
has	O
been	O
written	O
by	O
another	O
transaction	O
before	O
it	O
has	O
become	O
permanent	O
in	O
the	O
database	O
.	O
</s>
<s>
It	O
does	O
not	O
happen	O
if	O
serializability	B-Operating_System
is	O
maintained	O
.	O
</s>
<s>
If	O
any	O
specific	O
order	O
between	O
some	O
transactions	B-General_Concept
is	O
requested	O
by	O
an	O
application	O
,	O
then	O
it	O
is	O
enforced	O
independently	O
of	O
the	O
underlying	O
serializability	B-Operating_System
mechanisms	O
.	O
</s>
<s>
These	O
mechanisms	O
are	O
typically	O
indifferent	O
to	O
any	O
specific	O
order	O
,	O
and	O
generate	O
some	O
unpredictable	O
partial	O
order	O
that	O
is	O
typically	O
compatible	O
with	O
multiple	O
serial	O
orders	O
of	O
these	O
transactions	B-General_Concept
.	O
</s>
<s>
This	O
partial	O
order	O
results	O
from	O
the	O
scheduling	O
orders	O
of	O
concurrent	O
transactions	B-General_Concept
 '	O
data	O
access	O
operations	O
,	O
which	O
depend	O
on	O
many	O
factors	O
.	O
</s>
<s>
A	O
major	O
characteristic	O
of	O
a	O
database	B-General_Concept
transaction	I-General_Concept
is	O
atomicity	B-General_Concept
,	O
which	O
means	O
that	O
it	O
either	O
commits	O
,	O
i.e.	O
,	O
all	O
its	O
operations	O
 '	O
results	O
take	O
effect	O
in	O
the	O
database	O
,	O
or	O
aborts	O
(	O
rolled-back	O
)	O
,	O
all	O
its	O
operations	O
 '	O
results	O
do	O
not	O
have	O
any	O
effect	O
on	O
the	O
database	O
(	O
"	O
all	O
or	O
nothing	O
"	O
semantics	O
of	O
a	O
transaction	O
)	O
.	O
</s>
<s>
In	O
all	O
real	O
systems	O
transactions	B-General_Concept
can	O
abort	O
for	O
many	O
reasons	O
,	O
and	O
serializability	B-Operating_System
by	O
itself	O
is	O
not	O
sufficient	O
for	O
correctness	O
.	O
</s>
<s>
Recoverability	O
means	O
that	O
committed	O
transactions	B-General_Concept
have	O
not	O
read	O
data	O
written	O
by	O
aborted	O
transactions	B-General_Concept
(	O
whose	O
effects	O
do	O
not	O
exist	O
in	O
the	O
resulting	O
database	O
states	O
)	O
.	O
</s>
<s>
While	O
serializability	B-Operating_System
is	O
currently	O
compromised	O
on	O
purpose	O
in	O
many	O
applications	O
for	O
better	O
performance	O
(	O
only	O
in	O
cases	O
when	O
application	O
's	O
correctness	O
is	O
not	O
harmed	O
)	O
,	O
compromising	O
recoverability	O
would	O
quickly	O
violate	O
the	O
database	O
's	O
integrity	O
,	O
as	O
well	O
as	O
that	O
of	O
transactions	B-General_Concept
 '	O
results	O
external	O
to	O
the	O
database	O
.	O
</s>
<s>
A	O
schedule	B-General_Concept
with	O
the	O
recoverability	O
property	O
(	O
a	O
recoverable	O
schedule	B-General_Concept
)	O
"	O
recovers	O
"	O
from	O
aborts	O
by	O
itself	O
,	O
i.e.	O
,	O
aborts	O
do	O
not	O
harm	O
the	O
integrity	O
of	O
its	O
committed	O
transactions	B-General_Concept
and	O
resulting	O
database	O
.	O
</s>
<s>
This	O
results	O
in	O
a	O
waste	O
of	O
already	O
partially	O
executed	O
transactions	B-General_Concept
,	O
and	O
may	O
result	O
also	O
in	O
a	O
performance	O
penalty	O
.	O
</s>
<s>
Commercial	O
databases	O
provide	O
concurrency	B-Operating_System
control	I-Operating_System
with	O
a	O
whole	O
range	O
of	O
isolation	B-General_Concept
levels	I-General_Concept
which	O
are	O
in	O
fact	O
(	O
controlled	O
)	O
serializability	B-Operating_System
violations	O
in	O
order	O
to	O
achieve	O
higher	O
performance	O
.	O
</s>
<s>
Snapshot	B-Operating_System
isolation	I-Operating_System
is	O
an	O
example	O
of	O
a	O
popular	O
,	O
widely	O
utilized	O
efficient	O
relaxed	O
serializability	B-Operating_System
method	O
with	O
many	O
characteristics	O
of	O
full	O
serializability	B-Operating_System
