<s>
In	O
concurrency	B-Operating_System
control	I-Operating_System
of	O
databases	B-Application
,	O
transaction	B-General_Concept
processing	I-General_Concept
(	O
transaction	O
management	O
)	O
,	O
and	O
other	O
transactional	O
distributed	B-Architecture
applications	I-Architecture
,	O
global	B-General_Concept
serializability	B-Operating_System
(	O
or	O
modular	B-Operating_System
serializability	I-Operating_System
)	O
is	O
a	O
property	O
of	O
a	O
global	B-General_Concept
schedule	B-General_Concept
of	O
transactions	B-General_Concept
.	O
</s>
<s>
A	O
global	B-General_Concept
schedule	B-General_Concept
is	O
the	O
unified	O
schedule	B-General_Concept
of	O
all	O
the	O
individual	O
database	O
(	O
and	O
other	O
transactional	O
object	O
)	O
schedules	O
in	O
a	O
multidatabase	O
environment	O
(	O
e.g.	O
,	O
federated	B-General_Concept
database	I-General_Concept
)	O
.	O
</s>
<s>
Complying	O
with	O
global	B-General_Concept
serializability	B-Operating_System
means	O
that	O
the	O
global	B-General_Concept
schedule	B-General_Concept
is	O
serializable	B-Operating_System
,	O
has	O
the	O
serializability	B-Operating_System
property	O
,	O
while	O
each	O
component	O
database	O
(	O
module	B-Architecture
)	O
has	O
a	O
serializable	B-Operating_System
schedule	B-General_Concept
as	O
well	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
a	O
collection	O
of	O
serializable	B-Operating_System
components	O
provides	O
overall	O
system	O
serializability	B-Operating_System
,	O
which	O
is	O
usually	O
incorrect	O
.	O
</s>
<s>
A	O
need	O
in	O
correctness	O
across	O
databases	B-Application
in	O
multidatabase	O
systems	O
makes	O
global	B-General_Concept
serializability	B-Operating_System
a	O
major	O
goal	O
for	O
global	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
(	O
or	O
modular	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
)	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
as	O
well	O
as	O
increase	O
in	O
systems	B-Application
management	I-Application
sophistication	O
,	O
the	O
need	O
for	O
atomic	O
distributed	B-General_Concept
transactions	I-General_Concept
and	O
thus	O
effective	O
global	B-General_Concept
serializability	B-Operating_System
techniques	O
,	O
to	O
ensure	O
correctness	O
in	O
and	O
among	O
distributed	B-Operating_System
transactional	O
applications	O
,	O
seems	O
to	O
increase	O
.	O
</s>
<s>
In	O
a	O
federated	B-General_Concept
database	I-General_Concept
system	I-General_Concept
or	O
any	O
other	O
more	O
loosely	O
defined	O
multidatabase	O
system	O
,	O
which	O
are	O
typically	O
distributed	B-Operating_System
in	O
a	O
communication	O
network	O
,	O
transactions	B-General_Concept
span	O
multiple	O
(	O
and	O
possibly	O
distributed	B-Operating_System
)	O
databases	B-Application
.	O
</s>
<s>
Enforcing	O
global	B-General_Concept
serializability	B-Operating_System
in	O
such	O
system	O
,	O
where	O
different	O
databases	B-Application
may	O
use	O
different	O
types	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
is	O
problematic	O
.	O
</s>
<s>
Even	O
if	O
every	O
local	O
schedule	B-General_Concept
of	O
a	O
single	O
database	O
is	O
serializable	B-Operating_System
,	O
the	O
global	B-General_Concept
schedule	B-General_Concept
of	O
a	O
whole	O
system	O
is	O
not	O
necessarily	O
serializable	B-Operating_System
.	O
</s>
<s>
The	O
massive	O
communication	O
exchanges	O
of	O
conflict	O
information	O
needed	O
between	O
databases	B-Application
to	O
reach	O
conflict	O
serializability	B-Operating_System
globally	O
would	O
lead	O
to	O
unacceptable	O
performance	O
,	O
primarily	O
due	O
to	O
computer	O
and	O
communication	O
latency	O
.	O
</s>
<s>
Achieving	O
global	B-General_Concept
serializability	B-Operating_System
effectively	O
over	O
different	O
types	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
has	O
been	O
open	O
for	O
several	O
years	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
(	O
or	O
Commit	B-Architecture
ordering	I-Architecture
;	O
CO	O
)	O
,	O
a	O
serializability	B-Operating_System
technique	O
publicly	O
introduced	O
in	O
1991	O
by	O
Yoav	O
Raz	O
from	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
,	O
provides	O
an	O
effective	O
general	O
solution	O
for	O
global	B-General_Concept
(	O
conflict	O
)	O
serializability	B-Operating_System
across	O
any	O
collection	O
of	O
database	B-Application
systems	I-Application
and	O
other	O
transactional	O
objects	O
,	O
with	O
possibly	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	O
.	O
</s>
<s>
CO	O
does	O
not	O
need	O
the	O
distribution	O
of	O
conflict	O
information	O
,	O
but	O
rather	O
utilizes	O
the	O
already	O
needed	O
(	O
unmodified	O
)	O
atomic	O
commitment	O
protocol	O
messages	O
without	O
any	O
further	O
communication	O
between	O
databases	B-Application
.	O
</s>
<s>
It	O
also	O
allows	O
optimistic	B-Operating_System
(	O
non-blocking	O
)	O
implementations	O
.	O
</s>
<s>
CO	O
generalizes	O
