<s>
In	O
information	B-General_Concept
technology	I-General_Concept
and	O
computer	B-General_Concept
science	I-General_Concept
,	O
especially	O
in	O
the	O
fields	O
of	O
computer	B-General_Concept
programming	I-General_Concept
,	O
operating	B-General_Concept
systems	I-General_Concept
,	O
multiprocessors	B-Operating_System
,	O
and	O
databases	B-Application
,	O
concurrency	B-Architecture
control	O
ensures	O
that	O
correct	O
results	O
for	O
concurrent	B-Architecture
operations	O
are	O
generated	O
,	O
while	O
getting	O
those	O
results	O
as	O
quickly	O
as	O
possible	O
.	O
</s>
<s>
Computer	O
systems	O
,	O
both	O
software	O
and	O
hardware	B-Architecture
,	O
consist	O
of	O
modules	O
,	O
or	O
components	O
.	O
</s>
<s>
Each	O
component	O
is	O
designed	O
to	O
operate	O
correctly	O
,	O
i.e.	O
,	O
to	O
obey	O
or	O
to	O
meet	O
certain	O
consistency	B-General_Concept
rules	O
.	O
</s>
<s>
When	O
components	O
that	O
operate	O
concurrently	B-Operating_System
interact	O
by	O
messaging	O
or	O
by	O
sharing	O
accessed	O
data	O
(	O
in	O
memory	B-General_Concept
or	O
storage	B-General_Concept
)	O
,	O
a	O
certain	O
component	O
's	O
consistency	B-General_Concept
may	O
be	O
violated	O
by	O
another	O
component	O
.	O
</s>
<s>
The	O
general	O
area	O
of	O
concurrency	B-Architecture
control	O
provides	O
rules	O
,	O
methods	O
,	O
design	O
methodologies	O
,	O
and	O
theories	O
to	O
maintain	O
the	O
consistency	B-General_Concept
of	O
components	O
operating	O
concurrently	B-Operating_System
while	O
interacting	O
,	O
and	O
thus	O
the	O
consistency	B-General_Concept
and	O
correctness	O
of	O
the	O
whole	O
system	O
.	O
</s>
<s>
Introducing	O
concurrency	B-Architecture
control	O
into	O
a	O
system	O
means	O
applying	O
operation	O
constraints	O
which	O
typically	O
result	O
in	O
some	O
performance	O
reduction	O
.	O
</s>
<s>
Operation	O
consistency	B-General_Concept
and	O
correctness	O
should	O
be	O
achieved	O
with	O
as	O
good	O
as	O
possible	O
efficiency	O
,	O
without	O
reducing	O
performance	O
below	O
reasonable	O
levels	O
.	O
</s>
<s>
Concurrency	B-Architecture
control	O
can	O
require	O
significant	O
additional	O
complexity	O
and	O
overhead	O
in	O
a	O
concurrent	B-Architecture
algorithm	I-Architecture
compared	O
to	O
the	O
simpler	O
sequential	B-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
For	O
example	O
,	O
a	O
failure	O
in	O
concurrency	B-Architecture
control	O
can	O
result	O
in	O
data	O
corruption	O
from	O
torn	O
read	O
or	O
write	O
operations	O
.	O
</s>
<s>
This	O
section	O
is	O
applicable	O
to	O
all	O
transactional	B-General_Concept
systems	O
,	O
i.e.	O
,	O
to	O
all	O
systems	O
that	O
use	O
database	B-General_Concept
transactions	I-General_Concept
(	O
atomic	B-General_Concept
transactions	I-General_Concept
;	O
e.g.	O
,	O
transactional	B-General_Concept
objects	O
in	O
Systems	B-Application
management	I-Application
and	O
in	O
networks	B-Architecture
of	O
smartphones	B-Application
which	O
typically	O
implement	O
private	O
,	O
dedicated	O
database	B-Application
systems	I-Application
)	O
,	O
not	O
only	O
general-purpose	O
database	B-Application
management	I-Application
systems	I-Application
(	O
DBMSs	O
)	O
.	O
</s>
<s>
DBMSs	O
need	O
to	O
deal	O
also	O
with	O
concurrency	B-Architecture
control	O
issues	O
not	O
typical	O
just	O
to	O
database	B-General_Concept
transactions	I-General_Concept
but	O
rather	O
to	O
operating	B-General_Concept
systems	I-General_Concept
in	O
general	O
.	O
</s>
<s>
These	O
issues	O
(	O
e.g.	O
,	O
see	O
Concurrency	B-Architecture
control	O
in	O
operating	B-General_Concept
systems	I-General_Concept
below	O
)	O
are	O
out	O
of	O
the	O
scope	O
of	O
this	O
section	O
.	O
</s>
<s>
Concurrency	B-Architecture
control	O
in	O
Database	B-Application
management	I-Application
systems	I-Application
(	O
DBMS	O
;	O
e.g.	O
,	O
Bernstein	O
et	O
al	O
.	O
</s>
<s>
1987	O
,	O
Weikum	O
and	O
Vossen	O
2001	O
)	O
,	O
other	O
transactional	B-General_Concept
objects	O
,	O
and	O
related	O
distributed	B-Operating_System
applications	O
(	O
e.g.	O
,	O
Grid	B-Architecture
computing	I-Architecture
and	O
Cloud	B-Architecture
computing	I-Architecture
)	O
ensures	O
that	O
database	B-General_Concept
transactions	I-General_Concept
are	O
performed	O
concurrently	B-Operating_System
without	O
violating	O
the	O
data	O
integrity	O
of	O
the	O
respective	O
databases	B-Application
.	O
</s>
<s>
Thus	O
concurrency	B-Architecture
control	O
is	O
an	O
essential	O
element	O
for	O
correctness	O
in	O
any	O
system	O
where	O
two	O
database	B-General_Concept
transactions	I-General_Concept
or	O
more	O
,	O
executed	O
with	O
time	O
overlap	O
,	O
can	O
access	O
the	O
same	O
data	O
,	O
e.g.	O
,	O
virtually	O
in	O
any	O
general-purpose	O
database	B-Application
system	I-Application
.	O
</s>
<s>