,	O
but	O
still	O
short	O
of	O
some	O
,	O
and	O
unfit	O
in	O
many	O
situations	O
.	O
</s>
<s>
Another	O
common	O
reason	O
nowadays	O
for	O
distributed	B-Operating_System
serializability	I-Operating_System
relaxation	O
(	O
see	O
below	O
)	O
is	O
the	O
requirement	O
of	O
availability	O
of	O
internet	O
products	O
and	O
services	O
.	O
</s>
<s>
This	O
requirement	O
is	O
typically	O
answered	O
by	O
large-scale	O
data	B-Architecture
replication	I-Architecture
.	O
</s>
<s>
The	O
straightforward	O
solution	O
for	O
synchronizing	O
replicas	B-Architecture
 '	O
updates	O
of	O
the	O
same	O
database	O
object	O
is	O
including	O
all	O
these	O
updates	O
in	O
a	O
single	O
atomic	O
distributed	B-General_Concept
transaction	I-General_Concept
.	O
</s>
<s>
However	O
,	O
with	O
many	O
replicas	B-Architecture
such	O
a	O
transaction	O
is	O
very	O
large	O
,	O
and	O
may	O
span	O
enough	O
of	O
a	O
number	O
of	O
several	O
computers	O
and	O
networks	B-Architecture
that	O
some	O
of	O
them	O
are	O
likely	O
to	O
be	O
unavailable	O
.	O
</s>
<s>
Consequently	O
,	O
Optimistic	B-General_Concept
replication	I-General_Concept
(	O
Lazy	B-General_Concept
replication	I-General_Concept
)	O
is	O
often	O
utilized	O
(	O
e.g.	O
,	O
in	O
many	O
products	O
and	O
services	O
by	O
Google	B-Application
,	O
Amazon	B-Application
,	O
Yahoo	B-Application
,	O
and	O
the	O
like	O
)	O
,	O
while	O
serializability	B-Operating_System
is	O
relaxed	O
and	O
compromised	O
for	O
eventual	B-General_Concept
consistency	I-General_Concept
.	O
</s>
<s>
Classes	O
of	O
schedules	O
defined	O
by	O
relaxed	O
serializability	B-Operating_System
properties	O
either	O
contain	O
the	O
serializability	B-Operating_System
class	O
,	O
or	O
are	O
incomparable	O
with	O
it	O
.	O
</s>
<s>
Mechanisms	O
that	O
enforce	O
serializability	B-Operating_System
need	O
to	O
execute	O
in	O
real	B-General_Concept
time	I-General_Concept
,	O
or	O
almost	O
in	O
real	B-General_Concept
time	I-General_Concept
,	O
while	O
transactions	B-General_Concept
are	O
running	O
at	O
high	O
rates	O
.	O
</s>
<s>
In	O
order	O
to	O
meet	O
this	O
requirement	O
,	O
special	O
cases	O
of	O
serializability	B-Operating_System
,	O
sufficient	O
conditions	O
for	O
serializability	B-Operating_System
which	O
can	O
be	O
enforced	O
effectively	O
,	O
are	O
utilized	O
.	O
</s>
<s>
Two	O
major	O
types	O
of	O
serializability	B-Operating_System
exist	O
:	O
view-serializability	O
,	O
and	O
conflict-serializability	O
.	O
</s>
<s>
View-serializability	O
matches	O
the	O
general	O
definition	O
of	O
serializability	B-Operating_System
given	O
above	O
.	O
</s>
<s>
Conflict-serializability	O
is	O
a	O
broad	O
special	O
case	O
,	O
i.e.	O
,	O
any	O
schedule	B-General_Concept
that	O
is	O
conflict-serializable	O
is	O
also	O
view-serializable	O
,	O
but	O
not	O
necessarily	O
the	O
opposite	O
.	O
</s>
<s>
Conflict-serializability	O
is	O
widely	O
utilized	O
because	O
it	O
is	O
easier	O
to	O
determine	O
and	O
covers	O
a	O
substantial	O
portion	O
of	O
the	O
view-serializable	O
schedules	O
.	O
</s>
<s>
Determining	O
view-serializability	O
of	O
a	O
schedule	B-General_Concept
is	O
an	O
NP-complete	O
problem	O
(	O
a	O
class	O
of	O
problems	O
with	O
only	O
difficult-to-compute	O
,	O
excessively	O
time-consuming	O
known	O
solutions	O
)	O
.	O
</s>
<s>
View-serializability	O
of	O
a	O
schedule	B-General_Concept
is	O
defined	O
by	O
equivalence	O
to	O
a	O
serial	O
schedule	B-General_Concept
(	O
no	O
overlapping	O
transactions	B-General_Concept
)	O
with	O
the	O
same	O
