<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
consistency	B-General_Concept
model	I-General_Concept
specifies	O
a	O
contract	O
between	O
the	O
programmer	O
and	O
a	O
system	O
,	O
wherein	O
the	O
system	O
guarantees	O
that	O
if	O
the	O
programmer	O
follows	O
the	O
rules	O
for	O
operations	O
on	O
memory	O
,	O
memory	O
will	O
be	O
consistent	B-General_Concept
and	O
the	O
results	O
of	O
reading	O
,	O
writing	O
,	O
or	O
updating	O
memory	O
will	O
be	O
predictable	O
.	O
</s>
<s>
Consistency	B-General_Concept
models	I-General_Concept
are	O
used	O
in	O
distributed	B-Architecture
systems	I-Architecture
like	O
distributed	B-Operating_System
shared	I-Operating_System
memory	I-Operating_System
systems	O
or	O
distributed	O
data	O
stores	O
(	O
such	O
as	O
filesystems	B-Application
,	O
databases	O
,	O
optimistic	B-General_Concept
replication	I-General_Concept
systems	O
or	O
web	B-Protocol
caching	I-Protocol
)	O
.	O
</s>
<s>
Consistency	O
is	O
different	O
from	O
coherence	O
,	O
which	O
occurs	O
in	O
systems	O
that	O
are	O
cached	B-General_Concept
or	O
cache-less	O
,	O
and	O
is	O
consistency	B-General_Concept
of	I-General_Concept
data	I-General_Concept
with	O
respect	O
to	O
all	O
processors	O
.	O
</s>
<s>
High	B-Language
level	I-Language
languages	I-Language
,	O
such	O
as	O
C++	B-Language
and	O
Java	B-Language
,	O
maintain	O
the	O
consistency	O
contract	O
by	O
translating	O
memory	O
operations	O
into	O
low-level	O
operations	O
in	O
a	O
way	O
that	O
preserves	O
memory	B-General_Concept
semantics	I-General_Concept
,	O
reordering	O
some	O
memory	O
instructions	O
,	O
and	O
encapsulating	O
required	O
synchronization	O
with	O
library	O
calls	O
such	O
as	O
pthread_mutex_lock( )	O
.	O
</s>
<s>
The	O
consistency	B-General_Concept
model	I-General_Concept
determines	O
whether	O
client	O
B	O
will	O
definitely	O
see	O
the	O
write	O
performed	O
by	O
client	O
A	O
,	O
will	O
definitely	O
not	O
,	O
or	O
cannot	O
depend	O
on	O
seeing	O
the	O
write	O
.	O
</s>
<s>
Consistency	B-General_Concept
models	I-General_Concept
define	O
rules	O
for	O
the	O
apparent	O
order	O
and	O
visibility	O
of	O
updates	O
,	O
and	O
are	O
on	O
a	O
continuum	O
with	O
tradeoffs	O
.	O
</s>
<s>
There	O
are	O
two	O
methods	O
to	O
define	O
and	O
categorize	O
consistency	B-General_Concept
models	I-General_Concept
;	O
issue	O
and	O
view	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
consistency	B-General_Concept
model	I-General_Concept
can	O
define	O
that	O
a	O
process	O
is	O
not	O
allowed	O
to	O
issue	O
an	O
operation	O
until	O
all	O
previously	O
issued	O
operations	O
are	O
completed	O
.	O
</s>
<s>
Different	O
consistency	B-General_Concept
models	I-General_Concept
enforce	O
different	O
conditions	O
.	O
</s>
<s>
One	O
consistency	B-General_Concept
model	I-General_Concept
can	O
be	O
considered	O
stronger	O
than	O
another	O
if	O
it	O
requires	O
all	O
conditions	O
of	O
that	O
model	O
and	O
more	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
a	O
model	O
with	O
fewer	O
constraints	O
is	O
considered	O
a	O
weaker	O
consistency	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
Strict	O
consistency	O
is	O
the	O
strongest	O
consistency	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
The	O
sequential	B-General_Concept
consistency	I-General_Concept
model	O
was	O
proposed	O
by	O
Lamport(1979 )	O
.	O
</s>
<s>
It	O
