<s>
The	O
actor	B-Application
model	I-Application
in	O
computer	B-General_Concept
science	I-General_Concept
is	O
a	O
mathematical	O
model	O
of	O
concurrent	B-Architecture
computation	I-Architecture
that	O
treats	O
an	O
actor	O
as	O
the	O
basic	O
building	O
block	O
of	O
concurrent	B-Architecture
computation	I-Architecture
.	O
</s>
<s>
Actors	O
may	O
modify	O
their	O
own	O
private	B-Application
state	I-Application
,	O
but	O
can	O
only	O
affect	O
each	O
other	O
indirectly	O
through	O
messaging	O
(	O
removing	O
the	O
need	O
for	O
lock-based	B-Operating_System
synchronization	I-Operating_System
)	O
.	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
originated	O
in	O
1973	O
.	O
</s>
<s>
It	O
has	O
been	O
used	O
both	O
as	O
a	O
framework	O
for	O
a	O
theoretical	O
understanding	O
of	O
computation	B-Operating_System
and	O
as	O
the	O
theoretical	O
basis	O
for	O
several	O
practical	B-Application
implementations	I-Application
of	O
concurrent	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
The	O
relationship	O
of	O
the	O
model	O
to	O
other	O
work	O
is	O
discussed	O
in	O
actor	B-Application
model	I-Application
and	I-Application
process	I-Application
calculi	I-Application
.	O
</s>
<s>
According	O
to	O
Carl	O
Hewitt	O
,	O
unlike	O
previous	O
models	O
of	O
computation	B-Operating_System
,	O
the	O
actor	B-Application
model	I-Application
was	O
inspired	O
by	O
physics	O
,	O
including	O
general	O
relativity	O
and	O
quantum	O
mechanics	O
.	O
</s>
<s>
It	O
was	O
also	O
influenced	O
by	O
the	O
programming	O
languages	O
Lisp	B-Language
,	O
Simula	B-Language
,	O
early	O
versions	O
of	O
Smalltalk	B-Application
,	O
capability-based	O
systems	O
,	O
and	O
packet	B-Protocol
switching	I-Protocol
.	O
</s>
<s>
Since	O
that	O
time	O
,	O
the	O
advent	O
of	O
massive	O
concurrency	B-Operating_System
through	O
multi-core	B-Architecture
and	O
manycore	B-General_Concept
computer	O
architectures	O
has	O
revived	O
interest	O
in	O
the	O
actor	B-Application
model	I-Application
.	O
</s>
<s>
Following	O
Hewitt	O
,	O
Bishop	O
,	O
and	O
Steiger	O
's	O
1973	O
publication	O
,	O
Irene	O
Greif	O
developed	O
an	O
operational	O
semantics	O
for	O
the	O
actor	B-Application
model	I-Application
as	O
part	O
of	O
her	O
doctoral	O
research	O
.	O
</s>
<s>
Two	O
years	O
later	O
,	O
Henry	O
Baker	O
and	O
Hewitt	O
published	O
a	O
set	O
of	O
axiomatic	O
laws	O
for	O
actor	B-Application
systems	I-Application
.	O
</s>
<s>
Other	O
major	O
milestones	O
include	O
William	O
Clinger	O
's	O
1981	O
dissertation	O
introducing	O
a	O
denotational	B-Application
semantics	I-Application
based	O
on	O
power	O
domains	O
and	O
Gul	O
Agha	O
's	O
1985	O
dissertation	O
which	O
further	O
developed	O
a	O
transition-based	O
semantic	O
model	O
complementary	O
to	O
Clinger	O
's	O
.	O
</s>
<s>
This	O
resulted	O
in	O
the	O
full	O
development	O
of	O
actor	B-Application
model	I-Application
theory	O
.	O
</s>
<s>
Major	O
software	O
implementation	O
work	O
was	O
done	O
by	O
Russ	O
Atkinson	O
,	O
Giuseppe	O
Attardi	O
,	O
Henry	O
Baker	O
,	O
Gerry	O
Barber	O
,	O
Peter	O
Bishop	O
,	O
Peter	O
de	O
Jong	O
,	O
Ken	O
Kahn	O
,	O
Henry	O
Lieberman	O
,	O
Carl	O
Manning	O
,	O
Tom	O
Reinhardt	O
,	O
Richard	O
Steiger	O
and	O
Dan	O
Theriault	O
in	O
the	O
Message	B-Architecture
Passing	I-Architecture
Semantics	O
Group	O
at	O
Massachusetts	O
Institute	O
of	O
Technology	O
(	O
MIT	B-License
)	O
.	O
</s>
<s>
Research	O
groups	O
led	O
by	O
Chuck	O
Seitz	O
at	O
California	O
Institute	O