transactions	B-General_Concept
,	O
such	O
that	O
respective	O
transactions	B-General_Concept
in	O
the	O
two	O
schedules	O
read	O
and	O
write	O
the	O
same	O
data	O
values	O
(	O
"	O
view	O
"	O
the	O
same	O
data	O
values	O
)	O
.	O
</s>
<s>
Conflict-serializability	O
of	O
a	O
schedule	B-General_Concept
is	O
defined	O
by	O
equivalence	O
to	O
a	O
serial	O
schedule	B-General_Concept
(	O
no	O
overlapping	O
transactions	B-General_Concept
)	O
with	O
the	O
same	O
transactions	B-General_Concept
,	O
such	O
that	O
both	O
schedules	O
have	O
the	O
same	O
sets	O
of	O
respective	O
chronologically	O
ordered	O
pairs	O
of	O
conflicting	O
operations	O
(	O
same	O
precedence	O
relations	O
of	O
respective	O
conflicting	O
operations	O
)	O
.	O
</s>
<s>
Two	O
operations	O
are	O
conflicting	O
if	O
they	O
are	O
of	O
different	O
transactions	B-General_Concept
,	O
upon	O
the	O
same	O
datum	O
(	O
data	O
item	O
)	O
,	O
and	O
at	O
least	O
one	O
of	O
them	O
is	O
write	O
.	O
</s>
<s>
Another	O
more	O
complex	O
example	O
:	O
the	O
operations	O
increment	O
and	O
decrement	O
of	O
a	O
counter	O
are	O
both	O
write	O
operations	O
(	O
both	O
modify	O
the	O
counter	O
)	O
,	O
but	O
do	O
not	O
need	O
to	O
be	O
considered	O
conflicting	O
(	O
write-write	O
conflict	O
type	O
)	O
since	O
they	O
are	O
commutative	O
(	O
thus	O
increment	O
–	O
decrement	O
is	O
not	O
a	O
conflict	O
;	O
e.g.	O
,	O
already	O
has	O
been	O
supported	O
in	O
the	O
old	O
IBM	B-Application
's	I-Application
IMS	I-Application
"	I-Application
fast	I-Application
path	I-Application
"	I-Application
)	O
.	O
</s>
<s>
Only	O
precedence	O
(	O
time	O
order	O
)	O
in	O
pairs	O
of	O
conflicting	O
(	O
non-commutative	O
)	O
operations	O
is	O
important	O
when	O
checking	O
equivalence	O
to	O
a	O
serial	O
schedule	B-General_Concept
,	O
since	O
different	O
schedules	O
consisting	O
of	O
the	O
same	O
transactions	B-General_Concept
can	O
be	O
transformed	O
from	O
one	O
to	O
another	O
by	O
changing	O
orders	O
between	O
different	O
transactions	B-General_Concept
 '	O
operations	O
(	O
different	O
transactions	B-General_Concept
 '	O
interleaving	O
)	O
,	O
and	O
since	O
changing	O
orders	O
of	O
commutative	O
operations	O
(	O
non-conflicting	O
)	O
does	O
not	O
change	O
an	O
overall	O
operation	O
sequence	O
result	O
,	O
i.e.	O
,	O
a	O
schedule	B-General_Concept
outcome	O
(	O
the	O
outcome	O
is	O
preserved	O
through	O
order	O
change	O
between	O
non-conflicting	O
operations	O
,	O
but	O
typically	O
not	O
when	O
conflicting	O
operations	O
change	O
order	O
)	O
.	O
</s>
<s>
This	O
means	O
that	O
if	O
a	O
schedule	B-General_Concept
can	O
be	O
transformed	O
to	O
any	O
serial	O
schedule	B-General_Concept
without	O
changing	O
orders	O
of	O
conflicting	O
operations	O
(	O
but	O
changing	O
orders	O
of	O
non-conflicting	O
,	O
while	O
preserving	O
operation	O
order	O
inside	O
each	O
transaction	O
)	O
,	O
then	O
the	O
outcome	O
of	O
both	O
schedules	O
is	O
the	O
same	O
,	O
and	O
the	O
schedule	B-General_Concept
is	O
conflict-serializable	O
by	O
definition	O
.	O
</s>
<s>
Conflicts	O
are	O
the	O
reason	O
for	O
blocking	O
transactions	B-General_Concept
and	O
delays	O
(	O
non-materialized	O
conflicts	O
)	O
,	O
or	O
for	O
aborting	O
transactions	B-General_Concept
due	O
to	O
serializability	B-Operating_System
violation	O
prevention	O
.	O
</s>
<s>
A	O
transaction	O
can	O
issue/request	O
a	O
conflicting	O
operation	O
and	O
be	O
in	O
conflict	O
with	O
another	O
transaction	O
while	O
its	O
conflicting	O
operation	O
is	O
delayed	O
and	O
not	O
executed	O
(	O
e.g.	O