is	O
a	O
weaker	O
memory	O
model	O
than	O
strict	O
consistency	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
Sequential	B-General_Concept
consistency	I-General_Concept
is	O
met	O
if	O
"	O
the	O
result	O
of	O
any	O
execution	O
is	O
the	O
same	O
as	O
if	O
the	O
(	O
read	O
and	O
write	O
)	O
operations	O
of	O
all	O
processes	O
on	O
the	O
data	O
store	O
were	O
executed	O
in	O
some	O
sequential	O
order	O
,	O
and	O
the	O
operations	O
of	O
each	O
individual	O
processor	O
appear	O
in	O
this	O
sequence	O
in	O
the	O
order	O
specified	O
by	O
its	O
program.	O
"	O
</s>
<s>
Adve	O
and	O
Gharachorloo	O
,	O
1996	O
define	O
two	O
requirements	O
to	O
implement	O
the	O
sequential	B-General_Concept
consistency	I-General_Concept
;	O
program	O
order	O
and	O
write	O
atomicity	B-General_Concept
.	O
</s>
<s>
Write	O
atomicity	B-General_Concept
:	O
Write	O
atomicity	B-General_Concept
defines	O
that	O
memory	O
requests	O
are	O
serviced	O
based	O
on	O
the	O
order	O
of	O
a	O
single	O
FIFO	O
queue	O
.	O
</s>
<s>
In	O
sequential	B-General_Concept
consistency	I-General_Concept
,	O
there	O
is	O
no	O
notion	O
of	O
time	O
or	O
most	O
recent	O
write	O
operations	O
.	O
</s>
<s>
Cache-less	O
architectures	O
or	O
cached	B-General_Concept
architectures	O
with	O
interconnect	O
networks	O
that	O
are	O
not	O
instantaneous	O
can	O
contain	O
a	O
slow	O
path	O
between	O
processors	O
and	O
memories	O
.	O
</s>
<s>
Sequential	B-General_Concept
consistency	I-General_Concept
can	O
produce	O
non-deterministic	O
results	O
.	O
</s>
<s>
Linearizability	B-General_Concept
(	O
also	O
known	O
as	O
atomic	O
consistency	O
or	O
atomic	O
memory	O
)	O
can	O
be	O
defined	O
as	O
sequential	B-General_Concept
consistency	I-General_Concept
with	O
a	O
real-time	O
constraint	O
,	O
by	O
considering	O
a	O
begin	O
time	O
and	O
end	O
time	O
for	O
each	O
operation	O
.	O
</s>
<s>
An	O
execution	O
is	O
linearizable	B-General_Concept
if	O
each	O
operation	O
taking	O
place	O
in	O
linearizable	B-General_Concept
order	O
by	O
placing	O
a	O
point	O
between	O
its	O
begin	O
time	O
and	O
its	O
end	O
time	O
and	O
guarantees	O
sequential	B-General_Concept
consistency	I-General_Concept
.	O
</s>
<s>
Verifying	O
sequential	B-General_Concept
consistency	I-General_Concept
through	O
model	B-Application
checking	I-Application
is	O
undecidable	O
in	O
general	O
,	O
even	O
for	O
finite-state	O
cache	B-General_Concept
coherence	I-General_Concept
protocols	O
.	O
</s>
<s>
Causal	B-General_Concept
consistency	I-General_Concept
defined	O
by	O
Hutto	O
and	O
Ahamad	O
,	O
1990	O
,	O
is	O
a	O
weakening	O
of	O
the	O
sequential	B-General_Concept
consistency	I-General_Concept
model	O
by	O
categorizing	O
events	O
into	O
those	O
causally	O
related	O
and	O
those	O
that	O
are	O
not	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
an	O
event	O
b	O
takes	O
effect	O
from	O
an	O
earlier	O
event	O
a	O
,	O
the	O
causal	B-General_Concept
consistency	I-General_Concept
guarantees	O
that	O
all	O
processes	O
see	O
event	O
b	O
after	O
event	O
a	O
.	O
Tanenbaum	O
et	O
al.	O
,	O
2007	O
provide	O
a	O
stricter	O
definition	O
,	O
that	O
a	O
data	O
store	O
is	O
considered	O
causally	O
consistent	B-General_Concept
under	O
the	O
following	O
conditions	O
:	O
</s>
<s>
This	O
model	O
relaxes	O
sequential	B-General_Concept
consistency	I-General_Concept
on	O
concurrent	O
writes	O
by	O
a	O