strong	B-Operating_System
strict	I-Operating_System
two	I-Operating_System
phase	I-Operating_System
locking	I-Operating_System
(	O
SS2PL	B-Operating_System
)	O
,	O
which	O
in	O
conjunction	O
with	O
the	O
two-phase	B-General_Concept
commit	I-General_Concept
(	O
2PC	B-General_Concept
)	O
protocol	O
is	O
the	O
de	O
facto	O
standard	O
for	O
achieving	O
global	B-General_Concept
serializability	B-Operating_System
across	O
(	O
SS2PL	B-Operating_System
based	O
)	O
database	B-Application
systems	I-Application
.	O
</s>
<s>
As	O
a	O
result	O
,	O
CO	O
compliant	O
database	B-Application
systems	I-Application
(	O
with	O
any	O
,	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
types	O
)	O
can	O
transparently	O
join	O
existing	O
SS2PL	B-Operating_System
based	O
solutions	O
for	O
global	B-General_Concept
serializability	B-Operating_System
.	O
</s>
<s>
The	O
same	O
applies	O
also	O
to	O
all	O
other	O
multiple	O
(	O
transactional	O
)	O
object	O
systems	O
that	O
use	O
atomic	O
transactions	B-General_Concept
and	O
need	O
global	B-General_Concept
serializability	B-Operating_System
for	O
correctness	O
(	O
see	O
examples	O
above	O
;	O
nowadays	O
such	O
need	O
is	O
not	O
smaller	O
than	O
with	O
database	B-Application
systems	I-Application
,	O
the	O
origin	O
of	O
atomic	O
transactions	B-General_Concept
)	O
.	O
</s>
<s>
Seamless	O
,	O
low	O
overhead	O
integration	O
with	O
any	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
,	O
with	O
neither	O
changing	O
any	O
transaction	O
's	O
operation	O
scheduling	O
or	O
blocking	O
it	O
,	O
nor	O
adding	O
any	O
new	O
operation	O
.	O
</s>
<s>
Heterogeneity	B-General_Concept
:	O
Global	B-General_Concept
serializability	B-Operating_System
is	O
achieved	O
across	O
multiple	O
transactional	O
objects	O
(	O
e.g.	O
,	O
database	B-Application
management	I-Application
systems	I-Application
)	O
with	O
different	O
(	O
any	O
)	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	O
,	O
without	O
interfering	O
with	O
the	O
mechanisms	O
 '	O
operations	O
.	O
</s>
<s>
Modularity	B-Architecture
:	O
Transactional	O
objects	O
can	O
be	O
added	O
and	O
removed	O
transparently	O
.	O
</s>
<s>
Automatic	O
global	B-General_Concept
deadlock	B-Operating_System
resolution	O
.	O
</s>
<s>
All	O
these	O
aspects	O
,	O
except	O
the	O
first	O
two	O
,	O
are	O
also	O
possessed	O
by	O
the	O
popular	O
SS2PL	B-Operating_System
,	O
which	O
is	O
a	O
(	O
constrained	O
,	O
blocking	O
)	O
special	O
case	O
of	O
CO	O
and	O
inherits	O
many	O
of	O
CO	O
's	O
qualities	O
.	O
</s>
<s>
Find	O
an	O
efficient	O
(	O
high-performance	O
and	O
fault	B-General_Concept
tolerant	I-General_Concept
)	O
method	O
to	O
enforce	O
Global	B-General_Concept
serializability	B-Operating_System
(	O
global	B-General_Concept
conflict	O
serializability	B-Operating_System
)	O
in	O
a	O
heterogeneous	O
distributed	B-Operating_System
environment	O
of	O
multiple	O
autonomous	O
database	B-Application
systems	I-Application
.	O
</s>
<s>
The	O
database	B-Application
systems	I-Application
may	O
employ	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
methods	O
.	O
</s>
<s>
No	O
limitation	O
should	O
be	O
imposed	O
on	O
the	O
operations	O
of	O
either	O
local	O
transactions	B-General_Concept
(	O
confined	O
to	O
a	O
single	O
database	B-Application
system	I-Application
)	O
or	O
global	B-General_Concept
transactions	I-General_Concept
(	O
span	O
two	O
or	O
more	O
database	B-Application
systems	I-Application
)	O
.	O
</s>
<s>
Lack	O
of	O
an	O
appropriate	O
solution	O
for	O
the	O
global	B-General_Concept
serializability	B-Operating_System
problem	O
has	O
driven	O
researchers	O
to	O
look	O
for	O
alternatives	O
to	O
serializability	B-Operating_System
as	O
a	O
correctness	O
criterion	O
in	O
a	O
multidatabase	O
environment	O
(	O
e.g.	O
,	O
see	O
Relaxing	O
global	B-General_Concept
serializability	B-Operating_System
below	O
)	O
,	O
and	O
the	O
problem	O
has	O
been	O
characterized	O
as	O
difficult	O
and	O
open	O
.	O
</s>
<s>
"	O
Without	O
knowledge	O
about	O
local	O
as	O
well	O
as	O
global	B-General_Concept
transactions	I-General_Concept
,	O
it	O
is	O
highly	O
unlikely	O
that	O
efficient	O
global	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
can	O
be	O
provided	O
...	O
Additional	O
complications	O
occur	O
when	O
different	O
component	O
DBMSs	O
[	O
Database	O
Management	O
Systems ]	O
and	O
the	O
FDBMSs	O
[	O
Federated	B-General_Concept
Database	I-General_Concept
Management	O
Systems ]	O
support	O
different	O
concurrency	O
mechanisms	O
...	O
</s>
<s>
It	O
is	O
unlikely	O
that	O
a	O
theoretically	O
elegant	O
solution	O
that	O
provides	O
conflict	O
serializability	B-Operating_System
without	O
sacrificing	O
performance	O
(	O
i.e.	O
,	O
concurrency	O
and/or	O
response	O
time	O
)	O
and	O
availability	O
exists.	O
"	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
,	O
publicly	O
introduced	O
in	O
May	O
1991	O
(	O
see	O
below	O