,	O
blocked	O
by	O
a	O
lock	B-Operating_System
)	O
.	O
</s>
<s>
Only	O
executed	O
(	O
materialized	O
)	O
conflicting	O
operations	O
are	O
relevant	O
to	O
conflict	O
serializability	B-Operating_System
(	O
see	O
more	O
below	O
)	O
.	O
</s>
<s>
Schedule	B-General_Concept
compliance	O
with	O
conflict	O
serializability	B-Operating_System
can	O
be	O
tested	O
with	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
(	O
serializability	B-Operating_System
graph	O
,	O
serialization	O
graph	O
,	O
conflict	O
graph	O
)	O
for	O
committed	O
transactions	B-General_Concept
of	O
the	O
schedule	B-General_Concept
.	O
</s>
<s>
It	O
is	O
the	O
directed	O
graph	O
representing	O
precedence	O
of	O
transactions	B-General_Concept
in	O
the	O
schedule	B-General_Concept
,	O
as	O
reflected	O
by	O
precedence	O
of	O
conflicting	O
operations	O
in	O
the	O
transactions	B-General_Concept
.	O
</s>
<s>
In	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
transactions	B-General_Concept
are	O
nodes	O
and	O
precedence	O
relations	O
are	O
directed	O
edges	O
.	O
</s>
<s>
There	O
exists	O
an	O
edge	O
from	O
a	O
first	O
transaction	O
to	O
a	O
second	O
transaction	O
,	O
if	O
the	O
second	O
transaction	O
is	O
in	O
conflict	O
with	O
the	O
first	O
(	O
see	O
Conflict	O
serializability	B-Operating_System
above	O
)	O
,	O
and	O
the	O
conflict	O
is	O
materialized	O
(	O
i.e.	O
,	O
if	O
the	O
requested	O
conflicting	O
operation	O
is	O
actually	O
executed	O
:	O
in	O
many	O
cases	O
a	O
requested/issued	O
conflicting	O
operation	O
by	O
a	O
transaction	O
is	O
delayed	O
and	O
even	O
never	O
executed	O
,	O
typically	O
by	O
a	O
lock	B-Operating_System
on	O
the	O
operation	O
's	O
object	O
,	O
held	O
by	O
another	O
transaction	O
,	O
or	O
when	O
writing	O
to	O
a	O
transaction	O
's	O
temporary	O
private	O
workspace	O
and	O
materializing	O
,	O
copying	O
to	O
the	O
database	O
itself	O
,	O
upon	O
commit	O
;	O
as	O
long	O
as	O
a	O
requested/issued	O
conflicting	O
operation	O
is	O
not	O
executed	O
upon	O
the	O
database	O
itself	O
,	O
the	O
conflict	O
is	O
non-materialized	O
;	O
non-materialized	O
conflicts	O
are	O
not	O
represented	O
by	O
an	O
edge	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
)	O
.	O
</s>
<s>
Comment	O
:	O
In	O
many	O
text	O
books	O
only	O
committed	O
transactions	B-General_Concept
are	O
included	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
.	O
</s>
<s>
Here	O
all	O
transactions	B-General_Concept
are	O
included	O
for	O
convenience	O
in	O
later	O
discussions	O
.	O
</s>
<s>
The	O
following	O
observation	O
is	O
a	O
key	O
characterization	O
of	O
conflict	O
serializability	B-Operating_System
:	O
</s>
<s>
A	O
schedule	B-General_Concept
is	O
conflict-serializable	O
if	O
and	O
only	O
if	O
its	O
precedence	B-General_Concept
graph	I-General_Concept
of	O
committed	O
transactions	B-General_Concept
(	O
when	O
only	O
committed	O
transactions	B-General_Concept
are	O
considered	O
)	O
is	O
acyclic	O
.	O
</s>
<s>
This	O
means	O
that	O
a	O
cycle	O
consisting	O
of	O
committed	O
transactions	B-General_Concept
only	O
is	O
generated	O
in	O
the	O
(	O
general	O
)	O
precedence	B-General_Concept
graph	I-General_Concept
,	O
if	O
and	O
only	O
if	O
conflict-serializability	O
is	O
violated	O
.	O
</s>
<s>
Cycles	O
of	O
committed	O
transactions	B-General_Concept
can	O
be	O
prevented	O
by	O
aborting	O
an	O
undecided	O
(	O
neither	O
committed	O
,	O
nor	O
aborted	O
)	O
transaction	O
on	O
each	O
cycle	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
of	O
all	O
the	O
transactions	B-General_Concept