processor	O
and	O
on	O
writes	O
that	O
are	O
not	O
causally	O
related	O
.	O
</s>
<s>
As	O
in	O
sequential	B-General_Concept
consistency	I-General_Concept
,	O
reads	O
do	O
not	O
need	O
to	O
reflect	O
changes	O
instantaneously	O
,	O
however	O
,	O
they	O
need	O
to	O
reflect	O
all	O
changes	O
to	O
a	O
variable	O
sequentially	O
.	O
</s>
<s>
W(x )	O
2	O
happens	O
after	O
W(x )	O
1	O
due	O
to	O
the	O
read	O
made	O
by	O
P2	O
to	O
x	O
before	O
W(x )	O
2	O
,	O
hence	O
this	O
example	O
is	O
causally	O
consistent	B-General_Concept
under	O
Hutto	O
and	O
Ahamad	O
's	O
definition	O
(	O
although	O
not	O
under	O
Tanenbaum	O
et	O
al	O
.	O
</s>
<s>
In	O
order	O
for	O
consistency	O
in	O
data	O
to	O
be	O
maintained	O
and	O
to	O
attain	O
scalable	O
processor	O
systems	O
where	O
every	O
processor	O
has	O
its	O
own	O
memory	O
,	O
the	O
processor	B-General_Concept
consistency	I-General_Concept
model	O
was	O
derived	O
.	O
</s>
<s>
All	O
processors	O
need	O
to	O
be	O
consistent	B-General_Concept
in	O
the	O
order	O
in	O
which	O
they	O
see	O
writes	O
done	O
by	O
one	O
processor	O
and	O
in	O
the	O
way	O
they	O
see	O
writes	O
by	O
different	O
processors	O
to	O
the	O
same	O
location	O
(	O
coherence	O
is	O
maintained	O
)	O
.	O
</s>
<s>
However	O
,	O
they	O
do	O
not	O
need	O
to	O
be	O
consistent	B-General_Concept
when	O
the	O
writes	O
are	O
by	O
different	O
processors	O
to	O
different	O
locations	O
.	O
</s>
<s>
The	O
processor	B-General_Concept
consistency	I-General_Concept
model	O
is	O
similar	O
to	O
PRAM	B-General_Concept
consistency	I-General_Concept
model	O
with	O
a	O
stronger	O
condition	O
that	O
defines	O
all	O
writes	O
to	O
the	O
same	O
memory	O
location	O
must	O
be	O
seen	O
in	O
the	O
same	O
sequential	O
order	O
by	O
all	O
other	O
processes	O
.	O
</s>
<s>
Processor	B-General_Concept
consistency	I-General_Concept
is	O
weaker	O
than	O
sequential	B-General_Concept
consistency	I-General_Concept
but	O
stronger	O
than	O
PRAM	B-General_Concept
consistency	I-General_Concept
model	O
.	O
</s>
<s>
The	O
Stanford	B-Operating_System
DASH	I-Operating_System
multiprocessor	I-Operating_System
system	I-Operating_System
implements	O
a	O
variation	O
of	O
processor	B-General_Concept
consistency	I-General_Concept
which	O
is	O
incomparable	O
(	O
neither	O
weaker	O
nor	O
stronger	O
)	O
to	O
Goodman	O
's	O
definitions	O
.	O
</s>
<s>
All	O
processors	O
need	O
to	O
be	O
consistent	B-General_Concept
in	O
the	O
order	O
in	O
which	O
they	O
see	O
writes	O
by	O
one	O
processor	O
and	O
in	O
the	O
way	O
they	O
see	O
writes	O
by	O
different	O
processors	O
to	O
the	O
same	O
location	O
.	O
</s>
<s>
However	O
,	O
they	O
do	O
not	O
need	O
to	O
be	O
consistent	B-General_Concept
when	O
the	O
writes	O
are	O
by	O
different	O
processors	O
to	O
different	O
locations	O
.	O
</s>
<s>
Pipelined	B-General_Concept
RAM	I-General_Concept
consistency	I-General_Concept
(	O
PRAM	B-General_Concept
consistency	I-General_Concept
)	O
was	O
presented	O
by	O
Lipton	O
and	O
Sandberg	O
in	O
1988	O
as	O
one	O
of	O
the	O
first	O
described	O
consistency	B-General_Concept
models	I-General_Concept
.	O
</s>
<s>
In	O
PRAM	B-General_Concept
consistency	I-General_Concept
,	O
all	O
processes	O
view	O
the	O
operations	O
of	O
a	O
single	O
process	O
in	O
the	O
same	O
order	O
that	O
they	O
were	O
issued	O
by	O
that	O
process	O
,	O
while	O
operations	O
issued	O
by	O
