<s>
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
)	O
is	O
a	O
class	O
of	O
interoperable	O
serializability	B-Operating_System
techniques	O
in	O
concurrency	B-Operating_System
control	I-Operating_System
of	O
databases	O
,	O
transaction	B-General_Concept
processing	I-General_Concept
,	O
and	O
related	O
applications	O
.	O
</s>
<s>
With	O
the	O
proliferation	O
of	O
multi-core	B-Architecture
processors	I-Architecture
,	O
CO	O
has	O
also	O
been	O
increasingly	O
utilized	O
in	O
concurrent	B-Architecture
programming	I-Architecture
,	O
transactional	B-Operating_System
memory	I-Operating_System
,	O
and	O
software	B-Operating_System
transactional	I-Operating_System
memory	I-Operating_System
(	O
STM	O
)	O
to	O
achieve	O
serializability	B-Operating_System
optimistically	B-Operating_System
.	O
</s>
<s>
CO	O
is	O
also	O
the	O
name	O
of	O
the	O
resulting	O
transaction	B-General_Concept
schedule	B-General_Concept
(	O
history	O
)	O
property	O
,	O
defined	O
in	O
1988	O
with	O
the	O
name	O
dynamic	O
atomicity	O
.	O
</s>
<s>
In	O
a	O
CO	O
compliant	O
schedule	B-General_Concept
,	O
the	O
chronological	O
order	O
of	O
commitment	O
events	O
of	O
transactions	O
is	O
compatible	O
with	O
the	O
precedence	O
order	O
of	O
the	O
respective	O
transactions	O
.	O
</s>
<s>
CO	O
is	O
a	O
broad	O
special	O
case	O
of	O
conflict	O
serializability	B-Operating_System
and	O
effective	O
means	O
(	O
reliable	O
,	O
high-performance	O
,	O
distributed	O
,	O
and	O
scalable	B-Architecture
)	O
to	O
achieve	O
global	B-Operating_System
serializability	I-Operating_System
(	O
modular	B-Operating_System
serializability	I-Operating_System
)	O
across	O
any	O
collection	O
of	O
database	O
systems	O
that	O
possibly	O
use	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	O
(	O
CO	O
also	O
makes	O
each	O
system	O
serializability	B-Operating_System
compliant	O
,	O
if	O
not	O
already	O
)	O
.	O
</s>
<s>
Each	O
not-CO-compliant	O
database	O
system	O
is	O
augmented	O
with	O
a	O
CO	O
component	O
(	O
the	O
commitment	O
order	O
coordinator	O
—	O
COCO	O
)	O
which	O
orders	O
the	O
commitment	O
events	O
for	O
CO	O
compliance	O
,	O
with	O
neither	O
data-access	O
nor	O
any	O
other	O
transaction	B-General_Concept
operation	O
interference	O
.	O
</s>
<s>
As	O
such	O
,	O
CO	O
provides	O
a	O
low	O
overhead	O
,	O
general	O
solution	O
for	O
global	B-Operating_System
serializability	I-Operating_System
(	O
and	O
distributed	B-Operating_System
serializability	I-Operating_System
)	O
,	O
instrumental	O
for	O
global	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
(	O
and	O
distributed	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
)	O
of	O
multi-database	O
systems	O
and	O
other	O
transactional	O
objects	O
,	O
possibly	O
highly	O
distributed	O
(	O
e.g.	O
,	O
within	O
cloud	B-Architecture
computing	I-Architecture
,	O
grid	B-Architecture
computing	I-Architecture
,	O
and	O
networks	O
of	O
smartphones	B-Application
)	O
.	O
</s>
<s>
An	O
atomic	O
commitment	O
protocol	O
(	O
ACP	O
;	O
of	O
any	O
type	O
)	O
is	O
a	O
fundamental	O
part	O
of	O
the	O
solution	O
,	O
utilized	O
to	O
break	O
global	O
cycles	O
in	O
the	O
conflict	O
(	O
precedence	O
,	O
serializability	B-Operating_System
)	O
graph	O
.	O
</s>
<s>
CO	O
is	O
the	O
most	O
general	O
property	O
(	O
a	O
necessary	O
condition	O
)	O
that	O
guarantees	O
global	B-Operating_System
serializability	I-Operating_System
,	O
if	O
the	O
database	O
systems	O
involved	O
do	O
not	O
share	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
beyond	O
atomic	O
commitment	O
protocol	O
(	O
unmodified	O
)	O
messages	O
and	O
have	O
no	O
knowledge	O
of	O
whether	O
transactions	O
are	O
global	O
or	O
local	O
(	O
the	O
database	O
systems	O
are	O
autonomous	O
)	O
.	O
</s>
<s>
Thus	O
CO	O
(	O
with	O
its	O
variants	O
)	O
is	O
the	O
only	O
general	O
technique	O
that	O
does	O
not	O
require	O
the	O
typically	O
costly	O
distribution	O
of	O
local	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
(	O
e.g.	O
,	O
local	O
precedence	O
relations	O
,	O
locks	O
,	O
timestamps	B-Operating_System
,	O
or	O
tickets	O
)	O
.	O
</s>
<s>
It	O
generalizes	O
the	O
popular	O
strong	B-Operating_System
strict	I-Operating_System
two-phase	I-Operating_System
locking	I-Operating_System
(	O
SS2PL	O
)	O
property	O
,	O
which	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
is	O
the	O
de	O
facto	O
standard	O
to	O
achieve	O
global	B-Operating_System
serializability	I-Operating_System
across	O
(	O
SS2PL	O
based	O
)	O
database	O
systems	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
CO	O
compliant	O
database	O
systems	O
(	O
with	O
any	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
types	O
)	O
can	O
transparently	O
join	O
such	O
SS2PL	O
based	O
solutions	O
for	O
global	B-Operating_System
serializability	I-Operating_System
.	O
</s>
<s>
In	O
addition	O
,	O
locking	B-Operating_System
based	O
global	O
deadlocks	B-Operating_System
are	O
resolved	O
automatically	O
in	O
a	O
CO	O
based	O
multi-database	O
environment	O
,	O
a	O
vital	O
side-benefit	O
(	O
including	O
the	O
special	O
case	O
of	O
a	O
completely	O
SS2PL	O
based	O
environment	O
;	O
a	O
previously	O
unnoticed	O
fact	O
for	O
SS2PL	O
)	O
.	O
</s>
<s>
Furthermore	O
,	O
strict	O
commitment	B-Architecture
ordering	I-Architecture
(	O
SCO	O
;	O
Raz	O
1991c	O
)	O
,	O
the	O
intersection	O
of	O
Strictness	O
and	O
CO	O
,	O
provides	O
better	O
performance	O
(	O
shorter	O
average	O
transaction	B-General_Concept
completion	O
time	O
and	O
resulting	O
in	O
better	O
transaction	B-General_Concept
throughput	O
)	O
than	O
SS2PL	O
whenever	O
read-write	O
conflicts	O
are	O
present	O
(	O
identical	O
blocking	O
behavior	O
for	O
write-read	O
and	O
write-write	O
conflicts	O
;	O
comparable	O
locking	B-Operating_System
overhead	O
)	O
.	O
</s>
<s>
They	O
also	O
provide	O
global	B-Operating_System
serializability	I-Operating_System
without	O
local	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
distribution	O
,	O
can	O
be	O
combined	O
with	O
any	O
relevant	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
and	O
allow	O
optimistic	O
(	O
non-blocking	O
)	O
implementations	O
.	O
</s>
<s>
Both	O
use	O
additional	O
information	O
for	O
relaxing	O
CO	O
constraints	O
and	O
achieving	O
better	O
concurrency	B-Architecture
and	O
performance	O
.	O
</s>
<s>
Vote	B-Architecture
ordering	I-Architecture
(	O
VO	O
or	O
Generalized	O
CO	O
(	O
GCO	O
)	O
;	O
Raz	O
2009	O
)	O
is	O
a	O
container	O
schedule	B-General_Concept
set	O
(	O
property	O
)	O
and	O
technique	O
for	O
CO	O
and	O
all	O
its	O
variants	O
.	O
</s>
<s>
Local	O
VO	O
is	O
necessary	O
for	O
guaranteeing	O
global	B-Operating_System
serializability	I-Operating_System
if	O
the	O
atomic	O
commitment	O
protocol	O
(	O
ACP	O
)	O
participants	O
do	O
not	O
share	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
(	O
have	O
the	O
generalized	O
autonomy	O
property	O
)	O
.	O
</s>
<s>
CO	O
and	O
its	O
variants	O
inter-operate	O
transparently	O
,	O
guaranteeing	O
global	B-Operating_System
serializability	I-Operating_System
and	O
automatic	O
global	O
deadlock	B-Operating_System
resolution	O
together	O
in	O
a	O
mixed	O
,	O
heterogeneous	O
environment	O
with	O
different	O
variants	O
.	O
</s>
<s>
The	O
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
;	O
Raz	O
1990	O
,	O
1992	O
,	O
1994	O
,	O
2009	O
)	O
schedule	B-General_Concept
property	O
has	O
been	O
referred	O
to	O
also	O
as	O
Dynamic	O
atomicity	O
(	O
since	O
1988	O
)	O
,	O
commit	B-Architecture
ordering	I-Architecture
,	O
commit	B-Architecture
order	I-Architecture
serializability	I-Architecture
,	O
and	O
strong	B-Architecture
recoverability	I-Architecture
(	O
since	O
1991	O
)	O
.	O
</s>
<s>
In	O
2009	O
CO	O
has	O
been	O
characterized	O
as	O
a	O
major	O
concurrency	B-Operating_System
control	I-Operating_System
method	O
,	O
together	O
with	O
the	O
previously	O
known	O
(	O
since	O
the	O
1980s	O
)	O
three	O
major	O
methods	O
:	O
Locking	B-Operating_System
,	O
Time-stamp	O
ordering	O
,	O
and	O
Serialization	O
graph	O
testing	O
,	O
and	O
as	O
an	O
enabler	O
for	O
the	O
interoperability	O
of	O
systems	O
using	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	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	O
in	O
a	O
communication	O
network	O
,	O
transactions	O
span	O
multiple	O
and	O
possibly	O
Distributed	B-Operating_System
databases	I-Operating_System
.	O
</s>
<s>
Enforcing	O
global	B-Operating_System
serializability	I-Operating_System
in	O
such	O
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
still	O
serializable	B-Operating_System
,	O
the	O
global	O
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	O
to	O
reach	O
conflict	O
serializability	B-Operating_System
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>
The	O
problem	O
of	O
achieving	O
global	B-Operating_System
serializability	I-Operating_System
effectively	O
had	O
been	O
characterized	O
as	O
open	O
until	O
the	O
public	O
disclosure	O
of	O
CO	O
in	O
1991	O
by	O
its	O
inventor	O
Yoav	O
Raz	O
(	O
Raz	O
1991a	O
;	O
see	O
also	O
Global	B-Operating_System
serializability	I-Operating_System
)	O
.	O
</s>
<s>
Enforcing	O
CO	O
is	O
an	O
effective	O
way	O
to	O
enforce	O
conflict	O
serializability	B-Operating_System
globally	O
in	O
a	O
distributed	B-Architecture
system	I-Architecture
since	O
enforcing	O
CO	O
locally	O
in	O
each	O
database	O
(	O
or	O
other	O
transactional	O
objects	O
)	O
also	O
enforces	O
it	O
globally	O
.	O
</s>
<s>
Each	O
database	O
may	O
use	O
any	O
,	O
possibly	O
different	O
,	O
type	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
.	O
</s>
<s>
With	O
a	O
local	O
mechanism	O
that	O
already	O
provides	O
conflict	O
serializability	B-Operating_System
,	O
enforcing	O
CO	O
locally	O
does	O
not	O
cause	O
any	O
other	O
aborts	O
,	O
since	O
enforcing	O
CO	O
locally	O
does	O
not	O
affect	O
the	O
data	O
access	O
scheduling	O
strategy	O
of	O
the	O
mechanism	O
(	O
this	O
scheduling	O
determines	O
the	O
serializability	B-Operating_System
related	O
aborts	O
;	O
such	O
a	O
mechanism	O
typically	O
does	O
not	O
consider	O
the	O
commitment	O
events	O
or	O
their	O
order	O
)	O
.	O
</s>
<s>
The	O
CO	O
solution	O
requires	O
no	O
communication	O
overhead	O
since	O
it	O
uses	O
(	O
unmodified	O
)	O
atomic	O
commitment	O
protocol	O
messages	O
only	O
,	O
already	O
needed	O
by	O
each	O
distributed	B-General_Concept
transaction	I-General_Concept
to	O
reach	O
atomicity	O
.	O
</s>
<s>
CO	O
,	O
its	O
special	O
cases	O
,	O
and	O
its	O
generalizations	O
are	O
interoperable	O
and	O
achieve	O
global	B-Operating_System
serializability	I-Operating_System
while	O
transparently	O
being	O
utilized	O
together	O
in	O
a	O
single	O
heterogeneous	O
distributed	O
environment	O
comprising	O
objects	O
with	O
possibly	O
different	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	O
.	O
</s>
<s>
As	O
such	O
,	O
Commitment	B-Architecture
ordering	I-Architecture
,	O
including	O
its	O
special	O
cases	O
,	O
and	O
together	O
with	O
its	O
generalizations	O
(	O
see	O
CO	O
variants	O
below	O
)	O
,	O
provides	O
a	O
general	O
,	O
high	O
performance	O
,	O
fully	O
distributed	O
solution	O
(	O
no	O
central	O
processing	O
component	O
or	O
central	O
data	O
structure	O
are	O
needed	O
)	O
for	O
guaranteeing	O
global	B-Operating_System
serializability	I-Operating_System
in	O
heterogeneous	O
environments	O
of	O
multidatabase	O
systems	O
and	O
other	O
multiple	O
transactional	O
objects	O
(	O
objects	O
with	O
states	O
accessed	O
and	O
modified	O
only	O
by	O
transactions	O
;	O
e.g.	O
,	O
in	O
the	O
framework	O
of	O
transactional	O
processes	O
,	O
and	O
within	O
Cloud	B-Architecture
computing	I-Architecture
and	O
Grid	B-Architecture
computing	I-Architecture
)	O
.	O
</s>
<s>
The	O
CO	O
solution	O
scales	B-Architecture
up	I-Architecture
with	O
network	O
size	O
and	O
the	O
number	O
of	O
databases	O
without	O
any	O
negative	O
impact	O
on	O
performance	O
(	O
assuming	O
the	O
statistics	O
of	O
a	O
single	O
distributed	B-General_Concept
transaction	I-General_Concept
,	O
e.g.	O
,	O
the	O
average	O
number	O
of	O
databases	O
involved	O
with	O
a	O
single	O
transaction	B-General_Concept
,	O
are	O
unchanged	O
)	O
.	O
</s>
<s>
With	O
the	O
proliferation	O
of	O