Consequently	O
,	O
a	O
vast	O
body	O
of	O
related	O
research	O
has	O
been	O
accumulated	O
since	O
database	B-Application
systems	I-Application
emerged	O
in	O
the	O
early	O
1970s	O
.	O
</s>
<s>
A	O
well	O
established	O
concurrency	B-Architecture
control	O
theory	O
for	O
database	B-Application
systems	I-Application
is	O
outlined	O
in	O
the	O
references	O
mentioned	O
above	O
:	O
serializability	B-Operating_System
theory	I-Operating_System
,	O
which	O
allows	O
to	O
effectively	O
design	O
and	O
analyze	O
concurrency	B-Architecture
control	O
methods	O
and	O
mechanisms	O
.	O
</s>
<s>
An	O
alternative	O
theory	O
for	O
concurrency	B-Architecture
control	O
of	O
atomic	B-General_Concept
transactions	I-General_Concept
over	O
abstract	O
data	O
types	O
is	O
presented	O
in	O
(	O
Lynch	O
et	O
al	O
.	O
</s>
<s>
To	O
ensure	O
correctness	O
,	O
a	O
DBMS	O
usually	O
guarantees	O
that	O
only	O
serializable	B-Operating_System
transaction	B-General_Concept
schedules	O
are	O
generated	O
,	O
unless	O
serializability	B-Operating_System
is	O
intentionally	O
relaxed	O
to	O
increase	O
performance	O
,	O
but	O
only	O
in	O
cases	O
where	O
application	O
correctness	O
is	O
not	O
harmed	O
.	O
</s>
<s>
A	O
DBMS	O
also	O
guarantees	O
that	O
no	O
effect	O
of	O
committed	O
transactions	O
is	O
lost	O
,	O
and	O
no	O
effect	O
of	O
aborted	O
(	O
rolled	B-Application
back	I-Application
)	O
transactions	O
remains	O
in	O
the	O
related	O
database	O
.	O
</s>
<s>
Overall	O
transaction	B-General_Concept
characterization	O
is	O
usually	O
summarized	O
by	O
the	O
ACID	O
rules	O
below	O
.	O
</s>
<s>
As	O
databases	B-Application
have	O
become	O
distributed	B-Operating_System
,	O
or	O
needed	O
to	O
cooperate	O
in	O
distributed	B-Operating_System
environments	O
(	O
e.g.	O
,	O
Federated	B-General_Concept
databases	I-General_Concept
in	O
the	O
early	O
1990	O
,	O
and	O
Cloud	B-Architecture
computing	I-Architecture
currently	O
)	O
,	O
the	O
effective	O
distribution	O
of	O
concurrency	B-Architecture
control	O
mechanisms	O
has	O
received	O
special	O
attention	O
.	O
</s>
<s>
The	O
concept	O
of	O
a	O
database	B-General_Concept
transaction	I-General_Concept
(	O
or	O
atomic	B-General_Concept
transaction	I-General_Concept
)	O
has	O
evolved	O
in	O
order	O
to	O
enable	O
both	O
a	O
well	O
understood	O
database	B-Application
system	I-Application
behavior	O
in	O
a	O
faulty	O
environment	O
where	O
crashes	B-General_Concept
can	O
happen	O
any	O
time	O
,	O
and	O
recovery	B-Application
from	O
a	O
crash	B-General_Concept
to	O
a	O
well	O
understood	O
database	O
state	O
.	O
</s>
<s>
A	O
database	B-General_Concept
transaction	I-General_Concept
is	O
a	O
unit	B-General_Concept
of	I-General_Concept
work	I-General_Concept
,	O
typically	O
encapsulating	O
a	O
number	O
of	O
operations	O
over	O
a	O
database	O
(	O
e.g.	O
,	O
reading	O
a	O
database	O
object	O
,	O
writing	O
,	O
acquiring	O
lock	O
,	O
etc	O
.	O
</s>
<s>
Each	O
transaction	B-General_Concept
has	O
well	O
defined	O
boundaries	O
in	O
terms	O
of	O
which	O
program/code	O
executions	O
are	O
included	O
in	O
that	O
transaction	B-General_Concept
(	O
determined	O
by	O
the	O
transaction	B-General_Concept
's	O
programmer	O
via	O
special	O
transaction	B-General_Concept
commands	O
)	O
.	O
</s>
<s>
Every	O
database	B-General_Concept
transaction	I-General_Concept
obeys	O
the	O
following	O
rules	O
(	O
by	O
support	O
in	O
the	O
database	B-Application
system	I-Application
;	O
i.e.	O
,	O
a	O
database	B-Application
system	I-Application
is	O
designed	O
to	O
guarantee	O
them	O
for	O
the	O
transactions	O
it	O
runs	O
)	O
:	O
</s>
<s>
Atomicity	B-General_Concept
-	O
Either	O
the	O
effects	O
of	O
all	O
or	O
none	O
of	O
its	O
operations	O
remain	O
(	O
"	O
all	O
or	O
nothing	O
"	O
semantics	O
)	O
when	O
a	O
transaction	B-General_Concept
is	O
completed	O
(	O
committed	O
or	O
aborted	O
respectively	O
)	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
to	O
the	O
outside	O
world	O
a	O
committed	O
transaction	B-General_Concept
appears	O
(	O
by	O
its	O
effects	O
on	O
the	O
database	O
)	O
to	O
be	O
indivisible	O
(	O
atomic	O
)	O
,	O
and	O
an	O
aborted	O
transaction	B-General_Concept
does	O
not	O
affect	O
the	O
database	O
at	O
all	O
.	O
</s>
<s>
Consistency	B-General_Concept
-	O
Every	O
transaction	B-General_Concept
must	O
leave	O
the	O
database	O
in	O
a	O
consistent	O
(	O
correct	O
)	O
state	O
,	O
i.e.	O
,	O
maintain	O
the	O
predetermined	O
integrity	O
rules	O
of	O
the	O
database	O
(	O
constraints	O
upon	O
and	O
among	O
the	O
database	O
's	O
objects	O
)	O
.	O
</s>
<s>
A	O
transaction	B-General_Concept
must	O
transform	O
a	O
database	O
from	O
one	O
consistent	O
state	O
to	O
another	O
consistent	O
state	O
(	O
however	O
,	O
it	O
is	O
the	O
responsibility	O
of	O
the	O
transaction	B-General_Concept