)	O
,	O
provides	O
an	O
efficient	O
elegant	O
general	O
solution	O
,	O
from	O
both	O
practical	O
and	O
theoretical	O
points	O
of	O
view	O
,	O
to	O
the	O
global	B-General_Concept
serializability	B-Operating_System
problem	O
across	O
database	B-Application
systems	I-Application
with	O
possibly	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	O
.	O
</s>
<s>
It	O
provides	O
conflict	O
serializability	B-Operating_System
with	O
no	O
negative	O
effect	O
on	O
availability	O
,	O
and	O
with	O
no	O
worse	O
performance	O
than	O
the	O
de	O
facto	O
standard	O
for	O
global	B-General_Concept
serializability	B-Operating_System
,	O
CO	O
's	O
special	O
case	O
strong	B-Operating_System
strict	I-Operating_System
two-phase	I-Operating_System
locking	I-Operating_System
(	O
SS2PL	B-Operating_System
)	O
.	O
</s>
<s>
It	O
requires	O
knowledge	O
about	O
neither	O
local	O
nor	O
global	B-General_Concept
transactions	I-General_Concept
.	O
</s>
<s>
"	O
Transaction	O
management	O
in	O
a	O
heterogeneous	O
,	O
distributed	B-Operating_System
database	I-Operating_System
system	O
is	O
a	O
difficult	O
issue	O
.	O
</s>
<s>
The	O
main	O
problem	O
is	O
that	O
each	O
of	O
the	O
local	O
database	B-Application
management	I-Application
systems	I-Application
may	O
be	O
using	O
a	O
different	O
type	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
scheme	O
.	O
</s>
<s>
Integrating	O
this	O
is	O
a	O
challenging	O
problem	O
,	O
made	O
worse	O
if	O
we	O
wish	O
to	O
preserve	O
the	O
local	O
autonomy	O
of	O
each	O
of	O
the	O
local	O
databases	B-Application
,	O
and	O
allow	O
local	O
and	O
global	B-General_Concept
transactions	I-General_Concept
to	O
execute	O
in	O
parallel	O
.	O
</s>
<s>
One	O
simple	O
solution	O
is	O
to	O
restrict	O
global	B-General_Concept
transactions	I-General_Concept
to	O
retrieve-only	O
access	O
.	O
</s>
<s>
However	O
,	O
the	O
issue	O
of	O
reliable	O
transaction	O
management	O
in	O
the	O
general	O
case	O
,	O
where	O
global	B-General_Concept
and	O
local	O
transactions	B-General_Concept
are	O
allowed	O
to	O
both	O
read	O
and	O
write	O
data	O
,	O
is	O
still	O
open.	O
"	O
</s>
<s>
The	O
commitment	B-Architecture
ordering	I-Architecture
solution	O
comprises	O
effective	O
integration	O
of	O
autonomous	O
database	B-Application
management	I-Application
systems	I-Application
with	O
possibly	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	O
.	O
</s>
<s>
This	O
while	O
local	O
and	O
global	B-General_Concept
transactions	I-General_Concept
execute	O
in	O
parallel	O
without	O
restricting	O
any	O
read	O
or	O
write	O
operation	O
in	O
either	O
local	O
or	O
global	B-General_Concept
transactions	I-General_Concept
,	O
and	O
without	O
compromising	O
the	O
systems	O
 '	O
autonomy	O
.	O
</s>
<s>
Even	O
in	O
later	O
years	O
,	O
after	O
the	O
public	O
introduction	O
of	O
the	O
Commitment	B-Architecture
ordering	I-Architecture
general	O
solution	O
in	O
1991	O
,	O
the	O
problem	O
still	O
has	O
been	O
considered	O
by	O
many	O
unsolvable	O
:	O
</s>
<s>
"	O
We	O
present	O
a	O
transaction	O
model	O
for	O
multidatabase	O
systems	O
with	O
autonomous	O
component	O
systems	O
,	O
coined	O
heterogeneous	O
3-level	O
transactions	B-General_Concept
.	O
</s>
<s>
The	O
quotation	O
above	O
is	O
from	O
a	O
1997	O
article	O
proposing	O
a	O
relaxed	O
global	B-General_Concept
serializability	B-Operating_System
solution	O
(	O
see	O
Relaxing	O
global	B-General_Concept
serializability	B-Operating_System
below	O
)	O
,	O
and	O
referencing	O
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
)	O
articles	O
.	O
</s>
<s>
"	O
The	O
concept	O
of	O
serializability	B-Operating_System
has	O
been	O
the	O
traditionally	O
accepted	O
correctness	O
criterion	O
in	O
database	B-Application
systems	I-Application
.	O
</s>
<s>
However	O
in	O
multidatabase	O
systems	O
(	O
MDBSs	O
)	O
,	O
ensuring	O
global	B-General_Concept
serializability	B-Operating_System
is	O
a	O
difficult	O
task	O
.	O
</s>
<s>
The	O
difficulty	O
arises	O
due	O
to	O
the	O
heterogeneity	B-General_Concept
of	O
the	O
concurrency	B-Operating_System
control	I-Operating_System
protocols	O
used	O
by	O
the	O
participating	O
local	O
database	B-Application
management	I-Application
systems	I-Application
(	O
DBMSs	O
)	O
,	O
and	O
the	O
desire	O
to	O
preserve	O
the	O
autonomy	O
of	O
the	O
local	O
DBMSs	O
.	O
</s>
<s>
In	O
general	O
,	O
solutions	O
to	O
the	O
global	B-General_Concept
serializability	B-Operating_System
problem	O
result	O
in	O
executions	O
with	O
a	O
low	O
degree	O
of	O
concurrency	O
.	O
</s>
<s>
The	O
alternative	O
,	O
relaxed	O
serializability	B-Operating_System
,	O
may	O
result	O
in	O
data	O
inconsistency.	O
"	O
</s>
<s>
Also	O
the	O
above	O
quoted	O
article	O
proposes	O
a	O
relaxed	O
global	B-General_Concept
serializability	B-Operating_System
solution	O
,	O
while	O
referencing	O
the	O
CO	O
work	O
.	O
</s>
<s>
The	O
CO	O
solution	O
for	O
global	B-General_Concept