Multi-core	B-Architecture
processors	I-Architecture
,	O
Optimistic	O
CO	O
(	O
OCO	O
)	O
has	O
also	O
been	O
increasingly	O
utilized	O
to	O
achieve	O
serializability	B-Operating_System
in	O
software	B-Operating_System
transactional	I-Operating_System
memory	I-Operating_System
,	O
and	O
numerous	O
STM	O
articles	O
and	O
patents	O
utilizing	O
"	O
commit	B-Architecture
order	I-Architecture
"	O
have	O
already	O
been	O
published	O
(	O
e.g.	O
,	O
Zhang	O
et	O
al	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
)	O
is	O
a	O
special	O
case	O
of	O
conflict	O
serializability	B-Operating_System
.	O
</s>
<s>
CO	O
can	O
be	O
enforced	O
with	O
non-blocking	O
mechanisms	O
(	O
each	O
transaction	B-General_Concept
can	O
complete	O
its	O
task	O
without	O
having	O
its	O
data-access	O
blocked	O
,	O
which	O
allows	O
optimistic	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
;	O
however	O
,	O
commitment	O
could	O
be	O
blocked	O
)	O
.	O
</s>
<s>
In	O
a	O
CO	O
schedule	B-General_Concept
,	O
the	O
commitment	O
events	O
 '	O
(	O
partial	O
)	O
precedence	O
order	O
of	O
the	O
transactions	O
correspond	O
to	O
the	O
precedence	O
(	O
partial	O
)	O
order	O
of	O
the	O
respective	O
transactions	O
in	O
the	O
(	O
directed	O
)	O
conflict	O
graph	O
(	O
precedence	B-General_Concept
graph	I-General_Concept
,	O
serializability	B-Operating_System
graph	O
)	O
,	O
as	O
induced	O
by	O
their	O
conflicting	O
access	O
operations	O
(	O
usually	O
read	O
and	O
write	O
(	O
insert/modify/delete	O
)	O
operations	O
;	O
CO	O
also	O
applies	O
to	O
higher-level	O
operations	O
,	O
where	O
they	O
are	O
conflicting	O
if	O
noncommutative	O
,	O
as	O
well	O
as	O
to	O
conflicts	O
between	O
operations	O
upon	O
multi-version	O
data	O
)	O
.	O
</s>
<s>
Definition	O
commitment	B-Architecture
ordering	I-Architecture
Let	O
be	O
two	O
committed	O
transactions	O
in	O
a	O
schedule	B-General_Concept
,	O
such	O
that	O
is	O
in	O
a	O
conflict	O
with	O
(	O
precedes	O
)	O
.	O
</s>
<s>
The	O
schedule	B-General_Concept
has	O
the	O
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
)	O
property	O
,	O
if	O
for	O
every	O
two	O
such	O
transactions	O
commits	O
before	O
commits	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
the	O
conflict	O
graph	O
has	O
cycles	O
,	O
and	O
the	O
schedule	B-General_Concept
will	O
violate	O
serializability	B-Operating_System
when	O
all	O
the	O
transactions	O
on	O
a	O
cycle	O
are	O
committed	O
.	O
</s>
<s>
However	O
,	O
any	O
conflict	O
serializable	B-Operating_System
schedule	B-General_Concept
can	O
be	O
made	O
CO	O
without	O
aborting	O
any	O
transaction	B-General_Concept
by	O
properly	O
delaying	O
commit	O
events	O
to	O
comply	O
with	O
the	O
transactions	O
 '	O
precedence	O
partial	O
order	O
.	O
</s>
<s>
CO	O
enforcement	O
by	O
itself	O
is	O
not	O
sufficient	O
as	O
a	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
since	O
CO	O
lacks	O
the	O
recoverability	O
property	O
,	O
which	O
should	O
be	O
supported	O
as	O
well	O
.	O
</s>
<s>
A	O
fully	O
distributed	O
Global	O
commitment	B-Architecture
ordering	I-Architecture
enforcement	O
algorithm	O
exists	O
that	O
uses	O
local	O
CO	O
of	O
each	O
participating	O
database	O
,	O
and	O
needs	O
only	O
(	O
unmodified	O
)	O
Atomic	O
commitment	O
protocol	O
messages	O
with	O
no	O
further	O
communication	O
.	O
</s>
<s>
Atomic	O
commitment	O
protocol	O
is	O
essential	O
to	O
enforce	O
atomicity	O
of	O
each	O
distributed	B-General_Concept
transaction	I-General_Concept
(	O
to	O
decide	O
whether	O
to	O
commit	O
or	O
abort	O
it	O
;	O
this	O
procedure	O
is	O
always	O
carried	O
out	O
for	O
distributed	B-General_Concept
transactions	I-General_Concept
,	O
independently	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
and	O
CO	O
)	O
.	O
</s>
<s>
A	O
common	O
example	O
of	O
an	O
atomic	O
commitment	O
protocol	O
is	O
the	O
two-phase	B-General_Concept
commit	I-General_Concept
protocol	I-General_Concept
,	O
which	O
is	O
resilient	O
to	O
many	O
types	O
of	O
system	O
failure	O
.	O
</s>
<s>
In	O
a	O
reliable	O
environment	O
,	O
or	O
when	O
processes	O
usually	O
fail	O
together	O
(	O
e.g.	O
,	O
in	O
the	O
same	O
integrated	O
circuit	O
)	O
,	O
a	O
simpler	O
protocol	O
for	O
atomic	O
commitment	O
may	O
be	O
used	O
(	O
e.g.	O
,	O
a	O
simple	O
handshake	O
of	O
distributed	B-General_Concept
transaction	I-General_Concept
's	O
participating	O
processes	O
with	O
some	O
arbitrary	O
but	O
known	O
special	O
participant	O
,	O
the	O
transaction	B-General_Concept
's	O
coordinator	O
,	O
i.e.	O
,	O
a	O
type	O
of	O
one-phase	O
commit	O
protocol	O
)	O
.	O
</s>
<s>
An	O
atomic	O
commitment	O
protocol	O
reaches	O
consensus	O
among	O
participants	O
on	O
whether	O
to	O
commit	O
or	O
abort	O
a	O
distributed	O
(	O
global	O
)	O
transaction	B-General_Concept
that	O
spans	O
these	O
participants	O
.	O
</s>
<s>
Otherwise	O
,	O
a	O
participant	O
can	O
unilaterally	O
abort	O
the	O
transaction	B-General_Concept
by	O
an	O
explicit	O
NO	O
vote	O
.	O
</s>
<s>
The	O
protocol	O
commits	O
the	O
transaction	B-General_Concept
only	O
if	O
YES	O
votes	O
have	O
been	O
received	O
from	O
all	O
participants	O
(	O
thus	O
,	O
a	O
missing	O
vote	O
is	O
typically	O
considered	O
a	O
NO	O
)	O
.	O
</s>
<s>
Otherwise	O
,	O
the	O
protocol	O
aborts	O
the	O
transaction	B-General_Concept
.	O
</s>
<s>
The	O
entire	O
CO	O
solution	O
for	O
global	B-Operating_System
serializability	I-Operating_System
is	O
based	O
on	O
the	O
fact	O
that	O
the	O
atomic	O
commitment	O
protocol	O
eventually	O
aborts	O
this	O
transaction	B-General_Concept
in	O
case	O
of	O
a	O
missing	O
vote	O
for	O
a	O
distributed	B-General_Concept
transaction	I-General_Concept
.	O
</s>
<s>
Accordingly	O
,	O
YES	O
votes	O
in	O
the	O
atomic	O
commitment	O
protocol	O
are	O
scheduled	O
for	O
each	O
(	O
unaborted	O
)	O
distributed	B-General_Concept
transaction	I-General_Concept
(	O
in	O
what	O
follows	O
,	O
"	O
a	O
vote	O
"	O
means	O
a	O
YES	O
vote	O
)	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
the	O
second	O
will	O
not	O
be	O
voted	O
on	O
before	O
the	O
first	O
is	O
completed	O
(	O
either	O
committed	O
or	O
aborted	O
)	O
,	O
to	O
prevent	O
possible	O
commit	B-Architecture
order	I-Architecture
violation	O
by	O
the	O
atomic	O
commitment	O
protocol	O
.	O
</s>
<s>
Such	O
can	O
happen	O
since	O
the	O
commit	B-Architecture
order	I-Architecture
by	O
the	O
protocol	O
is	O
not	O
necessarily	O
the	O
same	O
as	O
the	O
voting	O
order	O
.	O
</s>
<s>
This	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
ensures	O
that	O
also	O
the	O
atomic	O
commitment	O
protocol	O
maintains	O
commitment	O
order	O
,	O
and	O
it	O
is	O
a	O
necessary	O
condition	O
for	O
guaranteeing	O
Global	O
CO	O
(	O
and	O
the	O
local	O
CO	O
of	O
a	O
database	O
;	O
without	O
it	O
both	O
Global	O
CO	O
and	O
Local	O
CO	O
(	O
a	O
property	O
meaning	O
that	O
each	O
database	O
is	O
CO	O
compliant	O
)	O
may	O
be	O
violated	O
)	O
.	O
</s>
<s>
However	O
,	O
since	O
database	O
systems	O
schedule	B-General_Concept
their	O
transactions	O
independently	O
,	O
it	O
is	O
possible	O
that	O
the	O
transactions	O
 '	O
precedence	O
orders	O
in	O
two	O
databases	O
or	O
more	O
are	O
not	O
compatible	O
(	O
no	O
global	O
partial	O
order	O
exists	O
that	O
can	O
embed	O
the	O
respective	O
local	O
partial	O
orders	O
together	O
)	O
.	O
</s>
<s>
When	O
participating	O
databases	O
in	O
the	O
same	O
distributed	B-General_Concept
transaction	I-General_Concept
do	O
not	O
have	O
compatible	O
local	O
precedence	O
orders	O
for	O
that	O
transaction	B-General_Concept
(	O
without	O
"	O
knowing	O
"	O
it	O
;	O
typically	O
no	O
coordination	O
between	O
database	O
systems	O
exists	O
on	O
conflicts	O
,	O
since	O
the	O
needed	O
communication	O
is	O
massive	O
and	O
unacceptably	O
degrades	O
performance	O
)	O
it	O
means	O
that	O
the	O
transaction	B-General_Concept
resides	O
on	O
a	O
global	O
cycle	O
(	O
involving	O
two	O
or	O
more	O
databases	O
)	O
in	O
the	O
global	O
conflict	O
graph	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
atomic	O
commitment	O
protocol	O
will	O
fail	O
to	O
collect	O
all	O
the	O
votes	O
needed	O
to	O
commit	O
that	O
transaction	B-General_Concept
:	O
By	O
the	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
above	O
,	O
at	O
least	O
one	O
database	O
will	O
delay	O
its	O
vote	O
for	O
that	O
transaction	B-General_Concept
indefinitely	O
to	O
comply	O
with	O
its	O
own	O
commitment	O
(	O
precedence	O
)	O
order	O
,	O
since	O
it	O
will	O
be	O
waiting	O
to	O
the	O
completion	O
of	O
another	O
,	O
preceding	O
transactions	O
on	O
that	O
global	O
cycle	O
delayed	O
indefinitely	O
by	O
another	O
database	O
with	O
a	O
different	O
order	O
.	O
</s>
<s>
This	O
means	O
a	O
voting-deadlock	O
situation	O
involving	O
the	O
databases	O
on	O
that	O
cycle	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
protocol	O
will	O
eventually	O
abort	O
some	O
deadlocked	O
transaction	B-General_Concept
on	O
this	O
global	O
cycle	O
,	O
since	O
each	O
such	O
transaction	B-General_Concept
is	O
missing	O
at	O
least	O
one	O
participant	O
's	O
vote	O
.	O
</s>
<s>
Selection	O
of	O
the	O
specific	O
transaction	B-General_Concept
on	O
the	O
cycle	O
to	O
be	O
aborted	O
depends	O
on	O
the	O
atomic	O
commitment	O
protocol	O
's	O
abort	O
policies	O
(	O
a	O
timeout	O
mechanism	O
is	O
common	O
,	O
but	O
it	O
may	O
result	O
in	O
more	O
than	O
one	O
needed	O
abort	O
per	O
cycle	O
;	O
both	O
preventing	O
unnecessary	O
aborts	O
and	O
abort	O
time	O
shortening	O
can	O
be	O
achieved	O
by	O
a	O
dedicated	O
abort	O
mechanism	O
for	O
CO	O
)	O
.	O
</s>
<s>
Such	O
abort	O
will	O
break	O
the	O
global	O
cycle	O
involving	O
that	O
distributed	B-General_Concept
transaction	I-General_Concept
.	O
</s>
<s>
It	O
is	O
worthwhile	O
noting	O
that	O
each	O
database	O
involved	O
with	O
the	O
voting-deadlock	O
continues	O
to	O
vote	O
regularly	O
on	O
transactions	O
that	O
are	O
not	O
in	O
conflict	O
with	O
its	O
deadlocked	O
transaction	B-General_Concept
,	O
typically	O
almost	O
all	O
the	O
outstanding	O
transactions	O
.	O
</s>
<s>
Thus	O
,	O
in	O
the	O
case	O
of	O
incompatible	O
local	O
(	O
partial	O
)	O
commitment	O
orders	O
,	O
no	O
action	O
is	O
needed	O
since	O
the	O
atomic	O
commitment	O
protocol	O
resolves	O
it	O
automatically	O
by	O
aborting	O
a	O
transaction	B-General_Concept
that	O
is	O
a	O
cause	O
of	O
incompatibility	O
.	O
</s>
<s>
This	O
means	O
that	O
the	O
above	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
is	O
also	O
a	O
sufficient	O
condition	O
for	O
guaranteeing	O
Global	O
CO	O
.	O
</s>
<s>
Then	O
,	O
having	O
ended	O
(	O
either	O
committed	O
or	O
aborted	O
)	O
before	O
is	O
voted	O
on	O
to	O
be	O
committed	O
(	O
the	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
)	O
,	O
in	O
each	O
such	O
database	O
system	O
in	O
a	O
multidatabase	O
environment	O
,	O
is	O
a	O
necessary	O
and	O
sufficient	O
condition	O
for	O
guaranteeing	O
Global	O
CO	O
(	O
the	O
condition	O
guarantees	O
Global	O
CO	O
,	O
which	O
may	O
be	O
violated	O
without	O
it	O
)	O
.	O
</s>
<s>
The	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
that	O
enforces	O
global	O
CO	O
is	O
referred	O
to	O
as	O
in	O
(	O
Raz	O
1992	O
)	O
.	O
</s>
<s>
The	O
Local	O
CO	O
property	O
of	O
a	O
global	O
schedule	B-General_Concept
means	O
that	O
each	O
database	O
is	O
CO	O
compliant	O
.	O
</s>
<s>
Together	O
it	O
means	O
that	O
Global	O
CO	O
is	O
guaranteed	O
if	O
and	O
only	O
if	O
Local	O
CO	O
is	O
guaranteed	O
(	O
which	O
is	O
untrue	O
for	O
Global	O
conflict	O
serializability	B-Operating_System
and	O
Local	O
conflict	O
serializability	B-Operating_System
:	O
Global	O
implies	O
Local	O
,	O
but	O
not	O
the	O
opposite	O
)	O
.	O
</s>
<s>
Global	O
CO	O
implies	O
Global	B-Operating_System
serializability	I-Operating_System
.	O
</s>
<s>
The	O
Global	O
CO	O
algorithm	O
comprises	O
enforcing	O
(	O
local	O
)	O
CO	O
in	O
each	O
participating	O
database	O
system	O
by	O
ordering	O
commits	O
of	O
local	O
transactions	O
(	O
see	O
Enforcing	O
CO	O
locally	O
below	O
)	O
and	O
enforcing	O
the	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
in	O
the	O
theorem	O
above	O
(	O
for	O
global	O
transactions	O
)	O
.	O
</s>
<s>
The	O
above	O
global	O
cycle	O
elimination	O
process	O
by	O
a	O
voting	O