,	O
which	O
can	O
otherwise	O
turn	O
into	O
a	O
cycle	O
of	O
committed	O
transactions	B-General_Concept
(	O
and	O
a	O
committed	O
transaction	O
cannot	O
be	O
aborted	O
)	O
.	O
</s>
<s>
One	O
transaction	O
aborted	O
per	O
cycle	O
is	O
both	O
required	O
and	O
sufficient	O
in	O
number	O
to	O
break	O
and	O
eliminate	O
the	O
cycle	O
(	O
more	O
aborts	O
are	O
possible	O
,	O
and	O
can	O
happen	O
under	O
some	O
mechanisms	O
,	O
but	O
are	O
unnecessary	O
for	O
serializability	B-Operating_System
)	O
.	O
</s>
<s>
Transactions	B-General_Concept
aborted	O
due	O
to	O
serializability	B-Operating_System
violation	O
prevention	O
are	O
restarted	O
and	O
executed	O
again	O
immediately	O
.	O
</s>
<s>
Serializability-enforcing	O
mechanisms	O
typically	O
do	O
not	O
maintain	O
a	O
precedence	B-General_Concept
graph	I-General_Concept
as	O
a	O
data	O
structure	O
,	O
but	O
rather	O
prevent	O
or	O
break	O
cycles	O
implicitly	O
(	O
e.g.	O
,	O
SS2PL	O
below	O
)	O
.	O
</s>
<s>
Strong	B-Operating_System
strict	I-Operating_System
two-phase	I-Operating_System
locking	I-Operating_System
(	O
SS2PL	O
)	O
is	O
a	O
common	O
mechanism	O
utilized	O
in	O
database	O
systems	O
since	O
their	O
early	O
days	O
in	O
the	O
1970s	O
(	O
the	O
"	O
SS	O
"	O
in	O
the	O
name	O
SS2PL	O
is	O
newer	O
,	O
though	O
)	O
to	O
enforce	O
both	O
conflict	O
serializability	B-Operating_System
and	O
strictness	O
(	O
a	O
special	O
case	O
of	O
recoverability	O
which	O
allows	O
effective	O
database	O
recovery	O
from	O
failure	O
)	O
of	O
a	O
schedule	B-General_Concept
.	O
</s>
<s>
Under	O
this	O
mechanism	O
,	O
each	O
datum	O
is	O
locked	O
by	O
a	O
transaction	O
before	O
its	O
accessing	O
it	O
(	O
in	O
any	O
read	O
or	O
write	O
operation	O
)	O
:	O
the	O
item	O
is	O
marked	O
by	O
and	O
associated	O
with	O
a	O
lock	B-Operating_System
of	O
a	O
certain	O
type	O
depending	O
on	O
the	O
operation	O
being	O
performed	O
(	O
and	O
the	O
specific	O
transaction	O
implementation	O
;	O
various	O
models	O
with	O
different	O
lock	B-Operating_System
types	O
exist	O
;	O
in	O
some	O
models	O
,	O
locks	O
may	O
change	O
type	O
during	O
the	O
transaction	O
's	O
life	O
)	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
access	O
by	O
another	O
transaction	O
may	O
be	O
blocked	O
,	O
typically	O
upon	O
a	O
conflict	O
(	O
the	O
lock	B-Operating_System
delays	O
or	O
completely	O
prevents	O
the	O
conflict	O
from	O
being	O
materialized	O
and	O
be	O
reflected	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
by	O
blocking	O
the	O
conflicting	O
operation	O
)	O
,	O
depending	O
on	O
lock	B-Operating_System
type	O
and	O
the	O
other	O
transaction	O
's	O
access	O
operation	O
type	O
.	O
</s>
<s>
SS2PL	O
is	O
the	O
name	O
of	O
the	O
resulting	O
schedule	B-General_Concept
property	O
as	O
well	O
,	O
which	O
is	O
also	O
called	O
rigorousness	B-Operating_System
.	O
</s>
<s>
Mutual	O
blocking	O
between	O
transactions	B-General_Concept
results	O
in	O
a	O
deadlock	O
,	O
where	O
execution	O
of	O
these	O
transactions	B-General_Concept
is	O
stalled	O
and	O
no	O
completion	O
can	O
be	O
reached	O
.	O
</s>
<s>
Thus	O
deadlocks	O
need	O
to	O
be	O
resolved	O
to	O
complete	O
these	O
transactions	B-General_Concept
 '	O
execution	O
and	O
release	O
related	O
computing	O
resources	O
.	O
</s>
<s>
A	O
deadlock	O
is	O
a	O
reflection	O
of	O
a	O
potential	O
cycle	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
that	O
would	O
occur	O
without	O
the	O
blocking	O
when	O
conflicts	O
are	O
materialized	O
.	O
</s>
<s>
It	O
is	O
often	O