different	O
processes	O
can	O
be	O
viewed	O
in	O
different	O
order	O
from	O
different	O
processes	O
.	O
</s>
<s>
PRAM	B-General_Concept
consistency	I-General_Concept
is	O
weaker	O
than	O
processor	B-General_Concept
consistency	I-General_Concept
.	O
</s>
<s>
Cache	B-General_Concept
consistency	I-General_Concept
requires	O
that	O
all	O
write	O
operations	O
to	O
the	O
same	O
memory	O
location	O
are	O
performed	O
in	O
some	O
sequential	O
order	O
.	O
</s>
<s>
Cache	B-General_Concept
consistency	I-General_Concept
is	O
weaker	O
than	O
processor	B-General_Concept
consistency	I-General_Concept
and	O
incomparable	O
with	O
PRAM	B-General_Concept
consistency	I-General_Concept
.	O
</s>
<s>
Slow	O
consistency	O
is	O
a	O
weaker	O
model	O
than	O
PRAM	O
and	O
cache	B-General_Concept
consistency	I-General_Concept
.	O
</s>
<s>
These	O
4	O
consistency	B-General_Concept
models	I-General_Concept
were	O
proposed	O
in	O
a	O
1994	O
paper	O
.	O
</s>
<s>
Program	O
order	O
and	O
atomicity	B-General_Concept
is	O
maintained	O
only	O
on	O
synchronisation	O
operations	O
and	O
not	O
on	O
all	O
reads	O
and	O
writes	O
.	O
</s>
<s>
This	O
model	O
ensures	O
that	O
write	O
atomicity	B-General_Concept
is	O
always	O
maintained	O
,	O
therefore	O
no	O
additional	O
safety	O
net	O
is	O
required	O
for	O
weak	O
ordering	O
.	O
</s>
<s>
Therefore	O
,	O
accesses	O
to	O
synchronization	O
variables	O
is	O
sequentially	B-General_Concept
consistent	I-General_Concept
and	O
any	O
read	O
or	O
write	O
should	O
be	O
performed	O
only	O
after	O
previous	O
synchronization	O
operations	O
have	O
completed	O
.	O
</s>
<s>
The	O
release	B-General_Concept
consistency	I-General_Concept
model	O
relaxes	O
the	O
weak	B-General_Concept
consistency	I-General_Concept
model	O
by	O
distinguishing	O
the	O
entrance	O
synchronization	O
operation	O
from	O
the	O
exit	O
synchronization	O
operation	O
.	O
</s>
<s>
However	O
,	O
under	O
release	B-General_Concept
consistency	I-General_Concept
model	O
,	O
during	O
the	O
entry	O
to	O
a	O
critical	O
section	O
,	O
termed	O
as	O
"	O
acquire	O
"	O
,	O
all	O
operations	O
with	O
respect	O
to	O
the	O
local	O
memory	O
variables	O
need	O
to	O
be	O
completed	O
.	O
</s>
<s>
Among	O
synchronization	O
variables	O
,	O
sequential	B-General_Concept
consistency	I-General_Concept
or	O
processor	B-General_Concept
consistency	I-General_Concept
can	O
be	O
maintained	O
.	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
release	B-General_Concept
consistency	I-General_Concept
,	O
release	B-General_Concept
consistency	I-General_Concept
with	O
sequential	B-General_Concept
consistency	I-General_Concept
(	O
RCsc	O
)	O
and	O
release	B-General_Concept
consistency	I-General_Concept
with	O
processor	B-General_Concept
consistency	I-General_Concept
(	O
RCpc	O
)	O
.	O
</s>
<s>
For	O
sequential	B-General_Concept
consistency	I-General_Concept
(	O
RCsc	O
)	O
,	O
the	O
constraints	O
are	O
:	O
</s>
<s>
For	O
processor	B-General_Concept
consistency	I-General_Concept
(	O
RCpc	O
)	O
the	O
write	O
to	O
read	O
program	O
order	O
is	O
relaxed	O
,	O
having	O
constraints	O
:	O
</s>
<s>
This	O
is	O
a	O
variant	O
of	O
the	O
release	B-General_Concept
consistency	I-General_Concept
model	O
.	O
</s>
<s>
Such	O
a	O
consistency	B-General_Concept
model	I-General_Concept
will	O
be	O
useful	O
when	O
different	O
matrix	O
elements	O
can	O
be	O
