<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
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>
This	O
article	O
reports	O
on	O
the	O
later	O
history	O
of	O
the	O
Actor	B-Application
model	I-Application
in	O
which	O
major	O
themes	O
were	O
investigation	O
of	O
the	O
basic	O
power	O
of	O
the	O
model	O
,	O
study	O
of	O
issues	O
of	O
compositionality	O
,	O
development	O
of	O
architectures	O
,	O
and	O
application	O
to	O
Open	O
systems	O
.	O
</s>
<s>
It	O
is	O
the	O
follow	O
on	O
article	O
to	O
Actor	B-Application
model	I-Application
middle	I-Application
history	I-Application
which	O
reports	O
on	O
the	O
initial	O
implementations	O
,	O
initial	O
applications	O
,	O
and	O
development	O
of	O
the	O
first	O
proof	O
theory	O
and	O
denotational	O
model	O
.	O
</s>
<s>
Investigations	O
began	O
into	O
the	O
basic	O
power	O
of	O
the	O
Actor	B-Application
model	I-Application
.	O
</s>
<s>
Carl	O
Hewitt	O
 [ 1985 ] 	O
argued	O
that	O
because	O
of	O
the	O
use	O
of	O
Arbiters	O
that	O
the	O
Actor	B-Application
model	I-Application
was	O
more	O
powerful	O
than	O
logic	B-Language
programming	I-Language
(	O
see	O
indeterminacy	B-Application
in	I-Application
concurrent	I-Application
computation	I-Application
)	O
.	O
</s>
<s>
A	O
family	O
of	O
Prolog-like	O
concurrent	O
message	O
passing	O
systems	O
using	O
unification	O
of	O
shared	O
variables	O
and	O
data	O
structure	O
streams	O
for	O
messages	O
were	O
developed	O
by	O
Keith	O
Clark	O
,	O
Hervé	O
Gallaire	O
,	O
Steve	O
Gregory	O
,	O
Vijay	O
Saraswat	O
,	O
Udi	O
Shapiro	O
,	O
Kazunori	O
Ueda	O
,	O
etc	O
.	O
</s>
<s>
However	O
,	O
like	O
the	O
Actor	B-Application
model	I-Application
,	O
the	O
Prolog-like	O
concurrent	O
systems	O
were	O
based	O
on	O
message	O
passing	O
and	O
consequently	O
were	O
subject	O
to	O
indeterminacy	O
in	O
the	O
ordering	O
of	O
messages	O
in	O
streams	O
that	O
was	O
similar	O
to	O
the	O
indeterminacy	O
in	O
arrival	O
ordering	O
of	O
messages	O
sent	O
to	O
Actors	O
.	O
</s>
<s>
Consequently	O
Carl	O
Hewitt	O
and	O
Gul	O
Agha	O
 [ 1991 ] 	O
concluded	O
that	O
the	O
Prolog-like	O
concurrent	O
systems	O
were	O
neither	O
deductive	O
nor	O
logical	O
.	O
</s>
<s>
Issues	O
of	O
compositionality	O
had	O
proven	O
to	O
be	O
serious	O
limitations	O
for	O
previous	O
theories	O
of	O
computation	O
including	O
the	O
lambda	B-Language
calculus	I-Language
and	O
Petri	O
nets	O
.	O
</s>
<s>
In	O
his	O
doctoral	O
dissertation	O
Gul	O
Agha	O
addressed	O
issues	O
of	O
compositionality	O
in	O
the	O
Actor	B-Application
model	I-Application
.	O
</s>
<s>
Therefore	O
the	O
time	O
that	O
it	O
will	O
take	O
to	O
complete	O
an	O
operation	O
cannot	O
be	O
bounded	O
in	O
advance	O
(	O
see	O
unbounded	B-Application
nondeterminism	I-Application
)	O
.	O
</s>
<s>
However	O
,	O
none	O
of	O
the	O
above	O
formalisms	O
addresses	O
the	O
crucial	O
property	O
of	O
guarantee	O
of	O
service	O
(	O
see	O
unbounded	B-Application
nondeterminism	I-Application
)	O
.	O
</s>
<s>
Recent	O
developments	O
in	O
the	O
Actor	B-Application
model	I-Application
have	O
come	O
from	O
several	O
sources	O
.	O
</s>
<s>
Hardware	O
development	O
is	O
furthering	O
both	O
local	O
and	O
nonlocal	O
massive	O
concurrency	B-Architecture
.	O
</s>
<s>
Local	O
concurrency	B-Architecture
is	O
being	O
enabled	O
by	O
new	O
hardware	O
for	O
64-bit	O
many-core	O
microprocessors	O
,	O
multi-chip	O
modules	O
,	O
and	O
high	O
performance	O
interconnect	O
.	O
</s>
<s>
Nonlocal	O
concurrency	B-Architecture
is	O
being	O
enabled	O
by	O
new	O
hardware	O
for	O
wired	O
and	O
wireless	O
broadband	O
packet	O
switched	O
communications	O
.	O
</s>
<s>
Hewitt	O
[	O
Hewitt	O
2006a	O
,	O
2006b ]	O
is	O
attempting	O
to	O
use	O
the	O
Actor	B-Application
model	I-Application
to	O
address	O
these	O
challenges	O
.	O
</s>
