<s>
The	O
actor	B-Application
model	I-Application
and	I-Application
process	I-Application
calculi	I-Application
share	O
an	O
interesting	O
history	O
and	O
co-evolution	O
.	O
</s>
<s>
The	O
Actor	B-Application
model	I-Application
,	O
first	O
published	O
in	O
1973	O
,	O
is	O
a	O
mathematical	O
model	O
of	O
concurrent	B-Architecture
computation	I-Architecture
.	O
</s>
<s>
The	O
Actor	B-Application
model	I-Application
treats	O
"	O
Actors	O
"	O
as	O
the	O
universal	O
primitives	O
of	O
concurrent	O
digital	O
computation	O
:	O
in	O
response	O
to	O
a	O
message	O
that	O
it	O
receives	O
,	O
an	O
Actor	O
can	O
make	O
local	O
decisions	O
,	O
create	O
more	O
Actors	O
,	O
send	O
more	O
messages	O
,	O
and	O
determine	O
how	O
to	O
respond	O
to	O
the	O
next	O
message	O
received	O
.	O
</s>
<s>
As	O
opposed	O
to	O
the	O
previous	O
approach	O
based	O
on	O
composing	O
sequential	O
processes	O
,	O
the	O
Actor	B-Application
model	I-Application
was	O
developed	O
as	O
an	O
inherently	O
concurrent	O
model	O
.	O
</s>
<s>
In	O
the	O
Actor	B-Application
model	I-Application
sequentiality	O
was	O
a	O
special	O
case	O
that	O
derived	O
from	O
concurrent	B-Architecture
computation	I-Architecture
as	O
explained	O
in	O
Actor	B-Application
model	I-Application
theory	O
.	O
</s>
<s>
Robin	O
Milner	O
's	O
initial	O
published	O
work	O
on	O
concurrency	B-Architecture
from	O
the	O
same	O
year	O
was	O
also	O
notable	O
in	O
that	O
it	O
positions	O
mathematical	B-Application
semantics	I-Application
of	O
communicating	O
processes	O
as	O
a	O
framework	O
to	O
understand	O
a	O
variety	O
of	O
interaction	O
agents	O
including	O
the	O
computer	O
's	O
interaction	O
with	O
memory	O
.	O
</s>
<s>
His	O
work	O
differed	O
from	O
the	O
Actor	B-Application
model	I-Application
in	O
the	O
following	O
ways	O
:	O
</s>
<s>
Communication	O
is	O
synchronous	O
as	O
opposed	O
to	O
the	O
Actor	B-Application
model	I-Application
in	O
which	O
an	O
unbounded	O
time	O
can	O
elapse	O
between	O
sending	O
and	O
receiving	O
a	O
message	O
.	O
</s>
<s>
The	O
semantics	O
provided	O
bounded	O
nondeterminism	O
unlike	O
the	O
Actor	B-Application
model	I-Application
with	O
unbounded	B-Application
nondeterminism	I-Application
.	O
</s>
<s>
However	O
,	O
with	O
bounded	O
nondeterminism	O
is	O
impossible	O
for	O
a	O
server	O
to	O
guarantee	O
service	O
to	O
its	O
clients	O
,	O
i.e.	O
,	O
a	O
client	O
might	O
starve	B-Operating_System
.	O
</s>
<s>
The	O
publication	O
by	O
Tony	O
Hoare	O
in	O
1978	O
of	O
the	O
original	O
Communicating	O
Sequential	O
Processes	O
was	O
different	O
from	O
the	O
Actor	B-Application
model	I-Application
which	O
states	O
:	O
</s>
<s>
This	O
paper	O
has	O
suggested	O
that	O
input	O
,	O
output	O
,	O
and	O
concurrency	B-Architecture
should	O
be	O
regarded	O
as	O
primitives	O
of	O
programming	O
,	O
which	O
underlie	O
many	O
familiar	O
and	O
less	O
familiar	O
programming	O
concepts	O
.	O
</s>
<s>
The	O
1978	O
version	O
of	O
CSP	O
differed	O
from	O
the	O
Actor	B-Application
model	I-Application
in	O
the	O
following	O
respects	O
[	O
Clinger	O
1981 ]	O
:	O
</s>
<s>
The	O
concurrency	B-Architecture
primitives	O
of	O
CSP	O
were	O
input	O
,	O
output	O
,	O
guarded	O
commands	O
,	O
and	O
parallel	O
composition	O
whereas	O
the	O
Actor	B-Application
model	I-Application
is	O
based	O
on	O
asynchronous	O
one-way	O
messaging	O
.	O
</s>
<s>
The	O
fundamental	O
unit	O
of	O
execution	O
was	O
a	O
sequential	O
process	O
in	O
contrast	O
to	O
the	O
Actor	B-Application
model	I-Application
in	O
which	O
execution	O
was	O
fundamentally	O
concurrent	O
.	O
</s>
<s>
The	O
processes	O
were	O
hierarchically	O
structured	O
using	O
parallel	O
composition	O
whereas	O
Actors	O