detected	O
using	O
a	O
wait-for	B-Operating_System
graph	I-Operating_System
(	O
a	O
graph	O
of	O
conflicts	O
blocked	O
by	O
locks	O
from	O
being	O
materialized	O
;	O
it	O
can	O
be	O
also	O
defined	O
as	O
the	O
graph	O
of	O
non-materialized	O
conflicts	O
;	O
conflicts	O
not	O
materialized	O
are	O
not	O
reflected	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
and	O
do	O
not	O
affect	O
serializability	B-Operating_System
)	O
,	O
which	O
indicates	O
which	O
transaction	O
is	O
"	O
waiting	O
for	O
"	O
the	O
release	O
of	O
one	O
of	O
more	O
locks	O
by	O
which	O
other	O
transaction	O
or	O
transactions	B-General_Concept
,	O
and	O
a	O
cycle	O
in	O
this	O
graph	O
means	O
a	O
deadlock	O
.	O
</s>
<s>
Transactions	B-General_Concept
aborted	O
due	O
to	O
deadlock	O
resolution	O
are	O
restarted	O
and	O
executed	O
again	O
immediately	O
.	O
</s>
<s>
The	O
above	O
(	O
conflict	O
)	O
serializability	B-Operating_System
techniques	O
in	O
their	O
general	O
form	O
do	O
not	O
provide	O
recoverability	O
.	O
</s>
<s>
Concurrency	B-Operating_System
control	I-Operating_System
techniques	O
are	O
of	O
three	O
major	O
types	O
:	O
</s>
<s>
Pessimistic	O
:	O
In	O
Pessimistic	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
a	O
transaction	O
blocks	O
the	O
data	O
access	O
operations	O
of	O
other	O
transactions	B-General_Concept
upon	O
conflicts	O
,	O
and	O
conflicts	O
are	O
non-materialized	O
until	O
blocking	O
is	O
removed	O
.	O
</s>
<s>
This	O
is	O
done	O
to	O
ensure	O
that	O
operations	O
that	O
may	O
violate	O
serializability	B-Operating_System
(	O
and	O
in	O
practice	O
also	O
recoverability	O
)	O
do	O
not	O
occur	O
.	O
</s>
<s>
Optimistic	O
:	O
In	O
Optimistic	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
,	O
the	O
data	O
access	O
operations	O
of	O
other	O
transactions	B-General_Concept
are	O
not	O
blocked	O
upon	O
conflicts	O
,	O
and	O
conflicts	O
are	O
immediately	O
materialized	O
.	O
</s>
<s>
When	O
the	O
transaction	O
reaches	O
the	O
ready	O
state	O
,	O
i.e.	O
,	O
its	O
running	O
state	O
has	O
been	O
completed	O
,	O
possible	O
serializability	B-Operating_System
(	O
and	O
in	O
practice	O
also	O
recoverability	O
)	O
violation	O
by	O
the	O
transaction	O
's	O
operations	O
(	O
relative	O
to	O
other	O
running	O
transactions	B-General_Concept
)	O
is	O
checked	O
:	O
if	O
violation	O
has	O
occurred	O
,	O
the	O
transaction	O
is	O
typically	O
aborted	O
(	O
sometimes	O
aborting	O
another	O
transaction	O
to	O
handle	O
serializability	B-Operating_System
violation	O
is	O
preferred	O
)	O
.	O
</s>
<s>
A	O
cycle	O
of	O
committed	O
transactions	B-General_Concept
(	O
with	O
materialized	O
conflicts	O
)	O
in	O
the	O
precedence	B-General_Concept
graph	I-General_Concept
(	O
conflict	O
graph	O
)	O
represents	O
a	O
serializability	B-Operating_System
violation	O
,	O
and	O
should	O
be	O
avoided	O
for	O
maintaining	O
serializability	B-Operating_System
.	O
</s>
<s>
A	O
cycle	O
of	O
(	O
non-materialized	O
)	O
conflicts	O
in	O
the	O
wait-for	B-Operating_System
graph	I-Operating_System
represents	O
a	O
deadlock	O
situation	O
,	O
which	O
should	O
be	O
resolved	O
by	O
breaking	O
the	O
cycle	O
.	O
</s>
<s>
Under	O
any	O
technique	O
type	O
,	O
conflicts	O
should	O
be	O
detected	O
and	O
considered	O
,	O
with	O
similar	O
overhead	O
for	O
both	O
materialized	O
and	O
non-materialized	O
conflicts	O
(	O
typically	O
by	O
using	O
mechanisms	O
like	O
locking	B-Operating_System
,	O
while	O
either	O
blocking	O
for	O
locks	O
or	O
not	O
blocking	O
but	O
recording	O
conflict	O