's	O
programmer	O
to	O
make	O
sure	O
that	O
the	O
transaction	B-General_Concept
itself	O
is	O
correct	O
,	O
i.e.	O
,	O
performs	O
correctly	O
what	O
it	O
intends	O
to	O
perform	O
(	O
from	O
the	O
application	O
's	O
point	O
of	O
view	O
)	O
while	O
the	O
predefined	O
integrity	O
rules	O
are	O
enforced	O
by	O
the	O
DBMS	O
)	O
.	O
</s>
<s>
Isolation	B-General_Concept
-	O
Transactions	O
cannot	O
interfere	O
with	O
each	O
other	O
(	O
as	O
an	O
end	O
result	O
of	O
their	O
executions	O
)	O
.	O
</s>
<s>
Moreover	O
,	O
usually	O
(	O
depending	O
on	O
concurrency	B-Architecture
control	O
method	O
)	O
the	O
effects	O
of	O
an	O
incomplete	O
transaction	B-General_Concept
are	O
not	O
even	O
visible	O
to	O
another	O
transaction	B-General_Concept
.	O
</s>
<s>
Providing	O
isolation	B-General_Concept
is	O
the	O
main	O
goal	O
of	O
concurrency	B-Architecture
control	O
.	O
</s>
<s>
Durability	B-General_Concept
-	O
Effects	O
of	O
successful	O
(	O
committed	O
)	O
transactions	O
must	O
persist	O
through	O
crashes	B-General_Concept
(	O
typically	O
by	O
recording	O
the	O
transaction	B-General_Concept
's	O
effects	O
and	O
its	O
commit	O
event	O
in	O
a	O
non-volatile	B-General_Concept
memory	I-General_Concept
)	O
.	O
</s>
<s>
The	O
concept	O
of	O
atomic	B-General_Concept
transaction	I-General_Concept
has	O
been	O
extended	O
during	O
the	O
years	O
to	O
what	O
has	O
become	O
Business	B-Application
transactions	I-Application
which	O
actually	O
implement	O
types	O
of	O
Workflow	B-Operating_System
and	O
are	O
not	O
atomic	O
.	O
</s>
<s>
However	O
also	O
such	O
enhanced	O
transactions	O
typically	O
utilize	O
atomic	B-General_Concept
transactions	I-General_Concept
as	O
components	O
.	O
</s>
<s>
If	O
transactions	O
are	O
executed	O
serially	O
,	O
i.e.	O
,	O
sequentially	O
with	O
no	O
overlap	O
in	O
time	O
,	O
no	O
transaction	B-General_Concept
concurrency	B-Architecture
exists	O
.	O
</s>
<s>
However	O
,	O
if	O
concurrent	B-Architecture
transactions	O
with	O
interleaving	O
operations	O
are	O
allowed	O
in	O
an	O
uncontrolled	O
manner	O
,	O
some	O
unexpected	O
,	O
undesirable	O
results	O
may	O
occur	O
,	O
such	O
as	O
:	O
</s>
<s>
The	O
lost	O
update	O
problem	O
:	O
A	O
second	O
transaction	B-General_Concept
writes	O
a	O
second	O
value	O
of	O
a	O
data-item	O
(	O
datum	O
)	O
on	O
top	O
of	O
a	O
first	O
value	O
written	O
by	O
a	O
first	O
concurrent	B-Architecture
transaction	B-General_Concept
,	O
and	O
the	O
first	O
value	O
is	O
lost	O
to	O
other	O
transactions	O
running	O
concurrently	B-Operating_System
which	O
need	O
,	O
by	O
their	O
precedence	O
,	O
to	O
read	O
the	O
first	O
value	O
.	O
</s>
<s>
The	O
dirty	O
read	O
problem	O
:	O
Transactions	O
read	O
a	O
value	O
written	O
by	O
a	O
transaction	B-General_Concept
that	O
has	O
been	O
later	O
aborted	O
.	O
</s>
<s>
This	O
value	O
disappears	O
from	O
the	O
database	O
upon	O
abort	O
,	O
and	O
should	O
not	O
have	O
been	O
read	O
by	O
any	O
transaction	B-General_Concept
(	O
"	O
dirty	O
read	O
"	O
)	O
.	O
</s>
<s>
The	O
incorrect	O
summary	O
problem	O
:	O
While	O
one	O
transaction	B-General_Concept
takes	O
a	O
summary	O
over	O
the	O
values	O
of	O
all	O
the	O
instances	O
of	O
a	O
repeated	O
data-item	O
,	O
a	O
second	O
transaction	B-General_Concept
updates	O
some	O
instances	O
of	O
that	O
data-item	O
.	O
</s>
<s>
Most	O
high-performance	O
transactional	B-General_Concept
systems	O
need	O
to	O
run	O
transactions	O
concurrently	B-Operating_System
to	O
meet	O
their	O
performance	O
requirements	O
.	O
</s>
<s>
Thus	O
,	O
without	O
concurrency	B-Architecture
control	O
such	O
systems	O
can	O
neither	O
provide	O
correct	O
results	O
nor	O
maintain	O
their	O
databases	B-Application
consistently	O
.	O
</s>
<s>
The	O
main	O
categories	O
of	O
concurrency	B-Architecture
control	O
mechanisms	O
are	O
:	O
</s>
<s>
Optimistic	B-Operating_System
-	O
Delay	O
the	O
checking	O
of	O
whether	O
a	O
transaction	B-General_Concept
meets	O
the	O
isolation	B-General_Concept
and	O
other	O
integrity	O
rules	O
(	O
e.g.	O
,	O
serializability	B-Operating_System
and	O
recoverability	O
)	O
until	O
its	O
end	O
,	O
without	O
blocking	O
any	O
of	O
its	O
(	O
read	O
,	O
write	O
)	O
operations	O
(	O
"	O
and	O
...	O
be	O
optimistic	B-Operating_System
about	O
the	O
rules	O
being	O
met	O
...	O
"	O
)	O
,	O
and	O
then	O
abort	O
a	O
transaction	B-General_Concept
to	O
prevent	O
the	O
violation	O
,	O
if	O
the	O
desired	O
rules	O
are	O
to	O
be	O
violated	O
upon	O
its	O
commit	O
.	O
</s>
<s>
An	O
aborted	O
transaction	B-General_Concept
is	O
immediately	O
restarted	O
and	O
re-executed	O
,	O
which	O
incurs	O
an	O
obvious	O
overhead	O
(	O
versus	O
executing	O
it	O
to	O
the	O
end	O
only	O
once	O
)	O
.	O
</s>
<s>
If	O
not	O
too	O
many	O
transactions	O
are	O
aborted	O
,	O
then	O