deadlock	B-Operating_System
can	O
be	O
explained	O
in	O
detail	O
by	O
the	O
following	O
observation	O
:	O
</s>
<s>
First	O
,	O
it	O
is	O
assumed	O
,	O
for	O
simplicity	O
,	O
that	O
every	O
transaction	B-General_Concept
reaches	O
the	O
ready-to-commit	O
state	O
and	O
is	O
voted	O
on	O
by	O
at	O
least	O
one	O
database	O
(	O
this	O
implies	O
that	O
no	O
blocking	O
by	O
locks	O
occurs	O
)	O
.	O
</s>
<s>
Define	O
a	O
"	O
wait	O
for	O
vote	O
to	O
commit	O
"	O
graph	O
as	O
a	O
directed	O
graph	O
with	O
transactions	O
as	O
nodes	O
and	O
a	O
directed	O
edge	O
from	O
any	O
first	O
transaction	B-General_Concept
to	O
a	O
second	O
transaction	B-General_Concept
if	O
the	O
first	O
transaction	B-General_Concept
blocks	O
the	O
vote	O
to	O
commit	O
of	O
the	O
second	O
transaction	B-General_Concept
(	O
opposite	O
to	O
conventional	O
edge	O
direction	O
in	O
a	O
wait-for	B-Operating_System
graph	I-Operating_System
)	O
.	O
</s>
<s>
Such	O
blocking	O
happens	O
only	O
if	O
the	O
second	O
transaction	B-General_Concept
conflicts	O
with	O
the	O
first	O
transaction	B-General_Concept
(	O
see	O
above	O
)	O
.	O
</s>
<s>
A	O
cycle	O
in	O
the	O
"	O
wait	O
for	O
vote	O
to	O
commit	O
"	O
graph	O
means	O
a	O
deadlock	B-Operating_System
in	O
voting	O
.	O
</s>
<s>
Hence	O
there	O
is	O
a	O
deadlock	B-Operating_System
in	O
voting	O
if	O
there	O
is	O
a	O
cycle	O
in	O
the	O
conflict	O
graph	O
.	O
</s>
<s>
The	O
local	O
serializability	B-Operating_System
mechanisms	O
eliminate	O
local	O
cycles	O
(	O
confined	O
to	O
a	O
single	O
database	O
)	O
.	O
</s>
<s>
Secondly	O
,	O
also	O
local	O
commits	O
are	O
dealt	O
with	O
:	O
Note	O
that	O
when	O
enforcing	O
CO	O
,	O
also	O
waiting	O
for	O
a	O
regular	O
local	O
commit	O
of	O
a	O
local	O
transaction	B-General_Concept
can	O
block	O
local	O
commits	O
and	O
votes	O
of	O
other	O
transactions	O
upon	O
conflicts	O
,	O
and	O
the	O
situation	O
for	O
global	O
transactions	O
does	O
not	O
also	O
change	O
without	O
the	O
simplifying	O
assumption	O
above	O
:	O
The	O
final	O
result	O
is	O
the	O
same	O
also	O
with	O
a	O
local	O
commitment	O
for	O
local	O
transactions	O
,	O
without	O
voting	O
in	O
atomic	O
commitment	O
for	O
them	O
.	O
</s>
<s>
Suppose	O
the	O
lock	O
is	O
released	O
only	O
after	O
the	O
transaction	B-General_Concept
end	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
it	O
may	O
block	O
indirectly	O
either	O
a	O
vote	O
or	O
a	O
local	O
commit	O
of	O
another	O
transaction	B-General_Concept
(	O
which	O
now	O
cannot	O
get	O
to	O
ready	O
state	O
)	O
,	O
with	O
the	O
same	O
effect	O
as	O
a	O
direct	O
blocking	O
of	O
a	O
vote	O
or	O
a	O
local	O
commit	O
.	O
</s>
<s>
An	O
augmented	O
conflict	O
graph	O
is	O
a	O
conflict	O
graph	O
with	O
added	O
edges	O
:	O
In	O
addition	O
to	O
the	O
original	O
edges	O
a	O
directed	O
edge	O
exists	O
from	O
transaction	B-General_Concept
to	O
transaction	B-General_Concept
if	O
two	O
conditions	O
are	O
met	O
:	O
</s>
<s>
Note	O
that	O
all	O
the	O
new	O
edges	O
are	O
all	O
the	O
(	O
reversed	O
to	O
the	O
conventional	O
)	O
edges	O
of	O
the	O
wait-for	B-Operating_System
graph	I-Operating_System
.	O
</s>
<s>
The	O
wait-for	B-Operating_System
graph	I-Operating_System
can	O
also	O
be	O
defined	O
as	O
the	O
graph	O
of	O
non-materialized	O
conflicts	O
.	O
</s>
<s>
By	O
the	O
common	O
conventions	O
,	O
edge	O
direction	O
in	O
a	O
conflict	O
graph	O
defines	O
time	O
order	O
between	O
conflicting	O
operations	O
,	O
which	O
is	O
opposite	O
to	O
the	O
time	O
order	O
defined	O
by	O
an	O
edge	O
in	O
a	O
wait-for	B-Operating_System
graph	I-Operating_System
.	O
</s>
<s>
Note	O
that	O
such	O
a	O
global	O
graph	O
contains	O
(	O
has	O
embedded	O
)	O
all	O
the	O
(	O
reversed	O
edge	O
)	O
regular	O
local	O
wait-for	B-Operating_System
graphs	I-Operating_System
and	O
also	O
may	O
include	O
locking	B-Operating_System
based	O
global	O
cycles	O
(	O
which	O
cannot	O
exist	O
in	O
the	O
local	O
graphs	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
all	O
the	O
databases	O
on	O
a	O
global	O
cycle	O
are	O
SS2PL	O
based	O
,	O
then	O
all	O
the	O
related	O
vote	O
blocking	O
situations	O
are	O
caused	O
by	O
locks	O
(	O
this	O
is	O
the	O
classical	O
and	O
probably	O
the	O
only	O
global	O
deadlock	B-Operating_System
situation	O
dealt	O
with	O
in	O
the	O
database	O
research	O
literature	O
)	O
.	O
</s>
<s>
This	O
is	O
a	O
global	O
deadlock	B-Operating_System
case	O
where	O
each	O
related	O
database	O
creates	O
a	O
portion	O
of	O
the	O
cycle	O
,	O
but	O
the	O
complete	O
cycle	O
does	O
not	O
reside	O
in	O
any	O
local	O
wait-for	B-Operating_System
graph	I-Operating_System
.	O
</s>
<s>
In	O
the	O
presence	O
of	O
CO	O
,	O
the	O
augmented	O
conflict	O
graph	O
is	O
,	O
in	O
fact	O
,	O
a	O
(	O
reversed	O
edge	O
)	O
local-commit	O
and	O
voting	O
wait-for	B-Operating_System
graph	I-Operating_System
:	O
An	O
edge	O
exists	O
from	O
a	O
first	O
transaction	B-General_Concept
,	O
either	O
local	O
or	O
global	O
,	O
to	O
a	O
second	O
,	O
if	O
the	O
second	O
is	O
waiting	O
for	O
the	O
first	O
to	O
end	O
in	O
order	O
to	O
be	O
either	O
voted	O
on	O
(	O
if	O
global	O
)	O
or	O
locally	O
committed	O
(	O
if	O
local	O
)	O
.	O
</s>
<s>
All	O
global	O
cycles	O
(	O
across	O
two	O
or	O
more	O
databases	O
)	O
in	O
this	O
graph	O
generate	O
voting-deadlocks	O
.	O
</s>
<s>
The	O
graph	O
's	O
global	O
cycles	O
provide	O
complete	O
characterization	O
for	O
voting	O
deadlocks	B-Operating_System
and	O
may	O
include	O
any	O
combination	O
of	O
materialized	O
and	O
non-materialized	O
conflicts	O
.	O
</s>
<s>
Only	O
cycles	O
of	O
(	O
only	O
)	O
materialized	O
conflicts	O
are	O
also	O
cycles	O
of	O
the	O
regular	O
conflict	O
graph	O
and	O
affect	O
serializability	B-Operating_System
.	O
</s>
<s>
One	O
or	O
more	O
(	O
lock	O
related	O
)	O
non-materialized	O
conflicts	O
on	O
a	O
cycle	O
prevent	O
it	O
from	O
being	O
a	O
cycle	O
in	O
the	O
regular	O
conflict	O
graph	O
and	O
make	O
it	O
a	O
locking	B-Operating_System
related	O
deadlock	B-Operating_System
.	O
</s>
<s>
All	O
the	O
global	O
cycles	O
(	O
voting-deadlocks	O
)	O
need	O
to	O
be	O
broken	O
(	O
resolved	O
)	O
to	O
both	O
maintain	O
global	B-Operating_System
serializability	I-Operating_System
and	O
resolve	O
global	O
deadlocks	B-Operating_System
involving	O
data	O
access	O
locking	B-Operating_System
.	O
</s>
<s>
Indeed	O
they	O
are	O
all	O
broken	O
by	O
the	O
atomic	O
commitment	O
protocol	O
due	O
to	O
missing	O
votes	O
upon	O
a	O
voting	O
deadlock	B-Operating_System
.	O
</s>
<s>
This	O
results	O
in	O
the	O
same	O
voting-deadlock	O
situations	O
and	O
resulting	O
global	O
cycle	O
elimination	O
process	O
for	O
ECO	O
.	O
</s>
<s>
The	O
voting-deadlock	O
situation	O
can	O
be	O
summarized	O
as	O
follows	O
:	O
</s>
<s>
A	O
voting-deadlock	O
occurs	O
if	O
and	O
only	O
if	O
a	O
global	O
cycle	O
(	O
spans	O
two	O
or	O
more	O
databases	O
)	O
exists	O
in	O
the	O
Global	O
augmented	O
conflict	O
graph	O
(	O
also	O
blocking	O
by	O
a	O
data-access	O
lock	O
is	O
represented	O
by	O
an	O
edge	O
)	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
all	O
the	O
global	O
transactions	O
on	O
it	O
are	O
involved	O
with	O
the	O
respective	O
voting-deadlock	O
.	O
</s>
<s>
Eventually	O
,	O
each	O
has	O
its	O
vote	O
blocked	O
(	O
either	O
directly	O
or	O
indirectly	O
by	O
a	O
data-access	O
lock	O
)	O
;	O
if	O
a	O
local	O
transaction	B-General_Concept
resides	O
on	O
the	O
cycle	O
,	O
eventually	O
,	O
it	O
has	O
its	O
(	O
local	O
)	O
commit	O
blocked	O
.	O
</s>
<s>
Comment	O
:	O
A	O
rare	O
situation	O
of	O
a	O
voting	O
deadlock	B-Operating_System
(	O
by	O
missing	O
blocked	O
votes	O
)	O
can	O
happen	O
,	O
with	O
no	O
voting	O
for	O
any	O
transaction	B-General_Concept
on	O
the	O
related	O
cycle	O
by	O
any	O
of	O
the	O
database	O
systems	O
involved	O
with	O
these	O
transactions	O
.	O
</s>
<s>
This	O
can	O
occur	O
when	O
local	O
sub-transactions	O
are	O
multi-threaded	B-Operating_System
.	O
</s>
<s>
Such	O
global	O
cycles	O
(	O
deadlocks	B-Operating_System
)	O
overlap	O
with	O
local	O
cycles	O
that	O
are	O
resolved	O
locally	O
and	O
are	O
typically	O
resolved	O
by	O
local	O
mechanisms	O
without	O
involving	O
atomic	O
commitment	O
.	O
</s>
<s>
Formally	O
it	O
is	O
also	O
a	O
global	O
cycle	O
,	O
but	O
practically	O
it	O
is	O
local	O
(	O
portions	O
of	O
local	O
cycles	O
generate	O
a	O
global	O
one	O
;	O
to	O
see	O
this	O
,	O
split	O
each	O
global	O
transaction	B-General_Concept
(	O
node	O
)	O
to	O
local	O
sub-transactions	O
(	O
its	O
portions	O
confined	O
each	O
to	O
a	O
single	O
database	O
)	O
;	O
a	O
directed	O
edge	O
exists	O
between	O
transactions	O
if	O
an	O
edge	O
exists	O
between	O
any	O
respective	O
local	O
sub-transactions	O
;	O
a	O
cycle	O
is	O
local	O
if	O
all	O
its	O
edges	O
originate	O
from	O
a	O
cycle	O
among	O
sub-transactions	O
of	O
the	O
same	O
database	O
,	O
and	O
global	O
if	O
not	O
;	O
global	O
and	O
local	O
can	O
overlap	O
:	O
the	O
same	O
cycle	O
among	O
transactions	O
can	O
result	O
from	O
several	O
different	O
cycles	O
among	O
sub-transactions	O
,	O
and	O
be	O
both	O
local	O
and	O
global	O
)	O
.	O
</s>
<s>
Also	O
,	O
the	O
following	O
locking	B-Operating_System
based	O
special	O
case	O
is	O
concluded	O
:	O
</s>
<s>
In	O
a	O
CO	O
compliant	O
multidatabase	O
system	O
,	O
a	O
locking-based	O
global-deadlock	O
,	O
involving	O
at	O
least	O
one	O
data-access	O
lock	O
(	O
non-materialized	O
conflict	O
)	O
,	O
and	O
two	O
or	O
more	O
database	O
systems	O
,	O
is	O
a	O
reflection	O
of	O
a	O
global	O
cycle	O
in	O
the	O
Global	O
augmented	O
conflict	O
graph	O
,	O
which	O
results	O
in	O
a	O
voting-deadlock	O
.	O
</s>
<s>
Such	O
a	O
cycle	O
is	O
not	O
a	O
cycle	O
in	O
the	O
(	O
regular	O
)	O
Global	O
conflict	O
graph	O
(	O
which	O
reflects	O
only	O
materialized	O
conflicts	O
,	O
so	O
such	O
a	O
cycle	O
does	O
not	O
affect	O
serializability	B-Operating_System
)	O
.	O
</s>
<s>
All	O
voting-deadlocks	O
are	O
resolved	O
(	O
almost	O
all	O
by	O
Atomic	O
commitment	O
;	O
see	O
comment	O
above	O
)	O
,	O
including	O
this	O
locking-based	O
type	O
.	O
</s>
<s>
Locking-based	O
global-deadlocks	O
can	O
also	O
be	O
generated	O
in	O
a	O
completely	O
SS2PL-based	O
distributed	O
environment	O
(	O
special	O
case	O
of	O
CO	O
based	O
)	O
.	O
</s>
<s>
All	O
the	O
vote	O
blocking	O
(	O
and	O
voting-deadlocks	O
)	O
are	O
caused	O
by	O
data-access	O
locks	O
.	O
</s>
<s>
Many	O
research	O
articles	O
have	O
dealt	O
for	O
years	O
with	O
resolving	O
such	O
global	O
deadlocks	B-Operating_System
,	O
but	O
none	O
(	O
except	O
the	O
CO	O
articles	O
)	O
is	O
known	O
(	O
as	O
of	O
2009	O
)	O
to	O
notice	O
that	O
atomic	O
commitment	O
automatically	O
resolves	O
them	O
.	O
</s>
<s>
Voting-deadlocks	O
are	O
the	O
key	O
to	O
the	O
operation	O
of	O
distributed	O
CO	O
.	O
</s>
<s>
Global	O
cycle	O
elimination	O
(	O
here	O
voting-deadlock	O
resolution	O
by	O
atomic	O
commitment	O
)	O
and	O
resulting	O
aborted	O
transactions	O
 '	O
re-executions	O
are	O
time-consuming	O
,	O
regardless	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
used	O
.	O
</s>
<s>
If	O
databases	O
schedule	B-General_Concept
transactions	O
independently	O
,	O
global	O
cycles	O
are	O
unavoidable	O
(	O
in	O
a	O
complete	O
analogy	O
to	O
cycles/deadlocks	O
generated	O
in	O
local	O
SS2PL	O
;	O
with	O
distribution	O
,	O
any	O
transaction	B-General_Concept
or	O
operation	O
scheduling	O
coordination	O
results	O
in	O
autonomy	O
violation	O
and	O
is	O
typically	O
in	O
substantial	O
performance	O
penalty	O
)	O
.	O
</s>
<s>
However	O
,	O
their	O
likelihood	O
can	O
be	O
made	O
very	O
low	O
in	O
many	O
cases	O
by	O
implementing	O
database	O
and	O
transaction	B-General_Concept
design	O
guidelines	O
that	O
reduce	O
the	O
number	O
of	O
conflicts	O
involving	O
a	O
global	O
transaction	B-General_Concept
.	O
</s>
<s>
This	O
,	O
primarily	O
by	O
properly	O
handling	O
hot	O
spots	O
(	O
database	O
objects	O
with	O
frequent	O
access	O
,	O
)	O
and	O
avoiding	O
conflicts	O
by	O
using	O
commutativity	O
when	O
possible	O
(	O
e.g.	O
,	O
when	O
extensively	O
using	O
counters	O
,	O
as	O
in	O
finances	O
,	O
and	O
especially	O
multi-transaction	O
accumulation	O
counters	O
,	O
which	O
are	O
typically	O
hot	O
spots	O
)	O
.	O
</s>
<s>
Atomic	O