serializability	B-Operating_System
both	O
bridges	O
between	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
protocols	O
with	O
no	O
substantial	O
concurrency	O
reduction	O
(	O
and	O
typically	O
minor	O
,	O
if	O
at	O
all	O
)	O
,	O
and	O
maintains	O
the	O
autonomy	O
of	O
local	O
DBMSs	O
.	O
</s>
<s>
This	O
misunderstanding	O
continues	O
to	O
2010	O
in	O
a	O
textbook	O
by	O
some	O
of	O
the	O
same	O
authors	O
,	O
where	O
the	O
same	O
relaxed	O
global	B-General_Concept
serializability	B-Operating_System
technique	O
,	O
Two	O
level	O
serializability	B-Operating_System
,	O
is	O
emphasized	O
and	O
described	O
in	O
detail	O
,	O
and	O
CO	O
is	O
not	O
mentioned	O
at	O
all	O
.	O
</s>
<s>
"	O
Not	O
all	O
concurrency	B-Operating_System
control	I-Operating_System
algorithms	O
use	O
locks	O
...	O
Three	O
other	O
techniques	O
are	O
timestamp	B-Operating_System
ordering	I-Operating_System
,	O
serialization	O
graph	O
testing	O
,	O
and	O
commit	B-Architecture
ordering	I-Architecture
.	O
</s>
<s>
Timestamp	B-Operating_System
ordering	I-Operating_System
assigns	O
each	O
transaction	O
a	O
timestamp	B-Operating_System
and	O
ensures	O
that	O
conflicting	O
operations	O
execute	O
in	O
timestamp	B-Operating_System
order	O
.	O
</s>
<s>
Commit	B-Architecture
ordering	I-Architecture
ensures	O
that	O
conflicting	O
operations	O
are	O
consistent	O
with	O
the	O
relative	O
order	O
in	O
which	O
their	O
transactions	B-General_Concept
commit	O
,	O
which	O
can	O
enable	O
interoperability	O
of	O
systems	O
using	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms.	O
"	O
</s>
<s>
Beyond	O
the	O
common	O
locking	B-Operating_System
based	O
algorithm	O
SS2PL	B-Operating_System
,	O
which	O
is	O
a	O
CO	O
variant	O
itself	O
,	O
also	O
additional	O
variants	O
of	O
CO	O
that	O
use	O
locks	O
exist	O
,	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
Since	O
CO	O
mechanisms	O
order	O
the	O
commit	O
events	O
according	O
to	O
conflicts	O
that	O
already	O
have	O
occurred	O
,	O
it	O
is	O
better	O
to	O
describe	O
CO	O
as	O
"	O
Commit	B-Architecture
ordering	I-Architecture
ensures	O
that	O
the	O
relative	O
order	O
in	O
which	O
transactions	B-General_Concept
commit	O
is	O
consistent	O
with	O
the	O
order	O
of	O
their	O
respective	O
conflicting	O
operations.	O
"	O
</s>
<s>
Several	O
solutions	O
,	O
some	O
partial	O
,	O
have	O
been	O
proposed	O
for	O
the	O
global	B-General_Concept
serializability	B-Operating_System
problem	O
.	O
</s>
<s>
The	O
problem	O
of	O
global	B-General_Concept
serializability	B-Operating_System
has	O
been	O
a	O
quite	O
intensively	O
researched	O
subject	O
in	O
the	O
late	O
1980s	O
and	O
early	O
1990s	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
)	O
has	O
provided	O
an	O
effective	O
general	O
solution	O
to	O
the	O
problem	O
,	O
insight	O
into	O
it	O
,	O
and	O
understanding	O
about	O
possible	O
generalizations	O
of	O
strong	B-Operating_System
strict	I-Operating_System
two	I-Operating_System
phase	I-Operating_System
locking	I-Operating_System
(	O
SS2PL	B-Operating_System
)	O
,	O
which	O
practically	O
and	O
almost	O
exclusively	O
has	O
been	O
utilized	O
(	O
in	O
conjunction	O
with	O
the	O
Two-phase	B-General_Concept
commit	I-General_Concept
protocol	I-General_Concept
(	O
2PC	B-General_Concept
)	O
)	O
since	O
the	O
1980s	O
to	O
achieve	O
global	B-General_Concept
serializability	B-Operating_System
across	O
databases	B-Application
.	O
</s>
<s>
An	O
important	O
side-benefit	O
of	O
CO	O
is	O
the	O
automatic	O
global	B-General_Concept
deadlock	B-Operating_System
resolution	O
that	O
it	O
provides	O
(	O
this	O
is	O
applicable	O
also	O
to	O
distributed	B-Operating_System
SS2PL	B-Operating_System
;	O
though	O
global	B-General_Concept
deadlocks	B-Operating_System
have	O
been	O
an	O
important	O
research	O
subject	O
for	O
SS2PL	B-Operating_System
,	O
automatic	O
resolution	O
has	O
been	O
overlooked	O
,	O
except	O
in	O
the	O
CO	O
articles	O
,	O
until	O
today	O
(	O
2009	O
)	O
)	O
.	O
</s>
<s>
At	O
that	O
time	O
quite	O
many	O
commercial	O
database	B-Application
system	I-Application
types	O
existed	O
,	O
many	O
non-relational	O
,	O
and	O
databases	B-Application
were	O
relatively	O
very	O
small	O
.	O
</s>
<s>
Multi	O
database	B-Application
systems	I-Application
were	O
considered	O
a	O
key	O
for	O
database	O
scalability	B-Architecture
by	O
database	B-Application
systems	I-Application
interoperability	O
,	O
and	O
global	B-General_Concept
serializability	B-Operating_System
was	O
urgently	O
needed	O
.	O
</s>
<s>
Since	O
then	O
the	O
tremendous	O
progress	O
in	O
computing	O
power	O
,	O
storage	O
,	O
and	O
communication	O
networks	B-Architecture
,	O
resulted	O
in	O
orders	O
of	O
magnitude	O
increases	O
in	O
both	O
centralized	O
databases	B-Application
 '	O
sizes	O
,	O
transaction	O
rates	O
,	O
and	O
remote	O
access	O
to	O
database	O
capabilities	O
,	O
as	O
well	O
as	O
blurring	O
the	O
boundaries	O
between	O