of	O
Technology	O
(	O
Caltech	O
)	O
and	O
Bill	O
Dally	O
at	O
MIT	B-License
constructed	O
computer	O
architectures	O
that	O
further	O
developed	O
the	O
message	B-Architecture
passing	I-Architecture
in	O
the	O
model	O
.	O
</s>
<s>
See	O
Actor	B-Application
model	I-Application
implementation	I-Application
.	O
</s>
<s>
Research	O
on	O
the	O
actor	B-Application
model	I-Application
has	O
been	O
carried	O
out	O
at	O
California	O
Institute	O
of	O
Technology	O
,	O
Kyoto	O
University	O
Tokoro	O
Laboratory	O
,	O
Microelectronics	O
and	O
Computer	O
Technology	O
Corporation	O
(	O
MCC	O
)	O
,	O
MIT	B-License
Artificial	O
Intelligence	O
Laboratory	O
,	O
SRI	O
,	O
Stanford	O
University	O
,	O
University	O
of	O
Illinois	O
at	O
Urbana	O
–	O
Champaign	O
,	O
Pierre	O
and	O
Marie	O
Curie	O
University	O
(	O
University	O
of	O
Paris	O
6	O
)	O
,	O
University	O
of	O
Pisa	O
,	O
University	O
of	O
Tokyo	O
Yonezawa	O
Laboratory	O
,	O
Centrum	O
Wiskunde	O
&	O
Informatica	O
(	O
CWI	O
)	O
and	O
elsewhere	O
.	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
adopts	O
the	O
philosophy	O
that	O
everything	O
is	O
an	O
actor	O
.	O
</s>
<s>
This	O
is	O
similar	O
to	O
the	O
everything	O
is	O
an	O
object	O
philosophy	O
used	O
by	O
some	O
object-oriented	B-Language
programming	I-Language
languages	I-Language
.	O
</s>
<s>
Decoupling	O
the	O
sender	O
from	O
communications	O
sent	O
was	O
a	O
fundamental	O
advance	O
of	O
the	O
actor	B-Application
model	I-Application
enabling	O
asynchronous	O
communication	O
and	O
control	O
structures	O
as	O
patterns	O
of	O
passing	B-Architecture
messages	I-Architecture
.	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
is	O
characterized	O
by	O
inherent	O
concurrency	B-Operating_System
of	O
computation	B-Operating_System
within	O
and	O
among	O
actors	O
,	O
dynamic	O
creation	O
of	O
actors	O
,	O
inclusion	O
of	O
actor	O
addresses	O
in	O
messages	O
,	O
and	O
interaction	O
only	O
through	O
direct	O
asynchronous	O
message	B-Architecture
passing	I-Architecture
with	O
no	O
restriction	O
on	O
message	O
arrival	O
order	O
.	O
</s>
<s>
Over	O
the	O
years	O
,	O
several	O
different	O
formal	O
systems	O
have	O
been	O
developed	O
which	O
permit	O
reasoning	O
about	O
systems	O
in	O
the	O
actor	B-Application
model	I-Application
.	O
</s>
<s>
There	O
are	O
also	O
formalisms	O
that	O
are	O
not	O
fully	O
faithful	O
to	O
the	O
actor	B-Application
model	I-Application
in	O
that	O
they	O
do	O
not	O
formalize	O
the	O
guaranteed	O
delivery	O
of	O
messages	O
including	O
the	O
following	O
(	O
See	O
Attempts	O
to	O
relate	O
actor	O
semantics	O
to	O
algebra	O
and	O
linear	O
logic	O
)	O
:	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
can	O
be	O
used	O
as	O
a	O
framework	O
for	O
modeling	O
,	O
understanding	O
,	O
and	O
reasoning	O
about	O
a	O
wide	O
range	O
of	O
concurrent	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Electronic	O
mail	O
(	O
email	O
)	O
can	O
be	O
modeled	O
as	O
an	O
actor	B-Application
system	I-Application
.	O
</s>
<s>
Web	O
services	O
can	O
be	O
modeled	O
with	O
Simple	O
Object	O
Access	O
Protocol	O
(	O
SOAP	B-Application
)	O
endpoints	O
modeled	O
as	O
actor	O
addresses	O
.	O
</s>
<s>
Objects	O
with	O
locks	O
(	O
e.g.	O
,	O
as	O
in	O
Java	B-Language
and	O
C#	B-Application
)	O
can	O
be	O
modeled	O
as	O
a	O
serializer	B-Application
,	O
provided	O
that	O
their	O
implementations	O
are	O
such	O
that	O
messages	O
can	O
continually	O
arrive	O
(	O
perhaps	O
by	O
being	O
stored	O