commitment	O
protocols	O
are	O
intended	O
and	O
designed	O
to	O
achieve	O
atomicity	O
without	O
considering	O
database	O
concurrency	B-Operating_System
control	I-Operating_System
.	O
</s>
<s>
They	O
abort	O
upon	O
detecting	O
or	O
heuristically	B-Algorithm
finding	O
(	O
e.g.	O
,	O
by	O
a	O
timeout	O
;	O
sometimes	O
mistakenly	O
,	O
unnecessarily	O
)	O
missing	O
votes	O
and	O
typically	O
unaware	O
of	O
global	O
cycles	O
.	O
</s>
<s>
These	O
protocols	O
can	O
be	O
especially	O
enhanced	O
for	O
CO	O
(	O
including	O
CO	O
's	O
variants	O
below	O
)	O
to	O
prevent	O
unnecessary	O
aborts	O
and	O
accelerate	O
aborts	O
used	O
for	O
breaking	O
global	O
cycles	O
in	O
the	O
global	O
augmented	O
conflict	O
graph	O
(	O
for	O
better	O
performance	O
by	O
earlier	O
release	O
upon	O
transaction-end	O
of	O
computing	O
resources	O
and	O
typically	O
locked	O
data	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
existing	O
locking	B-Operating_System
based	O
global	O
deadlock	B-Operating_System
detection	O
methods	O
,	O
other	O
than	O
the	O
timeout	O
,	O
can	O
be	O
generalized	O
also	O
to	O
consider	O
local	O
commit	O
and	O
vote	O
direct	O
blocking	O
,	O
besides	O
data	O
access	O
blocking	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
can	O
be	O
enforced	O
locally	O
(	O
in	O
a	O
single	O
database	O
)	O
by	O
a	O
dedicated	O
CO	O
algorithm	O
,	O
or	O
by	O
any	O
algorithm/protocol	O
that	O
provides	O
any	O
special	O
case	O
of	O
CO	O
.	O
An	O
important	O
such	O
protocol	O
,	O
being	O
utilized	O
extensively	O
in	O
database	O
systems	O
,	O
which	O
generates	O
a	O
CO	O
schedule	B-General_Concept
,	O
is	O
the	O
strong	O
strict	O
two	B-Operating_System
phase	I-Operating_System
locking	I-Operating_System
protocol	O
(	O
SS2PL	O
:	O
"	O
release	O
transaction	B-General_Concept
's	O
locks	O
only	O
after	O
the	O
transaction	B-General_Concept
has	O
been	O
either	O
committed	O
or	O
aborted	O
"	O
;	O
see	O
below	O
)	O
.	O
</s>
<s>
SS2PL	O
is	O
a	O
proper	O
subset	O
of	O
the	O
intersection	O
of	O
2PL	B-Operating_System
and	O
strictness	O
.	O
</s>
<s>
It	O
does	O
not	O
block	O
data	O
access	O
(	O
nonblocking	O
)	O
and	O
consists	O
of	O
aborting	O
a	O
certain	O
set	O
of	O
transactions	O
(	O
only	O
if	O
needed	O
)	O
upon	O
committing	O
a	O
transaction	B-General_Concept
.	O
</s>
<s>
It	O
aborts	O
a	O
(	O
uniquely	O
determined	O
at	O
any	O
given	O
time	O
)	O
minimal	O
set	O
of	O
other	O
undecided	O
(	O
neither	O
committed	O
,	O
nor	O
aborted	O
)	O
transactions	O
that	O
run	O
locally	O
and	O
can	O
cause	O
serializability	B-Operating_System
violation	O
in	O
the	O
future	O
(	O
can	O
later	O
generate	O
cycles	O
of	O
committed	O
transactions	O
in	O
the	O
conflict	O
graph	O
;	O
this	O
is	O
the	O
ABORT	O
set	O
of	O
a	O
committed	O
transaction	B-General_Concept
T	O
;	O
after	O
committing	O
T	O
no	O
transaction	B-General_Concept
in	O
ABORT	O
at	O
commit	O
time	O
can	O
be	O
committed	O
,	O
and	O
all	O
of	O
them	O
are	O
doomed	O
to	O
be	O
aborted	O
)	O
.	O
</s>
<s>
This	O
set	O
consists	O
of	O
all	O
undecided	O
transactions	O
with	O
directed	O
edges	O
in	O
the	O
conflict	O
graph	O
to	O
the	O
committed	O
transaction	B-General_Concept
.	O
</s>
<s>
The	O
size	O
of	O
this	O
set	O
cannot	O
increase	O
when	O
that	O
transaction	B-General_Concept
is	O
waiting	O
to	O
be	O
committed	O
(	O
in	O
the	O
ready	O
state	O
:	O
processing	O
has	O
ended	O
,	O
)	O
and	O
typically	O
decreases	O
in	O
time	O
as	O
its	O
transactions	O
are	O
being	O
decided	O
.	O
</s>
<s>
Thus	O
,	O
unless	O
real-time	B-General_Concept
constraints	O
exist	O
to	O
complete	O
that	O
transaction	B-General_Concept
,	O
it	O
is	O
preferred	O
to	O
wait	O
with	O
committing	O
that	O
transaction	B-General_Concept
and	O
let	O
this	O
set	O
decrease	O
in	O
size	O
.	O
</s>
<s>
If	O
another	O
serializability	B-Operating_System
mechanism	O
exists	O
locally	O
(	O
which	O
eliminates	O
cycles	O
in	O
the	O
local	O
conflict	O
graph	O
)	O
,	O
or	O
if	O
no	O
cycle	O
involving	O
that	O
transaction	B-General_Concept
exists	O
,	O
the	O
set	O
will	O
be	O
empty	O
eventually	O
,	O
and	O
no	O
abort	O
of	O
a	O
set	O
member	O
is	O
needed	O
.	O
</s>
<s>
Since	O
in	O
the	O
case	O
of	O
CO	O
conflicts	O
generate	O
blocking	O
on	O
commit	O
,	O
local	O
cycles	O
in	O
the	O
augments	O
conflict	O
graph	O
(	O
see	O
above	O
)	O
indicate	O
local	O
commit-deadlocks	O
,	O
and	O
deadlock	B-Operating_System
resolution	O
techniques	O
as	O
in	O
SS2PL	O
can	O
be	O
used	O
(	O
e.g.	O
,	O
like	O
timeout	O
and	O
wait-for	B-Operating_System
graph	I-Operating_System
)	O
.	O
</s>
<s>
A	O
local	O
cycle	O
in	O
the	O
augmented	O
conflict	O
graph	O
with	O
at	O
least	O
one	O
non-materialized	O
conflict	O
reflects	O
a	O
locking-based	O
deadlock	B-Operating_System
.	O
</s>
<s>
The	O
local	O
algorithm	O
above	O
,	O
applied	O
to	O
the	O
local	O
augmented	O
conflict	O
graph	O
rather	O
than	O
the	O
regular	O
local	O
conflict	O
graph	O
,	O
comprises	O
the	O
generic	O
enhanced	O
local	O
CO	O
algorithm	O
,	O
a	O
single	O
local	O
cycle	O
elimination	O
mechanism	O
,	O
for	O
both	O
guaranteeing	O
local	O
serializability	B-Operating_System
and	O
handling	O
locking	B-Operating_System
based	O
local	O
deadlocks	B-Operating_System
.	O
</s>
<s>
Practically	O
an	O
additional	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
is	O
always	O
utilized	O
,	O
even	O
solely	O
to	O
enforce	O
recoverability	O
.	O
</s>
<s>
The	O
generic	O
CO	O
algorithm	O
does	O
not	O
affect	O
the	O
local	O
data	O
access	O
scheduling	O
strategy	O
when	O
it	O
runs	O
alongside	O
any	O
other	O
local	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
.	O
</s>
<s>
It	O
affects	O
only	O
the	O
commit	B-Architecture
order	I-Architecture
,	O
and	O
for	O
this	O
reason	O
,	O
it	O
does	O
not	O
need	O
to	O
abort	O
more	O
transactions	O
than	O
those	O
needed	O
to	O
be	O
aborted	O
for	O
serializability	B-Operating_System
violation	O
prevention	O
by	O
any	O
combined	O
local	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
.	O
</s>
<s>
At	O
most	O
,	O
the	O
net	O
effect	O
of	O
CO	O
may	O
be	O
a	O
delay	O
of	O
commit	O
events	O
(	O
or	O
voting	O
in	O
a	O
distributed	O
environment	O
)	O
,	O
to	O
comply	O
with	O
the	O
needed	O
commit	B-Architecture
order	I-Architecture
(	O
but	O
not	O
more	O
delay	O
than	O
its	O
special	O
cases	O
,	O
for	O
example	O
,	O
SS2PL	O
,	O
and	O
on	O
average	O
significantly	O
less	O
)	O
.	O
</s>
<s>
The	O
Generic	O
local	O
CO	O
algorithm	O
guarantees	O
(	O
local	O
)	O
CO	O
(	O
a	O
CO	O
compliant	O
schedule	B-General_Concept
)	O
.	O
</s>
<s>
The	O
Generic	O
enhanced	O
local	O
CO	O
algorithm	O
guarantees	O
both	O
(	O
local	O
)	O
CO	O
and	O
(	O
local	O
)	O
locking	B-Operating_System
based	O
deadlock	B-Operating_System
resolution	O
.	O
</s>
<s>
And	O
(	O
when	O
not	O
using	O
a	O
timeout	O
,	O
and	O
no	O
real-time	B-General_Concept
transaction	B-General_Concept
completion	O
constraints	O
are	O
applied	O
)	O
neither	O
algorithm	O
aborts	O
more	O
transactions	O
than	O
the	O
minimum	O
needed	O
(	O
which	O
is	O
determined	O
by	O
the	O
transactions	O
 '	O
operations	O
scheduling	O
,	O
out	O
of	O
the	O
scope	O
of	O
the	O
algorithms	O
)	O
.	O
</s>
<s>
See	O
also	O
Concurrent	B-Architecture
programming	I-Architecture
and	O
Transactional	B-Operating_System
memory	I-Operating_System
.	O
</s>
<s>
With	O
the	O
proliferation	O
of	O
Multi-core	B-Architecture
processors	I-Architecture
,	O
variants	O
of	O
the	O
Generic	O
local	O
CO	O
algorithm	O
have	O
also	O
been	O
increasingly	O
utilized	O
in	O
Concurrent	B-Architecture
programming	I-Architecture
,	O
Transactional	B-Operating_System
memory	I-Operating_System
,	O
and	O
especially	O
in	O
Software	B-Operating_System
transactional	I-Operating_System
memory	I-Operating_System
for	O
achieving	O
serializability	B-Operating_System
optimistically	B-Operating_System
by	O
"	O
commit	B-Architecture
order	I-Architecture
"	O
(	O
e.g.	O
,	O
Ramadan	O
et	O
al	O
.	O
</s>
<s>
From	O
a	O
software	B-Architecture
architecture	I-Architecture
point	O
of	O
view	O
,	O
a	O
CO	O
component	O
that	O
implements	O
the	O
generic	O
CO	O
algorithm	O
locally	O
,	O
the	O
Commitment	O
Order	O
Coordinator	O
(	O
COCO	O
)	O
,	O
can	O
be	O
designed	O
straightforwardly	O
as	O
a	O
mediator	O
between	O
a	O
(	O
single	O
)	O
database	O
system	O
and	O
an	O
atomic	O
commitment	O
protocol	O
component	O
(	O
Raz	O
1991b	O
)	O
.	O
</s>
<s>
The	O
COCO	O
's	O
functions	O
are	O
to	O
vote	O
to	O
commit	O
on	O
ready	O
global	O
transactions	O
(	O
the	O
processing	O
has	O
ended	O
)	O
according	O
to	O
the	O
local	O
commitment	O
order	O
,	O
to	O
vote	O
to	O
abort	O
on	O
transactions	O
for	O
which	O
the	O
database	O
system	O
has	O
initiated	O
an	O
abort	O
(	O
the	O
database	O
system	O
can	O
initiate	O
abort	O
for	O
any	O
transaction	B-General_Concept
,	O
for	O
many	O
reasons	O
)	O
,	O
and	O
to	O
pass	O
the	O
atomic	O
commitment	O
decision	O
to	O
the	O
database	O
system	O
.	O
</s>
<s>
For	O
determining	O
the	O
commitment	O
order	O
,	O
the	O
COCO	O
maintains	O
an	O
updated	O
representation	O
of	O
the	O
local	O
conflict	O
graph	O
(	O
or	O
local	O
augmented	O
conflict	O
graph	O
for	O
capturing	O
also	O
locking	B-Operating_System
deadlocks	B-Operating_System
)	O
of	O
the	O
undecided	O
(	O
neither	O
committed	O
nor	O
aborted	O
)	O
transactions	O
as	O
a	O
data	O
structure	O
(	O
e.g.	O
,	O
utilizing	O
mechanisms	O
similar	O
to	O
locking	B-Operating_System
for	O
capturing	O
conflicts	O
,	O
but	O
with	O
no	O
data-access	O
blocking	O
)	O
.	O
</s>
<s>
The	O
COCO	O
component	O
has	O
an	O
interface	B-Application
with	O
its	O
database	O
system	O
to	O
receive	O
"	O
conflict	O
,	O
"	O
"	O
ready	O
"	O
(	O
the	O
processing	O
has	O
ended	O
;	O
readiness	O
to	O
vote	O
on	O
a	O
global	O
transaction	B-General_Concept
or	O
commit	O
a	O
local	O
one	O
)	O
,	O
and	O
"	O
abort	O
"	O
notifications	O
from	O
the	O
database	O
system	O
.	O
</s>
<s>
It	O
also	O
interfaces	B-Application
with	O
the	O
atomic	O
commitment	O
protocol	O
to	O
vote	O
and	O
receive	O
the	O
atomic	O
commitment	O
protocol	O
's	O
decision	O
on	O
each	O
global	O
transaction	B-General_Concept
.	O
</s>
<s>
The	O
decisions	O
are	O
delivered	O
from	O
the	O
COCO	O
to	O
the	O
database	O
system	O
through	O
their	O
interface	B-Application
,	O
as	O
well	O
as	O
local	O
transactions	O
 '	O
commit	O
notifications	O
,	O
at	O
a	O
proper	O
commit	B-Architecture
order	I-Architecture
.	O
</s>
<s>
The	O
COCO	O
,	O
including	O
its	O
interfaces	B-Application
,	O
can	O
be	O
enhanced	O
,	O
if	O
it	O
implements	O
another	O
variant	O
of	O
CO	O
(	O
see	O
below	O
)	O
or	O
plays	O
a	O
role	O
in	O
the	O
database	O
's	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
beyond	O
voting	O
in	O
atomic	O
commitment	O
.	O
</s>
<s>
The	O
COCO	O
also	O
guarantees	O
CO	O
locally	O
in	O
a	O
single	O
,	O
isolated	O
database	O
system	O
with	O
no	O
interface	B-Application
with	O
an	O
atomic	O
commitment	O
protocol	O
.	O
</s>
<s>
Suppose	O
databases	O
that	O
participate	O
in	O
distributed	B-General_Concept
transactions	I-General_Concept
(	O
i.e.	O
,	O
transactions	O
that	O
span	O
more	O
than	O
a	O
single	O
database	O
)	O
do	O
not	O
use	O
any	O
shared	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
and	O
use	O
unmodified	O
atomic	O
commitment	O
protocol	O
messages	O
(	O
for	O
reaching	O
atomicity	O
)	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
maintaining	O
(	O
local	O
)	O
commitment	B-Architecture
ordering	I-Architecture
or	O
one	O
of	O
its	O
generalizing	O
variants	O
(	O
see	O
below	O
)	O
is	O
a	O
necessary	O
condition	O
for	O
guaranteeing	O
global	B-Operating_System
serializability	I-Operating_System
(	O
a	O
proof	O
technique	O
can	O
be	O
found	O
in	O
(	O
Raz	O
1992	O
)	O
,	O
and	O
a	O
different	O
proof	O
method	O
for	O
this	O
in	O
(	O
Raz	O
1993a	O
)	O
)	O
;	O
it	O
is	O
also	O
a	O
sufficient	O
condition	O
.	O
</s>
<s>
This	O
is	O
a	O
mathematical	O
fact	O
derived	O
from	O
the	O
definitions	O
of	O
serializability	B-Operating_System
and	O
a	O
transaction	B-General_Concept
.	O
</s>
<s>
It	O
means	O
that	O
if	O
not	O
complying	O
with	O
CO	O
,	O
then	O
global	B-Operating_System
serializability	I-Operating_System
cannot	O
be	O