centralized	O
computing	O
and	O
distributed	B-Operating_System
one	O
over	O
fast	O
,	O
low-latency	O
local	O
networks	B-Architecture
(	O
e.g.	O
,	O
Infiniband	B-Architecture
)	O
.	O
</s>
<s>
These	O
,	O
together	O
with	O
progress	O
in	O
database	O
vendors	O
 '	O
distributed	B-Operating_System
solutions	O
(	O
primarily	O
the	O
popular	O
SS2PL	B-Operating_System
with	O
2PC	B-General_Concept
based	O
,	O
a	O
de	O
facto	O
standard	O
that	O
allows	O
interoperability	O
among	O
different	O
vendors	O
 '	O
(	O
SS2PL-based	O
)	O
databases	B-Application
;	O
both	O
SS2PL	B-Operating_System
and	O
2PC	B-General_Concept
technologies	O
have	O
gained	O
substantial	O
expertise	O
and	O
efficiency	O
)	O
,	O
workflow	B-Operating_System
management	O
systems	O
,	O
and	O
database	B-General_Concept
replication	I-General_Concept
technology	O
,	O
in	O
most	O
cases	O
have	O
provided	O
satisfactory	O
and	O
sometimes	O
better	O
information	B-General_Concept
technology	I-General_Concept
solutions	O
without	O
multi	O
database	O
atomic	O
distributed	B-General_Concept
transactions	I-General_Concept
over	O
databases	B-Application
with	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
(	O
bypassing	O
the	O
problem	O
above	O
)	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
sense	O
of	O
urgency	O
that	O
existed	O
with	O
the	O
problem	O
at	O
that	O
period	O
,	O
and	O
in	O
general	O
with	O
high-performance	O
distributed	B-Operating_System
atomic	O
transactions	B-General_Concept
over	O
databases	B-Application
with	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
types	O
,	O
has	O
reduced	O
.	O
</s>
<s>
However	O
,	O
the	O
need	O
in	O
concurrent	O
distributed	B-Operating_System
atomic	O
transactions	B-General_Concept
as	O
a	O
fundamental	O
element	O
of	O
reliability	O
exists	O
in	O
distributed	B-Architecture
systems	I-Architecture
also	O
beyond	O
database	B-Application
systems	I-Application
,	O
and	O
so	O
the	O
need	O
in	O
global	B-General_Concept
serializability	B-Operating_System
as	O
a	O
fundamental	O
correctness	O
criterion	O
for	O
such	O
transactional	O
systems	O
(	O
see	O
also	O
Distributed	B-Operating_System
serializability	I-Operating_System
in	O
Serializability	B-Operating_System
)	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
small	O
,	O
portable	O
,	O
powerful	O
computing	O
devices	O
(	O
e.g.	O
,	O
smartphones	B-Application
)	O
,	O
and	O
sophisticated	O
systems	B-Application
management	I-Application
the	O
need	O
for	O
effective	O
global	B-General_Concept
serializability	B-Operating_System
techniques	O
to	O
ensure	O
correctness	O
in	O
and	O
among	O
distributed	B-Operating_System
transactional	O
applications	O
seems	O
to	O
increase	O
,	O
and	O
thus	O
also	O
the	O
need	O
in	O
Commitment	B-Architecture
ordering	I-Architecture
(	O
including	O
the	O
popular	O
for	O
databases	B-Application
special	O
case	O
SS2PL	B-Operating_System
;	O
SS2PL	B-Operating_System
,	O
though	O
,	O
does	O
not	O
meet	O
the	O
requirements	O
of	O
many	O
other	O
transactional	O
objects	O
)	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
(	O
or	O
Commit	B-Architecture
ordering	I-Architecture
;	O
CO	O
)	O
is	O
the	O
only	O
high-performance	O
,	O
fault	B-General_Concept
tolerant	I-General_Concept
,	O
conflict	O
serializability	B-Operating_System
providing	O
solution	O
that	O
has	O
been	O
proposed	O
as	O
a	O
fully	O
distributed	B-Operating_System
(	O
no	O
central	O
computing	O
component	O
or	O
data-structure	O
are	O
needed	O
)	O
,	O
general	O
mechanism	O
that	O
can	O
be	O
combined	O
seamlessly	O
with	O
any	O
local	O
(	O
to	O
a	O
database	O
)	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
(	O
see	O
technical	O
summary	O
)	O
.	O
</s>
<s>
Since	O
the	O
CO	O
property	O
of	O
a	O
schedule	B-General_Concept
is	O
a	O
necessary	O
condition	O
for	O
global	B-General_Concept
serializability	B-Operating_System
of	O
autonomous	O
databases	B-Application
(	O
in	O
the	O
context	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
)	O
,	O
it	O
provides	O
the	O
only	O
general	O
solution	O
for	O
autonomous	O
databases	B-Application
(	O
i.e.	O
,	O
if	O
autonomous	O
databases	B-Application
do	O
not	O
comply	O
with	O
CO	O
,	O
then	O
global	B-General_Concept
serializability	B-Operating_System
may	O
be	O
violated	O
)	O
.	O
</s>
<s>
does	O
not	O
interfere	O
with	O
any	O
transaction	O
's	O
operation	O
,	O
particularly	O
neither	O
block	O
,	O
restrict	O
nor	O
delay	O
any	O
data-access	O
operation	O
(	O
read	O
or	O
write	O
)	O
for	O
either	O
local	O
or	O
global	B-General_Concept
transactions	I-General_Concept
(	O
and	O
thus	O
does	O
not	O
cause	O
any	O
extra	O
aborts	O
)	O
;	O
thus	O
allows	O
seamless	O
integration	O
with	O
any	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
.	O
</s>
<s>
allows	O
optimistic	B-Operating_System
implementations	O
(	O
non-blocking	O
,	O
i.e.	O
,	O
non	O
data	O
access	O
blocking	O