in	O
an	O
internal	O
queue	B-Application
)	O
.	O
</s>
<s>
A	O
serializer	B-Application
is	O
an	O
important	O
kind	O
of	O
actor	O
defined	O
by	O
the	O
property	O
that	O
it	O
is	O
continually	O
available	O
to	O
the	O
arrival	O
of	O
new	O
messages	O
;	O
every	O
message	O
sent	O
to	O
a	O
serializer	B-Application
is	O
guaranteed	O
to	O
arrive	O
.	O
</s>
<s>
Testing	O
and	O
Test	O
Control	O
Notation	O
(	O
TTCN	O
)	O
,	O
both	O
TTCN-2	O
and	O
TTCN-3	B-Protocol
,	O
follows	O
actor	B-Application
model	I-Application
rather	O
closely	O
.	O
</s>
<s>
Built-in	O
language	O
constructs	O
allow	O
the	O
definition	O
of	O
actions	O
to	O
be	O
taken	O
when	O
an	O
expected	O
message	O
is	O
received	O
from	O
the	O
internal	O
message	O
queue	B-Application
,	O
like	O
sending	O
a	O
message	O
to	O
another	O
peer	O
entity	O
or	O
creating	O
new	O
test	O
components	O
.	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
is	O
about	O
the	O
semantics	O
of	O
message	B-Architecture
passing	I-Architecture
.	O
</s>
<s>
Arguably	O
,	O
the	O
first	O
concurrent	B-Architecture
programs	I-Architecture
were	O
interrupt	B-General_Concept
handlers	I-General_Concept
.	O
</s>
<s>
During	O
the	O
course	O
of	O
its	O
normal	O
operation	O
a	O
computer	O
needed	O
to	O
be	O
able	O
to	O
receive	O
information	O
from	O
outside	O
(	O
characters	O
from	O
a	O
keyboard	O
,	O
packets	B-Protocol
from	O
a	O
network	O
,	O
etc	O
)	O
.	O
</s>
<s>
So	O
when	O
the	O
information	O
arrived	O
the	O
execution	O
of	O
the	O
computer	O
was	O
interrupted	O
and	O
special	O
code	O
(	O
called	O
an	O
interrupt	B-General_Concept
handler	I-General_Concept
)	O
was	O
called	O
to	O
put	O
the	O
information	O
in	O
a	O
data	B-General_Concept
buffer	I-General_Concept
where	O
it	O
could	O
be	O
subsequently	O
retrieved	O
.	O
</s>
<s>
Having	O
concurrency	B-Operating_System
with	O
shared	B-Operating_System
memory	I-Operating_System
gave	O
rise	O
to	O
the	O
problem	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
.	O
</s>
<s>
Originally	O
,	O
this	O
problem	O
was	O
conceived	O
as	O
being	O
one	O
of	O
mutual	B-Operating_System
exclusion	I-Operating_System
on	O
a	O
single	O
computer	O
.	O
</s>
<s>
Edsger	O
Dijkstra	O
developed	O
semaphores	B-Operating_System
and	O
later	O
,	O
between	O
1971	O
and	O
1973	O
,	O
Tony	O
Hoare	O
and	O
Per	O
Brinch	O
Hansen	O
developed	O
monitors	O
to	O
solve	O
the	O
mutual	B-Operating_System
exclusion	I-Operating_System
problem	O
.	O
</s>
<s>
This	O
encapsulation	B-Application
was	O
later	O
accomplished	O
by	O
the	O
serializer	B-Application
construct	O
( [	O
Hewitt	O
and	O
Atkinson	O
1977	O
,	O
1979 ]	O
and	O
[	O
Atkinson	O
1980 ]	O
)	O
.	O
</s>
<s>
The	O
first	O
models	O
of	O
computation	B-Operating_System
(	O
e.g.	O
,	O
Turing	B-Architecture
machines	I-Architecture
,	O
Post	O
productions	O
,	O
the	O
lambda	B-Language
calculus	I-Language
,	O
etc	O
.	O
)	O
</s>
<s>
Each	O
computational	O
step	O
was	O
from	O
one	O
global	O
state	O
of	O
the	O
computation	B-Operating_System
to	O
the	O
next	O
global	O
state	O
.	O
</s>
<s>
The	O
global	O
state	O
approach	O
was	O
continued	O
in	O
automata	B-Application
theory	I-Application
for	O
finite-state	B-Architecture
machines	I-Architecture
and	O
push	O
down	O
stack	B-Application
machines	I-Application
,	O
including	O
their	O
nondeterministic	B-General_Concept
versions	O
.	O
</s>
<s>
Such	O
nondeterministic	B-General_Concept
automata	O
have	O
the	O
property	O
of	O
bounded	B-Application