guaranteed	O
under	O
this	O
condition	O
(	O
the	O
condition	O
of	O
no	O
local	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
sharing	O
between	O
databases	O
beyond	O
atomic	O
commit	O
protocol	O
messages	O
)	O
.	O
</s>
<s>
Atomic	O
commitment	O
is	O
a	O
minimal	O
requirement	O
for	O
a	O
distributed	B-General_Concept
transaction	I-General_Concept
since	O
it	O
is	O
always	O
needed	O
,	O
which	O
is	O
implied	O
by	O
the	O
transaction	B-General_Concept
definition	O
.	O
</s>
<s>
A	O
database	O
system	O
is	O
Autonomous	O
,	O
if	O
it	O
does	O
not	O
share	O
any	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
beyond	O
unmodified	O
atomic	O
commitment	O
protocol	O
messages	O
with	O
any	O
other	O
entity	O
.	O
</s>
<s>
In	O
addition	O
,	O
it	O
does	O
not	O
use	O
for	O
concurrency	B-Operating_System
control	I-Operating_System
any	O
additional	O
local	O
information	O
beyond	O
conflicts	O
(	O
the	O
last	O
sentence	O
does	O
not	O
appear	O
explicitly	O
but	O
rather	O
implied	O
by	O
further	O
discussion	O
in	O
Raz	O
1992	O
)	O
.	O
</s>
<s>
CO	O
compliance	O
of	O
every	O
autonomous	O
database	O
system	O
(	O
or	O
transactional	O
object	O
)	O
in	O
a	O
multidatabase	O
environment	O
is	O
a	O
necessary	O
condition	O
for	O
guaranteeing	O
Global	B-Operating_System
serializability	I-Operating_System
(	O
without	O
CO	O
,	O
Global	B-Operating_System
serializability	I-Operating_System
may	O
be	O
violated	O
)	O
.	O
</s>
<s>
CO	O
compliance	O
with	O
every	O
database	O
system	O
is	O
a	O
sufficient	O
condition	O
for	O
guaranteeing	O
Global	B-Operating_System
serializability	I-Operating_System
.	O
</s>
<s>
If	O
autonomy	O
is	O
augmented	O
with	O
the	O
ability	O
to	O
identify	O
local	O
transactions	O
,	O
then	O
compliance	O
with	O
a	O
more	O
general	O
property	O
,	O
Extended	O
commitment	B-Architecture
ordering	I-Architecture
(	O
ECO	O
,	O
see	O
below	O
)	O
,	O
makes	O
ECO	O
the	O
necessary	O
condition	O
.	O
</s>
<s>
A	O
database	O
system	O
has	O
the	O
Generalized	O
autonomy	O
property	O
if	O
it	O
does	O
not	O
share	O
any	O
other	O
database	O
system	O
any	O
local	O
concurrency	B-Architecture
information	O
beyond	O
(	O
unmodified	O
)	O
atomic	O
commit	O
protocol	O
messages	O
(	O
however	O
,	O
any	O
local	O
information	O
can	O
be	O
utilized	O
)	O
.	O
</s>
<s>
This	O
definition	O
is	O
probably	O
the	O
broadest	O
such	O
definition	O
possible	O
in	O
the	O
context	O
of	O
database	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
and	O
it	O
makes	O
CO	O
together	O
with	O
any	O
of	O
its	O
(	O
useful	O
:	O
No	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
distribution	O
)	O
generalizing	O
variants	O
(	O
Vote	B-Architecture
ordering	I-Architecture
(	O
VO	O
)	O
;	O
see	O
CO	O
variants	O
below	O
)	O
the	O
necessary	O
condition	O
for	O
Global	B-Operating_System
serializability	I-Operating_System
(	O
i.e.	O
,	O
the	O
union	O
of	O
CO	O
and	O
its	O
generalizing	O
variants	O
is	O
the	O
necessary	O
set	O
VO	O
,	O
which	O
may	O
also	O
include	O
new	O
unknown	O
useful	O
generalizing	O
variants	O
)	O
.	O
</s>
<s>
The	O
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
)	O
solution	O
(	O
technique	O
)	O
for	O
global	B-Operating_System
serializability	I-Operating_System
can	O
be	O
summarized	O
as	O
follows	O
:	O
</s>
<s>
If	O
each	O
database	O
(	O
or	O
any	O
other	O
transactional	O
object	O
)	O
in	O
a	O
multidatabase	O
environment	O
complies	O
with	O
CO	O
,	O
i.e.	O
,	O
arranges	O
its	O
local	O
transactions	O
 '	O
commitments	O
and	O
its	O
votes	O
on	O
(	O
global	O
,	O
distributed	O
)	O
transactions	O
to	O
the	O
atomic	O
commitment	O
protocol	O
according	O
to	O
the	O
local	O
(	O
to	O
the	O
database	O
)	O
partial	O
order	O
induced	O
by	O
the	O
local	O
conflict	O
graph	O
(	O
serializability	B-Operating_System
graph	O
)	O
for	O
the	O
respective	O
transactions	O
,	O
then	O
Global	O
CO	O
and	O
Global	B-Operating_System
serializability	I-Operating_System
are	O
guaranteed	O
.	O
</s>
<s>
A	O
database	O
's	O
CO	O
compliance	O
can	O
be	O
achieved	O
effectively	O
with	O
any	O
local	O
conflict	O
serializability	B-Operating_System
based	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
,	O
neither	O
affecting	O
any	O
transaction	B-General_Concept
's	O
execution	O
process	O
or	O
scheduling	O
nor	O
aborting	O
it	O
.	O
</s>
<s>
The	O
only	O
low	O
overhead	O
incurred	O
is	O
detecting	O
conflicts	O
(	O
e.g.	O
,	O
with	O
locking	B-Operating_System
,	O
but	O
with	O
no	O
data-access	O
blocking	O
;	O
if	O
not	O
already	O
detected	O
for	O
other	O
purposes	O
)	O
and	O
ordering	O
votes	O
and	O
local	O
transactions	O
 '	O
commits	O
according	O
to	O
the	O
conflicts	O
.	O
</s>
<s>
A	O
voting-deadlock	O
occurs	O
for	O
the	O
databases	O
on	O
that	O
cycle	O
(	O
however	O
,	O
allowed	O
concurrent	O
voting	O
in	O
each	O
database	O
,	O
typically	O
for	O
almost	O
all	O
the	O
outstanding	O
votes	O
,	O
continue	O
to	O
execute	O
)	O
.	O
</s>
<s>
This	O
breaks	O
the	O
global	O
cycle	O
,	O
the	O
voting-deadlock	O
is	O
resolved	O
,	O
and	O
the	O
related	O
blocked	O
votes	O
are	O
free	O
to	O
be	O
executed	O
.	O
</s>
<s>
Breaking	O
the	O
global	O
cycle	O
in	O
the	O
global	O
conflict	O
graph	O
ensures	O
that	O
global	O
CO	O
and	O
global	B-Operating_System
serializability	I-Operating_System
are	O
maintained	O
.	O
</s>
<s>
Thus	O
,	O
in	O
the	O
case	O
of	O
incompatible	O
local	O
(	O
partial	O
)	O
commitment	O
orders	O
,	O
no	O
action	O
is	O
needed	O
since	O
the	O
atomic	O
commitment	O
protocol	O
resolves	O
it	O
automatically	O
by	O
aborting	O
a	O
transaction	B-General_Concept
that	O
is	O
a	O
cause	O
for	O
the	O
incompatibility	O
.	O
</s>
<s>
Furthermore	O
,	O
global	O
deadlocks	B-Operating_System
due	O
to	O
locking	B-Operating_System
(	O
global	O
cycles	O
in	O
the	O
augmented	O
conflict	O
graph	O
with	O
at	O
least	O
one	O
data	O
access	O
blocking	O
)	O
result	O
in	O
voting	O
deadlocks	B-Operating_System
and	O
are	O
resolved	O
automatically	O
by	O
the	O
same	O
mechanism	O
.	O
</s>
<s>
Local	O
CO	O
is	O
a	O
necessary	O
condition	O
for	O
guaranteeing	O
Global	B-Operating_System
serializability	I-Operating_System
if	O
the	O
databases	O
involved	O
do	O
not	O
share	O
any	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
beyond	O
(	O
unmodified	O
)	O
atomic	O
commitment	O
protocol	O
messages	O
,	O
i.e.	O
,	O
if	O
the	O
databases	O
are	O
autonomous	O
in	O
the	O
context	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
.	O
</s>
<s>
This	O
means	O
that	O
every	O
global	B-Operating_System
serializability	I-Operating_System
solution	O
for	O
autonomous	O
databases	O
must	O
comply	O
with	O
CO	O
.	O
</s>
<s>
Otherwise	O
,	O
global	B-Operating_System
serializability	I-Operating_System
may	O
be	O
violated	O
(	O
and	O
thus	O
,	O
is	O
likely	O
to	O
be	O
violated	O
very	O
quickly	O
in	O
a	O
high-performance	O
environment	O
)	O
.	O
</s>
<s>
The	O
CO	O
solution	O
scales	B-Architecture
up	I-Architecture
with	O
network	O
size	O
and	O
the	O
number	O
of	O
databases	O
without	O
performance	O
penalty	O
when	O
it	O
utilizes	O
common	O
distributed	O
atomic	O
commitment	O
architecture	O
.	O
</s>
<s>
A	O
distinguishing	O
characteristic	O
of	O
the	O
CO	O
solution	O
to	O
distributed	B-Operating_System
serializability	I-Operating_System
from	O
other	O
techniques	O
is	O
the	O
fact	O
that	O
it	O
requires	O
no	O
conflict	O
information	O
distributed	O
(	O
e.g.	O
,	O
local	O
precedence	O
relations	O
,	O
locks	O
,	O
timestamps	B-Operating_System
,	O
tickets	O
)	O
,	O
which	O
makes	O
it	O
uniquely	O
effective	O
.	O
</s>
<s>
A	O
common	O
way	O
to	O
achieve	O
distributed	B-Operating_System
serializability	I-Operating_System
in	O
a	O
(	O
distributed	O
)	O
system	O
is	O
by	O
a	O
distributed	B-Operating_System
lock	I-Operating_System
manager	I-Operating_System
(	O
DLM	O
)	O
.	O
</s>
<s>
CO	O
allows	O
to	O
achieve	O
distributed	B-Operating_System
serializability	I-Operating_System
under	O
very	O
general	O
conditions	O
,	O
without	O
a	O
distributed	B-Operating_System
lock	I-Operating_System
manager	I-Operating_System
,	O
exhibiting	O
the	O
benefits	O
already	O
explored	O
above	O
for	O
multidatabase	O
environments	O
;	O
in	O
particular	O
:	O
reliability	O
,	O
high	O
performance	O
,	O
scalability	B-Architecture
,	O
the	O
possibility	O
of	O
using	O
optimistic	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
when	O
desired	O
,	O
no	O
conflict	O
information	O
related	O
communications	O
over	O
the	O
network	O
(	O
which	O
have	O
incurred	O
overhead	O
and	O
delays	O
)	O
,	O
and	O
automatic	O
distributed	O
deadlock	B-Operating_System
resolution	O
.	O
</s>
<s>
All	O
distributed	O
transactional	O
systems	O
rely	O
on	O
some	O
atomic	O
commitment	O
protocol	O
to	O
coordinate	O
atomicity	O
(	O
whether	O
to	O
commit	O
or	O
abort	O
)	O
among	O
processes	O
in	O
a	O
distributed	B-General_Concept
transaction	I-General_Concept
.	O
</s>
<s>
Also	O
,	O
typically	O
recoverable	O
data	O
(	O
i.e.	O
,	O
data	O
under	O
transactions	O
 '	O
control	O
,	O
e.g.	O
,	O
database	O
data	O
;	O
not	O
to	O
be	O
confused	O
with	O
the	O
recoverability	O
property	O
of	O
a	O
schedule	B-General_Concept
)	O
are	O
directly	O
accessed	O
by	O
a	O
single	O
transactional	O
data	O
manager	O
component	O
(	O
also	O
referred	O
to	O
as	O
a	O
resource	O
manager	O
)	O
that	O
handles	O
local	O
sub-transactions	O
(	O
the	O
distributed	B-General_Concept
transaction	I-General_Concept
's	O
portion	O
in	O
a	O
single	O
location	O
,	O
e.g.	O
,	O
network	O
node	O
)	O
,	O
even	O
if	O
these	O
data	O
are	O
accessed	O
indirectly	O
by	O
other	O
entities	O
in	O
the	O
distributed	B-Architecture
system	I-Architecture
during	O
a	O
transaction	B-General_Concept
(	O
i.e.	O
,	O
indirect	O
access	O
requires	O
a	O
direct	O
access	O
through	O
a	O
local	O
sub-transaction	O
)	O
.	O
</s>
<s>
If	O
each	O
participant	O
complies	O
with	O
CO	O
(	O
e.g.	O
,	O
by	O
using	O
SS2PL	O
,	O
or	O
COCOs	O
,	O
or	O
a	O
combination	O
;	O
see	O
above	O
)	O
,	O
then	O
the	O
entire	O
distributed	B-Architecture
system	I-Architecture
provides	O
CO	O
(	O
by	O
the	O
theorems	O
above	O
;	O
each	O
participant	O
can	O
be	O
considered	O
a	O
separate	O
transactional	O
object	O
)	O
,	O
and	O
thus	O
(	O
distributed	O
)	O
serializability	B-Operating_System
.	O
</s>
<s>
Furthermore	O
:	O
When	O
CO	O
is	O
utilized	O
together	O
with	O
an	O
atomic	O
commitment	O
protocol	O
also	O
distributed	O
deadlocks	B-Operating_System
(	O
i.e.	O
,	O
deadlocks	B-Operating_System
that	O
span	O
two	O
or	O
more	O
data	O
managers	O
)	O
caused	O
by	O
data-access	O
locking	B-Operating_System
are	O
resolved	O
automatically	O
.	O
</s>
<s>
Let	O
a	O
distributed	O
transactional	O
system	O
(	O
e.g.	O
,	O
a	O
distributed	B-Operating_System
database	I-Operating_System
system	O
)	O
comprise	O
transactional	O
data	O
managers	O
(	O
also	O
called	O
resource	O
managers	O
)	O
that	O
manage	O
all	O
the	O
system	O
's	O
recoverable	O
data	O
.	O
</s>
<s>
Data	O
partition	O
:	O
Recoverable	O
data	O
are	O
partitioned	O
among	O
the	O
data	O
managers	O
,	O
i.e.	O
,	O
each	O
recoverable	O
datum	O
(	O
data	O
item	O
)	O
is	O
controlled	O
by	O
a	O
single	O
data	O
manager	O
(	O
e.g.	O
,	O
as	O
common	O
in	O
a	O
Shared	B-Operating_System
nothing	I-Operating_System
architecture	I-Operating_System
;	O
even	O
copies	O
of	O
the	O
same	O
datum	O
under	O
different	O
data	O
managers	O
are	O
physically	O
distinct	O
,	O
replicated	O
)	O
.	O
</s>
<s>
Participants	O
in	O
atomic	O
commitment	O
protocol	O
:	O
These	O
data	O
managers	O
are	O
the	O
participants	O
in	O
the	O
system	O
's	O
atomic	O
commitment	O
protocol	O
for	O
coordinating	O
distributed	B-General_Concept
transactions	I-General_Concept
 '	O
atomicity	O
.	O
</s>
<s>
Data-access-based	O
distributed	O
deadlocks	B-Operating_System
(	O
deadlocks	B-Operating_System
involving	O
two	O
or	O
more	O
data	O
managers	O
with	O
at	O
least	O
one	O
non-materialized	O
conflict	O
)	O
are	O
resolved	O
automatically	O
.	O
</s>
<s>
Furthermore	O
:	O
The	O
data	O
managers	O
being	O
CO	O
compliant	O
is	O
a	O
necessary	O
condition	O
for	O
(	O
distributed	O
)	O
serializability	B-Operating_System
in	O
a	O
system	O
meeting	O
conditions	O
1	O
,	O
2	O
above	O
,	O
when	O
the	O
data	O