being	O
optimistic	B-Operating_System
is	O
usually	O
a	O
good	O
strategy	O
.	O
</s>
<s>
Pessimistic	O
-	O
Block	O
an	O
operation	O
of	O
a	O
transaction	B-General_Concept
,	O
if	O
it	O
may	O
cause	O
violation	O
of	O
the	O
rules	O
,	O
until	O
the	O
possibility	O
of	O
violation	O
disappears	O
.	O
</s>
<s>
Semi-optimistic	O
-	O
Block	O
operations	O
in	O
some	O
situations	O
,	O
if	O
they	O
may	O
cause	O
violation	O
of	O
some	O
rules	O
,	O
and	O
do	O
not	O
block	O
in	O
other	O
situations	O
while	O
delaying	O
rules	O
checking	O
(	O
if	O
needed	O
)	O
to	O
transaction	B-General_Concept
's	O
end	O
,	O
as	O
done	O
with	O
optimistic	B-Operating_System
.	O
</s>
<s>
Different	O
categories	O
provide	O
different	O
performance	O
,	O
i.e.	O
,	O
different	O
average	O
transaction	B-General_Concept
completion	O
rates	O
(	O
throughput	O
)	O
,	O
depending	O
on	O
transaction	B-General_Concept
types	O
mix	O
,	O
computing	O
level	O
of	O
parallelism	O
,	O
and	O
other	O
factors	O
.	O
</s>
<s>
The	O
mutual	O
blocking	O
between	O
two	O
transactions	O
(	O
where	O
each	O
one	O
blocks	O
the	O
other	O
)	O
or	O
more	O
results	O
in	O
a	O
deadlock	B-Operating_System
,	O
where	O
the	O
transactions	O
involved	O
are	O
stalled	O
and	O
cannot	O
reach	O
completion	O
.	O
</s>
<s>
Most	O
non-optimistic	O
mechanisms	O
(	O
with	O
blocking	O
)	O
are	O
prone	O
to	O
deadlocks	B-Operating_System
which	O
are	O
resolved	O
by	O
an	O
intentional	O
abort	O
of	O
a	O
stalled	O
transaction	B-General_Concept
(	O
which	O
releases	O
the	O
other	O
transactions	O
in	O
that	O
deadlock	B-Operating_System
)	O
,	O
and	O
its	O
immediate	O
restart	O
and	O
re-execution	O
.	O
</s>
<s>
The	O
likelihood	O
of	O
a	O
deadlock	B-Operating_System
is	O
typically	O
low	O
.	O
</s>
<s>
Blocking	O
,	O
deadlocks	B-Operating_System
,	O
and	O
aborts	O
all	O
result	O
in	O
performance	O
reduction	O
,	O
and	O
hence	O
the	O
trade-offs	O
between	O
the	O
categories	O
.	O
</s>
<s>
Many	O
methods	O
for	O
concurrency	B-Architecture
control	O
exist	O
.	O
</s>
<s>
Locking	B-Operating_System
(	O
e.g.	O
,	O
Two-phase	B-Operating_System
locking	I-Operating_System
-	O
2PL	O
)	O
-	O
Controlling	O
access	O
to	O
data	O
by	O
locks	B-Operating_System
assigned	O
to	O
the	O
data	O
.	O
</s>
<s>
Access	O
of	O
a	O
transaction	B-General_Concept
to	O
a	O
data	O
item	O
(	O
database	O
object	O
)	O
locked	O
by	O
another	O
transaction	B-General_Concept
may	O
be	O
blocked	O
(	O
depending	O
on	O
lock	O
type	O
and	O
access	O
operation	O
type	O
)	O
until	O
lock	O
release	O
.	O
</s>
<s>
Serialization	O
graph	O
checking	O
(	O
also	O
called	O
Serializability	B-Operating_System
,	O
or	O
Conflict	O
,	O
or	O
Precedence	O
graph	O
checking	O
)	O
-	O
Checking	O
for	O
cycles	O
in	O
the	O
schedule	B-General_Concept
's	O
graph	O
and	O
breaking	O
them	O
by	O
aborts	O
.	O
</s>
<s>
Timestamp	B-Operating_System
ordering	I-Operating_System
(	O
TO	O
)	O
-	O
Assigning	O
timestamps	O
to	O
transactions	O
,	O
and	O
controlling	O
or	O
checking	O
access	O
to	O
data	O
by	O
timestamp	O
order	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
(	O
or	O
Commit	B-Architecture
ordering	I-Architecture
;	O
CO	O
)	O
-	O
Controlling	O
or	O
checking	O
transactions	O
 '	O
chronological	O
order	O
of	O
commit	O
events	O
to	O
be	O
compatible	O
with	O
their	O
respective	O
precedence	O
order	O
.	O
</s>
<s>
Other	O
major	O
concurrency	B-Architecture
control	O
types	O
that	O
are	O
utilized	O
in	O
conjunction	O
with	O
the	O
methods	O
above	O
include	O
:	O
</s>
<s>
Multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
(	O
MVCC	O
)	O
-	O
Increasing	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
,	O
and	O
allowing	O
transactions	O
 '	O
read	O
operations	O
of	O
several	O
last	O
relevant	O
versions	O
(	O
of	O
each	O
object	O
)	O
depending	O
on	O
scheduling	O
method	O
.	O
</s>
<s>
Index	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
-	O
Synchronizing	O
access	O
operations	O
to	O
indexes	O
,	O
rather	O
than	O
to	O
user	O
data	O
.	O
</s>
<s>
Private	O
workspace	O
model	O
(	O
Deferred	O
update	O
)	O
-	O
Each	O
transaction	B-General_Concept
maintains	O
a	O
private	O
workspace	O
for	O
its	O
accessed	O
data	O
,	O
and	O
its	O
changed	O
data	O
become	O
visible	O
outside	O
the	O
transaction	B-General_Concept
only	O
upon	O
its	O
commit	O
(	O
e.g.	O
,	O
Weikum	O
and	O
Vossen	O
2001	O
)	O
.	O
</s>
<s>
This	O
model	O
provides	O
a	O
different	O
concurrency	B-Architecture
control	O
behavior	O
with	O
benefits	O
in	O
many	O
cases	O
.	O
</s>
<s>
The	O
most	O
common	O
mechanism	O
type	O
in	O
database	B-Application
systems	I-Application
since	O
their	O
early	O
days	O
in	O
the	O
1970s	O
has	O
been	O
Strong	B-Operating_System
strict	I-Operating_System
Two-phase	I-Operating_System
locking	I-Operating_System
