<s>
TLA+	B-Application
is	O
a	O
formal	B-Application
specification	I-Application
language	O
developed	O
by	O
Leslie	O
Lamport	O
.	O
</s>
<s>
It	O
is	O
used	O
for	O
designing	O
,	O
modelling	O
,	O
documentation	O
,	O
and	O
verification	O
of	O
programs	O
,	O
especially	O
concurrent	B-Operating_System
systems	I-Operating_System
and	O
distributed	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
TLA+	B-Application
is	O
considered	O
to	O
be	O
exhaustively-testable	O
pseudocode	B-Language
,	O
and	O
its	O
use	O
likened	O
to	O
drawing	B-Application
blueprints	I-Application
for	O
software	O
systems	O
;	O
TLA	O
is	O
an	O
acronym	O
for	O
Temporal	O
Logic	O
of	O
Actions	B-Application
.	O
</s>
<s>
For	O
design	O
and	O
documentation	O
,	O
TLA+	B-Application
fulfills	O
the	O
same	O
purpose	O
as	O
informal	O
technical	O
specifications	B-Application
.	O
</s>
<s>
However	O
,	O
TLA+	B-Application
specifications	B-Application
are	O
written	O
in	O
a	O
formal	O
language	O
of	O
logic	O
and	O
mathematics	O
,	O
and	O
the	O
precision	O
of	O
specifications	B-Application
written	O
in	O
this	O
language	O
is	O
intended	O
to	O
uncover	O
design	O
flaws	O
before	O
system	O
implementation	O
is	O
underway	O
.	O
</s>
<s>
Since	O
TLA+	B-Application
specifications	B-Application
are	O
written	O
in	O
a	O
formal	O
language	O
,	O
they	O
are	O
amenable	O
to	O
finite	O
model	B-Application
checking	I-Application
.	O
</s>
<s>
The	O
model	B-Application
checker	I-Application
finds	O
all	O
possible	O
system	O
behaviours	O
up	O
to	O
some	O
number	O
of	O
execution	O
steps	O
,	O
and	O
examines	O
them	O
for	O
violations	O
of	O
desired	O
invariance	B-Application
properties	I-Application
such	O
as	O
safety	B-Application
and	O
liveness	B-Application
.	O
</s>
<s>
TLA+	B-Application
specifications	B-Application
use	O
basic	O
set	O
theory	O
to	O
define	O
safety	B-Application
(	O
bad	O
things	O
wo	O
n't	O
happen	O
)	O
and	O
temporal	O
logic	O
to	O
define	O
liveness	B-Application
(	O
good	O
things	O
eventually	O
happen	O
)	O
.	O
</s>
<s>
TLA+	B-Application
is	O
also	O
used	O
to	O
write	O
machine-checked	B-Application
proofs	I-Application
of	I-Application
correctness	I-Application
both	O
for	O
algorithms	O
and	O
mathematical	O
theorems	O
.	O
</s>
<s>
Both	O
formal	O
and	O
informal	O
structured	O
mathematical	O
proofs	O
can	O
be	O
written	O
in	O
TLA+	B-Application
;	O
the	O
language	O
is	O
similar	O
to	O
LaTeX	B-Application
,	O
and	O
tools	O
exist	O
to	O
translate	O
TLA+	B-Application
specifications	B-Application
to	O
LaTeX	B-Application
documents	O
.	O
</s>
<s>
TLA+	B-Application
was	O
introduced	O
in	O
1999	O
,	O
following	O
several	O
decades	O
of	O
research	O
into	O
a	O
verification	O
method	O
for	O
concurrent	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Ever	O
since	O
,	O
a	O
toolchain	O
has	O
been	O
developed	O
,	O
including	O
an	O
IDE	B-Application
and	O
a	O
distributed	O
model	B-Application
checker	I-Application
.	O
</s>
<s>
The	O
pseudocode-like	O
language	O