processed	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Local	O
consistency	O
is	O
the	O
weakest	O
consistency	B-General_Concept
model	I-General_Concept
in	O
shared	O
memory	O
systems	O
.	O
</s>
<s>
An	O
eventual	B-General_Concept
consistency	I-General_Concept
is	O
a	O
weak	B-General_Concept
consistency	I-General_Concept
model	O
in	O
the	O
system	O
with	O
the	O
lack	O
of	O
simultaneous	O
updates	O
.	O
</s>
<s>
It	O
defines	O
that	O
if	O
no	O
update	O
takes	O
a	O
very	O
long	O
time	O
,	O
all	O
replicas	O
eventually	O
become	O
consistent	B-General_Concept
.	O
</s>
<s>
Most	O
shared	O
decentralized	O
databases	O
have	O
an	O
eventual	B-General_Concept
consistency	I-General_Concept
model	O
,	O
either	O
BASE	O
:	O
basically	O
available	O
;	O
soft	O
state	O
;	O
eventually	B-General_Concept
consistent	I-General_Concept
,	O
or	O
a	O
combination	O
of	O
ACID	B-Operating_System
and	O
BASE	O
sometimes	O
called	O
SALT	O
:	O
sequential	O
;	O
agreed	O
;	O
ledgered	O
;	O
tamper-resistant	O
,	O
and	O
also	O
symmetric	O
;	O
admin-free	O
;	O
ledgered	O
;	O
and	O
time-consensual	O
.	O
</s>
<s>
Some	O
different	O
consistency	B-General_Concept
models	I-General_Concept
can	O
be	O
defined	O
by	O
relaxing	O
one	O
or	O
more	O
requirements	O
in	O
sequential	B-General_Concept
consistency	I-General_Concept
called	O
relaxed	O
consistency	B-General_Concept
models	I-General_Concept
.	O
</s>
<s>
These	O
consistency	B-General_Concept
models	I-General_Concept
do	O
not	O
provide	O
memory	B-General_Concept
consistency	I-General_Concept
at	O
the	O
hardware	O
level	O
.	O
</s>
<s>
In	O
fact	O
,	O
the	O
programmers	O
are	O
responsible	O
for	O
implementing	O
the	O
memory	B-General_Concept
consistency	I-General_Concept
by	O
applying	O
synchronization	O
techniques	O
.	O
</s>
<s>
Relaxation	O
One	O
way	O
to	O
categorize	O
the	O
relaxed	O
consistency	O
is	O
to	O
define	O
which	O
sequential	B-General_Concept
consistency	I-General_Concept
requirements	O
are	O
relaxed	O
.	O
</s>
<s>
We	O
can	O
have	O
less	O
strict	O
models	O
by	O
relaxing	O
either	O
program	O
order	O
or	O
write	O
atomicity	B-General_Concept
requirements	O
defined	O
by	O
Adve	O
and	O
Gharachorloo	O
,	O
1996	O
.	O
</s>
<s>
Program	O
order	O
guarantees	O
that	O
each	O
process	O
issues	O
a	O
memory	O
request	O
ordered	O
by	O
its	O
program	O
and	O
write	O
atomicity	B-General_Concept
defines	O
that	O
memory	O
requests	O
are	O
serviced	O
based	O
on	O
the	O
order	O
of	O
a	O
single	O
FIFO	O
queue	O
.	O
</s>
<s>
In	O
the	O
relaxed	O
write	O
atomicity	B-General_Concept
model	O
,	O
a	O
process	O
can	O
view	O
its	O
own	O
writes	O
before	O
any	O
other	O
processors	O
.	O
</s>
<s>
Synchronizing	O
vs.	O
non-synchronizing	O
A	O
synchronizing	O
model	O
can	O
be	O
defined	O
by	O
dividing	O
the	O
memory	O
accesses	O
into	O
two	O
groups	O
and	O
assigning	O
different	O
consistency	O
restrictions	O
to	O
each	O
group	O
considering	O
that	O
one	O
group	O
can	O
have	O
a	O
weak	B-General_Concept
consistency	B-General_Concept
model	I-General_Concept
while	O
the	O
other	O
one	O
needs	O
a	O
more	O
restrictive	O
consistency	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
In	O
contrast	O
,	O
a	O
non-synchronizing	O
model	O
assigns	O
the	O
same	O
consistency	B-General_Concept
model	I-General_Concept
to	O
the	O
memory	O
access	O
types	O
.	O
</s>
<s>
Issue	O
vs.	O
view-based	O
Issue	O