)	O
.	O
</s>
<s>
allows	O
heterogeneity	B-General_Concept
:	O
Global	B-General_Concept
serializability	B-Operating_System
is	O
achieved	O
across	O
multiple	O
transactional	O
objects	O
with	O
different	O
(	O
any	O
)	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	O
,	O
without	O
interfering	O
with	O
the	O
mechanisms	O
 '	O
operations	O
.	O
</s>
<s>
allows	O
modularity	B-Architecture
:	O
Transactional	O
objects	O
can	O
be	O
added	O
and	O
removed	O
transparently	O
.	O
</s>
<s>
maintains	O
each	O
database	O
's	O
autonomy	O
,	O
and	O
does	O
not	O
need	O
any	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
distribution	O
(	O
e.g.	O
,	O
local	O
precedence	O
relations	O
,	O
locks	O
,	O
timestamps	O
,	O
or	O
tickets	O
)	O
.	O
</s>
<s>
does	O
not	O
need	O
any	O
knowledge	O
about	O
the	O
transactions	B-General_Concept
.	O
</s>
<s>
requires	O
no	O
communication	O
overhead	O
since	O
it	O
only	O
uses	O
already	O
needed	O
,	O
unmodified	O
atomic	O
commitment	O
protocol	O
messages	O
(	O
any	O
such	O
protocol	O
;	O
using	O
fault	B-General_Concept
tolerant	I-General_Concept
atomic	O
commitment	O
protocols	O
and	O
database	B-Application
systems	I-Application
makes	O
the	O
CO	O
solution	O
fault	B-General_Concept
tolerant	I-General_Concept
)	O
.	O
</s>
<s>
automatically	O
resolves	O
global	B-General_Concept
deadlocks	B-Operating_System
due	O
to	O
locking	B-Operating_System
.	O
</s>
<s>
scales	B-Architecture
up	I-Architecture
effectively	O
with	O
computer	B-Architecture
network	I-Architecture
size	O
and	O
number	O
of	O
databases	B-Application
,	O
almost	O
without	O
any	O
negative	O
impact	O
on	O
performance	O
,	O
since	O
each	O
global	B-General_Concept
transaction	O
is	O
typically	O
confined	O
to	O
certain	O
relatively	O
small	O
numbers	O
of	O
databases	B-Application
and	O
network	O
nodes	O
.	O
</s>
<s>
requires	O
no	O
additional	O
,	O
artificial	O
transaction	O
access	O
operations	O
(	O
e.g.	O
,	O
"	O
take	O
timestamp	B-Operating_System
"	O
or	O
"	O
take	O
ticket	O
"	O
)	O
,	O
which	O
typically	O
result	O
in	O
additional	O
,	O
artificial	O
conflicts	O
that	O
reduce	O
concurrency	O
.	O
</s>
<s>
The	O
only	O
overhead	O
incurred	O
by	O
the	O
CO	O
solution	O
is	O
locally	O
detecting	O
conflicts	O
(	O
which	O
is	O
already	O
done	O
by	O
any	O
known	O
serializability	B-Operating_System
mechanism	O
,	O
both	O
pessimistic	O
and	O
optimistic	B-Operating_System
)	O
and	O
locally	O
ordering	O
in	O
each	O
database	B-Application
system	I-Application
both	O
the	O
(	O
local	O
)	O
commits	O
of	O
local	O
transactions	B-General_Concept
and	O
the	O
voting	O
for	O
atomic	O
commitment	O
of	O
global	B-General_Concept
transactions	I-General_Concept
.	O
</s>
<s>
The	O
net	O
effect	O
of	O
CO	O
may	O
be	O
some	O
delays	O
of	O
commit	O
events	O
(	O
but	O
never	O
more	O
delay	O
than	O
SS2PL	B-Operating_System
,	O
and	O
on	O
the	O
average	O
less	O
)	O
.	O
</s>
<s>
This	O
makes	O
CO	O
instrumental	O
for	O
global	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
of	O
multidatabase	O
systems	O
(	O
e.g.	O
,	O
federated	B-General_Concept
database	I-General_Concept
systems	I-General_Concept
)	O
.	O
</s>
<s>
The	O
underlying	O
Theory	O
of	O
Commitment	B-Architecture
ordering	I-Architecture
,	O
part	O
of	O
Serializability	B-Operating_System
theory	I-Operating_System
,	O
is	O
both	O
sound	O
and	O
elegant	O
(	O
and	O
even	O
"	O
mathematically	O
beautiful	O
"	O
;	O
referring	O
to	O
structure	O
and	O
dynamics	O
of	O
conflicts	O
,	O
graph	O
cycles	O
,	O
and	O
deadlocks	B-Operating_System
)	O
,	O
with	O
interesting	O
implications	O
for	O
transactional	O
distributed	B-Architecture
applications	I-Architecture
.	O
</s>
<s>
All	O
the	O
qualities	O
of	O
CO	O
in	O
the	O
list	O
above	O
,	O
except	O
the	O
first	O
three	O
,	O
are	O
also	O
possessed	O
by	O
SS2PL	B-Operating_System
,	O
which	O
is	O
a	O
special	O
case	O
of	O
CO	O
,	O
but	O
blocking	O
and	O
constraining	O
.	O
</s>
<s>
This	O
partially	O
explains	O
the	O
popularity	O
of	O
SS2PL	B-Operating_System
as	O
a	O
solution	O
(	O
practically	O
,	O
the	O
only	O
solution	O
,	O
for	O
many	O
years	O
)	O
for	O
achieving	O
global	B-General_Concept
serializability	B-Operating_System
.	O
</s>
<s>
However	O
,	O
property	O
9	O
above	O
,	O
automatic	O
resolution	O
of	O
global	B-General_Concept
deadlocks	B-Operating_System
,	O
has	O
not	O
been	O
noticed	O
for	O
SS2PL	B-Operating_System
in	O
the	O
database	O
research	O
literature	O
until	O
today	O
(	O
2009	O
;	O
except	O
in	O
the	O
CO	O
publications	O
)	O
.	O
</s>
<s>
This	O
,	O
since	O
the	O
phenomenon	O
of	O
voting-deadlocks	O
in	O
such	O
environments	O
and	O
their	O
automatic	O
resolution	O
by	O
the	O
atomic	O
commitment	O
protocol	O
has	O
been	O
overlooked	O
.	O
</s>
<s>
Most	O
existing	O
database	B-Application
systems	I-Application
,	O
including	O
all	O
major	O
commercial	O
database	B-Application
systems	I-Application