(	O
SS2PL	B-Operating_System
;	O
also	O
called	O
Rigorous	O
scheduling	O
or	O
Rigorous	O
2PL	O
)	O
which	O
is	O
a	O
special	O
case	O
(	O
variant	O
)	O
of	O
both	O
Two-phase	B-Operating_System
locking	I-Operating_System
(	O
2PL	O
)	O
and	O
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
)	O
.	O
</s>
<s>
In	O
spite	O
of	O
its	O
long	O
name	O
(	O
for	O
historical	O
reasons	O
)	O
the	O
idea	O
of	O
the	O
SS2PL	B-Operating_System
mechanism	O
is	O
simple	O
:	O
"	O
Release	O
all	O
locks	B-Operating_System
applied	O
by	O
a	O
transaction	B-General_Concept
only	O
after	O
the	O
transaction	B-General_Concept
has	O
ended.	O
"	O
</s>
<s>
SS2PL	B-Operating_System
(	O
or	O
Rigorousness	B-Operating_System
)	O
is	O
also	O
the	O
name	O
of	O
the	O
set	O
of	O
all	O
schedules	O
that	O
can	O
be	O
generated	O
by	O
this	O
mechanism	O
,	O
i.e.	O
,	O
these	O
SS2PL	B-Operating_System
(	O
or	O
Rigorous	O
)	O
schedules	O
have	O
the	O
SS2PL	B-Operating_System
(	O
or	O
Rigorousness	B-Operating_System
)	O
property	O
.	O
</s>
<s>
Concurrency	B-Architecture
control	O
mechanisms	O
firstly	O
need	O
to	O
operate	O
correctly	O
,	O
i.e.	O
,	O
to	O
maintain	O
each	O
transaction	B-General_Concept
's	O
integrity	O
rules	O
(	O
as	O
related	O
to	O
concurrency	B-Architecture
;	O
application-specific	O
integrity	O
rule	O
are	O
out	O
of	O
the	O
scope	O
here	O
)	O
while	O
transactions	O
are	O
running	O
concurrently	B-Operating_System
,	O
and	O
thus	O
the	O
integrity	O
of	O
the	O
entire	O
transactional	B-General_Concept
system	O
.	O
</s>
<s>
In	O
addition	O
,	O
increasingly	O
a	O
need	O
exists	O
to	O
operate	O
effectively	O
while	O
transactions	O
are	O
distributed	B-Operating_System
over	O
processes	B-Operating_System
,	O
computers	O
,	O
and	O
computer	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
Other	O
subjects	O
that	O
may	O
affect	O
concurrency	B-Architecture
control	O
are	O
recovery	B-Application
and	O
replication	B-Architecture
.	O
</s>
<s>
For	O
correctness	O
,	O
a	O
common	O
major	O
goal	O
of	O
most	O
concurrency	B-Architecture
control	O
mechanisms	O
is	O
generating	O
schedules	O
with	O
the	O
Serializability	B-Operating_System
property	O
.	O
</s>
<s>
Without	O
serializability	B-Operating_System
undesirable	O
phenomena	O
may	O
occur	O
,	O
e.g.	O
,	O
money	O
may	O
disappear	O
from	O
accounts	O
,	O
or	O
be	O
generated	O
from	O
nowhere	O
.	O
</s>
<s>
Serializability	B-Operating_System
of	O
a	O
schedule	B-General_Concept
means	O
equivalence	O
(	O
in	O
the	O
resulting	O
database	O
values	O
)	O
to	O
some	O
serial	O
schedule	B-General_Concept
with	O
the	O
same	O
transactions	O
(	O
i.e.	O
,	O
in	O
which	O
transactions	O
are	O
sequential	O
with	O
no	O
overlap	O
in	O
time	O
,	O
and	O
thus	O
completely	O
isolated	O
from	O
each	O
other	O
:	O
No	O
concurrent	B-Architecture
access	O
by	O
any	O
two	O
transactions	O
to	O
the	O
same	O
data	O
is	O
possible	O
)	O
.	O
</s>
<s>
Serializability	B-Operating_System
is	O
considered	O
the	O
highest	O
level	O
of	O
isolation	B-General_Concept
among	O
database	B-General_Concept
transactions	I-General_Concept
,	O
and	O
the	O
major	O
correctness	O
criterion	O
for	O
concurrent	B-Architecture
transactions	O
.	O
</s>
<s>
In	O
some	O
cases	O
compromised	O
,	O
relaxed	O
forms	O
of	O
serializability	B-Operating_System
are	O
allowed	O
for	O
better	O
performance	O
(	O
e.g.	O
,	O
the	O
popular	O
Snapshot	B-Operating_System
isolation	I-Operating_System
mechanism	O
)	O
or	O
to	O
meet	O
availability	O
requirements	O
in	O
highly	O
distributed	B-Operating_System
systems	O
(	O
see	O
Eventual	B-General_Concept
consistency	I-General_Concept
)	O
,	O
but	O
only	O
if	O
application	O
's	O
correctness	O
is	O
not	O
violated	O
by	O
the	O
relaxation	O
(	O
e.g.	O
,	O
no	O
relaxation	O
is	O
allowed	O
for	O
money	O
transactions	O
,	O
since	O
by	O
relaxation	O
money	O
can	O
disappear	O
,	O
or	O
appear	O
from	O
nowhere	O
)	O
.	O
</s>
<s>
Almost	O
all	O
implemented	O
concurrency	B-Architecture
control	O
mechanisms	O
achieve	O
serializability	B-Operating_System
by	O
providing	O
Conflict	O
serializablity	O
,	O
a	O
broad	O
special	O
case	O
of	O
serializability	B-Operating_System
(	O
i.e.	O
,	O
it	O
covers	O
,	O
enables	O
most	O
serializable	B-Operating_System
schedules	O
,	O
and	O
does	O
not	O
impose	O
significant	O
additional	O
delay-causing	O
constraints	O
)	O
which	O
can	O
be	O
implemented	O
efficiently	O
.	O
</s>
<s>
Comment	O
:	O
While	O
in	O
the	O
general	O
area	O
of	O
systems	O
the	O
term	O
"	O
recoverability	O
"	O
may	O
refer	O
to	O
the	O
ability	O
of	O
a	O
system	O
to	O
recover	O
from	O
failure	O
or	O
from	O
an	O
incorrect/forbidden	O
state	O
,	O
within	O
concurrency	B-Architecture
control	O
of	O
database	B-Application
systems	I-Application
this	O
term	O