nondeterminism	I-Application
;	O
that	O
is	O
,	O
if	O
a	O
machine	O
always	O
halts	O
when	O
started	O
in	O
its	O
initial	O
state	O
,	O
then	O
there	O
is	O
a	O
bound	O
on	O
the	O
number	O
of	O
states	O
in	O
which	O
it	O
halts	O
.	O
</s>
<s>
Edsger	O
Dijkstra	O
further	O
developed	O
the	O
nondeterministic	B-General_Concept
global	O
state	O
approach	O
.	O
</s>
<s>
Dijkstra	O
's	O
model	O
gave	O
rise	O
to	O
a	O
controversy	O
concerning	O
unbounded	B-Application
nondeterminism	I-Application
(	O
also	O
called	O
unbounded	B-Application
indeterminacy	I-Application
)	O
,	O
a	O
property	O
of	O
concurrency	B-Operating_System
by	O
which	O
the	O
amount	O
of	O
delay	O
in	O
servicing	O
a	O
request	O
can	O
become	O
unbounded	O
as	O
a	O
result	O
of	O
arbitration	O
of	O
contention	O
for	O
shared	O
resources	O
while	O
still	O
guaranteeing	O
that	O
the	O
request	O
will	O
eventually	O
be	O
serviced	O
.	O
</s>
<s>
Hewitt	O
argued	O
that	O
the	O
actor	B-Application
model	I-Application
should	O
provide	O
the	O
guarantee	O
of	O
service	O
.	O
</s>
<s>
Dijkstra	O
argued	O
that	O
it	O
was	O
impossible	O
to	O
implement	O
unbounded	B-Application
nondeterminism	I-Application
.	O
</s>
<s>
Arbiters	O
are	O
used	O
in	O
computers	O
to	O
deal	O
with	O
the	O
circumstance	O
that	O
computer	O
clocks	O
operate	O
asynchronously	O
with	O
respect	O
to	O
input	O
from	O
outside	O
,	O
e.g.	O
,	O
keyboard	O
input	O
,	O
disk	O
access	O
,	O
network	O
input	O
,	O
etc	O
.	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
features	O
unbounded	B-Application
nondeterminism	I-Application
which	O
was	O
captured	O
in	O
a	O
mathematical	O
model	O
by	O
Will	O
Clinger	O
using	O
domain	O
theory	O
.	O
</s>
<s>
In	O
the	O
actor	B-Application
model	I-Application
,	O
there	O
is	O
no	O
global	O
state	O
.	O
</s>
<s>
Messages	O
in	O
the	O
actor	B-Application
model	I-Application
are	O
not	O
necessarily	O
buffered	O
.	O
</s>
<s>
This	O
was	O
a	O
sharp	O
break	O
with	O
previous	O
approaches	O
to	O
models	O
of	O
concurrent	B-Architecture
computation	I-Architecture
.	O
</s>
<s>
The	O
lack	O
of	O
buffering	O
caused	O
a	O
great	O
deal	O
of	O
misunderstanding	O
at	O
the	O
time	O
of	O
the	O
development	O
of	O
the	O
actor	B-Application
model	I-Application
and	O
is	O
still	O
a	O
controversial	O
issue	O
.	O
</s>
<s>
Also	O
,	O
messages	O
in	O
the	O
actor	B-Application
model	I-Application
are	O
simply	O
sent	O
(	O
like	O
packets	B-Protocol
in	O
IP	B-Protocol
)	O
;	O
there	O
is	O
no	O
requirement	O
for	O
a	O
synchronous	O
handshake	O
with	O
the	O
recipient	O
.	O
</s>
<s>
A	O
natural	O
development	O
of	O
the	O
actor	B-Application
model	I-Application
was	O
to	O
allow	O
addresses	O
in	O
messages	O
.	O
</s>
<s>
Influenced	O
by	O
packet	B-Protocol
switched	I-Protocol
networks	I-Protocol
[	O
1961	O
and	O
1964 ]	O
,	O
Hewitt	O
proposed	O
the	O
development	O
of	O
a	O
new	O
model	O
of	O
concurrent	B-Architecture
computation	I-Architecture
in	O
which	O
communications	O
would	O
not	O
have	O
any	O
required	O
fields	O
at	O
all	O
:	O
they	O
could	O
be	O
empty	O
.	O
</s>
<s>
For	O
example	O
,	O
an	O
actor	O
might	O
need	O
to	O
send	O
a	O
message	O
to	O
a	O
recipient	O
actor	O
from	O
which	O
it	O
later	O
expects	O
to	O
receive	O
a	O
response	O
,	O
but	O
the	O
response	O
will	O
actually	O
be	O
handled	O
by	O
a	O
third	O
actor	O
component	O
that	O
has	O
been	O
configured	O
to	O
receive	O
and	O
handle	O
the	O
response	O
(	O
for	O
example	O
,	O
a	O
different	O
actor	O
implementing	O