method	O
provides	O
sequential	B-General_Concept
consistency	I-General_Concept
simulation	O
by	O
defining	O
the	O
restrictions	O
for	O
processes	O
to	O
issue	O
memory	O
operations	O
.	O
</s>
<s>
Relative	O
model	O
strength	O
Some	O
consistency	B-General_Concept
models	I-General_Concept
are	O
more	O
restrictive	O
than	O
others	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
strict	O
consistency	B-General_Concept
models	I-General_Concept
enforce	O
more	O
constraints	O
as	O
consistency	O
requirements	O
.	O
</s>
<s>
The	O
strength	O
of	O
a	O
model	O
can	O
be	O
defined	O
by	O
the	O
program	O
order	O
or	O
atomicity	B-General_Concept
relaxations	O
and	O
the	O
strength	O
of	O
models	O
can	O
also	O
be	O
compared	O
.	O
</s>
<s>
Sequential	B-General_Concept
consistency	I-General_Concept
has	O
two	O
requirements	O
,	O
program	O
order	O
and	O
write	O
atomicity	B-General_Concept
.	O
</s>
<s>
Different	O
relaxed	O
consistency	B-General_Concept
models	I-General_Concept
can	O
be	O
obtained	O
by	O
relaxing	O
these	O
requirements	O
.	O
</s>
<s>
This	O
is	O
done	O
so	O
that	O
,	O
along	O
with	O
relaxed	O
constraints	O
,	O
the	O
performance	O
increases	O
,	O
but	O
the	O
programmer	O
is	O
responsible	O
for	O
implementing	O
the	O
memory	B-General_Concept
consistency	I-General_Concept
by	O
applying	O
synchronisation	O
techniques	O
and	O
must	O
have	O
a	O
good	O
understanding	O
of	O
the	O
hardware	O
.	O
</s>
<s>
Whereas	O
,	O
programs	O
like	O
YYY	O
are	O
still	O
expected	O
to	O
give	O
consistent	B-General_Concept
results	O
because	O
of	O
the	O
enforcement	O
of	O
the	O
remaining	O
program	O
order	O
constraints	O
.	O
</s>
<s>
The	O
processor	B-General_Concept
consistency	I-General_Concept
model	O
(	O
PC	O
)	O
is	O
the	O
most	O
relaxed	O
of	O
the	O
three	O
models	O
and	O
relaxes	O
both	O
the	O
constraints	O
such	O
that	O
a	O
read	O
can	O
complete	O
before	O
an	O
earlier	O
write	O
even	O
before	O
it	O
is	O
made	O
visible	O
to	O
other	O
processors	O
.	O
</s>
<s>
To	O
ensure	O
sequential	B-General_Concept
consistency	I-General_Concept
in	O
the	O
above	O
models	O
,	O
safety	O
nets	O
or	O
fences	O
are	O
used	O
to	O
manually	O
enforce	O
the	O
constraint	O
.	O
</s>
<s>
PSO	O
is	O
similar	O
to	O
TSO	O
in	O
terms	O
of	O
atomicity	B-General_Concept
requirements	O
,	O
in	O
that	O
,	O
it	O
allows	O
a	O
processor	O
to	O
read	O
the	O
value	O
of	O
its	O
own	O
write	O
and	O
preventing	O
other	O
processors	O
from	O
reading	O
another	O
processor	O
's	O
write	O
before	O
the	O
write	O
is	O
visible	O
to	O
all	O
other	O
processors	O
.	O
</s>
<s>
The	O
safety	O
net	O
that	O
PSO	O
provides	O
is	O
similar	O
to	O
TSO	O
's	O
,	O
it	O
imposes	O
program	O
order	O
from	O
a	O
write	O
to	O
a	O
read	O
and	O
enforces	O
write	O
atomicity	B-General_Concept
.	O
</s>
<s>
The	O
weak	O
ordering	O
may	O
be	O
classified	O
under	O
this	O
category	O
and	O
two	O
types	O
of	O
release	B-General_Concept
consistency	I-General_Concept
models	O
(	O
RCsc	O
and	O
RCpc	O
)	O
also	O
come	O
under	O
this	O
model	O
.	O
</s>
<s>
This	O
model	O
also	O
differs	O
from	O
Alpha	O
and	O
RMO	O
in	O
terms	O
of	O
atomicity	B-General_Concept
.	O
</s>
<s>
A	O
combination	O
of	O
read	O
modify	O
write	O
operations	O
may	O
be	O
required	O
to	O
make	O
an	O
illusion	O
of	O
write	O
atomicity	B-General_Concept
.	O
</s>
<s>
From	O
a	O
programmer	O
's	O
perspective	O