has	O
received	O
a	O
specific	O
meaning	O
.	O
</s>
<s>
Concurrency	B-Architecture
control	O
typically	O
also	O
ensures	O
the	O
Recoverability	O
property	O
of	O
schedules	O
for	O
maintaining	O
correctness	O
in	O
cases	O
of	O
aborted	O
transactions	O
(	O
which	O
can	O
always	O
happen	O
for	O
many	O
reasons	O
)	O
.	O
</s>
<s>
Recoverability	O
(	O
from	O
abort	O
)	O
means	O
that	O
no	O
committed	O
transaction	B-General_Concept
in	O
a	O
schedule	B-General_Concept
has	O
read	O
data	O
written	O
by	O
an	O
aborted	O
transaction	B-General_Concept
.	O
</s>
<s>
Reading	O
such	O
data	O
violates	O
the	O
consistency	B-General_Concept
rule	O
of	O
ACID	O
.	O
</s>
<s>
Unlike	O
Serializability	B-Operating_System
,	O
Recoverability	O
cannot	O
be	O
compromised	O
,	O
relaxed	O
at	O
any	O
case	O
,	O
since	O
any	O
relaxation	O
results	O
in	O
quick	O
database	O
integrity	O
violation	O
upon	O
aborts	O
.	O
</s>
<s>
The	O
major	O
methods	O
listed	O
above	O
provide	O
serializability	B-Operating_System
mechanisms	O
.	O
</s>
<s>
A	O
commonly	O
utilized	O
special	O
case	O
of	O
recoverability	O
is	O
Strictness	O
,	O
which	O
allows	O
efficient	O
database	O
recovery	B-Application
from	O
failure	O
(	O
but	O
excludes	O
optimistic	B-Operating_System
implementations	O
;	O
e.g.	O
,	O
Strict	O
CO	O
(	O
SCO	O
)	O
cannot	O
have	O
an	O
optimistic	B-Operating_System
implementation	O
,	O
but	O
has	O
semi-optimistic	O
ones	O
)	O
.	O
</s>
<s>
Comment	O
:	O
Note	O
that	O
the	O
Recoverability	O
property	O
is	O
needed	O
even	O
if	O
no	O
database	O
failure	O
occurs	O
and	O
no	O
database	O
recovery	B-Application
from	O
failure	O
is	O
needed	O
.	O
</s>
<s>
It	O
is	O
rather	O
needed	O
to	O
correctly	O
automatically	O
handle	O
transaction	B-General_Concept
aborts	O
,	O
which	O
may	O
be	O
unrelated	O
to	O
database	O
failure	O
and	O
recovery	B-Application
from	O
it	O
.	O
</s>
<s>
With	O
the	O
fast	O
technological	O
development	O
of	O
computing	O
the	O
difference	O
between	O
local	O
and	O
distributed	B-Operating_System
computing	O
over	O
low	O
latency	O
networks	B-Architecture
or	O
buses	B-General_Concept
is	O
blurring	O
.	O
</s>
<s>
Thus	O
the	O
quite	O
effective	O
utilization	O
of	O
local	O
techniques	O
in	O
such	O
distributed	B-Operating_System
environments	O
is	O
common	O
,	O
e.g.	O
,	O
in	O
computer	B-Architecture
clusters	I-Architecture
and	O
multi-core	B-Architecture
processors	I-Architecture
.	O
</s>
<s>
However	O
the	O
local	O
techniques	O
have	O
their	O
limitations	O
and	O
use	O
multi-processes	O
(	O
or	O
threads	O
)	O
supported	O
by	O
multi-processors	B-Operating_System
(	O
or	O
multi-cores	B-Architecture
)	O
to	O
scale	O
.	O
</s>
<s>
This	O
often	O
turns	O
transactions	O
into	O
distributed	B-Operating_System
ones	O
,	O
if	O
they	O
themselves	O
need	O
to	O
span	O
multi-processes	O
.	O
</s>
<s>
In	O
these	O
cases	O
most	O
local	O
concurrency	B-Architecture
control	O
techniques	O
do	B-Architecture
not	I-Architecture
scale	I-Architecture
well	O
.	O
</s>
<s>
As	O
database	B-Application
systems	I-Application
have	O
become	O
distributed	B-Operating_System
,	O
or	O
started	O
to	O
cooperate	O
in	O
distributed	B-Operating_System
environments	O
(	O
e.g.	O
,	O
Federated	B-General_Concept
databases	I-General_Concept
in	O
the	O
early	O
1990s	O
,	O
and	O
nowadays	O
Grid	B-Architecture
computing	I-Architecture
,	O
Cloud	B-Architecture
computing	I-Architecture
,	O
and	O
networks	B-Architecture
with	O
smartphones	B-Application
)	O
,	O
some	O
transactions	O
have	O
become	O
distributed	B-Operating_System
.	O
</s>
<s>
A	O
distributed	B-General_Concept
transaction	I-General_Concept
means	O
that	O
the	O
transaction	B-General_Concept
spans	O
processes	B-Operating_System
,	O
and	O
may	O
span	O
computers	O
and	O
geographical	O
sites	O
.	O
</s>
<s>
This	O
generates	O
a	O
need	O
in	O
effective	O
distributed	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
mechanisms	O
.	O
</s>
<s>
Achieving	O
the	O
Serializability	B-Operating_System
property	O
of	O
a	O
distributed	B-Operating_System
system	O
's	O
schedule	B-General_Concept
(	O
see	O
Distributed	B-Operating_System
serializability	I-Operating_System
and	O
Global	B-Operating_System
serializability	I-Operating_System
(	O
Modular	B-Operating_System
serializability	I-Operating_System
)	O
)	O
effectively	O
poses	O
special	O
challenges	O
typically	O
not	O
met	O
by	O
most	O
of	O
the	O
regular	O
serializability	B-Operating_System
mechanisms	O
,	O
originally	O
designed	O
to	O
operate	O
locally	O
.	O
</s>
<s>
This	O
is	O
especially	O
due	O
to	O
a	O
need	O
in	O
costly	O
distribution	O
of	O
concurrency	B-Architecture
control	O
information	O
amid	O
communication	O
and	O
computer	O
latency	O
.	O
</s>
<s>
The	O
only	O
known	O
general	O
effective	O
technique	O