the	O
observer	B-Language
pattern	I-Language
)	O
.	O
</s>
<s>
So	O
,	O
the	O
ability	O
of	O
actors	O
to	O
create	O
new	O
actors	O
with	O
which	O
they	O
can	O
exchange	O
communications	O
,	O
along	O
with	O
the	O
ability	O
to	O
include	O
the	O
addresses	O
of	O
other	O
actors	O
in	O
messages	O
,	O
gives	O
actors	O
the	O
ability	O
to	O
create	O
and	O
participate	O
in	O
arbitrarily	O
variable	O
topological	O
relationships	O
with	O
one	O
another	O
,	O
much	O
as	O
the	O
objects	O
in	O
Simula	B-Language
and	O
other	O
object-oriented	B-Language
languages	I-Language
may	O
also	O
be	O
relationally	O
composed	O
into	O
variable	O
topologies	O
of	O
message-exchanging	O
objects	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>
If	O
output	O
message	O
ordering	O
is	O
desired	O
,	O
then	O
it	O
can	O
be	O
modeled	O
by	O
a	O
queue	B-Application
actor	O
that	O
provides	O
this	O
functionality	O
.	O
</s>
<s>
Such	O
a	O
queue	B-Application
actor	O
would	O
queue	B-Application
the	O
messages	O
that	O
arrived	O
so	O
that	O
they	O
could	O
be	O
retrieved	O
in	O
FIFO	B-Operating_System
order	O
.	O
</s>
<s>
In	O
this	O
respect	O
the	O
actor	B-Application
model	I-Application
mirrors	O
packet	B-Protocol
switching	I-Protocol
systems	O
which	O
do	O
not	O
guarantee	O
that	O
packets	B-Protocol
must	O
be	O
received	O
in	O
the	O
order	O
sent	O
.	O
</s>
<s>
Not	O
providing	O
the	O
order	O
of	O
delivery	O
guarantee	O
allows	O
packet	B-Protocol
switching	I-Protocol
to	O
buffer	B-General_Concept
packets	B-Protocol
,	O
use	O
multiple	O
paths	O
to	O
send	O
packets	B-Protocol
,	O
resend	O
damaged	O
packets	B-Protocol
,	O
and	O
to	O
provide	O
other	O
optimizations	O
.	O
</s>
<s>
Another	O
important	O
characteristic	O
of	O
the	O
actor	B-Application
model	I-Application
is	O
locality	O
.	O
</s>
<s>
In	O
this	O
way	O
it	O
differs	O
from	O
some	O
other	O
models	O
of	O
concurrency	B-Operating_System
,	O
e.g.	O
,	O
the	O
Petri	B-Operating_System
net	I-Operating_System
model	O
in	O
which	O
tokens	O
are	O
simultaneously	O
removed	O
from	O
multiple	O
locations	O
and	O
placed	O
in	O
other	O
locations	O
.	O
</s>
<s>
The	O
idea	O
of	O
composing	O
actor	B-Application
systems	I-Application
into	O
larger	O
ones	O
is	O
an	O
important	O
aspect	O
of	O
modularity	B-Architecture
that	O
was	O
developed	O
in	O
Gul	O
Agha	O
's	O
doctoral	O
dissertation	O
,	O
developed	O
later	O
by	O
Gul	O
Agha	O
,	O
Ian	O
Mason	O
,	O
Scott	O
Smith	O
,	O
and	O
Carolyn	O
Talcott	O
.	O
</s>
<s>
Behaviors	O
provided	O
a	O
mechanism	O
to	O
mathematically	O
model	O
the	O
sharing	O
in	O
concurrency	B-Operating_System
.	O
</s>
<s>
Behaviors	O
also	O
freed	O
the	O
actor	B-Application
model	I-Application
from	O
implementation	O
details	O
,	O
e.g.	O
,	O
the	O
Smalltalk-72	B-Application
token	O
stream	O
interpreter	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
critical	O
to	O
understand	O
that	O
the	O
efficient	O
implementation	O
of	O
systems	O
described	O
by	O
the	O
actor	B-Application
model	I-Application
require	O
extensive	O
optimization	O
.	O
</s>
<s>
See	O
Actor	B-Application
model	I-Application
implementation	I-Application
for	O
details	O
.	O
</s>
<s>
Other	O
concurrency	B-Operating_System
systems	O
(	O
e.g.	O
,	O
process	O
calculi	O
)	O
can	O
be	O
modeled	O
in	O
the	O
actor	B-Application
model	I-Application
using	O
a	O
two-phase	B-General_Concept
commit	I-General_Concept
protocol	I-General_Concept
.	O
</s>
<s>
There	O
is	O
a	O
Computational	O