PlusCal	B-Application
was	O
created	O
in	O
2009	O
;	O
it	O
transpiles	B-Language
to	O
TLA+	B-Application
and	O
is	O
useful	O
for	O
specifying	O
sequential	O
algorithms	O
.	O
</s>
<s>
The	O
current	O
TLA+	B-Application
reference	O
is	O
by	O
Leslie	O
Lamport	O
.	O
</s>
<s>
Although	O
Amir	O
Pnueli	O
was	O
the	O
first	O
to	O
seriously	O
study	O
the	O
applications	O
of	O
temporal	O
logic	O
to	O
computer	B-General_Concept
science	I-General_Concept
,	O
Prior	O
speculated	O
on	O
its	O
use	O
a	O
decade	O
earlier	O
in	O
1967	O
:	O
</s>
<s>
LTL	O
became	O
an	O
important	O
tool	O
for	O
analysis	O
of	O
concurrent	B-Operating_System
programs	O
,	O
easily	O
expressing	O
properties	O
such	O
as	O
mutual	B-Operating_System
exclusion	I-Operating_System
and	O
freedom	O
from	O
deadlock	B-Operating_System
.	O
</s>
<s>
Concurrent	B-Operating_System
with	O
Pnueli	O
's	O
work	O
on	O
LTL	O
,	O
academics	O
were	O
working	O
to	O
generalize	O
Hoare	O
logic	O
for	O
verification	O
of	O
multiprocess	O
programs	O
.	O
</s>
<s>
Leslie	O
Lamport	O
became	O
interested	O
in	O
the	O
problem	O
after	O
peer	O
review	O
found	O
an	O
error	O
in	O
a	O
paper	O
he	O
submitted	O
on	O
mutual	B-Operating_System
exclusion	I-Operating_System
.	O
</s>
<s>
Ed	O
Ashcroft	O
introduced	O
invariance	B-Application
in	O
his	O
1975	O
paper	O
"	O
Proving	O
Assertions	O
About	O
Parallel	B-Operating_System
Programs	I-Operating_System
"	O
,	O
which	O
Lamport	O
used	O
to	O
generalize	O
Floyd	O
's	O
method	O
in	O
his	O
1977	O
paper	O
"	O
Proving	O
Correctness	O
of	O
Multiprocess	O
Programs	O
"	O
.	O
</s>
<s>
Lamport	O
's	O
paper	O
also	O
introduced	O
safety	B-Application
and	O
liveness	B-Application
as	O
generalizations	O
of	O
partial	O
correctness	O
and	O
termination	B-Application
,	O
respectively	O
.	O
</s>
<s>
This	O
method	O
was	O
used	O
to	O
verify	O
the	O
first	O
concurrent	B-Operating_System
garbage	B-General_Concept
collection	I-General_Concept
algorithm	O
in	O
a	O
1978	O
paper	O
with	O
Edsger	O
Dijkstra	O
.	O
</s>
<s>
Lamport	O
worked	O
on	O
writing	O
temporal	O
logic	O
specifications	B-Application
during	O
his	O
time	O
at	O
SRI	O
,	O
but	O
found	O
the	O
approach	O
to	O
be	O
impractical	O
:	O
</s>
<s>
His	O
search	O
for	O
a	O
practical	O
method	O
of	O
specification	B-Application
resulted	O
in	O
the	O
1983	O
paper	O
"	O
Specifying	O
Concurrent	B-Operating_System
Programming	O
Modules	O
"	O
,	O
which	O
introduced	O
the	O
idea	O
of	O
describing	O
state	O
transitions	O
as	O
boolean-valued	O
functions	O
of	O
primed	O
and	O
unprimed	O
variables	O
.	O
</s>
<s>
Work	O
continued	O
throughout	O
the	O
1980s	O
,	O
and	O
Lamport	O
began	O
publishing	O
papers	O
on	O
the	O
temporal	O
logic	O
of	O
actions	B-Application
in	O
1990	O
;	O
however	O
,	O
it	O
was	O
not	O
formally	O
introduced	O
until	O
"	O
The	O
Temporal	O
Logic	O
of	O
Actions	B-Application
"	O
was	O
published	O
in	O
1994	O
.	O
</s>
<s>
TLA	O
enabled	O
the	O
use	O
of	O
actions	B-Application
in	O
temporal	O
formulas	O
,	O
which	O
according	O