these	O
models	O
must	O
maintain	O
the	O
illusion	O
of	O
write	O
atomicity	B-General_Concept
even	O
though	O
they	O
allow	O
the	O
processor	O
to	O
read	O
its	O
own	O
write	O
early	O
.	O
</s>
<s>
Transactional	O
memory	O
model	O
is	O
the	O
combination	O
of	O
cache	B-General_Concept
coherency	I-General_Concept
and	O
memory	B-General_Concept
consistency	I-General_Concept
models	I-General_Concept
as	O
a	O
communication	O
model	O
for	O
shared	O
memory	O
systems	O
supported	O
by	O
software	O
or	O
hardware	O
;	O
a	O
transactional	O
memory	O
model	O
provides	O
both	O
memory	B-General_Concept
consistency	I-General_Concept
and	O
cache	B-General_Concept
coherency	I-General_Concept
.	O
</s>
<s>
A	O
transaction	O
is	O
a	O
sequence	O
of	O
operations	O
executed	O
by	O
a	O
process	O
that	O
transforms	O
data	O
from	O
one	O
consistent	B-General_Concept
state	O
to	O
another	O
.	O
</s>
<s>
Compared	O
to	O
relaxed	O
consistency	B-General_Concept
models	I-General_Concept
,	O
a	O
transactional	O
model	O
is	O
easier	O
to	O
use	O
and	O
can	O
provide	O
the	O
higher	O
performance	O
than	O
a	O
sequential	B-General_Concept
consistency	I-General_Concept
model	O
.	O
</s>
<s>
Some	O
other	O
consistency	B-General_Concept
models	I-General_Concept
are	O
as	O
follows	O
:	O
</s>
<s>
Several	O
other	O
consistency	B-General_Concept
models	I-General_Concept
have	O
been	O
conceived	O
to	O
express	O
restrictions	O
with	O
respect	O
to	O
ordering	O
or	O
visibility	O
of	O
operations	O
,	O
or	O
to	O
deal	O
with	O
specific	O
fault	O
assumptions	O
.	O
</s>
<s>
Reliability	O
can	O
be	O
achieved	O
in	O
a	O
replicated	O
file	B-Application
system	I-Application
by	O
switching	O
to	O
another	O
replica	O
in	O
the	O
case	O
of	O
the	O
current	O
replica	O
failure	O
.	O
</s>
<s>
The	O
multiple	O
copies	O
are	O
consistent	B-General_Concept
if	O
a	O
read	O
operation	O
returns	O
the	O
same	O
value	O
from	O
all	O
copies	O
and	O
a	O
write	O
operation	O
as	O
a	O
single	O
atomic	B-General_Concept
operation	I-General_Concept
(	O
transaction	O
)	O
updates	O
all	O
copies	O
before	O
any	O
other	O
operation	O
takes	O
place	O
.	O
</s>
<s>
However	O
,	O
applying	O
global	O
synchronizations	O
to	O
keep	O
all	O
copies	O
consistent	B-General_Concept
is	O
costly	O
.	O
</s>
<s>
Tanenbaum	O
et	O
al.	O
,	O
2007	O
defines	O
the	O
consistency	B-General_Concept
model	I-General_Concept
as	O
a	O
contract	O
between	O
the	O
software	O
(	O
processes	O
)	O
and	O
memory	O
implementation	O
(	O
data	O
store	O
)	O
.	O
</s>
<s>
The	O
goal	O
of	O
data-centric	O
consistency	B-General_Concept
models	I-General_Concept
is	O
to	O
provide	O
a	O
consistent	B-General_Concept
view	O
on	O
a	O
data	O
store	O
where	O
processes	O
may	O
carry	O
out	O
concurrent	O
updates	O
.	O
</s>
<s>
Some	O
consistency	B-General_Concept
models	I-General_Concept
such	O
as	O
sequential	O
and	O
also	O
causal	B-General_Concept
consistency	I-General_Concept
models	I-General_Concept
deal	O
with	O
the	O
order	O
of	O
operations	O
on	O
shared	O
replicated	O
data	O
in	O
order	O
to	O
provide	O
consistency	O
.	O
</s>
<s>
In	O
these	O
models	O
,	O
all	O
replicas	O
must	O
agree	O
on	O
a	O
consistent	B-General_Concept
global	O
ordering	O
of	O
updates	O
.	O
</s>
<s>
In	O
grouping	O
operation	O
,	O
accesses	O
to	O
the	O
synchronization	O
variables	O
are	O
sequentially	B-General_Concept
consistent	I-General_Concept
