<s>
An	O
Active	B-Operating_System
message	I-Operating_System
(	O
in	O
computing	O
)	O
is	O
a	O
messaging	O
object	O
capable	O
of	O
performing	O
processing	O
on	O
its	O
own	O
.	O
</s>
<s>
This	O
contrasts	O
with	O
traditional	O
computer-based	O
messaging	B-Operating_System
systems	I-Operating_System
in	O
which	O
messages	O
are	O
passive	O
entities	O
with	O
no	O
processing	O
power	O
.	O
</s>
<s>
Active	B-Operating_System
messages	I-Operating_System
are	O
communications	O
primitive	O
for	O
exploiting	O
the	O
full	O
performance	O
and	O
flexibility	O
of	O
modern	O
computer	O
interconnects	O
.	O
</s>
<s>
They	O
are	O
often	O
classified	O
as	O
one	O
of	O
the	O
three	O
main	O
types	O
of	O
distributed	O
memory	O
programming	O
,	O
the	O
other	O
two	O
being	O
data	B-Operating_System
parallel	I-Operating_System
and	O
message	B-Architecture
passing	I-Architecture
.	O
</s>
<s>
The	O
view	O
is	O
that	O
Active	B-Operating_System
Messages	I-Operating_System
are	O
actually	O
a	O
lower-level	O
mechanism	O
that	O
can	O
be	O
used	O
to	O
implement	O
data	B-Operating_System
parallel	I-Operating_System
or	O
message	B-Architecture
passing	I-Architecture
efficiently	O
.	O
</s>
<s>
Early	O
active	B-Operating_System
message	I-Operating_System
systems	O
passed	O
the	O
actual	O
remote	O
code	O
address	O
across	O
the	O
network	O
,	O
however	O
this	O
approach	O
required	O
the	O
initiator	O
to	O
know	O
the	O
address	O
of	O
the	O
remote	O
handler	O
function	O
when	O
composing	O
a	O
message	O
,	O
which	O
can	O
be	O
quite	O
limiting	O
even	O
within	O
the	O
context	O
of	O
a	O
SPMD	B-Operating_System
programming	O
model	O
(	O
and	O
generally	O
relies	O
upon	O
address	O
space	O
uniformity	O
which	O
is	O
absent	O
in	O
many	O
modern	O
systems	O
)	O
.	O
</s>
<s>
Newer	O
active	B-Operating_System
message	I-Operating_System
interfaces	O
require	O
the	O
client	O
to	O
register	O
a	O
table	O
with	O
the	O
software	O
at	O
initialization	O
time	O
that	O
maps	O
an	O
integer	O
index	O
to	O
the	O
local	O
address	O
of	O
a	O
handler	O
function	O
;	O
in	O
these	O
systems	O
the	O
sender	O
of	O
an	O
active	B-Operating_System
message	I-Operating_System
provides	O
an	O
index	O
into	O
the	O
remote	O
handler	O
table	O
,	O
and	O
upon	O
arrival	O
of	O
the	O
active	B-Operating_System
message	I-Operating_System
the	O
table	O
is	O
used	O
to	O
map	O
this	O
index	O
to	O
the	O
handler	O
address	O
that	O
is	O
invoked	O
to	O
handle	O
the	O
message	O
.	O
</s>
<s>
Other	O
variations	O
of	O
active	B-Operating_System
messages	I-Operating_System
carry	O
the	O
actual	O
code	O
itself	O
,	O
not	O
a	O
pointer	O
to	O
the	O
code	O
.	O
</s>
<s>
On	O
arrival	O
at	O
the	O
receiving	O
end	O
,	O
more	O
data	O
is	O
acquired	O
,	O
and	O
the	O
computation	O
in	O
the	O
active	B-Operating_System
message	I-Operating_System
is	O
performed	O
,	O
making	O
use	O
of	O
data	O
in	O
the	O
message	O
as	O
well	O
as	O
data	O
in	O
the	O
receiving	O
node	O
.	O
</s>
<s>
This	O
form	O
of	O
active	O
messaging	O
is	O
not	O
restricted	O
to	O
SPMD	B-Operating_System
,	O
although	O
originator	O
and	O
receiver	O
must	O
share	O
some	O
notions	O
as	O
to	O
what	O
data	O
can	O
be	O
accessed	O
at	O
the	O
receiving	O
node	O
.	O
</s>
<s>
A	O
higher	O
level	O
implementation	O
for	O
active	B-Operating_System
messages	I-Operating_System
is	O
also	O
named	O
Swarm	O
communication	O
in	O
the	O
SwarmESB	O
project	O
.	O
</s>
<s>
The	O
basic	O
model	O
of	O
the	O
active	B-Operating_System
messages	I-Operating_System
is	O
extend	O
with	O
new	O
concepts	O
and	O
Java	O
Script	O
is	O
used	O
to	O
express	O
the	O
code	O
of	O
the	O
active	B-Operating_System
messages	I-Operating_System
.	O
</s>