to	O
Lamport	O
"	O
provides	O
an	O
elegant	O
way	O
to	O
formalize	O
and	O
systematize	O
all	O
the	O
reasoning	O
used	O
in	O
concurrent	B-Operating_System
system	I-Operating_System
verification.	O
"	O
</s>
<s>
TLA	O
specifications	B-Application
mostly	O
consisted	O
of	O
ordinary	O
non-temporal	O
mathematics	O
,	O
which	O
Lamport	O
found	O
less	O
cumbersome	O
than	O
a	O
purely	O
temporal	O
specification	B-Application
.	O
</s>
<s>
TLA	O
provided	O
a	O
mathematical	O
foundation	O
to	O
the	O
specification	B-Application
language	O
TLA+	B-Application
,	O
introduced	O
with	O
the	O
paper	O
"	O
Specifying	O
Concurrent	B-Operating_System
Systems	I-Operating_System
with	O
TLA+	B-Application
"	O
in	O
1999	O
.	O
</s>
<s>
Later	O
that	O
same	O
year	O
,	O
Yuan	O
Yu	O
wrote	O
the	O
TLC	O
model	B-Application
checker	I-Application
for	O
TLA+	B-Application
specifications	B-Application
;	O
TLC	O
was	O
used	O
to	O
find	O
errors	O
in	O
the	O
cache	B-General_Concept
coherence	I-General_Concept
protocol	O
for	O
a	O
Compaq	O
multiprocessor	O
.	O
</s>
<s>
Lamport	O
published	O
a	O
full	O
textbook	O
on	O
TLA+	B-Application
in	O
2002	O
,	O
titled	O
"	O
Specifying	O
Systems	O
:	O
The	O
TLA+	B-Application
Language	O
and	O
Tools	O
for	O
Software	O
Engineers	O
"	O
.	O
</s>
<s>
PlusCal	B-Application
was	O
introduced	O
in	O
2009	O
,	O
and	O
the	O
TLA+	B-Application
proof	O
system	O
(	O
TLAPS	O
)	O
in	O
2012	O
.	O
</s>
<s>
Lamport	O
is	O
engaged	O
in	O
creating	O
an	O
updated	O
TLA+	B-Application
reference	O
,	O
"	O
The	O
TLA+	B-Application
Hyperbook	O
"	O
.	O
</s>
<s>
Lamport	O
is	O
also	O
creating	O
,	O
described	O
therein	O
as	O
"	O
a	O
work	O
in	O
progress	O
that	O
consists	O
of	O
the	O
beginning	O
of	O
a	O
series	O
of	O
video	O
lectures	O
to	O
teach	O
programmers	O
and	O
software	O
engineers	O
how	O
to	O
write	O
their	O
own	O
TLA+	B-Application
specifications	B-Application
"	O
.	O
</s>
<s>
TLA+	B-Application
specifications	B-Application
are	O
organized	O
into	O
modules	O
.	O
</s>
<s>
Although	O
the	O
TLA+	B-Application
standard	O
is	O
specified	O
in	O
typeset	O
mathematical	O
symbols	O
,	O
existing	O
TLA+	B-Application
tools	O
use	O
LaTeX-like	O
symbol	O
definitions	O
in	O
ASCII	B-Protocol
.	O
</s>
<s>
TLA+	B-Application
uses	O
several	O
terms	O
which	O
require	O
definition	O
:	O
</s>
<s>
TLA+	B-Application
concerns	O
itself	O
with	O
defining	O
the	O
set	O
of	O
all	O
correct	O
system	O
behaviours	O
.	O
</s>
<s>
The	O
safety	B-Application
properties	I-Application
of	O
the	O
one-bit	O
clock	O
–	O
the	O
set	O
of	O
reachable	O
system	O
states	O
–	O
are	O
adequately	O
described	O
by	O
the	O
spec	O
.	O
</s>
<s>
One	O
solution	O
is	O
to	O
disable	O
stuttering	O
,	O
but	O
TLA+	B-Application
requires	O
stuttering	O
always	O
be	O
enabled	O
;	O
a	O
stuttering	O
step	O
represents	O
a	O
change	O
to	O
some	O
part	O
of	O
the	O
system	O
not	O
described	O
in	O
the	O
spec	O
,	O
and	O
is	O
useful	O
for	O
refinement	O
.	O
</s>
<s>
Weak	O
fairness	O
over	O
an	O