.	O
</s>
<s>
In	O
distributed	B-Architecture
systems	I-Architecture
,	O
maintaining	O
sequential	B-General_Concept
consistency	I-General_Concept
in	O
order	O
to	O
control	O
the	O
concurrent	O
operations	O
is	O
essential	O
.	O
</s>
<s>
In	O
some	O
special	O
data	O
stores	O
without	O
simultaneous	O
updates	O
,	O
client-centric	O
consistency	B-General_Concept
models	I-General_Concept
can	O
deal	O
with	O
inconsistencies	O
in	O
a	O
less	O
costly	O
way	O
.	O
</s>
<s>
The	O
following	O
models	O
are	O
some	O
client-centric	O
consistency	B-General_Concept
models	I-General_Concept
:	O
</s>
<s>
The	O
implementation	O
of	O
a	O
consistency	B-General_Concept
model	I-General_Concept
is	O
defined	O
by	O
a	O
consistency	O
protocol	O
.	O
</s>
<s>
Since	O
the	O
consistency	O
requirements	O
can	O
differ	O
based	O
on	O
application	O
semantics	O
,	O
Yu	O
and	O
Vahdat	O
(	O
2000	O
)	O
believe	O
that	O
a	O
predefined	O
uniform	O
consistency	B-General_Concept
model	I-General_Concept
may	O
not	O
be	O
an	O
appropriate	O
approach	O
.	O
</s>
<s>
If	O
all	O
three	O
deviation	O
bounds	O
set	O
to	O
zero	O
,	O
the	O
continuous	O
consistency	B-General_Concept
model	I-General_Concept
is	O
the	O
strong	B-General_Concept
consistency	I-General_Concept
.	O
</s>
<s>
For	O
instance	O
,	O
sequential	O
ordering	O
is	O
a	O
popular	O
consistency	B-General_Concept
model	I-General_Concept
when	O
consistent	B-General_Concept
ordering	O
of	O
operations	O
is	O
considered	O
.	O
</s>
<s>
There	O
is	O
a	O
scalability	O
issue	O
in	O
implementing	O
such	O
a	O
multicasting	O
mechanism	O
in	O
large	O
distributed	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
As	O
an	O
example	O
,	O
suppose	O
in	O
a	O
distributed	O
file	B-Application
system	I-Application
,	O
a	O
file	O
is	O
replicated	O
on	O
N	O
servers	O
.	O
</s>
<s>
In	O
a	O
replicated	O
file	B-Application
system	I-Application
,	O
a	O
cache-coherence	O
protocol	O
provides	O
the	O
cache	B-General_Concept
consistency	I-General_Concept
while	O
caches	O
are	O
generally	O
controlled	O
by	O
clients	O
.	O
</s>
<s>
In	O
many	O
approaches	O
,	O
cache	B-General_Concept
consistency	I-General_Concept
is	O
provided	O
by	O
the	O
underlying	O
hardware	O
.	O
</s>
<s>
Some	O
other	O
approaches	O
in	O
middleware-based	O
distributed	B-Architecture
systems	I-Architecture
apply	O
software-based	O
solutions	O
to	O
provide	O
the	O
cache	B-General_Concept
consistency	I-General_Concept
.	O
</s>
<s>
Cache	B-General_Concept
consistency	I-General_Concept
models	O
can	O
differ	O
in	O
their	O
coherence	O
detection	O
strategies	O
that	O
define	O
when	O
inconsistencies	O
occur	O
.	O
</s>
<s>
In	O
the	O
dynamic	O
solution	O
,	O
the	O
server	O
checks	O
for	O
inconsistencies	O
at	O
run	O
time	O
to	O
control	O
the	O
consistency	O
of	O
the	O
cached	B-General_Concept
data	O
that	O
has	O
changed	O
after	O
it	O
was	O
cached	B-General_Concept
.	O
</s>
<s>
One	O
way	O
to	O
keep	O
the	O
data	O
consistent	B-General_Concept
is	O
to	O
never	O
cache	O
the	O
shared	O
data	O
.	O
</s>
<s>
In	O
this	O
solution	O
,	O
only	O
private	O
data	O
can	O
be	O
cached	B-General_Concept
by	O
clients	O
.	O
</s>
<s>
In	O
the	O
case	O
that	O
shared	O
data	O
are	O
cached	B-General_Concept
,	O
there	O
are	O
two	O
approaches	O
in	O
order	O
to	O
enforce	O
the	O
cache	B-General_Concept
coherence	I-General_Concept
.	O
</s>