,	O
are	O
strong	B-Operating_System
strict	I-Operating_System
two	I-Operating_System
phase	I-Operating_System
locking	I-Operating_System
(	O
SS2PL	B-Operating_System
)	O
based	O
and	O
already	O
CO	O
compliant	O
.	O
</s>
<s>
Thus	O
they	O
can	O
participate	O
in	O
a	O
CO	O
based	O
solution	O
for	O
global	B-General_Concept
serializability	B-Operating_System
in	O
multidatabase	O
environments	O
without	O
any	O
modification	O
(	O
except	O
for	O
the	O
popular	O
multiversioning	B-Operating_System
,	O
where	O
additional	O
CO	O
aspects	O
should	O
be	O
considered	O
)	O
.	O
</s>
<s>
Achieving	O
global	B-General_Concept
serializability	B-Operating_System
across	O
SS2PL	B-Operating_System
based	O
databases	B-Application
using	O
atomic	O
commitment	O
(	O
primarily	O
using	O
two	B-General_Concept
phase	I-General_Concept
commit	I-General_Concept
,	O
2PC	B-General_Concept
)	O
has	O
been	O
employed	O
for	O
many	O
years	O
(	O
i.e.	O
,	O
using	O
the	O
same	O
CO	O
solution	O
for	O
a	O
specific	O
special	O
case	O
;	O
however	O
,	O
no	O
reference	O
is	O
known	O
prior	O
to	O
CO	O
,	O
that	O
notices	O
this	O
special	O
case	O
's	O
automatic	O
global	B-General_Concept
deadlock	B-Operating_System
resolution	O
by	O
the	O
atomic	O
commitment	O
protocol	O
's	O
augmented-conflict-graph	O
global	B-General_Concept
cycle	O
elimination	O
process	O
)	O
.	O
</s>
<s>
Virtually	O
all	O
existing	O
distributed	B-General_Concept
transaction	I-General_Concept
processing	I-General_Concept
environments	O
and	O
supporting	O
products	O
rely	O
on	O
SS2PL	B-Operating_System
and	O
provide	O
2PC	B-General_Concept
.	O
</s>
<s>
As	O
a	O
matter	O
of	O
fact	O
SS2PL	B-Operating_System
together	O
with	O
2PC	B-General_Concept
have	O
become	O
a	O
de	O
facto	O
standard	O
.	O
</s>
<s>
This	O
solution	O
is	O
a	O
homogeneous	B-General_Concept
concurrency	B-Operating_System
control	I-Operating_System
one	O
,	O
suboptimal	O
(	O
when	O
both	O
Serializability	B-Operating_System
and	O
Strictness	O
are	O
needed	O
;	O
see	O
Strict	O
commitment	B-Architecture
ordering	I-Architecture
;	O
SCO	O
)	O
but	O
still	O
quite	O
effective	O
in	O
most	O
cases	O
,	O
sometimes	O
at	O
the	O
cost	O
of	O
increased	O
computing	O
power	O
needed	O
relatively	O
to	O
the	O
optimum	O
.	O
</s>
<s>
(	O
However	O
,	O
for	O
better	O
performance	O
relaxed	O
serializability	B-Operating_System
is	O
used	O
whenever	O
applications	O
allow	O
)	O
.	O
</s>
<s>
It	O
allows	O
inter-operation	O
among	O
SS2PL-compliant	O
different	O
database	B-Application
system	I-Application
types	O
,	O
i.e.	O
,	O
allows	O
heterogeneity	B-General_Concept
in	O
aspects	O
other	O
than	O
concurrency	B-Operating_System
control	I-Operating_System
.	O
</s>
<s>
SS2PL	B-Operating_System
is	O
a	O
very	O
constraining	O
schedule	B-General_Concept
property	O
,	O
and	O
"	O
takes	O
over	O
"	O
when	O
combined	O
with	O
any	O
other	O
property	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
combined	O
with	O
any	O
optimistic	B-Operating_System
property	O
,	O
the	O
result	O
is	O
not	O
optimistic	B-Operating_System
anymore	O
,	O
but	O
rather	O
characteristically	O
SS2PL	B-Operating_System
.	O
</s>
<s>
Since	O
also	O
CO	O
uses	O
atomic	O
commitment	O
(	O
e.g.	O
,	O
2PC	B-General_Concept
)	O
for	O
achieving	O
global	B-General_Concept
serializability	B-Operating_System
,	O
as	O
SS2PL	B-Operating_System
does	O
,	O
any	O
CO	O
compliant	O
database	B-Application
system	I-Application
or	O
transactional	O
object	O
can	O
transparently	O
join	O
existing	O
SS2PL	B-Operating_System
based	O
environments	O
,	O
use	O
2PC	B-General_Concept
,	O
and	O
maintain	O
global	B-General_Concept
serializability	B-Operating_System
without	O
any	O
environment	O
change	O
.	O
</s>
<s>
This	O
makes	O
CO	O
a	O
straightforward	O
,	O
natural	O
generalization	O
of	O
SS2PL	B-Operating_System
for	O
any	O
conflict	O
serializability	B-Operating_System
based	O
database	B-Application
system	I-Application
,	O
for	O
all	O
practical	O
purposes	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
has	O
been	O
quite	O
widely	O
known	O
inside	O
the	O
transaction	B-General_Concept
processing	I-General_Concept
and	O
databases	B-Application
communities	O
at	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
since	O
1990	O
.	O
</s>
<s>
It	O
has	O
been	O
misunderstood	O
by	O
many	O
database	O
researchers	O
years	O
after	O
its	O
introduction	O
,	O
which	O
is	O
evident	O
by	O
the	O
quotes	O
above	O
from	O
articles	O
in	O
1997-1998	O
referencing	O
Commitment	B-Architecture
ordering	I-Architecture
articles	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
CO	O
has	O
been	O
utilized	O
extensively	O
as	O
a	O
solution	O
for	O
global	B-General_Concept
serializability	B-Operating_System
in	O
works	O
on	O
Transactional	O
processes	O
,	O
</s>
<s>
which	O
is	O
an	O
approach	O
for	O
transaction	O
management	O
in	O
the	O
converging	O
Grid	B-Architecture