Representation	O
Theorem	O
in	O
the	O
actor	B-Application
model	I-Application
for	O
systems	O
which	O
are	O
closed	O
in	O
the	O
sense	O
that	O
they	O
do	O
not	O
receive	O
communications	O
from	O
outside	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
can	O
be	O
mathematically	O
characterized	O
in	O
terms	O
of	O
all	O
its	O
possible	O
behaviors	O
(	O
including	O
those	O
involving	O
unbounded	B-Application
nondeterminism	I-Application
)	O
.	O
</s>
<s>
One	O
of	O
the	O
key	O
motivations	O
for	O
the	O
development	O
of	O
the	O
actor	B-Application
model	I-Application
was	O
to	O
understand	O
and	O
deal	O
with	O
the	O
control	O
structure	O
issues	O
that	O
arose	O
in	O
development	O
of	O
the	O
Planner	B-Application
programming	I-Application
language	I-Application
.	O
</s>
<s>
Once	O
the	O
actor	B-Application
model	I-Application
was	O
initially	O
defined	O
,	O
an	O
important	O
challenge	O
was	O
to	O
understand	O
the	O
power	O
of	O
the	O
model	O
relative	O
to	O
Robert	O
Kowalski	O
's	O
thesis	O
that	O
"	O
computation	B-Operating_System
can	O
be	O
subsumed	O
by	O
deduction	O
"	O
.	O
</s>
<s>
Hewitt	O
argued	O
that	O
Kowalski	O
's	O
thesis	O
turned	O
out	O
to	O
be	O
false	O
for	O
the	O
concurrent	B-Architecture
computation	I-Architecture
in	O
the	O
actor	B-Application
model	I-Application
(	O
see	O
Indeterminacy	B-Application
in	I-Application
concurrent	I-Application
computation	I-Application
)	O
.	O
</s>
<s>
Nevertheless	O
,	O
attempts	O
were	O
made	O
to	O
extend	O
logic	B-Language
programming	I-Language
to	O
concurrent	B-Architecture
computation	I-Architecture
.	O
</s>
<s>
However	O
,	O
Hewitt	O
and	O
Agha	O
 [ 1991 ] 	O
claimed	O
that	O
the	O
resulting	O
systems	O
were	O
not	O
deductive	O
in	O
the	O
following	O
sense	O
:	O
computational	O
steps	O
of	O
the	O
concurrent	O
logic	B-Language
programming	I-Language
systems	O
do	O
not	O
follow	O
deductively	O
from	O
previous	O
steps	O
(	O
see	O
Indeterminacy	B-Application
in	I-Application
concurrent	I-Application
computation	I-Application
)	O
.	O
</s>
<s>
Recently	O
,	O
logic	B-Language
programming	I-Language
has	O
been	O
integrated	O
into	O
the	O
actor	B-Application
model	I-Application
in	O
a	O
way	O
that	O
maintains	O
logical	O
semantics	O
.	O
</s>
<s>
Migration	O
in	O
the	O
actor	B-Application
model	I-Application
is	O
the	O
ability	O
of	O
actors	O
to	O
change	O
locations	O
.	O
</s>
<s>
E.g.	O
,	O
in	O
his	O
dissertation	O
,	O
Aki	O
Yonezawa	O
modeled	O
a	O
post	O
office	O
that	O
customer	O
actors	O
could	O
enter	O
,	O
change	O
locations	O
within	O
while	O
operating	O
,	O
and	O
exit	O
.	O
</s>
<s>
computer	B-Architecture
hardware	I-Architecture
as	O
in	O
Burroughs	B-Device
B5000	I-Device
,	O
Lisp	B-Operating_System
machine	I-Operating_System
,	O
etc	O
.	O
</s>
<s>
virtual	B-Architecture
machines	I-Architecture
as	O
in	O
Java	B-Language
virtual	I-Language
machine	I-Language
,	O
Common	O
Language	O
Runtime	O
,	O
etc	O
.	O
</s>
<s>
A	O
delicate	O
point	O
in	O
the	O
actor	B-Application
model	I-Application
is	O
the	O
ability	O
to	O
synthesize	O
the	O
address	O
of	O
an	O
actor	O
.	O
</s>
<s>
SOAP	B-Application
uses	O
a	O
URL	O
for	O
the	O
address	O
of	O
an	O
endpoint	O
where	O
an	O
actor	O
can	O
be	O
reached	O
.	O
</s>
<s>
The	O
idea	O
is	O
to	O
use	O
an	O
actor	B-Application
system	I-Application
to	O
perform	O
the	O
mapping	O
to	O
the	O
actual	O
actor	O
addresses	O
.	O
</s>
<s>
For	O
example	O
,	O
on	O
a	O
computer	O
the	O
memory	O
structure	O
of	O