managers	O
are	O
autonomous	O
,	O
i.e.	O
,	O
do	O
not	O
share	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
beyond	O
unmodified	O
messages	O
of	O
atomic	O
commitment	O
protocol	O
.	O
</s>
<s>
This	O
theorem	O
also	O
means	O
that	O
when	O
SS2PL	O
(	O
or	O
any	O
other	O
CO	O
variant	O
)	O
is	O
used	O
locally	O
in	O
each	O
transactional	O
data	O
manager	O
,	O
and	O
each	O
data	O
manager	O
has	O
exclusive	O
control	O
of	O
its	O
data	O
,	O
no	O
distributed	B-Operating_System
lock	I-Operating_System
manager	I-Operating_System
(	O
which	O
is	O
often	O
utilized	O
to	O
enforce	O
distributed	O
SS2PL	O
)	O
is	O
needed	O
for	O
distributed	O
SS2PL	O
and	O
serializability	B-Operating_System
.	O
</s>
<s>
For	O
implementing	O
Distributed	O
Optimistic	O
CO	O
(	O
DOCO	O
)	O
,	O
the	O
generic	O
local	O
CO	O
algorithm	O
is	O
utilized	O
in	O
all	O
the	O
atomic	O
commitment	O
protocol	O
participants	O
in	O
the	O
system	O
with	O
no	O
data	O
access	O
blocking	O
and	O
thus	O
with	O
no	O
local	O
deadlocks	B-Operating_System
.	O
</s>
<s>
Global	O
(	O
voting	O
)	O
deadlocks	B-Operating_System
are	O
resolved	O
automatically	O
(	O
and	O
all	O
are	O
serializability	B-Operating_System
related	O
(	O
with	O
non-blocking	O
conflicts	O
)	O
rather	O
than	O
locking	B-Operating_System
related	O
(	O
with	O
blocking	O
and	O
possibly	O
also	O
non-blocking	O
conflicts	O
)	O
)	O
.	O
</s>
<s>
Thus	O
,	O
no	O
deadlock	B-Operating_System
handling	O
is	O
needed	O
.	O
</s>
<s>
A	O
distributed	B-Operating_System
database	I-Operating_System
system	O
that	O
utilizes	O
SS2PL	O
resides	O
on	O
two	O
remote	O
nodes	O
,	O
A	O
and	O
B	O
.	O
</s>
<s>
For	O
each	O
distributed	B-General_Concept
transaction	I-General_Concept
such	O
data	O
managers	O
need	O
to	O
execute	O
the	O
available	O
atomic	O
commitment	O
protocol	O
.	O
</s>
<s>
Two	O
distributed	B-General_Concept
transactions	I-General_Concept
,	O
and	O
,	O
are	O
running	O
concurrently	O
,	O
and	O
both	O
access	O
data	O
x	O
and	O
y	O
.	O
x	O
is	O
under	O
the	O
exclusive	O
control	O
of	O
the	O
data	O
manager	O
on	O
A	O
(	O
B	O
's	O
manager	O
cannot	O
access	O
x	O
)	O
,	O
and	O
y	O
under	O
that	O
on	O
B	O
.	O
</s>
<s>
reads	O
x	O
on	O
A	O
and	O
writes	O
y	O
on	O
B	O
,	O
i.e.	O
,	O
when	O
using	O
notation	O
common	O
for	O
concurrency	B-Operating_System
control	I-Operating_System
.	O
</s>
<s>
The	O
database	O
system	O
's	O
schedule	B-General_Concept
at	O
a	O
certain	O
point	O
in	O
time	O
is	O
the	O
following	O
:	O
</s>
<s>
This	O
is	O
a	O
distributed	O
deadlock	B-Operating_System
situation	O
,	O
which	O
is	O
also	O
a	O
voting-deadlock	O
(	O
see	O
below	O
)	O
with	O
a	O
distributed	O
(	O
global	O
)	O
cycle	O
of	O
length	O
2	O
(	O
number	O
of	O
edges	O
,	O
conflicts	O
;	O
2	O
is	O
the	O
most	O
frequent	O
length	O
)	O
.	O
</s>
<s>
Since	O
the	O
atomic	O
commitment	O
protocol	O
cannot	O
receive	O
votes	O
for	O
blocked	O
sub-transactions	O
(	O
a	O
voting-deadlock	O
)	O
,	O
it	O
will	O
eventually	O
abort	O
some	O
transaction	B-General_Concept
with	O
a	O
missing	O
vote(s )	O
by	O
timeout	O
,	O
either	O
,	O
or	O
,	O
(	O
or	O
both	O
,	O
if	O
the	O
timeouts	O
fall	O
very	O
close	O
)	O
.	O
</s>
<s>
This	O
will	O
resolve	O
the	O
global	O
deadlock	B-Operating_System
.	O
</s>
<s>
The	O
remaining	O
transaction	B-General_Concept
will	O
complete	O
running	O
,	O
be	O
voted	O
on	O
,	O
and	O
committed	O
.	O
</s>
<s>
An	O
aborted	O
transaction	B-General_Concept
is	O
immediately	O
restarted	O
and	O
re-executed	O
.	O
</s>
<s>
If	O
a	O
transaction	B-General_Concept
is	O
running	O
on	O
B	O
concurrently	O
with	O
and	O
and	O
directly	O
writes	O
x	O
,	O
then	O
,	O
without	O
a	O
distributed	B-Operating_System
lock	I-Operating_System
manager	I-Operating_System
the	O
read-lock	O
for	O
x	O
held	O
by	O
on	O
A	O
is	O
not	O
visible	O
on	O
B	O
and	O
cannot	O
block	O
the	O
write	O
of	O
(	O
or	O
signal	O
a	O
materialized	O
conflict	O
for	O
a	O
non-blocking	O
CO	O
variant	O
;	O
see	O
below	O
)	O
.	O
</s>
<s>
Thus	O
serializability	B-Operating_System
can	O
be	O
violated	O
.	O
</s>
<s>
However	O
,	O
functionality	O
is	O
not	O
limited	O
,	O
and	O
a	O
transaction	B-General_Concept
running	O
on	O
B	O
still	O
can	O
issue	O
a	O
write	O
or	O
read	O
request	O
of	O
x	O
(	O
not	O
common	O
)	O
.	O
</s>
<s>
This	O
request	O
is	O
communicated	O
to	O
the	O
transaction	B-General_Concept
's	O
local	O
sub-transaction	O
on	O
A	O
(	O
which	O
is	O
generated	O
,	O
if	O
does	O
not	O
exist	O
already	O
)	O
which	O
issues	O
this	O
request	O
to	O
the	O
local	O
data	O
manager	O
on	O
A	O
.	O
</s>
<s>
In	O
the	O
scenario	O
above	O
both	O
conflicts	O
are	O
non-materialized	O
,	O
and	O
the	O
global	O
voting-deadlock	O
is	O
reflected	O
as	O
a	O
cycle	O
in	O
the	O
global	O
wait-for	B-Operating_System
graph	I-Operating_System
(	O
but	O
not	O
in	O
the	O
global	O
conflict	O
graph	O
;	O
see	O
Exact	O
characterization	O
of	O
voting-deadlocks	O
by	O
global	O
cycles	O
above	O
)	O
.	O
</s>
<s>
However	O
the	O
database	O
system	O
can	O
utilize	O
any	O
CO	O
variant	O
with	O
exactly	O
the	O
same	O
conflicts	O
and	O
voting-deadlock	O
situation	O
,	O
and	O
same	O
resolution	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
SCO	O
(	O
below	O
)	O
is	O
used	O
by	O
the	O
distributed	B-Operating_System
database	I-Operating_System
system	O
instead	O
of	O
SS2PL	O
,	O
then	O
the	O
two	O
conflicts	O
in	O
the	O
example	O
are	O
materialized	O
,	O
all	O
local	O
sub-transactions	O
are	O
in	O
ready	O
states	O
,	O
and	O
vote	O
blocking	O
occurs	O
in	O
the	O
two	O
transactions	O
,	O
one	O
on	O
each	O
node	O
,	O
because	O
of	O
the	O
CO	O
voting	O
rule	O
applied	O
independently	O
on	O
both	O
A	O
and	O
B	O
:	O
due	O
to	O
conflicts	O
is	O
not	O
voted	O
on	O
before	O
ends	O
,	O
and	O
is	O
not	O
voted	O
on	O
before	O
ends	O
,	O
which	O
is	O
a	O
voting-deadlock	O
.	O
</s>
<s>
Now	O
the	O
conflict	O
graph	O
has	O
the	O
global	O
cycle	O
(	O
all	O
conflicts	O
are	O
materialized	O
)	O
,	O
and	O
again	O
it	O
is	O
resolved	O
by	O
the	O
atomic	O
commitment	O
protocol	O
,	O
and	O
distributed	B-Operating_System
serializability	I-Operating_System
is	O
maintained	O
.	O
</s>
<s>
Unlikely	O
for	O
a	O
distributed	B-Operating_System
database	I-Operating_System
system	O
,	O
but	O
possible	O
in	O
principle	O
(	O
and	O
occurs	O
in	O
a	O
multi-database	O
)	O
,	O
A	O
can	O
employ	O
SS2PL	O
while	O
B	O
employs	O
SCO	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
global	O
cycle	O
is	O
neither	O
in	O
the	O
wait-for	B-Operating_System
graph	I-Operating_System
nor	O
in	O
the	O
serializability	B-Operating_System
graph	O
,	O
but	O
still	O
in	O
the	O
augmented	O
conflict	O
graph	O
(	O
the	O
union	O
of	O
the	O
two	O
)	O
.	O
</s>
<s>
Conflicts	O
and	O
thus	O
cycles	O
in	O
the	O
augmented	O
conflict	O
graph	O
are	O
determined	O
by	O
the	O
transactions	O
and	O
their	O
initial	O
scheduling	O
only	O
,	O
independently	O
of	O
the	O
concurrency	B-Operating_System
control	I-Operating_System
utilized	O
.	O
</s>
<s>
With	O
any	O
variant	O
of	O
CO	O
,	O
any	O
global	O
cycle	O
(	O
i.e.	O
,	O
spans	O
two	O
databases	O
or	O
more	O
)	O
causes	O
a	O
voting	O
deadlock	B-Operating_System
.	O
</s>
<s>
As	O
noted	O
above	O
,	O
only	O
case	O
4	O
describes	O
a	O
cycle	O
in	O
the	O
(	O
regular	O
)	O
conflict	O
graph	O
which	O
affects	O
serializability	B-Operating_System
.	O
</s>
<s>
Cases	O
1-3	O
describe	O
cycles	O
of	O
locking	B-Operating_System
based	O
global	O
deadlocks	B-Operating_System
(	O
at	O
least	O
one	O
lock	O
blocking	O
exists	O
)	O
.	O
</s>
<s>
However	O
,	O
no	O
research	O
article	O
,	O
except	O
the	O
CO	O
articles	O
,	O
is	O
known	O
to	O
notice	O
this	O
automatic	O
locking	B-Operating_System
global	O
deadlock	B-Operating_System
resolution	O
as	O
of	O
2009	O
.	O
</s>
<s>
Such	O
global	O
deadlocks	B-Operating_System
typically	O
have	O
been	O
dealt	O
with	O
by	O
dedicated	O
mechanisms	O
.	O
</s>
<s>
Case	O
4	O
above	O
is	O
also	O
an	O
example	O
for	O
a	O
typical	O
voting-deadlock	O
when	O
Distributed	O
optimistic	O
CO	O
(	O
DOCO	O
)	O
is	O
used	O
(	O
i.e.	O
,	O
Case	O
4	O
is	O
unchanged	O
when	O
Optimistic	O
CO	O
(	O
OCO	O
;	O
see	O
below	O
)	O
replaces	O
SCO	O
on	O
both	O
A	O
and	O
B	O
)	O
:	O
No	O
data-access	O
blocking	O
occurs	O
,	O
and	O
only	O
materialized	O
conflicts	O
exist	O
.	O
</s>
<s>
Certain	O
experimental	O
distributed	O
memory-resident	O
databases	O
advocate	O
multi	O
single-threaded	B-Operating_System
core	O
(	O
MuSiC	O
)	O
transactional	O
environments	O
.	O
</s>
<s>
"	O
Single-threaded	B-Operating_System
"	O
refers	O
to	O
transaction	B-General_Concept
threads	B-Operating_System
only	O
,	O
and	O
to	O
serial	O
execution	O
of	O
transactions	O
.	O
</s>
<s>
The	O
purpose	O
is	O
possible	O
orders	O
of	O
magnitude	O
gain	O
in	O
performance	O
(	O
e.g.	O
,	O
H-Store	O
and	O
VoltDB	B-Application
)	O
relatively	O
to	O
conventional	O
transaction	B-General_Concept
execution	O
in	O
multiple	O
threads	B-Operating_System
on	O
a	O
same	O
core	O
.	O
</s>
<s>
In	O
such	O
an	O
environment	O
,	O
if	O
recoverable	O
(	O
transactional	O
)	O
data	O
are	O
partitioned	O
among	O
threads	B-Operating_System
(	O
cores	O
)	O
,	O
and	O
it	O
is	O
implemented	O
in	O
the	O
conventional	O
way	O
for	O
distributed	O
CO	O
,	O
as	O
described	O
in	O
previous	O
sections	O
,	O
then	O
DOCO	O
and	O
Strictness	O
exist	O
automatically	O
.	O
</s>
<s>
Comment	O
:	O
The	O
MuSiC	O
straightforward	O
implementation	O
described	O
here	O
(	O
which	O
uses	O
,	O
for	O
example	O
,	O
as	O
usual	O
in	O
distributed	O
CO	O
,	O
voting	O
(	O
and	O
transaction	B-General_Concept
thread	B-Operating_System
)	O
blocking	O
in	O
atomic	O
commitment	O
protocol	O
when	O
needed	O
)	O
is	O
for	O
demonstration	O
only	O
,	O
and	O
has	O
no	O
connection	O
to	O
the	O
implementation	O
in	O
H-Store	O
or	O
any	O
other	O
project	O
.	O
</s>
<s>
Thus	O
both	O
local	O
Optimistic	O
CO	O
(	O
OCO	O
;	O
see	O
below	O
)	O
and	O
the	O
Global	O
CO	O
enforcement	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
condition	O
for	O
the	O
atomic	O
commitment	O
protocol	O
are	O
met	O
automatically	O
.	O
</s>
<s>
This	O
results	O
in	O
both	O
distributed	O
CO	O
compliance	O
(	O
and	O
thus	O
distributed	B-Operating_System
serializability	I-Operating_System
)	O
and	O
automatic	O
global	O
(	O
voting	O
)	O
deadlock	B-Operating_System
resolution	O
.	O
</s>
<s>
Furthermore	O
,	O
also	O
local	O
Strictness	O
follows	O
automatically	O
in	O
a	O
serial	O
schedule	B-General_Concept
.	O
</s>
<s>
By	O
Theorem	O
5.2	O
in	O
(	O
Raz	O
1992	O
;	O
page	O
307	O
)	O
,	O
when	O
the	O
CO	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
is	O
applied	O
,	O
also	O
Global	O
Strictness	O
is	O
guaranteed	O
.	O
</s>
<s>
automatically	O
exist	O
globally	O
with	O
unbounded	O
scalability	B-Architecture
in	O
number	O
of	O
cores	O
used	O
.	O
</s>
<s>
Local	O
sub-transactions	O
of	O
a	O
global	O
transaction	B-General_Concept
are	O
blocked	O
until	O
commit	O
,	O
which	O
makes	O
the	O
respective	O
cores	O
idle	O
.	O
</s>
<s>
Also	O
a	O
local	O
(	O
to	O
a	O
core	O
)	O
transaction	B-General_Concept
may	O
become	O
global	O
(	O
multi-core	B-Architecture
)	O
to	O
reach	O
its	O
needed	O
data	O
,	O
with	O
additional	O
incurred	O
overhead	O
.	O
</s>
<s>
Another	O
consideration	O
is	O
putting	O
in	O
a	O
same	O
core	O
partition	O
all	O
the	O
data	O
that	O
are	O
usually	O
accessed	O
by	O
a	O
same	O
transaction	B-General_Concept
(	O
if	O
possible	O
)	O
,	O
to	O
maximize	O
the	O
number	O
of	O
local	O
transactions	O
(	O
and	O
minimize	O
the	O
number	O
of	O
global	O
,	O
distributed	B-General_Concept
transactions	I-General_Concept
)	O
.	O
</s>
<s>
Special	O
case	O
schedule	B-General_Concept
property	O
classes	O
(	O
e.g.	O
,	O
SS2PL	O
and	O
SCO	O
below	O
)	O
are	O
strictly	O
contained	O
in	O
the	O
CO	O
class	O
.	O
</s>
<s>
The	O
generalizing	O
variants	O
also	O
guarantee	O
global	B-Operating_System
serializability	I-Operating_System
without	O
distributing	O
local	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
(	O
each	O
database	O
has	O
the	O
generalized	O
autonomy	O
property	O
:	O
it	O
uses	O
only	O
local	O
information	O
)	O
,	O
while	O
relaxing	O
CO	O
constraints	O
and	O
utilizing	O
additional	O