for	O
materialized	O
conflicts	O
)	O
.	O
</s>
<s>
In	O
a	O
blocking	O
method	O
,	O
typically	O
a	O
context	B-Operating_System
switching	I-Operating_System
occurs	O
upon	O
conflict	O
,	O
with	O
(	O
additional	O
)	O
incurred	O
overhead	O
.	O
</s>
<s>
Otherwise	O
,	O
blocked	O
transactions	B-General_Concept
 '	O
related	O
computing	O
resources	O
remain	O
idle	O
,	O
unutilized	O
,	O
which	O
may	O
be	O
a	O
worse	O
alternative	O
.	O
</s>
<s>
Unless	O
schedule	B-General_Concept
classes	O
are	O
inherently	O
blocking	O
(	O
i.e.	O
,	O
they	O
cannot	O
be	O
implemented	O
without	O
data-access	O
operations	O
blocking	O
;	O
e.g.	O
,	O
2PL	O
,	O
SS2PL	O
and	O
SCO	O
above	O
;	O
see	O
chart	O
)	O
,	O
they	O
can	O
also	O
be	O
implemented	O
using	O
optimistic	O
techniques	O
(	O
e.g.	O
,	O
Serializability	B-Operating_System
,	O
Recoverability	O
)	O
.	O
</s>
<s>
Multi-version	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
(	O
MVCC	O
)	O
is	O
a	O
common	O
way	O
today	O
to	O
increase	O
concurrency	B-Architecture
and	O
performance	O
by	O
generating	O
a	O
new	O
version	O
of	O
a	O
database	O
object	O
each	O
time	O
the	O
object	O
is	O
written	O
and	O
allowing	O
transactions	B-General_Concept
 '	O
read	O
operations	O
of	O
several	O
last	O
relevant	O
versions	O
(	O
of	O
each	O
object	O
)	O
,	O
depending	O
on	O
scheduling	O
method	O
.	O
</s>
<s>
MVCC	O
can	O
be	O
combined	O
with	O
all	O
the	O
serializability	B-Operating_System
techniques	O
listed	O
above	O
(	O
except	O
SerializableSI	O
,	O
which	O
is	O
originally	O
MVCC-based	O
)	O
.	O
</s>
<s>
MVCC	O
is	O
especially	O
popular	O
nowadays	O
through	O
the	O
relaxed	O
serializability	B-Operating_System
(	O
see	O
above	O
)	O
method	O
Snapshot	B-Operating_System
isolation	I-Operating_System
(	O
SI	O
)	O
which	O
provides	O
better	O
performance	O
than	O
most	O
known	O
serializability	B-Operating_System
mechanisms	O
(	O
at	O
the	O
cost	O
of	O
possible	O
serializability	B-Operating_System
violation	O
in	O
certain	O
cases	O
)	O
.	O
</s>
<s>
SerializableSI	O
,	O
which	O
is	O
an	O
efficient	O
enhancement	O
of	O
SI	O
to	O
make	O
it	O
serializable	B-Operating_System
,	O
is	O
intended	O
to	O
provide	O
an	O
efficient	O
serializable	B-Operating_System
solution	O
.	O
</s>
<s>
Distributed	B-Operating_System
serializability	I-Operating_System
is	O
the	O
serializability	B-Operating_System
of	O
a	O
schedule	B-General_Concept
of	O
a	O
transactional	B-General_Concept
distributed	B-Architecture
system	I-Architecture
(	O
e.g.	O
,	O
a	O
distributed	B-Operating_System
database	I-Operating_System
system	O
)	O
.	O
</s>
<s>
Such	O
a	O
system	O
is	O
characterized	O
by	O
distributed	B-General_Concept
transactions	I-General_Concept
(	O
also	O
called	O
global	O
transactions	B-General_Concept
)	O
,	O
i.e.	O
,	O
transactions	B-General_Concept
that	O
span	O
computer	O
processes	O
(	O
a	O
process	O
abstraction	O
in	O
a	O
general	O
sense	O
,	O
depending	O
on	O
computing	O
environment	O
;	O
e.g.	O
,	O
operating	B-General_Concept
system	I-General_Concept
's	O
thread	B-Operating_System
)	O
and	O
possibly	O
network	O
nodes	O
.	O
</s>
<s>
A	O
distributed	B-General_Concept
transaction	I-General_Concept
comprises	O
more	O
than	O
one	O
of	O
several	O
local	O
sub-transactions	O
that	O
each	O
has	O
states	O
as	O
described	O
above	O
for	O
a	O
database	B-General_Concept
transaction	I-General_Concept
.	O
</s>
<s>
Distributed	B-General_Concept
transactions	I-General_Concept
imply	O
a	O
need	O
for	O
an	O
atomic	B-Operating_System
commit	I-Operating_System