the	O
computer	O
can	O
be	O
modeled	O
as	O
an	O
actor	B-Application
system	I-Application
that	O
does	O
the	O
mapping	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
SOAP	B-Application
addresses	O
,	O
it	O
's	O
modeling	O
the	O
DNS	B-Protocol
and	O
the	O
rest	O
of	O
the	O
URL	O
mapping	O
.	O
</s>
<s>
Robin	O
Milner	O
's	O
initial	O
published	O
work	O
on	O
concurrency	B-Operating_System
was	O
also	O
notable	O
in	O
that	O
it	O
was	O
not	O
based	O
on	O
composing	O
sequential	O
processes	O
.	O
</s>
<s>
His	O
work	O
differed	O
from	O
the	O
actor	B-Application
model	I-Application
because	O
it	O
was	O
based	O
on	O
a	O
fixed	O
number	O
of	O
processes	O
of	O
fixed	O
topology	O
communicating	O
numbers	O
and	O
strings	O
using	O
synchronous	O
communication	O
.	O
</s>
<s>
The	O
original	O
communicating	O
sequential	O
processes	O
(	O
CSP	O
)	O
model	O
published	O
by	O
Tony	O
Hoare	O
differed	O
from	O
the	O
actor	B-Application
model	I-Application
because	O
it	O
was	O
based	O
on	O
the	O
parallel	O
composition	O
of	O
a	O
fixed	O
number	O
of	O
sequential	O
processes	O
connected	O
in	O
a	O
fixed	O
topology	O
,	O
and	O
communicating	O
using	O
synchronous	O
message-passing	B-Architecture
based	O
on	O
process	O
names	O
(	O
see	O
Actor	B-Application
model	I-Application
and	I-Application
process	I-Application
calculi	I-Application
history	O
)	O
.	O
</s>
<s>
These	O
early	O
models	O
by	O
Milner	O
and	O
Hoare	O
both	O
had	O
the	O
property	O
of	O
bounded	B-Application
nondeterminism	I-Application
.	O
</s>
<s>
Modern	O
,	O
theoretical	O
CSP	O
( [	O
Hoare	O
1985 ]	O
and	O
[	O
Roscoe	O
2005 ]	O
)	O
explicitly	O
provides	O
unbounded	B-Application
nondeterminism	I-Application
.	O
</s>
<s>
Petri	B-Operating_System
nets	I-Operating_System
and	O
their	O
extensions	O
(	O
e.g.	O
,	O
coloured	O
Petri	B-Operating_System
nets	I-Operating_System
)	O
are	O
like	O
actors	O
in	O
that	O
they	O
are	O
based	O
on	O
asynchronous	O
message	B-Architecture
passing	I-Architecture
and	O
unbounded	B-Application
nondeterminism	I-Application
,	O
while	O
they	O
are	O
like	O
early	O
CSP	O
in	O
that	O
they	O
define	O
fixed	O
topologies	O
of	O
elementary	O
processing	O
steps	O
(	O
transitions	O
)	O
and	O
message	O
repositories	O
(	O
places	O
)	O
.	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
has	O
been	O
influential	O
on	O
both	O
theory	O
development	O
and	O
practical	O
software	O
development	O
.	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
has	O
influenced	O
the	O
development	O
of	O
the	O
π-calculus	O
and	O
subsequent	O
process	O
calculi	O
.	O
</s>
<s>
Now	O
,	O
the	O
pure	O
lambda-calculus	B-Language
is	O
built	O
with	O
just	O
two	O
kinds	O
of	O
thing	O
:	O
terms	O
and	O
variables	O
.	O
</s>
<s>
The	O
actor	B-Application
model	I-Application
has	O
had	O
extensive	O
influence	O
on	O
commercial	O
practice	O
.	O
</s>
<s>
For	O
example	O
,	O
Twitter	O
has	O
used	O
actors	O
for	O
scalability	B-Architecture
.	O
</s>
<s>
Also	O
,	O
Microsoft	O
has	O
used	O
the	O
actor	B-Application
model	I-Application
in	O
the	O
development	O
of	O
its	O
Asynchronous	O
Agents	O
Library	O
.	O
</s>
<s>
According	O
to	O
Hewitt	O
[2006],	O
the	O
actor	B-Application
model	I-Application
addresses	O
issues	O
in	O
computer	O
and	O
communications	O
architecture	O
,	O
concurrent	B-Architecture
programming	I-Architecture
languages	O
,	O
and	O
Web	O
services	O
including	O
the	O
following	O
:	O
</s>
<s>
Scalability	B-Architecture
:	O
the	O
challenge	O