(	O
local	O
)	O
information	O
for	O
better	O
concurrency	B-Architecture
and	O
performance	O
:	O
ECO	O
uses	O
knowledge	O
about	O
transactions	O
being	O
local	O
(	O
i.e.	O
,	O
confined	O
to	O
a	O
single	O
database	O
)	O
,	O
and	O
MVCO	O
uses	O
availability	O
of	O
data	O
versions	O
values	O
.	O
</s>
<s>
Like	O
CO	O
,	O
both	O
generalizing	O
variants	O
are	O
non-blocking	O
,	O
do	O
not	O
interfere	O
with	O
any	O
transaction	B-General_Concept
's	O
operation	O
scheduling	O
,	O
and	O
can	O
be	O
seamlessly	O
combined	O
with	O
any	O
relevant	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
.	O
</s>
<s>
The	O
term	O
CO	O
variant	O
refers	O
in	O
general	O
to	O
CO	O
,	O
ECO	O
,	O
MVCO	O
,	O
or	O
a	O
combination	O
of	O
each	O
of	O
them	O
with	O
any	O
relevant	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
or	O
property	O
(	O
including	O
Multi-version	O
based	O
ECO	O
,	O
MVECO	O
)	O
.	O
</s>
<s>
No	O
other	O
generalizing	O
variants	O
(	O
which	O
guarantee	O
global	B-Operating_System
serializability	I-Operating_System
with	O
no	O
local	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
distribution	O
)	O
are	O
known	O
,	O
but	O
may	O
be	O
discovered	O
.	O
</s>
<s>
Strong	O
Strict	O
Two	B-Operating_System
Phase	I-Operating_System
Locking	I-Operating_System
(	O
SS2PL	O
;	O
also	O
referred	O
to	O
as	O
Rigorousness	B-Operating_System
or	O
Rigorous	O
scheduling	O
)	O
means	O
that	O
both	O
read	O
and	O
write	O
locks	O
of	O
a	O
transaction	B-General_Concept
are	O
released	O
only	O
after	O
the	O
transaction	B-General_Concept
has	O
ended	O
(	O
either	O
committed	O
or	O
aborted	O
)	O
.	O
</s>
<s>
This	O
property	O
is	O
widely	O
utilized	O
in	O
database	O
systems	O
,	O
and	O
since	O
it	O
implies	O
CO	O
,	O
databases	O
that	O
use	O
it	O
and	O
participate	O
in	O
global	O
transactions	O
generate	O
together	O
a	O
serializable	B-Operating_System
global	O
schedule	B-General_Concept
(	O
when	O
using	O
any	O
atomic	O
commitment	O
protocol	O
,	O
which	O
is	O
needed	O
for	O
atomicity	O
in	O
a	O
multi-database	O
environment	O
)	O
.	O
</s>
<s>
A	O
transaction	B-General_Concept
can	O
be	O
voted	O
on	O
by	O
a	O
database	O
system	O
immediately	O
after	O
entering	O
a	O
"	O
ready	O
"	O
state	O
,	O
i.e.	O
,	O
completing	O
running	O
its	O
task	O
locally	O
.	O
</s>
<s>
If	O
a	O
local	O
timeout	O
mechanism	O
is	O
used	O
by	O
a	O
database	O
system	O
to	O
resolve	O
(	O
local	O
)	O
SS2PL	O
deadlocks	B-Operating_System
,	O
then	O
aborting	O
blocked	O
transactions	O
breaks	O
not	O
only	O
potential	O
local	O
cycles	O
in	O
the	O
global	O
conflict	O
graph	O
(	O
real	O
cycles	O
in	O
the	O
augmented	O
conflict	O
graph	O
)	O
,	O
but	O
also	O
database	O
system	O
's	O
potential	O
global	O
cycles	O
as	O
a	O
side	O
effect	O
,	O
if	O
the	O
atomic	O
commitment	O
protocol	O
's	O
abort	O
mechanism	O
is	O
relatively	O
slow	O
.	O
</s>
<s>
Such	O
independent	O
aborts	O
by	O
several	O
entities	O
typically	O
may	O
result	O
in	O
unnecessary	O
aborts	O
for	O
more	O
than	O
one	O
transaction	B-General_Concept
per	O
global	O
cycle	O
.	O
</s>
<s>
The	O
situation	O
is	O
different	O
for	O
a	O
local	O
wait-for	B-Operating_System
graph	I-Operating_System
based	O
mechanisms	O
:	O
Such	O
cannot	O
identify	O
global	O
cycles	O
,	O
and	O
the	O
atomic	O
commitment	O
protocol	O
will	O
break	O
the	O
global	O
cycle	O
,	O
if	O
the	O
resulting	O
voting	O
deadlock	B-Operating_System
is	O
not	O
resolved	O
earlier	O
in	O
another	O
database	O
.	O
</s>
<s>
Local	O
SS2PL	O
together	O
with	O
atomic	O
commitment	O
implying	O
global	B-Operating_System
serializability	I-Operating_System
can	O
also	O
be	O
deduced	O
directly	O
:	O
All	O
transactions	O
,	O
including	O
distributed	O
,	O
obey	O
the	O
2PL	B-Operating_System
(	O
SS2PL	O
)	O
rules	O
.	O
</s>
<s>
Probably	O
for	O
this	O
reason	O
,	O
without	O
considering	O
the	O
atomic	O
commitment	O
voting	O
mechanism	O
,	O
automatic	O
global	O
deadlock	B-Operating_System
resolution	O
has	O
not	O
been	O
noticed	O
before	O
CO	O
.	O
</s>
<s>
Strict	O
Commitment	B-Architecture
Ordering	I-Architecture
(	O
SCO	O
;	O
(	O
Raz	O
1991c	O
)	O
)	O
is	O
the	O
intersection	O
of	O
strictness	O
(	O
a	O
special	O
case	O
of	O
recoverability	O
)	O
and	O
CO	O
,	O
and	O
provides	O
an	O
upper	O
bound	O
for	O
a	O
schedule	B-General_Concept
's	O
concurrency	B-Architecture
when	O
both	O
properties	O
exist	O
.	O
</s>
<s>
It	O
can	O
be	O
implemented	O
using	O
blocking	O
mechanisms	O
(	O
locking	B-Operating_System
)	O
similar	O
to	O
those	O
used	O
for	O
the	O
popular	O
SS2PL	O
with	O
similar	O
overheads	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
SCO	O
has	O
shorter	O
average	O
blocking	O
periods	O
,	O
and	O
more	O
concurrency	B-Architecture
(	O
e.g.	O
,	O
performance	O
simulations	O
of	O
a	O
single	O
database	O
for	O
the	O
most	O
significant	O
variant	O
of	O
locks	B-Operating_System
with	I-Operating_System
ordered	I-Operating_System
sharing	I-Operating_System
,	O
which	O
is	O
identical	O
to	O
SCO	O
,	O
clearly	O
show	O
this	O
,	O
with	O
approximately	O
100%	O
gain	O
for	O
some	O
transaction	B-General_Concept
loads	O
;	O
also	O
for	O
identical	O
transaction	B-General_Concept
loads	O
SCO	O
can	O
reach	O
higher	O
transaction	B-General_Concept
rates	O
than	O
SS2PL	O
before	O
lock	O
thrashing	B-General_Concept
occurs	O
)	O
.	O
</s>
<s>
More	O
concurrency	B-Architecture
means	O
that	O
with	O
given	O
computing	O
resources	O
more	O
transactions	O
are	O
completed	O
in	O
time	O
unit	O
(	O
higher	O
transaction	B-General_Concept
rate	O
,	O
throughput	O
)	O
,	O
and	O
the	O
average	O
duration	O
of	O
a	O
transaction	B-General_Concept
is	O
shorter	O
(	O
faster	O
completion	O
;	O
see	O
chart	O
)	O
.	O
</s>
<s>
SCO	O
provides	O
shorter	O
average	O
transaction	B-General_Concept
completion	O
time	O
than	O
SS2PL	O
,	O
if	O
read-write	O
conflicts	O
exist	O
.	O
</s>
<s>
SCO	O
is	O
as	O
practical	O
as	O
SS2PL	O
since	O
as	O
SS2PL	O
it	O
provides	O
besides	O
serializability	B-Operating_System
also	O
strictness	O
,	O
which	O
is	O
widely	O
utilized	O
as	O
a	O
basis	O
for	O
efficient	O
recovery	O
of	O
databases	O
from	O
failure	O
.	O
</s>
<s>
SS2PL	O
is	O
a	O
proper	O
subset	O
of	O
SCO	O
(	O
which	O
is	O
another	O
explanation	O
why	O
SCO	O
is	O
less	O
constraining	O
and	O
provides	O
more	O
concurrency	B-Architecture
than	O
SS2PL	O
)	O
.	O
</s>
<s>
For	O
implementing	O
Optimistic	O
commitment	B-Architecture
ordering	I-Architecture
(	O
OCO	O
)	O
the	O
generic	O
local	O
CO	O
algorithm	O
is	O
utilized	O
without	O
data	O
access	O
blocking	O
,	O
and	O
thus	O
without	O
local	O
deadlocks	B-Operating_System
.	O
</s>
<s>
OCO	O
without	O
transaction	B-General_Concept
or	O
operation	O
scheduling	O
constraints	O
covers	O
the	O
entire	O
CO	O
class	O
,	O
and	O
is	O
not	O
a	O
special	O
case	O
of	O
the	O
CO	O
class	O
,	O
but	O
rather	O
a	O
useful	O
CO	O
variant	O
and	O
mechanism	O
characterization	O
.	O
</s>
<s>
Extended	O
Commitment	B-Architecture
Ordering	I-Architecture
(	O
ECO	O
;	O
(	O
Raz	O
1993a	O
)	O
)	O
generalizes	O
CO	O
.	O
</s>
<s>
Thus	O
,	O
for	O
a	O
local	O
(	O
to	O
a	O
database	O
)	O
schedule	B-General_Concept
to	O
have	O
the	O
ECO	O
property	O
,	O
the	O
chronological	O
(	O
partial	O
)	O
order	O
of	O
commit	O
events	O
of	O
global	O
transactions	O
only	O
(	O
unimportant	O
for	O
local	O
transactions	O
)	O
is	O
consistent	O
with	O
their	O
order	O
on	O
the	O
respective	O
local	O
conflict	O
graph	O
.	O
</s>
<s>
Let	O
be	O
two	O
committed	O
global	O
transactions	O
in	O
a	O
schedule	B-General_Concept
,	O
such	O
that	O
a	O
directed	O
path	O
of	O
unaborted	O
transactions	O
exists	O
in	O
the	O
conflict	O
graph	O
(	O
precedence	B-General_Concept
graph	I-General_Concept
)	O
from	O
to	O
(	O
precedes	O
,	O
possibly	O
transitively	O
,	O
indirectly	O
)	O
.	O
</s>
<s>
The	O
schedule	B-General_Concept
has	O
the	O
Extended	O
commitment	B-Architecture
ordering	I-Architecture
(	O
ECO	O
)	O
property	O
,	O
if	O
for	O
every	O
two	O
such	O
transactions	O
commits	O
before	O
commits	O
.	O
</s>
<s>
In	O
order	O
to	O
guarantee	O
global	B-Operating_System
serializability	I-Operating_System
,	O
each	O
database	O
needs	O
to	O
guarantee	O
also	O
the	O
conflict	O
serializability	B-Operating_System
of	O
its	O
own	O
transactions	O
by	O
any	O
(	O
local	O
)	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
.	O
</s>
<s>
(	O
Local	O
,	O
which	O
implies	O
global	O
)	O
ECO	O
together	O
with	O
local	O
conflict	O
serializability	B-Operating_System
,	O
is	O
a	O
sufficient	O
condition	O
to	O
guarantee	O
global	O
conflict	O
serializability	B-Operating_System
.	O
</s>
<s>
When	O
no	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
beyond	O
atomic	O
commitment	O
messages	O
is	O
shared	O
outside	O
a	O
database	O
(	O
autonomy	O
)	O
,	O
and	O
local	O
transactions	O
can	O
be	O
identified	O
,	O
it	O
is	O
also	O
a	O
necessary	O
condition	O
.	O
</s>
<s>
This	O
condition	O
(	O
ECO	O
with	O
local	O
serializability	B-Operating_System
)	O
is	O
weaker	O
than	O
CO	O
,	O
and	O
allows	O
more	O
concurrency	B-Architecture
at	O
the	O
cost	O
of	O
a	O
little	O
more	O
complicated	O
local	O
algorithm	O
(	O
however	O
,	O
no	O
practical	O
overhead	O
difference	O
with	O
CO	O
exists	O
)	O
.	O
</s>
<s>
Before	O
a	O
global	O
transaction	B-General_Concept
is	O
committed	O
,	O
a	O
generic	O
local	O
(	O
to	O
a	O
database	O
)	O
ECO	O
algorithm	O
aborts	O
a	O
minimal	O
set	O
of	O
undecided	O
transactions	O
(	O
neither	O
committed	O
,	O
nor	O
aborted	O
;	O
either	O
local	O
transactions	O
,	O
or	O
global	O
that	O
run	O
locally	O
)	O
,	O
that	O
can	O
cause	O
later	O
a	O
cycle	O
in	O
the	O
conflict	O
graph	O
.	O
</s>
<s>
This	O
set	O
of	O
aborted	O
transactions	O
(	O
not	O
unique	O
,	O
contrary	O
to	O
CO	O
)	O
can	O
be	O
optimized	O
,	O
if	O
each	O
transaction	B-General_Concept
is	O
assigned	O
with	O
a	O
weight	O
(	O
that	O
can	O
be	O
determined	O
by	O
transaction	B-General_Concept
's	O
importance	O
and	O
by	O
the	O
computing	O
resources	O
already	O
invested	O
in	O
the	O
running	O
transaction	B-General_Concept
;	O
optimization	O
can	O
be	O
carried	O
out	O
,	O
for	O
example	O
,	O
by	O
a	O
reduction	O
from	O
the	O
Max	B-Algorithm
flow	I-Algorithm
in	I-Algorithm
networks	I-Algorithm
problem	O
(	O
Raz	O
1993a	O
)	O
)	O
.	O
</s>
<s>
Practically	O
,	O
almost	O
in	O
all	O
needed	O
implementations	O
a	O
transaction	B-General_Concept
should	O
be	O
committed	O
only	O
when	O
the	O
set	O
is	O
empty	O
(	O
and	O
no	O
set	O
optimization	O
is	O
applicable	O
)	O
.	O
</s>
<s>
The	O
local	O
(	O
to	O
the	O
database	O
)	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
(	O
separate	O
from	O
the	O
ECO	O
algorithm	O
)	O
ensures	O
that	O
local	O
cycles	O
are	O
eliminated	O
(	O
unlike	O
with	O
CO	O
,	O
which	O
implies	O
serializability	B-Operating_System
by	O
itself	O
;	O
however	O
,	O
practically	O
also	O
for	O
CO	O
a	O
local	O
concurrency	B-Architecture
mechanism	O
is	O
utilized	O
,	O
at	O
least	O
to	O
ensure	O
Recoverability	O
)	O
.	O
</s>
<s>
When	O
the	O
overall	O
transactions	O
 '	O
local	O
partial	O
order	O
(	O
which	O
is	O
determined	O
by	O
the	O
local	O
conflict	O
graph	O
,	O
now	O
only	O
with	O
possible	O
temporary	O
local	O
cycles	O
,	O
since	O
cycles	O
are	O
eliminated	O
by	O
a	O
local	O
serializability	B-Operating_System
mechanism	O
)	O
allows	O
,	O
also	O
global	O
transactions	O
can	O
be	O
voted	O
on	O
to	O
be	O
committed	O
concurrently	O
(	O
when	O
all	O
their	O
transitively	O
(	O
indirect	O
)	O
preceding	O
(	O
via	O
conflict	O
)	O
global	O
transactions	O
are	O
committed	O
,	O
while	O
transitively	O
preceding	O
local	O
transactions	O
can	O
be	O
at	O
any	O
state	O
.	O
</s>
<s>
Let	O
be	O
undecided	O
(	O
neither	O
committed	O
nor	O
aborted	O
)	O
global	O
transactions	O
in	O
a	O
database	O
system	O
that	O
ensures	O
serializability	B-Operating_System
locally	O
,	O
such	O
that	O
a	O
directed	O
path	O
of	O
unaborted	O
transactions	O
exists	O
in	O
the	O
local	O
conflict	O
graph	O
(	O
that	O
of	O
the	O
database	O
itself	O
)	O
from	O
to	O
.	O
</s>
<s>
Global	O
ECO	O
(	O
all	O
global	O
cycles	O
in	O
the	O
global	O
conflict	O
graph	O
are	O
eliminated	O
by	O
atomic	O
commitment	O
)	O
together	O
with	O
Local	O