action	B-Application
means	O
if	O
that	O
action	B-Application
is	O
continuously	O
enabled	O
,	O
it	O
must	O
eventually	O
be	O
taken	O
.	O
</s>
<s>
This	O
temporal	O
logical	O
statement	O
about	O
Tick	O
is	O
called	O
a	O
liveness	B-Application
assertion	O
.	O
</s>
<s>
In	O
general	O
,	O
a	O
liveness	B-Application
assertion	O
should	O
be	O
machine-closed	O
:	O
it	O
should	O
n't	O
constrain	O
the	O
set	O
of	O
reachable	O
states	O
,	O
only	O
the	O
set	O
of	O
possible	O
behaviours	O
.	O
</s>
<s>
Most	O
specifications	B-Application
do	O
not	O
require	O
assertion	O
of	O
liveness	B-Application
properties	I-Application
.	O
</s>
<s>
Safety	B-Application
properties	I-Application
suffice	O
both	O
for	O
model	B-Application
checking	I-Application
and	O
guidance	O
in	O
system	O
implementation	O
.	O
</s>
<s>
TLA+	B-Application
is	O
based	O
on	O
ZF	O
,	O
so	O
operations	O
on	O
variables	O
involve	O
set	O
manipulation	O
.	O
</s>
<s>
The	O
language	O
includes	O
set	O
membership	O
,	O
union	O
,	O
intersection	O
,	O
difference	O
,	O
powerset	B-General_Concept
,	O
and	O
subset	O
operators	O
.	O
</s>
<s>
First-order	O
logic	O
operators	O
such	O
as	O
,	O
,	O
,	O
,	O
,	O
are	O
also	O
included	O
,	O
as	O
well	O
as	O
universal	O
and	O
existential	B-Algorithm
quantifiers	I-Algorithm
and	O
.	O
</s>
<s>
Temporal	O
logic	O
operators	O
are	O
built	O
into	O
TLA+	B-Application
.	O
</s>
<s>
Weak	O
fairness	O
WFe(A )	O
means	O
if	O
action	B-Application
A	O
is	O
enabled	O
continuously	O
(	O
i.e.	O
</s>
<s>
Strong	O
fairness	O
SFe(A )	O
means	O
if	O
action	B-Application
A	O
is	O
enabled	O
continually	O
(	O
repeatedly	O
,	O
with	O
or	O
without	O
interruptions	O
)	O
,	O
it	O
must	O
eventually	O
be	O
taken	O
.	O
</s>
<s>
Temporal	O
existential	B-Algorithm
and	O
universal	O
quantification	O
are	O
included	O
in	O
TLA+	B-Application
,	O
although	O
without	O
support	O
from	O
the	O
tools	O
.	O
</s>
<s>
Operators	O
differ	O
from	O
functions	O
in	O
that	O
their	O
domain	B-Algorithm
need	O
not	O
be	O
a	O
set	O
:	O
for	O
example	O
,	O
the	O
set	O
membership	O
operator	O
has	O
the	O
category	O
of	O
sets	O
as	O
its	O
domain	B-Algorithm
,	O
which	O
is	O
not	O
a	O
valid	O
set	O
in	O
ZFC	O
(	O
since	O
its	O
existence	O
leads	O
to	O
Russell	O
's	O
paradox	O
)	O
.	O
</s>
<s>
The	O
foundational	O
data	O
structure	O
of	O
TLA+	B-Application
is	O
the	O
set	O
.	O
</s>
<s>
Functions	O
in	O
TLA+	B-Application
assign	O
a	O
value	O
to	O
each	O
element	O
in	O
their	O
domain	B-Algorithm
,	O
a	O
set	O
.	O
</s>
<s>
[	O
S	O
->	O
T ]	O
is	O
the	O
set	O
of	O
all	O
functions	O
with	O
f[x]	O
in	O
T	O
,	O
for	O
each	O
x	O
in	O
the	O
domain	B-Algorithm
set	O
S	O
.	O
For	O
example	O
,	O
the	O
TLA+	B-Application
function	O
Double[ x	O
\in	O
Nat ]	O
==	O
x*2	O
is	O
an	O
element	O
of	O
the	O
set	O
[	O
Nat	O
->	O
Nat ]	O
so	O
Double	O
\in	O
[	O
Nat	O
->	O
Nat ]	O
is	O
a	O
true	O
statement	O
in	O
TLA+	B-Application
.	O
</s>
<s>
Records	O
are	O
a	O
type	O
of	O
function	O
in	O
TLA+	B-Application
.	O
</s>
<s>