computing	I-Architecture
and	O
Cloud	B-Architecture
computing	I-Architecture
.	O
</s>
<s>
See	O
more	O
in	O
The	O
History	O
of	O
Commitment	B-Architecture
Ordering	I-Architecture
.	O
</s>
<s>
Some	O
techniques	O
have	O
been	O
developed	O
for	O
relaxed	O
global	B-General_Concept
serializability	B-Operating_System
(	O
i.e.	O
,	O
they	O
do	O
not	O
guarantee	O
global	B-General_Concept
serializability	B-Operating_System
;	O
see	O
also	O
Relaxing	O
serializability	B-Operating_System
)	O
.	O
</s>
<s>
While	O
local	O
(	O
to	O
a	O
database	B-Application
system	I-Application
)	O
relaxed	O
serializability	B-Operating_System
methods	O
compromise	O
serializability	B-Operating_System
for	O
performance	O
gain	O
(	O
and	O
are	O
utilized	O
only	O
when	O
the	O
application	O
can	O
tolerate	O
possible	O
resulting	O
inaccuracies	O
,	O
or	O
its	O
integrity	O
is	O
unharmed	O
)	O
,	O
it	O
is	O
unclear	O
that	O
various	O
proposed	O
relaxed	O
global	B-General_Concept
serializability	B-Operating_System
methods	O
which	O
compromise	O
global	B-General_Concept
serializability	B-Operating_System
,	O
provide	O
any	O
performance	O
gain	O
over	O
commitment	B-Architecture
ordering	I-Architecture
which	O
guarantees	O
global	B-General_Concept
serializability	B-Operating_System
.	O
</s>
<s>
Typically	O
,	O
the	O
declared	O
intention	O
of	O
such	O
methods	O
has	O
not	O
been	O
performance	O
gain	O
over	O
effective	O
global	B-General_Concept
serializability	B-Operating_System
methods	O
(	O
which	O
apparently	O
have	O
been	O
unknown	O
to	O
the	O
inventors	O
)	O
,	O
but	O
rather	O
correctness	O
criteria	O
alternatives	O
due	O
to	O
lack	O
of	O
a	O
known	O
effective	O
global	B-General_Concept
serializability	B-Operating_System
method	O
.	O
</s>
<s>
Oddly	O
,	O
some	O
of	O
them	O
were	O
introduced	O
years	O
after	O
CO	O
had	O
been	O
introduced	O
,	O
and	O
some	O
even	O
quote	O
CO	O
without	O
realizing	O
that	O
it	O
provides	O
an	O
effective	O
global	B-General_Concept
serializability	B-Operating_System
solution	O
,	O
and	O
thus	O
without	O
providing	O
any	O
performance	O
comparison	O
with	O
CO	O
to	O
justify	O
them	O
as	O
alternatives	O
to	O
global	B-General_Concept
serializability	B-Operating_System
for	O
some	O
applications	O
(	O
e.g.	O
,	O
Two-level	O
serializability	B-Operating_System
)	O
.	O
</s>
<s>
Two-level	O
serializability	B-Operating_System
is	O
even	O
presented	O
as	O
a	O
major	O
global	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
method	O
in	O
a	O
2010	O
edition	O
of	O
a	O
text-book	O
on	O
databases	B-Application
(	O
authored	O
by	O
two	O
of	O
the	O
original	O
authors	O
of	O
Two-level	O
serializability	B-Operating_System
,	O
where	O
one	O
of	O
them	O
,	O
Avi	O
Silberschatz	O
,	O
is	O
also	O
an	O
author	O
of	O
the	O
original	O
Strong	B-Architecture
recoverability	I-Architecture
articles	O
)	O
.	O
</s>
<s>
This	O
book	O
neither	O
mentions	O
CO	O
nor	O
references	O
it	O
,	O
and	O
strangely	O
,	O
apparently	O
does	O
not	O
consider	O
CO	O
a	O
valid	O
Global	B-General_Concept
serializability	B-Operating_System
solution	O
.	O
</s>
<s>
Another	O
common	O
reason	O
nowadays	O
for	O
Global	B-General_Concept
serializability	B-Operating_System
relaxation	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	O
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
a	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
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
alike	O
)	O
,	O
while	O
global	B-General_Concept
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
global	B-General_Concept
serializability	B-Operating_System
properties	O
either	O
contain	O
the	O
global	B-General_Concept
serializability	B-Operating_System
class	O
,	O
or	O
are	O
incomparable	O
with	O
it	O
.	O
</s>
<s>
What	O
differentiates	O
techniques	O
for	O
relaxed	O
global	B-General_Concept
conflict	O
serializability	B-Operating_System
(	O
RGCSR	O
)	O
properties	O
from	O
those	O
of	O
relaxed	O
conflict	O
serializability	B-Operating_System
(	O
RCSR	O
)	O
properties	O
that	O
are	O
not	O
RGCSR	O
is	O
typically	O
the	O
different	O
way	O
global	B-General_Concept
cycles	O
(	O
span	O
two	O
or	O
more	O
databases	B-Application
)	O
in	O
the	O
global	B-General_Concept
conflict	O
graph	O
are	O
handled	O
.	O
</s>
<s>
No	O
distinction	O
between	O
global	B-General_Concept
and	O
local	O
cycles	O
exists	O
for	O
RCSR	O
properties	O
that	O
are	O
not	O
RGCSR	O
.	O
</s>
<s>
Typically	O
RGCSR	O
techniques	O
eliminate	O
local	O
cycles	O
,	O
i.e.	O
,	O
provide	O
local	O
serializability	B-Operating_System
(	O
which	O
can	O
be	O
achieved	O
effectively	O
by	O
regular	O
,	O
known	O
concurrency	B-Operating_System
control	I-Operating_System
methods	O
)	O
;	O
however	O
,	O
obviously	O
they	O
do	O
not	O
eliminate	O
all	O
global	B-General_Concept
cycles	O
(	O
which	O
would	O
achieve	O
global	B-General_Concept
serializability	B-Operating_System
)	O
.	O
</s>