for	O
distribution	O
is	O
Commitment	B-Architecture
ordering	I-Architecture
,	O
which	O
was	O
disclosed	O
publicly	O
in	O
1991	O
(	O
after	O
being	O
patented	O
)	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
(	O
Commit	B-Architecture
ordering	I-Architecture
,	O
CO	O
;	O
Raz	O
1992	O
)	O
means	O
that	O
transactions	O
 '	O
chronological	O
order	O
of	O
commit	O
events	O
is	O
kept	O
compatible	O
with	O
their	O
respective	O
precedence	O
order	O
.	O
</s>
<s>
CO	O
does	O
not	O
require	O
the	O
distribution	O
of	O
concurrency	B-Architecture
control	O
information	O
and	O
provides	O
a	O
general	O
effective	O
solution	O
(	O
reliable	O
,	O
high-performance	O
,	O
and	O
scalable	B-Architecture
)	O
for	O
both	O
distributed	B-Operating_System
and	O
global	B-Operating_System
serializability	I-Operating_System
,	O
also	O
in	O
a	O
heterogeneous	O
environment	O
with	O
database	B-Application
systems	I-Application
(	O
or	O
other	O
transactional	B-General_Concept
objects	O
)	O
with	O
different	O
(	O
any	O
)	O
concurrency	B-Architecture
control	O
mechanisms	O
.	O
</s>
<s>
CO	O
is	O
indifferent	O
to	O
which	O
mechanism	O
is	O
utilized	O
,	O
since	O
it	O
does	O
not	O
interfere	O
with	O
any	O
transaction	B-General_Concept
operation	O
scheduling	O
(	O
which	O
most	O
mechanisms	O
control	O
)	O
,	O
and	O
only	O
determines	O
the	O
order	O
of	O
commit	O
events	O
.	O
</s>
<s>
Thus	O
,	O
CO	O
enables	O
the	O
efficient	O
distribution	O
of	O
all	O
other	O
mechanisms	O
,	O
and	O
also	O
the	O
distribution	O
of	O
a	O
mix	O
of	O
different	O
(	O
any	O
)	O
local	O
mechanisms	O
,	O
for	O
achieving	O
distributed	B-Operating_System
and	O
global	B-Operating_System
serializability	I-Operating_System
.	O
</s>
<s>
The	O
existence	O
of	O
such	O
a	O
solution	O
has	O
been	O
considered	O
"	O
unlikely	O
"	O
until	O
1991	O
,	O
and	O
by	O
many	O
experts	O
also	O
later	O
,	O
due	O
to	O
misunderstanding	O
of	O
the	O
CO	O
solution	O
(	O
see	O
Quotations	O
in	O
Global	B-Operating_System
serializability	I-Operating_System
)	O
.	O
</s>
<s>
An	O
important	O
side-benefit	O
of	O
CO	O
is	O
automatic	O
distributed	B-Operating_System
deadlock	B-Operating_System
resolution	O
.	O
</s>
<s>
Contrary	O
to	O
CO	O
,	O
virtually	O
all	O
other	O
techniques	O
(	O
when	O
not	O
combined	O
with	O
CO	O
)	O
are	O
prone	O
to	O
distributed	B-Operating_System
deadlocks	B-Operating_System
(	O
also	O
called	O
global	O
deadlocks	B-Operating_System
)	O
which	O
need	O
special	O
handling	O
.	O
</s>
<s>
CO	O
is	O
also	O
the	O
name	O
of	O
the	O
resulting	O
schedule	B-General_Concept
property	O
:	O
A	O
schedule	B-General_Concept
has	O
the	O
CO	O
property	O
if	O
the	O
chronological	O
order	O
of	O
its	O
transactions	O
 '	O
commit	O
events	O
is	O
compatible	O
with	O
the	O
respective	O
transactions	O
 '	O
precedence	O
(	O
partial	O
)	O
order	O
.	O
</s>
<s>
SS2PL	B-Operating_System
mentioned	O
above	O
is	O
a	O
variant	O
(	O
special	O
case	O
)	O
of	O
CO	O
and	O
thus	O
also	O
effective	O
to	O
achieve	O
distributed	B-Operating_System
and	O
global	B-Operating_System
serializability	I-Operating_System
.	O
</s>
<s>
It	O
also	O
provides	O
automatic	O
distributed	B-Operating_System
deadlock	B-Operating_System
resolution	O
(	O
a	O
fact	O
overlooked	O
in	O
the	O
research	O
literature	O
even	O
after	O
CO	O
's	O
publication	O
)	O
,	O
as	O
well	O
as	O
Strictness	O
and	O
thus	O
Recoverability	O
.	O
</s>
<s>
Possessing	O
these	O
desired	O
properties	O
together	O
with	O
known	O
efficient	O
locking	B-Operating_System
based	O
implementations	O
explains	O
SS2PL	B-Operating_System
's	O
popularity	O
.	O
</s>
<s>
SS2PL	B-Operating_System
has	O
been	O
utilized	O
to	O
efficiently	O
achieve	O
Distributed	B-Operating_System
and	O
Global	B-Operating_System
serializability	I-Operating_System
since	O
the	O
1980	O
,	O
and	O
has	O
become	O
the	O
de	O
facto	O
standard	O
for	O
it	O
.	O
</s>
<s>
However	O
,	O
SS2PL	B-Operating_System
is	O
blocking	O
and	O
constraining	O
(	O
pessimistic	O
)	O
,	O
and	O
with	O
the	O
proliferation	O
of	O
distribution	O
and	O
utilization	O
of	O
systems	O
different	O
from	O
traditional	O
database	B-Application
systems	I-Application
(	O
e.g.	O
,	O
as	O
in	O
Cloud	B-Architecture
computing	I-Architecture
)	O
,	O
less	O
constraining	O
types	O
of	O
CO	O
(	O
e.g.	O
,	O
Optimistic	B-Operating_System
CO	O
)	O
may	O
be	O
needed	O
for	O
better	O
performance	O
.	O
</s>
<s>
The	O
Distributed	B-Operating_System
conflict	O
serializability	B-Operating_System
property	O
in	O
its	O
general	O
form	O
is	O
difficult	O
to	O
achieve	O
efficiently	O
,	O
but	O
it	O
is	O
achieved	O
efficiently	O
via	O
its	O
special	O
case	O
Distributed	B-Operating_System
CO	O
:	O
Each	O
local	O
component	O
(	O
e.g.	O
,	O
a	O
local	O
DBMS	O
)	O
needs	O
both	O
to	O