of	O
scaling	B-Architecture
up	O
concurrency	B-Operating_System
both	O
locally	O
and	O
nonlocally	O
.	O
</s>
<s>
Transparency	B-Protocol
:	O
bridging	O
the	O
chasm	O
between	O
local	O
and	O
nonlocal	O
concurrency	B-Operating_System
.	O
</s>
<s>
Transparency	B-Protocol
is	O
currently	O
a	O
controversial	O
issue	O
.	O
</s>
<s>
Some	O
researchers	O
have	O
advocated	O
a	O
strict	O
separation	O
between	O
local	O
concurrency	B-Operating_System
using	O
concurrent	B-Architecture
programming	I-Architecture
languages	O
(	O
e.g.	O
,	O
Java	B-Language
and	O
C#	B-Application
)	O
from	O
nonlocal	O
concurrency	B-Operating_System
using	O
SOAP	B-Application
for	O
Web	O
services	O
.	O
</s>
<s>
Strict	O
separation	O
produces	O
a	O
lack	O
of	O
transparency	B-Protocol
that	O
causes	O
problems	O
when	O
it	O
is	O
desirable/necessary	O
to	O
change	O
between	O
local	O
and	O
nonlocal	O
access	O
to	O
Web	O
services	O
(	O
see	O
Distributed	B-Architecture
computing	I-Architecture
)	O
.	O
</s>
<s>
This	O
inconsistency	O
extends	O
to	O
the	O
documentation	O
and	O
specifications	O
of	O
very	O
large	O
systems	O
(	O
e.g.	O
,	O
Microsoft	O
Windows	O
software	O
,	O
etc	O
.	O
</s>
<s>
Many	O
of	O
the	O
ideas	O
introduced	O
in	O
the	O
actor	B-Application
model	I-Application
are	O
now	O
also	O
finding	O
application	O
in	O
multi-agent	O
systems	O
for	O
these	O
same	O
reasons	O
[	O
Hewitt	O
2006b	O
2007b ]	O
.	O
</s>
<s>
A	O
number	O
of	O
different	O
programming	O
languages	O
employ	O
the	O
actor	B-Application
model	I-Application
or	O
some	O
variation	O
of	O
it	O
.	O
</s>
<s>
Name	O
Status	O
Latest	O
release	O
License	B-License
Languages	O
Xcraft	O
Goblins	O
2022-08-30	O
MIT	B-License
JavaScript	O
ReActed	O
2022-11-30	O
Apache	B-Application
2.0	I-Application
Java	B-Language
Acteur	O
2020-04-16Apache-2.0	O
/	O
MIT	B-License
RustBastion	O
2020-08-12Apache-2.0	O
/	O
MITRust	O
Actix	O
2020-09-11	O
MIT	B-License
Rust	O
Aojet	O
2016-10-17	O
MIT	B-License
Swift	B-Application
Actor	O
2017-03-09	O
MIT	B-License
Java	B-Language
Actor4j	O
2020-01-31	O
Apache	B-Application
2.0	I-Application
Java	B-Language
Actr	O
2019-04-09	O
Apache	B-Application
2.0	I-Application
Java	B-Language
Vert.x	O
2018-02-13	O
Apache	B-Application
2.0	I-Application
Java	B-Language
,	O
Groovy	O
,	O
Javascript	O
,	O
Ruby	B-Language
,	O
Scala	B-Application
,	O
Kotlin	O
,	O
Ceylon	O
ActorFx	O
2013-11-13	O
Apache	B-Application
2.0	I-Application
.NET	O
Akka	B-Language
(	O
toolkit	O
)	O
2022-09-06	O
Commercial	O
(	O
from	O
2.7.0	O
,	O
Apache	B-Application
2.0	I-Application
up	O
to	O
2.6.20	O
)	O
Java	B-Language
and	O
Scala	B-Application
Akka.NET	O
2020-08-20Akka.NET	O
v1.4.10	O
Stable	O
Release	O
Apache	B-Application
2.0	I-Application
.NET	O
Dapr	O
2019-10-16	O
Apache	B-Application
2.0	I-Application
Java	B-Language
,	O
.NET	O
Core	O
,	O
Go	O
,	O
Javascript	O
,	O
Python	B-Language
,	O
Rust	O
and	O
C++	O
DOTNETACTORS	O
2021-06-14	O
MIT	B-License
.NET	O
,	O
C#	B-Application
,	O
Azure	O
Service	O
Bus	O
Remact.Net	O
2016-06-26	O
MIT	B-License
.NET	O
,	O
Javascript	O
Ateji	O
PX	O
?	O
</s>
<s>
Java	B-Language
czmq	O
2016-11-10	O
MPL-2	B-License
C	O
F#	O
MailboxProcessor	O
same	O
as	O
F#	O
(	O
built-in	O
core	O
library	O
)	O
Apache	B-Application
License	I-Application
F#	O
Korus	O
2010-02-04	O
GPL	B-License
3	I-License
Java	B-Language
Kilim	O
2018-11-09	O
MIT	B-License
Java	B-Language
ActorFoundry	B-Application
(	O
based	O
on	O
Kilim	O
)	O
2008-12-28	O
?	O
</s>
