<s>
ProActive	B-Operating_System
Parallel	O
Suite	O
is	O
an	O
open-source	B-Application
software	I-Application
for	O
enterprise	O
workload	O
orchestration	B-Application
,	O
part	O
of	O
the	O
OW2	B-Architecture
community	O
.	O
</s>
<s>
A	O
workflow	B-Operating_System
model	O
allows	O
a	O
set	O
of	O
executables	B-Application
or	O
scripts	B-Language
,	O
written	O
in	O
any	O
language	O
,	O
to	O
be	O
defined	O
along	O
with	O
their	O
dependencies	O
,	O
so	O
ProActive	B-Operating_System
Parallel	O
Suite	O
can	O
schedule	B-General_Concept
and	O
orchestrate	B-Application
executions	O
while	O
optimising	O
the	O
use	O
of	O
computational	O
resources	O
.	O
</s>
<s>
ProActive	B-Operating_System
Parallel	O
Suite	O
is	O
based	O
on	O
the	O
"	O
active	B-Operating_System
object	I-Operating_System
"	O
design	O
pattern	O
(	O
see	O
Active	B-Operating_System
objects	I-Operating_System
)	O
to	O
optimise	O
task	O
distribution	O
and	O
fault-tolerance	O
.	O
</s>
<s>
Workflows	B-Operating_System
ease	O
task	O
parallelization	B-Operating_System
(	O
Java	B-Language
,	O
scripts	B-Language
,	O
or	O
native	O
executables	B-Application
)	O
,	O
running	O
them	O
on	O
resources	O
matching	O
various	O
constraints	O
(	O
like	O
GPU	O
acceleration	O
,	O
library	O
or	O
data	O
locality	O
)	O
.	O
</s>
<s>
Web	O
interfaces	O
are	O
provided	O
to	O
design	O
and	O
execute	O
job	O
workflows	B-Operating_System
and	O
manage	O
computing	O
resources	O
.	O
</s>
<s>
It	O
provides	O
auto-scaling	B-General_Concept
and	O
ease	O
resource	O
management	O
strategies	O
.	O
</s>
<s>
Interoperability	O
is	O
provided	O
with	O
heterogenous	O
workflows	B-Operating_System
,	O
where	O
tasks	O
can	O
run	O
on	O
various	O
platforms	O
,	O
including	O
Windows	O
,	O
Mac	O
and	O
Linux	O
.	O
</s>
<s>
The	O
book	O
A	O
Theory	O
of	O
Distributed	B-Architecture
Objects	O
presents	O
the	O
ASP	O
calculus	O
that	O
formalizes	O
ProActive	B-Operating_System
features	O
,	O
and	O
provides	O
formal	B-Application
semantics	I-Application
to	O
the	O
calculus	O
,	O
together	O
with	O
properties	O
of	O
ProActive	B-Operating_System
program	O
execution	O
.	O
</s>
<s>
Active	B-Operating_System
objects	I-Operating_System
are	O
the	O
basic	O
units	O
of	O
activity	O
and	O
distribution	O
used	O
for	O
building	O
concurrent	B-Operating_System
applications	O
using	O
ProActive	B-Operating_System
.	O
</s>
<s>
An	O
active	B-Operating_System
object	I-Operating_System
runs	O
with	O
its	O
own	O
thread	B-Operating_System
.	O
</s>
<s>
This	O
thread	B-Operating_System
only	O
executes	O
the	O
methods	O
invoked	O
on	O
this	O
active	B-Operating_System
object	I-Operating_System
by	O
other	O
active	B-Operating_System
objects	I-Operating_System
,	O
and	O
those	O
of	O
the	O
passive	O
objects	O
of	O
the	O
subsystem	O
that	O
belongs	O
to	O
this	O
active	B-Operating_System
object	I-Operating_System
.	O
</s>
<s>
With	O
ProActive	B-Operating_System
,	O
the	O
programmer	O
does	O
not	O
have	O
to	O
explicitly	O
manipulate	O
Thread	B-Operating_System
objects	O
,	O
unlike	O
in	O
standard	O
Java	B-Language
.	O
</s>
<s>
Active	B-Operating_System
objects	I-Operating_System
can	O
be	O
created	O
on	O
any	O
of	O
the	O
hosts	O
involved	O
in	O
the	O
computation	O
.	O
</s>
<s>
Once	O
an	O
active	B-Operating_System
object	I-Operating_System
is	O
created	O
,	O
its	O
activity	O