Tuples	B-Application
are	O
included	O
in	O
TLA+	B-Application
.	O
</s>
<s>
Sets	O
of	O
tuples	B-Application
are	O
defined	O
by	O
Cartesian	O
product	O
;	O
for	O
example	O
,	O
the	O
set	O
of	O
all	O
pairs	O
of	O
natural	O
numbers	O
is	O
defined	O
Nat	O
\X	O
Nat	O
.	O
</s>
<s>
TLA+	B-Application
has	O
a	O
set	O
of	O
standard	O
modules	O
containing	O
common	O
operators	O
.	O
</s>
<s>
The	O
TLC	O
model	B-Application
checker	I-Application
uses	O
Java	B-Language
implementations	O
for	O
improved	O
performance	O
.	O
</s>
<s>
Sequences	O
:	O
Defines	O
operators	O
on	O
tuples	B-Application
such	O
as	O
Len(S )	O
,	O
Head(S )	O
,	O
Tail(S )	O
,	O
Append(S, E )	O
,	O
concatenation	O
,	O
and	O
filter	B-Application
.	O
</s>
<s>
Bags	O
:	O
Module	O
for	O
working	O
with	O
multisets	B-Language
.	O
</s>
<s>
Reals	O
:	O
Defines	O
the	O
Real	O
numbers	O
along	O
with	O
division	O
and	O
infinity	B-Application
.	O
</s>
<s>
RealTime	O
:	O
Provides	O
definitions	O
useful	O
in	O
real-time	B-General_Concept
system	I-General_Concept
specifications	B-Application
.	O
</s>
<s>
TLC	O
:	O
Provides	O
utility	O
functions	O
for	O
model-checked	O
specifications	B-Application
,	O
such	O
as	O
logging	O
and	O
assertions	O
.	O
</s>
<s>
An	O
integrated	B-Application
development	I-Application
environment	I-Application
is	O
implemented	O
on	O
top	O
of	O
Eclipse	B-Application
.	O
</s>
<s>
It	O
includes	O
an	O
editor	O
with	O
error	O
and	O
syntax	O
highlighting	O
,	O
plus	O
a	O
GUI	B-Application
front-end	O
to	O
several	O
other	O
TLA+	B-Application
tools	O
:	O
</s>
<s>
The	O
LaTeX	B-Application
translator	O
,	O
to	O
generate	O
pretty-printed	B-Application
specs	O
.	O
</s>
<s>
The	O
PlusCal	B-Application
translator	O
.	O
</s>
<s>
The	O
TLC	O
model	B-Application
checker	I-Application
.	O
</s>
<s>
The	O
IDE	B-Application
is	O
distributed	O
in	O
.	O
</s>
<s>
The	O
TLC	O
model	B-Application
checker	I-Application
builds	O
a	O
finite	B-Architecture
state	I-Architecture
model	O
of	O
TLA+	B-Application
specifications	B-Application
for	O
checking	O
invariance	B-Application
properties	I-Application
.	O
</s>
<s>
TLC	O
generates	O
a	O
set	O
of	O
initial	O
states	O
satisfying	O
the	O
spec	O
,	O
then	O
performs	O
a	O
breadth-first	B-Algorithm
search	I-Algorithm
over	O
all	O
defined	O
state	O
transitions	O
.	O
</s>
<s>
If	O
TLC	O
discovers	O
a	O
state	O
which	O
violates	O
a	O
system	O
invariant	B-Application
,	O
it	O
halts	O
and	O
provides	O
a	O
state	O
trace	O
path	O
to	O
the	O
offending	O
state	O
.	O
</s>
<s>
It	O
also	O
parallelizes	B-Operating_System
the	O
state	O
exploration	O
step	O
,	O
and	O
can	O
run	O
in	O
distributed	O
mode	O
to	O
spread	O
the	O
workload	O
across	O
a	O
large	O
number	O
of	O
computers	O
.	O
</s>
<s>
As	O
an	O
alternative	O
to	O
exhaustive	O
breadth-first	B-Algorithm
search	I-Algorithm
,	O
TLC	O
can	O
use	O
depth-first	O
search	O
or	O
generate	O
random	O
behaviours	O
.	O
</s>
<s>
TLC	O
operates	O
on	O
a	O
subset	O
of	O
TLA+	B-Application
;	O
the	O
model	O
must	O
be	O