provide	O
some	O
form	O
of	O
CO	O
,	O
and	O
enforce	O
a	O
special	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
for	O
the	O
Two-phase	B-General_Concept
commit	I-General_Concept
protocol	I-General_Concept
(	O
2PC	B-General_Concept
:	O
utilized	O
to	O
commit	O
distributed	B-General_Concept
transactions	I-General_Concept
)	O
.	O
</s>
<s>
Differently	O
from	O
the	O
general	O
Distributed	B-Operating_System
CO	O
,	O
Distributed	B-Operating_System
SS2PL	B-Operating_System
exists	O
automatically	O
when	O
all	O
local	O
components	O
are	O
SS2PL	B-Operating_System
based	O
(	O
in	O
each	O
component	O
CO	O
exists	O
,	O
implied	O
,	O
and	O
the	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
is	O
now	O
met	O
automatically	O
)	O
.	O
</s>
<s>
This	O
fact	O
has	O
been	O
known	O
and	O
utilized	O
since	O
the	O
1980s	O
(	O
i.e.	O
,	O
that	O
SS2PL	B-Operating_System
exists	O
globally	O
,	O
without	O
knowing	O
about	O
CO	O
)	O
for	O
efficient	O
Distributed	B-Operating_System
SS2PL	B-Operating_System
,	O
which	O
implies	O
Distributed	B-Operating_System
serializability	I-Operating_System
and	O
strictness	O
(	O
e.g.	O
,	O
see	O
Raz	O
1992	O
,	O
page	O
293	O
;	O
it	O
is	O
also	O
implied	O
in	O
Bernstein	O
et	O
al	O
.	O
</s>
<s>
Less	O
constrained	O
Distributed	B-Operating_System
serializability	I-Operating_System
and	O
strictness	O
can	O
be	O
efficiently	O
achieved	O
by	O
Distributed	B-Operating_System
Strict	O
CO	O
(	O
SCO	O
)	O
,	O
or	O
by	O
a	O
mix	O
of	O
SS2PL	B-Operating_System
based	O
and	O
SCO	O
based	O
local	O
components	O
.	O
</s>
<s>
About	O
the	O
references	O
and	O
Commitment	B-Architecture
ordering	I-Architecture
:	O
(	O
Bernstein	O
et	O
al	O
.	O
</s>
<s>
The	O
CO	O
schedule	B-General_Concept
property	O
is	O
called	O
Dynamic	O
atomicity	B-General_Concept
in	O
(	O
Lynch	O
et	O
al	O
.	O
</s>
<s>
(	O
Raz	O
1992	O
)	O
was	O
the	O
first	O
refereed	O
and	O
accepted	O
for	O
publication	O
article	O
about	O
CO	O
algorithms	O
(	O
however	O
,	O
publications	O
about	O
an	O
equivalent	O
Dynamic	O
atomicity	B-General_Concept
property	O
can	O
be	O
traced	O
to	O
1988	O
)	O
.	O
</s>
<s>
(	O
Bernstein	O
and	O
Newcomer	O
2009	O
)	O
note	O
CO	O
as	O
one	O
of	O
the	O
four	O
major	O
concurrency	B-Architecture
control	O
methods	O
,	O
and	O
CO	O
's	O
ability	O
to	O
provide	O
interoperability	O
among	O
other	O
methods	O
.	O
</s>
<s>
Unlike	O
Serializability	B-Operating_System
,	O
Distributed	B-Operating_System
recoverability	O
and	O
Distributed	B-Operating_System
strictness	O
can	O
be	O
achieved	O
efficiently	O
in	O
a	O
straightforward	O
way	O
,	O
similarly	O
to	O
the	O
way	O
Distributed	B-Operating_System
CO	O
is	O
achieved	O
:	O
In	O
each	O
database	B-Application
system	I-Application
they	O
have	O
to	O
be	O
applied	O
locally	O
,	O
and	O
employ	O
a	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
for	O
the	O
Two-phase	B-General_Concept
commit	I-General_Concept
protocol	I-General_Concept
(	O
2PC	B-General_Concept
;	O
Raz	O
1992	O
,	O
page	O
307	O
)	O
.	O
</s>
<s>
As	O
has	O
been	O
mentioned	O
above	O
,	O
Distributed	B-Operating_System
SS2PL	B-Operating_System
,	O
including	O
Distributed	B-Operating_System
strictness	O
(	O
recoverability	O
)	O
and	O
Distributed	B-Operating_System
commitment	B-Architecture
ordering	I-Architecture
(	O
serializability	B-Operating_System
)	O
,	O
automatically	O
employs	O
the	O
needed	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
,	O
and	O
is	O
achieved	O
(	O
globally	O
)	O
when	O
employed	O
locally	O
in	O
each	O
(	O
local	O
)	O
database	B-Application
system	I-Application
(	O
as	O
has	O
been	O
known	O
and	O
utilized	O
for	O
many	O
years	O
;	O
as	O
a	O
matter	O
of	O
fact	O
locality	O
is	O
defined	O
by	O
the	O
boundary	O
of	O
a	O
2PC	B-General_Concept
participant	O
(	O
Raz	O
1992	O
)	O
)	O
.	O
</s>
<s>
All	O
systems	O
are	O
prone	O
to	O
failures	O
,	O
and	O
handling	O
recovery	B-Application
from	O
failure	O
is	O
a	O
must	O
.	O
</s>
<s>
The	O
properties	O
of	O
the	O
generated	O
schedules	O
,	O
which	O
are	O
dictated	O
by	O
the	O
concurrency	B-Architecture
control	O
mechanism	O
,	O
may	O
affect	O
the	O
effectiveness	O
and	O
efficiency	O
of	O
recovery	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Strictness	O
property	O
(	O
mentioned	O
in	O
the	O
section	O
Recoverability	O
above	O
)	O
is	O
often	O
desirable	O
for	O
an	O
efficient	O
recovery	B-Application
.	O
</s>
<s>
For	O
high	O
availability	O
database	O
objects	O
are	O
often	O
replicated	B-Architecture
.	O
</s>
<s>
Updates	O
of	O
replicas	B-Architecture
of	O
a	O
same	O
database	O
object	O
need	O
to	O
be	O
kept	O
synchronized	O
.	O
</s>
<s>
This	O
may	O
affect	O
the	O
way	O
concurrency	B-Architecture
control	O
is	O
done	O
(	O
e.g.	O
,	O
Gray	O
et	O
al	O
.	O
</s>