serializability	B-Operating_System
(	O
i.e.	O
,	O
each	O
database	O
system	O
maintains	O
serializability	B-Operating_System
locally	O
;	O
all	O
local	O
cycles	O
are	O
eliminated	O
)	O
imply	O
Global	B-Operating_System
serializability	I-Operating_System
(	O
all	O
cycles	O
are	O
eliminated	O
)	O
.	O
</s>
<s>
This	O
means	O
that	O
if	O
each	O
database	O
system	O
in	O
a	O
multidatabase	O
environment	O
provides	O
local	O
serializability	B-Operating_System
(	O
by	O
any	O
mechanism	O
)	O
and	O
enforces	O
the	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
in	O
the	O
theorem	O
above	O
(	O
a	O
generalization	O
of	O
CO	O
's	O
vote	B-Architecture
ordering	I-Architecture
strategy	O
)	O
,	O
then	O
Global	B-Operating_System
serializability	I-Operating_System
is	O
guaranteed	O
(	O
no	O
local	O
CO	O
is	O
needed	O
anymore	O
)	O
.	O
</s>
<s>
Similarly	O
to	O
CO	O
as	O
well	O
,	O
the	O
ECO	O
voting-deadlock	O
situation	O
can	O
be	O
summarized	O
as	O
follows	O
:	O
</s>
<s>
Let	O
a	O
multidatabase	O
environment	O
comprise	O
database	O
systems	O
that	O
enforce	O
,	O
each	O
,	O
both	O
Global	O
ECO	O
(	O
using	O
the	O
condition	O
in	O
the	O
theorem	O
above	O
)	O
and	O
local	O
conflict	O
serializability	B-Operating_System
(	O
which	O
eliminates	O
local	O
cycles	O
in	O
the	O
global	O
conflict	O
graph	O
)	O
.	O
</s>
<s>
Then	O
,	O
a	O
voting-deadlock	O
occurs	O
if	O
and	O
only	O
if	O
a	O
global	O
cycle	O
(	O
spans	O
two	O
or	O
more	O
databases	O
)	O
exists	O
in	O
the	O
Global	O
augmented	O
conflict	O
graph	O
(	O
also	O
blocking	O
by	O
a	O
data-access	O
lock	O
is	O
represented	O
by	O
an	O
edge	O
)	O
.	O
</s>
<s>
If	O
the	O
cycle	O
does	O
not	O
break	O
by	O
any	O
abort	O
,	O
then	O
all	O
the	O
global	O
transactions	O
on	O
it	O
are	O
involved	O
with	O
the	O
respective	O
voting-deadlock	O
,	O
and	O
eventually	O
each	O
has	O
its	O
vote	O
blocked	O
(	O
either	O
directly	O
,	O
or	O
indirectly	O
by	O
a	O
data-access	O
lock	O
)	O
.	O
</s>
<s>
If	O
a	O
local	O
transaction	B-General_Concept
resides	O
on	O
the	O
cycle	O
,	O
it	O
may	O
be	O
in	O
any	O
unaborted	O
state	O
(	O
running	O
,	O
ready	O
,	O
or	O
committed	O
;	O
unlike	O
CO	O
no	O
local	O
commit	O
blocking	O
is	O
needed	O
)	O
.	O
</s>
<s>
As	O
with	O
CO	O
this	O
means	O
that	O
also	O
global	O
deadlocks	B-Operating_System
due	O
to	O
data-access	O
locking	B-Operating_System
(	O
with	O
at	O
least	O
one	O
lock	O
blocking	O
)	O
are	O
voting	O
deadlocks	B-Operating_System
,	O
and	O
are	O
automatically	O
resolved	O
by	O
atomic	O
commitment	O
.	O
</s>
<s>
Multi-version	O
Commitment	B-Architecture
Ordering	I-Architecture
(	O
MVCO	O
;	O
(	O
Raz	O
1993b	O
)	O
)	O
is	O
a	O
generalization	O
of	O
CO	O
for	O
databases	O
with	O
multi-version	B-Operating_System
resources	I-Operating_System
.	O
</s>
<s>
Utilizing	O
such	O
resources	O
is	O
a	O
common	O
way	O
nowadays	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
,	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
.	O
</s>
<s>
MVCO	O
implies	O
One-copy-serializability	O
(	O
1SER	O
or	O
1SR	O
)	O
which	O
is	O
the	O
generalization	O
of	O
serializability	B-Operating_System
for	O
multi-version	B-Operating_System
resources	I-Operating_System
.	O
</s>
<s>
Like	O
CO	O
,	O
MVCO	O
is	O
non-blocking	O
,	O
and	O
can	O
be	O
combined	O
with	O
any	O
relevant	O
multi-version	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
mechanism	O
without	O
interfering	O
with	O
it	O
.	O
</s>
<s>
Results	O
for	O
the	O
regular	O
and	O
augmented	O
conflict	O
graphs	O
remain	O
unchanged	O
,	O
and	O
similarly	O
to	O
CO	O
a	O
distributed	O
MVCO	O
enforcing	O
algorithm	O
exists	O
,	O
now	O
for	O
a	O
mixed	O
environment	O
with	O
both	O
single-version	O
and	O
multi-version	B-Operating_System
resources	I-Operating_System
(	O
now	O
single-version	O
is	O
a	O
special	O
case	O
of	O
multi-version	O
)	O
.	O
</s>
<s>
Locking-based	O
global	O
deadlocks	B-Operating_System
translate	O
to	O
voting	O
deadlocks	B-Operating_System
and	O
are	O
resolved	O
automatically	O
.	O
</s>
<s>
MVCO	O
compliance	O
of	O
every	O
autonomous	O
database	O
system	O
(	O
or	O
transactional	O
object	O
)	O
in	O
a	O
mixed	O
multidatabase	O
environment	O
of	O
single-version	O
and	O
multi-version	O
databases	O
is	O
a	O
necessary	O
condition	O
for	O
guaranteeing	O
Global	O
one-copy-serializability	O
(	O
1SER	O
)	O
.	O
</s>
<s>
Locking-based	O
global	O
deadlocks	B-Operating_System
are	O
resolved	O
automatically	O
.	O
</s>
<s>
CO	O
based	O
snapshot	B-Operating_System
isolation	I-Operating_System
(	O
COSI	O
)	O
is	O
the	O
intersection	O
of	O
Snapshot	B-Operating_System
isolation	I-Operating_System
(	O
SI	O
)	O
with	O
MVCO	O
.	O
</s>
<s>
SI	O
is	O
a	O
multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
method	O
widely	O
utilized	O
due	O
to	O
good	O
performance	O
and	O
similarity	O
to	O
serializability	B-Operating_System
(	O
1SER	O
)	O
in	O
several	O
aspects	O
.	O
</s>
<s>
2008	O
)	O
;	O
see	O
also	O
Making	O
snapshot	B-Operating_System
isolation	I-Operating_System
serializable	B-Operating_System
and	O
the	O
references	O
there	O
)	O
,	O
for	O
analyzing	O
conflicts	O
in	O
SI	O
in	O
order	O
to	O
make	O
it	O
serializable	B-Operating_System
.	O
</s>
<s>
2008	O
)	O
,	O
Serializable	B-Operating_System
snapshot	B-Operating_System
isolation	I-Operating_System
(	O
SerializableSI	O
)	O
,	O
a	O
low	O
overhead	O
modification	O
of	O
SI	O
,	O
provides	O
good	O
performance	O
results	O
versus	O
SI	O
,	O
with	O
only	O
small	O
penalty	O
for	O
enforcing	O
serializability	B-Operating_System
.	O
</s>
<s>
A	O
different	O
method	O
,	O
by	O
combining	O
SI	O
with	O
MVCO	O
(	O
COSI	O
)	O
,	O
makes	O
SI	O
serializable	B-Operating_System
as	O
well	O
,	O
with	O
a	O
relatively	O
low	O
overhead	O
,	O
similarly	O
to	O
combining	O
the	O
generic	O
CO	O
algorithm	O
with	O
single-version	O
mechanisms	O
.	O
</s>
<s>
On	O
one	O
hand	O
,	O
all	O
serializable	B-Operating_System
SI	O
schedules	O
can	O
be	O
made	O
MVCO	O
by	O
COSI	O
(	O
by	O
possible	O
commit	O
delays	O
when	O
needed	O
)	O
without	O
aborting	O
transactions	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
SerializableSI	O
is	O
known	O
to	O
unnecessarily	O
abort	O
and	O
restart	O
certain	O
percentages	O
of	O
transactions	O
also	O
in	O
serializable	B-Operating_System
SI	O
schedules	O
.	O
</s>
<s>
With	O
CO	O
and	O
its	O
variants	O
(	O
e.g.	O
,	O
SS2PL	O
,	O
SCO	O
,	O
OCO	O
,	O
ECO	O
,	O
and	O
MVCO	O
above	O
)	O
global	B-Operating_System
serializability	I-Operating_System
is	O
achieved	O
via	O
atomic	O
commitment	O
protocol	O
based	O
distributed	O
algorithms	O
.	O
</s>
<s>
In	O
cases	O
of	O
either	O
incompatible	O
local	O
commitment	O
orders	O
in	O
two	O
or	O
more	O
databases	O
(	O
when	O
no	O
global	O
partial	O
order	O
can	O
embed	O
the	O
respective	O
local	O
partial	O
orders	O
together	O
)	O
,	O
or	O
a	O
data-access	O
locking	B-Operating_System
related	O
voting	O
deadlock	B-Operating_System
,	O
both	O
implying	O
a	O
global	O
cycle	O
in	O
the	O
global	O
augmented	O
conflict	O
graph	O
and	O
missing	O
votes	O
,	O
the	O
atomic	O
commitment	O
protocol	O
breaks	O
such	O
cycle	O
by	O
aborting	O
an	O
undecided	O
transaction	B-General_Concept
on	O
it	O
(	O
see	O
The	O
distributed	O
CO	O
algorithm	O
above	O
)	O
.	O
</s>
<s>
Each	O
local	O
CO	O
instance	O
of	O
any	O
variant	O
has	O
the	O
same	O
role	O
,	O
to	O
determine	O
the	O
position	O
of	O
every	O
global	O
transaction	B-General_Concept
(	O
a	O
transaction	B-General_Concept
that	O
spans	O
two	O
or	O
more	O
databases	O
)	O
within	O
the	O
local	O
commitment	O
order	O
,	O
i.e.	O
,	O
to	O
determine	O
when	O
it	O
is	O
the	O
transaction	B-General_Concept
's	O
turn	O
to	O
be	O
voted	O
on	O
locally	O
in	O
the	O
atomic	O
commitment	O
protocol	O
.	O
</s>
<s>
This	O
means	O
that	O
they	O
are	O
all	O
interoperable	O
via	O
atomic	O
commitment	O
(	O
using	O
the	O
same	O
software	O
interfaces	B-Application
,	O
typically	O
provided	O
as	O
services	O
,	O
some	O
already	O
standardized	O
for	O
atomic	O
commitment	O
,	O
primarily	O
for	O
the	O
two	B-General_Concept
phase	I-General_Concept
commit	I-General_Concept
protocol	O
,	O
e.g.	O
,	O
X/Open	O
XA	O
)	O
and	O
transparently	O
can	O
be	O
utilized	O
together	O
in	O
any	O
distributed	O
environment	O
(	O
while	O
each	O
CO	O
variant	O
instance	O
is	O
possibly	O
associated	O
with	O
any	O
relevant	O
local	O
concurrency	B-Operating_System
control	I-Operating_System
mechanism	O
type	O
)	O
.	O
</s>
<s>
In	O
summary	O
,	O
any	O
single	O
global	O
transaction	B-General_Concept
can	O
participate	O
simultaneously	O
in	O
databases	O
that	O
may	O
employ	O
each	O
any	O
,	O
possibly	O
different	O
,	O
CO	O
variant	O
(	O
while	O
concurrently	O
running	O
processes	O
in	O
each	O
such	O
database	O
,	O
and	O
running	O
concurrently	O
with	O
local	O
and	O
other	O
global	O
transactions	O
in	O
each	O
such	O
database	O
)	O
.	O
</s>
<s>
Any	O
global	O
cycle	O
generated	O
in	O
the	O
augmented	O
global	O
conflict	O
graph	O
may	O
span	O
databases	O
of	O
different	O
CO	O
variants	O
,	O
and	O
generate	O
(	O
if	O
not	O
broken	O
by	O
any	O
local	O
abort	O
)	O
a	O
voting	O
deadlock	B-Operating_System
that	O
is	O
resolved	O
by	O
atomic	O
commitment	O
exactly	O
the	O
same	O
way	O
as	O
in	O
a	O
single	O
CO	O
variant	O
environment	O
.	O
</s>
<s>
local	O
cycles	O
(	O
now	O
possibly	O
with	O
mixed	O
materialized	O
and	O
non-materialized	O
conflicts	O
,	O
both	O
serializability	B-Operating_System
and	O
data-access-locking	O
deadlock	B-Operating_System
related	O
,	O
e.g.	O
,	O
SCO	O
)	O
are	O
resolved	O
locally	O
(	O
each	O
by	O
its	O
respective	O
variant	O
instance	O
's	O
own	O
local	O
mechanisms	O
)	O
.	O
</s>
<s>
Vote	B-Architecture
ordering	I-Architecture
(	O
VO	O
or	O
Generalized	O
CO	O
(	O
GCO	O
)	O
;	O
Raz	O
2009	O
)	O
,	O
the	O
union	O
of	O
CO	O
and	O
all	O
its	O
above	O
variants	O
,	O
is	O
a	O
useful	O
concept	O
and	O
global	B-Operating_System
serializability	I-Operating_System
technique	O
.	O
</s>
<s>
To	O
comply	O
with	O
VO	O
,	O
local	O
serializability	B-Operating_System
(	O
in	O
it	O
most	O
general	O
form	O
,	O
commutativity	O
based	O
,	O
and	O
including	O
multi-versioning	O
)	O
and	O
the	O
vote	O
order	O
strategy	O
(	O
voting	O
by	O
local	O
precedence	O
order	O
)	O
are	O
needed	O
.	O
</s>
<s>
In	O
a	O
multi-database	O
environment	O
,	O
where	O
each	O
database	O
system	O
(	O
transactional	O
object	O
)	O
is	O
compliant	O
with	O
some	O
CO	O
variant	O
property	O
(	O
VO	O
compliant	O
)	O
,	O
any	O
global	O
transaction	B-General_Concept
can	O
participate	O
simultaneously	O
in	O
databases	O
of	O
possibly	O
different	O
CO	O
variants	O
,	O
and	O
Global	B-Operating_System
serializability	I-Operating_System
is	O
guaranteed	O
(	O
sufficient	O
condition	O
for	O
Global	B-Operating_System
serializability	I-Operating_System
;	O
and	O
Global	O
one-copy-serializability	O
(	O
1SER	O
)	O
,	O
for	O
a	O
case	O
when	O
a	O
multi-version	O
database	O
exists	O
)	O
.	O
</s>
<s>
If	O
only	O
local	O
(	O
to	O
a	O
database	O
system	O
)	O
concurrency	B-Operating_System
control	I-Operating_System
information	O
is	O
utilized	O
by	O
every	O
database	O
system	O
(	O
each	O
has	O
the	O
generalized	O
autonomy	O
property	O
,	O
a	O
generalization	O
of	O
autonomy	O
)	O
,	O
then	O
compliance	O
of	O
each	O
with	O
some	O
(	O
any	O
)	O
CO	O
variant	O
property	O
(	O
VO	O
compliance	O
)	O
is	O
a	O
necessary	O
condition	O
for	O
guaranteeing	O
Global	B-Operating_System
serializability	I-Operating_System
(	O
and	O
Global	O
1SER	O
;	O
otherwise	O
they	O
may	O
be	O
violated	O
)	O
.	O
</s>
<s>
Furthermore	O
,	O
in	O
such	O
environment	O
data-access-locking	O
related	O
global	O
deadlocks	B-Operating_System
are	O
resolved	O
automatically	O
(	O
each	O
such	O
deadlock	B-Operating_System
is	O
generated	O
by	O
a	O
global	O
cycle	O
in	O
the	O
augmented	O
conflict	O
graph	O
(	O
i.e.	O
,	O
a	O
voting	O
deadlock	B-Operating_System
;	O
see	O
above	O
)	O
,	O
involving	O
at	O
least	O
one	O
data-access	O
lock	O
(	O
non-materialized	O
conflict	O
)	O
and	O
two	O
database	O
systems	O
;	O
thus	O
,	O
not	O
a	O
cycle	O
in	O
the	O
regular	O
conflict	O
graph	O
and	O
does	O
not	O
affect	O
serializability	B-Operating_System
)	O
.	O
</s>