finite	O
and	O
enumerable	O
,	O
and	O
some	O
temporal	O
operators	O
are	O
not	O
supported	O
.	O
</s>
<s>
In	O
distributed	O
mode	O
TLC	O
cannot	O
check	O
liveness	B-Application
properties	I-Application
,	O
nor	O
check	O
random	O
or	O
depth-first	O
behaviours	O
.	O
</s>
<s>
The	O
TLA+	B-Application
Proof	O
System	O
,	O
or	O
TLAPS	O
,	O
mechanically	B-Application
checks	I-Application
proofs	O
written	O
in	O
TLA+	B-Application
.	O
</s>
<s>
It	O
was	O
developed	O
at	O
the	O
Microsoft	O
Research-INRIA	O
Joint	O
Centre	O
to	O
prove	O
correctness	O
of	O
concurrent	B-Operating_System
and	O
distributed	O
algorithms	O
.	O
</s>
<s>
The	O
primary	O
back-end	O
provers	O
are	O
Isabelle	B-Application
and	O
Zenon	O
,	O
with	O
fallback	O
to	O
SMT	B-Application
solvers	I-Application
CVC3	O
,	O
Yices	O
,	O
and	O
Z3	B-Language
.	O
</s>
<s>
TLAPS	O
works	O
well	O
with	O
TLC	O
,	O
as	O
the	O
model	B-Application
checker	I-Application
quickly	O
finds	O
small	O
errors	O
before	O
verification	O
is	O
begun	O
.	O
</s>
<s>
In	O
turn	O
,	O
TLAPS	O
can	O
prove	O
system	O
properties	O
which	O
are	O
beyond	O
the	O
capabilities	O
of	O
finite	O
model	B-Application
checking	I-Application
.	O
</s>
<s>
Isabelle	B-Application
and	O
Zenon	O
generally	O
cannot	O
prove	O
arithmetic	O
proof	O
obligations	O
,	O
requiring	O
use	O
of	O
the	O
SMT	B-Application
solvers	I-Application
.	O
</s>
<s>
It	O
is	O
distributed	O
separately	O
from	O
the	O
rest	O
of	O
the	O
TLA+	B-Application
tools	O
and	O
is	O
free	O
software	O
,	O
distributed	O
under	O
the	O
BSD	B-Operating_System
license	I-Operating_System
.	O
</s>
<s>
At	O
Microsoft	O
,	O
a	O
critical	O
bug	O
was	O
discovered	O
in	O
the	B-Operating_System
Xbox	I-Operating_System
360	I-Operating_System
memory	O
module	O
during	O
the	O
process	O
of	O
writing	O
a	O
specification	B-Application
in	O
TLA+	B-Application
.	O
</s>
<s>
TLA+	B-Application
was	O
used	O
to	O
write	O
formal	O
proofs	O
of	O
correctness	O
for	O
Byzantine	O
Paxos	O
and	O
components	O
of	O
the	O
Pastry	O
distributed	O
hash	O
table	O
.	O
</s>
<s>
Amazon	B-Application
Web	I-Application
Services	I-Application
has	O
used	O
TLA+	B-Application
since	O
2011	O
.	O
</s>
<s>
TLA+	B-Application
model	B-Application
checking	I-Application
uncovered	O
bugs	O
in	O
DynamoDB	B-Application
,	O
S3	B-General_Concept
,	O
EBS	O
,	O
and	O
an	O
internal	O
distributed	O
lock	O
manager	O
;	O
some	O
bugs	O
required	O
state	O
traces	O
of	O
35	O
steps	O
.	O
</s>
<s>
Model	B-Application
checking	I-Application
was	O
also	O
used	O
to	O
verify	O
aggressive	O
optimizations	O
.	O
</s>
<s>
In	O
addition	O
,	O
TLA+	B-Application
specifications	B-Application
were	O
found	O
to	O
hold	O
value	O
as	O
documentation	O
and	O
design	O
aids	O
.	O
</s>
<s>
Microsoft	B-Application
Azure	I-Application
used	O
TLA+	B-Application
to	O
design	O
Cosmos	B-Application
DB	I-Application
,	O
a	O
globally-distributed	O
database	O
with	O
five	O
different	O
consistency	B-General_Concept
models	I-General_Concept
.	O
</s>