protocol	O
to	O
reach	O
consensus	O
among	O
its	O
local	O
sub-transactions	O
on	O
whether	O
to	O
commit	O
or	O
abort	O
.	O
</s>
<s>
Such	O
protocols	O
can	O
vary	O
from	O
a	O
simple	O
(	O
one-phase	O
)	O
handshake	O
among	O
processes	O
that	O
fail	O
together	O
to	O
more	O
sophisticated	O
protocols	O
,	O
like	O
two-phase	B-General_Concept
commit	I-General_Concept
,	O
to	O
handle	O
more	O
complicated	O
cases	O
of	O
failure	O
(	O
e.g.	O
,	O
process	O
,	O
node	O
,	O
communication	O
,	O
etc	O
.	O
</s>
<s>
Distributed	B-Operating_System
serializability	I-Operating_System
is	O
a	O
major	O
goal	O
of	O
distributed	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
for	O
correctness	O
.	O
</s>
<s>
With	O
the	O
proliferation	O
of	O
the	O
Internet	O
,	O
cloud	B-Architecture
computing	I-Architecture
,	O
grid	B-Architecture
computing	I-Architecture
,	O
and	O
small	O
,	O
portable	O
,	O
powerful	O
computing	O
devices	O
(	O
e.g.	O
,	O
smartphones	B-Application
,	O
)	O
the	O
need	O
for	O
effective	O
distributed	B-Operating_System
serializability	I-Operating_System
techniques	O
to	O
ensure	O
correctness	O
in	O
and	O
among	O
distributed	B-Architecture
applications	I-Architecture
seems	O
to	O
increase	O
.	O
</s>
<s>
Distributed	B-Operating_System
serializability	I-Operating_System
is	O
achieved	O
by	O
implementing	O
distributed	B-Architecture
versions	O
of	O
the	O
known	O
centralized	O
techniques	O
.	O
</s>
<s>
Typically	O
,	O
all	O
such	O
distributed	B-Architecture
versions	O
require	O
utilizing	O
conflict	O
information	O
(	O
of	O
either	O
materialized	O
or	O
non-materialized	O
conflicts	O
,	O
or	O
,	O
equivalently	O
,	O
transaction	O
precedence	O
or	O
blocking	O
information	O
;	O
conflict	O
serializability	B-Operating_System
is	O
usually	O
utilized	O
)	O
that	O
is	O
not	O
generated	O
locally	O
,	O
but	O
rather	O
in	O
different	O
processes	O
,	O
and	O
remote	O
locations	O
.	O
</s>
<s>
Thus	O
information	O
distribution	O
is	O
needed	O
(	O
e.g.	O
,	O
precedence	O
relations	O
,	O
lock	B-Operating_System
information	O
,	O
timestamps	O
,	O
or	O
tickets	O
)	O
.	O
</s>
<s>
When	O
the	O
distributed	B-Architecture
system	I-Architecture
is	O
of	O
a	O
relatively	O
small	O
scale	O
and	O
message	O
delays	O
across	O
the	O
system	O
are	O
small	O
,	O
the	O
centralized	O
concurrency	B-Operating_System
control	I-Operating_System
methods	O
can	O
be	O
used	O
unchanged	O
while	O
certain	O
processes	O
or	O
nodes	O
in	O
the	O
system	O
manage	O
the	O
related	O
algorithms	O
.	O
</s>
<s>
However	O
,	O
in	O
a	O
large-scale	O
system	O
(	O
e.g.	O
,	O
grid	O
and	O
cloud	O
)	O
,	O
due	O
to	O
the	O
distribution	O
of	O
such	O
information	O
,	O
a	O
substantial	O
performance	O
penalty	O
is	O
typically	O
incurred	O
,	O
even	O
when	O
distributed	B-Architecture
versions	O
of	O
the	O
methods	O
(	O
vs	O
.	O
the	O
centralized	O
ones	O
)	O
are	O
used	O
,	O
primarily	O
due	O
to	O
computer	O
and	O
communication	O
latency	O
.	O
</s>
<s>
Also	O
,	O
when	O
such	O
information	O
is	O
distributed	B-Architecture
,	O
related	O
techniques	O
typically	O
do	O
not	O
scale	O
well	O
.	O
</s>
<s>
A	O
well-known	O
example	O
with	O
respect	O
to	O
scalability	O
problems	O
is	O
a	O
distributed	B-Operating_System
lock	I-Operating_System
manager	I-Operating_System
,	O
which	O
distributes	O
lock	B-Operating_System
(	O
non-materialized	O
conflict	O
)	O
information	O
across	O
the	O
distributed	B-Architecture
system	I-Architecture
to	O
implement	O
locking	B-Operating_System
techniques	O
.	O
</s>