(	O
the	O
fact	O
that	O
it	O
runs	O
with	O
its	O
own	O
thread	B-Operating_System
)	O
and	O
its	O
location	O
(	O
local	O
or	O
remote	O
)	O
are	O
perfectly	O
transparent	O
.	O
</s>
<s>
Any	O
active	B-Operating_System
object	I-Operating_System
can	O
be	O
manipulated	O
as	O
if	O
it	O
were	O
a	O
passive	O
instance	O
of	O
the	O
same	O
class	O
.	O
</s>
<s>
An	O
active	B-Operating_System
object	I-Operating_System
is	O
composed	O
of	O
two	O
objects	O
:	O
a	O
body	O
,	O
and	O
a	O
standard	O
Java	B-Language
object	O
.	O
</s>
<s>
The	O
body	O
is	O
not	O
visible	O
from	O
the	O
outside	O
of	O
the	O
active	B-Operating_System
object	I-Operating_System
.	O
</s>
<s>
The	O
body	O
is	O
responsible	O
for	O
receiving	O
calls	O
(	O
or	O
requests	O
)	O
on	O
the	O
active	B-Operating_System
object	I-Operating_System
and	O
storing	O
them	O
in	O
a	O
queue	O
of	O
pending	O
calls	O
.	O
</s>
<s>
If	O
a	O
synchronization	O
policy	O
is	O
not	O
specified	O
,	O
calls	O
are	O
managed	O
in	O
a	O
"	O
First	B-Operating_System
in	I-Operating_System
,	I-Operating_System
first	I-Operating_System
out	I-Operating_System
"	O
(	O
FIFO	B-Operating_System
)	O
manner	O
.	O
</s>
<s>
The	O
thread	B-Operating_System
of	O
an	O
active	B-Operating_System
object	I-Operating_System
then	O
chooses	O
a	O
method	O
in	O
the	O
queue	O
of	O
pending	O
requests	O
and	O
executes	O
it	O
.	O
</s>
<s>
No	O
parallelism	B-Operating_System
is	O
provided	O
inside	O
an	O
active	B-Operating_System
object	I-Operating_System
;	O
this	O
is	O
an	O
important	O
decision	O
in	O
ProActive	B-Operating_System
's	O
design	O
,	O
enabling	O
the	O
use	O
of	O
"	O
pre-post	O
"	O
conditions	O
and	O
class	B-Application
invariants	I-Application
.	O
</s>
<s>
On	O
the	O
side	O
of	O
the	O
subsystem	O
that	O
sends	O
a	O
call	O
to	O
an	O
active	B-Operating_System
object	I-Operating_System
,	O
the	O
active	B-Operating_System
object	I-Operating_System
is	O
represented	O
by	O
a	O
proxy	O
.	O
</s>
<s>
ProActive	B-Operating_System
is	O
a	O
library	O
designed	O
for	O
developing	O
applications	O
in	O
the	O
model	O
introduced	O
by	O
Eiffel//	O
,	O
a	O
parallel	O
extension	O
of	O
the	O
Eiffel	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
There	O
is	O
one	O
active	B-Operating_System
object	I-Operating_System
(	O
and	O
therefore	O
one	O
thread	B-Operating_System
)	O
for	O
each	O
subsystem	O
,	O
and	O
one	O
subsystem	O
for	O
each	O
active	B-Operating_System
object	I-Operating_System
(	O
or	O
thread	B-Operating_System
)	O
.	O
</s>
<s>
Each	O
subsystem	O
is	O
thus	O
composed	O
of	O
one	O
active	B-Operating_System
object	I-Operating_System
and	O
any	O
number	O
of	O
passive	O
objects	O
—	O
possibly	O
no	O
passive	O
objects	O
.	O
</s>
<s>
The	O
thread	B-Operating_System
of	O
one	O
subsystem	O
only	O
executes	O
methods	O
in	O
the	O
objects	O
of	O
this	O
subsystem	O
.	O
</s>
<s>
Of	O
all	O
the	O
objects	O
that	O
make	O
up	O
a	O
subsystem	O
—	O
the	O
active	B-Operating_System
object	I-Operating_System
and	O
the	O
passive	O
objects	O
—	O
only	O
the	O
active	B-Operating_System
object	I-Operating_System
is	O
known	O
to	O
objects	O
outside	O
of	O
the	O
subsystem	O
.	O
</s>
<s>
All	O
objects	O
,	O
both	O
active	O
and	O
passive	O
,	O
may	O
have	O
references	O
onto	O
active	B-Operating_System
objects	I-Operating_System
.	O
</s>
<s>
This	O
has	O
also	O