allowed	O
the	O
creation	O
of	O
non-hierarchical	O
execution	O
using	O
futures	B-Operating_System
[	O
Baker	O
and	O
Hewitt	O
1977 ]	O
.	O
</s>
<s>
Also	O
message	O
passing	O
is	O
the	O
fundamental	O
mechanism	O
for	O
generating	O
parallelism	O
in	O
the	O
Actor	B-Application
model	I-Application
;	O
sending	O
more	O
messages	O
generates	O
the	O
possibility	O
of	O
more	O
parallelism	O
.	O
</s>
<s>
Communication	O
was	O
between	O
processes	O
whereas	O
in	O
the	O
Actor	B-Application
model	I-Application
communications	O
are	O
one-way	O
to	O
Actors	O
.	O
</s>
<s>
Data	O
structures	O
consisted	O
of	O
numbers	O
,	O
strings	O
,	O
and	O
arrays	O
whereas	O
in	O
the	O
Actor	B-Application
model	I-Application
data	O
structures	O
were	O
Actors	O
.	O
</s>
<s>
Messages	O
contain	O
numbers	O
and	O
strings	O
whereas	O
in	O
the	O
Actor	B-Application
model	I-Application
messages	O
could	O
include	O
the	O
addresses	O
of	O
Actors	O
.	O
</s>
<s>
The	O
model	O
of	O
CSP	O
deliberately	O
had	O
bounded	O
nondeterminism	O
[	O
Francez	O
,	O
Hoare	O
,	O
Lehmann	O
,	O
and	O
de	O
Roever	O
1979 ]	O
whereas	O
the	O
Actor	B-Application
model	I-Application
had	O
unbounded	B-Application
nondeterminism	I-Application
.	O
</s>
<s>
Dijkstra	O
 [ 1976 ] 	O
had	O
convinced	O
Hoare	O
that	O
a	O
programming	O
language	O
with	O
unbounded	B-Application
nondeterminism	I-Application
could	O
not	O
be	O
implemented	O
.	O
</s>
<s>
Will	O
Clinger	O
 [ 1981 ] 	O
developed	O
the	O
first	O
denotational	O
Actor	B-Application
model	I-Application
for	O
concurrent	B-Architecture
computation	I-Architecture
that	O
embodied	O
unbounded	B-Application
nondeterminism	I-Application
.	O
</s>
<s>
Bill	O
Kornfeld	O
and	O
Carl	O
Hewitt	O
 [ 1981 ] 	O
showed	O
that	O
the	O
Actor	B-Application
model	I-Application
could	O
encompass	O
large-scale	O
concurrency	B-Architecture
.	O
</s>
<s>
Agha	O
developed	O
Actors	O
as	O
a	O
fundamental	O
model	O
for	O
concurrent	B-Architecture
computation	I-Architecture
.	O
</s>
<s>
The	O
π-calculus	O
,	O
partially	O
inspired	O
by	O
the	O
Actor	B-Application
model	I-Application
as	O
described	O
by	O
Milner	O
above	O
,	O
introduced	O
dynamic	O
topology	O
into	O
the	O
process	O
calculi	O
by	O
allowing	O
dynamic	O
creation	O
of	O
processes	O
and	O
for	O
the	O
names	O
to	O
be	O
passed	O
among	O
different	O
processes	O
.	O
</s>
<s>
However	O
,	O
the	O
goal	O
of	O
Milner	O
and	O
Hoare	O
to	O
attain	O
an	O
algebraic	O
calculus	O
led	O
to	O
a	O
critical	O
divergence	O
from	O
the	O
Actor	B-Application
model	I-Application
:	O
communication	O
in	O
the	O
process	O
calculi	O
is	O
not	O
direct	O
as	O
in	O
the	O
Actor	B-Application
model	I-Application
but	O
rather	O
indirectly	O
through	O
channels	O
(	O
see	O
Actor	B-Application
model	I-Application
and	I-Application
process	I-Application
calculi	I-Application
)	O
.	O
</s>
<s>
In	O
contrast	O
,	O
recent	O
work	O
on	O
the	O
Actor	B-Application
model	I-Application
[	O
Hewitt	O
2006	O
,	O
2007a ]	O
has	O
emphasized	O
denotational	O
models	O
and	O
the	O
Representation	O
Theorem	O
.	O
</s>
<s>
Nevertheless	O
there	O
are	O
interesting	O
co-evolutions	O
between	O
the	O
Actor	B-Application
Model	I-Application
and	I-Application
Process	I-Application
Calculi	I-Application
.	O
</s>
<s>
Montanari	O
and	O
Talcott	O
discussed	O
whether	O
the	O
Actor	B-Application
Model	I-Application
and	O
π-calculus	O
were	O
compatible	O
with	O
each	O
other	O
.	O
</s>
<s>
Although	O
algebraic	O
laws	O
have	O
been	O
developed	O
for	O
the	O
Actor	B-Application
model	I-Application
,	O
they	O
do	O
not	O
capture	O
the	O
crucial	O
property	O
of	O
guaranteed	O
delivery	O
of	O
messages	O
sent	O
to	O
Serializers	O
.	O
</s>