consequences	O
on	O
the	O
semantics	B-Application
of	O
message-passing	O
between	O
subsystems	O
.	O
</s>
<s>
When	O
an	O
object	O
in	O
a	O
subsystem	O
calls	O
a	O
method	O
on	O
an	O
active	B-Operating_System
object	I-Operating_System
,	O
the	O
parameters	O
of	O
the	O
call	O
may	O
be	O
references	O
on	O
passive	O
objects	O
of	O
the	O
subsystem	O
,	O
which	O
would	O
lead	O
to	O
shared	O
passive	O
objects	O
.	O
</s>
<s>
This	O
is	O
why	O
passive	O
objects	O
passed	O
as	O
parameters	O
of	O
calls	O
on	O
active	B-Operating_System
objects	I-Operating_System
are	O
always	O
passed	O
by	O
deep-copy	O
.	O
</s>
<s>
Active	B-Operating_System
objects	I-Operating_System
,	O
on	O
the	O
other	O
hand	O
,	O
are	O
always	O
passed	O
by	O
reference	O
.	O
</s>
<s>
Symmetrically	O
,	O
this	O
also	O
applies	O
to	O
objects	O
returned	O
from	O
methods	O
called	O
on	O
active	B-Operating_System
objects	I-Operating_System
.	O
</s>
<s>
Thanks	O
to	O
the	O
concepts	O
of	O
asynchronous	B-Operating_System
calls	I-Operating_System
,	O
futures	O
,	O
and	O
no	O
data	O
sharing	O
,	O
an	O
application	O
written	O
with	O
ProActive	B-Operating_System
does	O
n't	O
need	O
any	O
structural	O
change	O
—	O
actually	O
,	O
hardly	O
any	O
change	O
at	O
all	O
—	O
whether	O
it	O
runs	O
in	O
a	O
sequential	O
,	O
multi-threaded	B-Operating_System
,	O
or	O
distributed	B-Architecture
environment	O
.	O
</s>
<s>
Whenever	O
possible	O
,	O
a	O
method	O
call	O
on	O
an	O
active	B-Operating_System
object	I-Operating_System
is	O
reified	O
as	O
an	O
asynchronous	O
request	O
.	O
</s>
<s>
If	O
not	O
possible	O
,	O
the	O
call	O
is	O
synchronous	O
,	O
and	O
blocks	B-Operating_System
until	O
the	O
reply	O
is	O
received	O
.	O
</s>
<s>
As	O
a	O
consequence	O
,	O
the	O
calling	O
thread	B-Operating_System
can	O
go	O
on	O
with	O
executing	O
its	O
code	O
,	O
as	O
long	O
as	O
it	O
does	O
n't	O
need	O
to	O
invoke	O
methods	O
on	O
the	O
returned	O
object	O
.	O
</s>
<s>
If	O
the	O
need	O
arises	O
,	O
the	O
calling	O
thread	B-Operating_System
is	O
automatically	O
blocked	O
if	O
the	O
result	O
of	O
the	O
method	O
invocation	O
is	O
not	O
yet	O
available	O
.	O
</s>
<s>
Although	O
a	O
future	O
object	O
has	O
structure	O
similar	O
to	O
that	O
of	O
an	O
active	B-Operating_System
object	I-Operating_System
,	O
a	O
future	O
object	O
is	O
not	O
active	O
.	O
</s>
<s>
Suppose	O
a	O
user	O
calls	O
a	O
method	O
and	O
a	O
method	O
from	O
an	O
active	B-Operating_System
object	I-Operating_System
;	O
the	O
method	O
returns	O
void	O
and	O
the	O
method	O
returns	O
an	O
object	O
of	O
class	O
:	O
</s>
<s>
When	O
is	O
called	O
on	O
an	O
active	B-Operating_System
object	I-Operating_System
,	O
it	O
returns	O
immediately	O
(	O
as	O
the	O
current	O
thread	B-Operating_System
cannot	O
execute	O
methods	O
in	O
the	O
other	O
subsystem	O
)	O
.	O
</s>
<s>
After	O
both	O
methods	O
have	O
returned	O
,	O
the	O
calling	O
thread	B-Operating_System
continues	O
executing	O
its	O
code	O
as	O
if	O
the	O
call	O
had	O
been	O
effectively	O
performed	O
.	O
</s>
<s>
The	O
role	O
of	O
the	O
future	O
mechanism	O
is	O
to	O
block	O
the	O
caller	O
thread	B-Operating_System
when	O
the	O
method	O
is	O
called	O
on	O
and	O
the	O
result	O
has	O
not	O
yet	O
been	O
set	O
:	O
this	O
inter-object	O
synchronization	O
policy	O
is	O
known	O
as	O
wait-by-necessity	O
.	O
</s>
