<s>
Portable	B-Application
Distributed	I-Application
Objects	I-Application
(	O
PDO	O
)	O
is	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
for	O
creating	O
object-oriented	B-Language
code	I-Language
that	O
can	O
be	O
executed	O
remotely	O
on	O
a	O
network	B-Architecture
of	O
computers	O
.	O
</s>
<s>
It	O
was	O
created	O
by	O
NeXT	O
Computer	O
,	O
Inc	O
.	O
using	O
their	O
OpenStep	B-Operating_System
system	O
,	O
whose	O
use	O
of	O
Objective-C	B-Language
made	O
the	O
package	O
very	O
easy	O
to	O
write	O
.	O
</s>
<s>
It	O
was	O
characterized	O
by	O
its	O
very	O
light	O
weight	O
and	O
high	O
speed	O
in	O
comparison	O
to	O
similar	O
systems	O
such	O
as	O
CORBA	B-Architecture
.	O
</s>
<s>
Versions	O
of	O
PDO	O
were	O
available	O
for	O
Solaris	B-Application
,	O
HP-UX	B-Application
and	O
all	O
versions	O
of	O
the	O
OPENSTEP	B-Operating_System
system	O
,	O
although	O
an	O
agreement	O
was	O
also	O
announced	O
for	O
a	O
version	O
to	O
be	O
made	O
for	O
Digital	B-Operating_System
Unix	I-Operating_System
,	O
then	O
still	O
known	O
as	O
OSF/1	O
,	O
with	O
delivery	O
anticipated	O
after	O
versions	O
for	O
SunOS	O
and	O
Solaris	B-Application
had	O
been	O
released	O
.	O
</s>
<s>
A	O
version	O
that	O
worked	O
with	O
Microsoft	O
OLE	B-Operating_System
was	O
also	O
available	O
called	O
D'OLE	O
,	O
allowing	O
distributed	O
code	O
written	O
using	O
PDO	O
on	O
any	O
platform	O
to	O
be	O
presented	O
on	O
Microsoft	O
systems	O
as	O
if	O
they	O
were	O
local	O
OLE	B-Operating_System
objects	I-Operating_System
.	O
</s>
<s>
PDO	O
was	O
one	O
of	O
a	O
number	O
of	O
distributed	B-Operating_System
object	I-Operating_System
systems	I-Operating_System
created	O
in	O
the	O
early	O
1990s	O
,	O
a	O
design	O
model	O
where	O
"	O
front	O
end	O
"	O
applications	O
on	O
GUI-based	O
microcomputers	O
would	O
call	O
code	O
running	O
on	O
mainframe	B-Architecture
and	O
minicomputers	B-Architecture
for	O
their	O
processing	O
and	O
data	O
storage	O
.	O
</s>
<s>
Microsoft	O
was	O
evolving	O
OLE	B-Operating_System
into	O
the	O
Component	B-Application
Object	I-Application
Model	I-Application
(	O
COM	O
)	O
and	O
a	O
similar	O
distributed	O
version	O
called	O
DCOM	O
,	O
IBM	O
had	O
their	O
System	B-Application
Object	I-Application
Model	I-Application
(	O
SOM/DSOM	O
)	O
,	O
Sun	O
Microsystems	O
was	O
promoting	O
their	O
Distributed	B-Application
Objects	I-Application
Everywhere	I-Application
,	O
and	O
there	O
were	O
a	O
host	O
of	O
smaller	O
players	O
as	O
well	O
.	O
</s>
<s>
PDO	O
,	O
on	O
the	O
other	O
hand	O
,	O
relied	O
on	O
a	O
small	O
number	O
of	O
features	O
in	O
the	O
Objective-C	B-Language
runtime	B-Device
to	O
handle	O
both	O
portability	O
as	O
well	O
as	O
distribution	O
.	O
</s>
<s>
The	O
key	O
feature	O
was	O
the	O
language	O
's	O
support	O
for	O
a	O
"	O
second	O
chance	O
"	O
method	O
in	O
all	O
classes	O
;	O
if	O
a	O
method	O
call	O
on	O
an	O
object	O
failed	O
because	O
the	O
object	O
did	O
n't	O
support	O
it	O
(	O
normally	O
not	O
allowed	O
in	O
most	O
languages	O
due	O
to	O
strong	O
typing	O
)	O
,	O
the	O
runtime	B-Device
would	O
then	O
bundle	O
the	O
message	O
into	O
a	O
compact	O
format	O
and	O
pass	O
it	O
back	O
into	O
the	O
object	O
's	O
forwardInvocation	O
method	O
.	O
</s>
<s>
PDO	O
instead	O
supplied	O
a	O
number	O
of	O
new	O
objects	O
with	O
forwardInvocation	O
methods	O
that	O
passed	O
the	O
invocation	O
object	O
to	O
another	O
machine	O
on	O
the	O
network	B-Architecture
,	O
with	O
various	O
versions	O
to	O
support	O
different	O
networks	O
and	O
platforms	O
.	O
</s>
<s>
Calling	O
methods	O
on	O
remote	O
objects	O
was	O
almost	O
invisible	O
;	O
after	O
some	O
network	B-Architecture
setup	O
(	O
a	O
few	O
lines	O
typically	O
)	O
PDO	O
objects	O
were	O
instantiated	O
locally	O
and	O
called	O
the	O
same	O
way	O
as	O
any	O
other	O
object	O
on	O
the	O
system	O
.	O
</s>
<s>
In	O
comparison	O
with	O
CORBA	B-Architecture
,	O
PDO	O
programs	O
were	O
typically	O
1/10	O
or	O
less	O
in	O
size	O
;	O
it	O
was	O
common	O
for	O
NeXT	O
staffers	O
to	O
write	O
into	O
magazines	O
showing	O
how	O
to	O
re-implement	O
a	O
multi-page	O
CORBA	B-Architecture
article	O
in	O
perhaps	O
15	O
lines	O
of	O
code	O
.	O
</s>
<s>
However	O
,	O
PDO	O
was	O
also	O
reliant	O
entirely	O
on	O
Objective-C	B-Language
to	O
function	O
.	O
</s>
<s>
This	O
was	O
a	O
price	O
most	O
were	O
unwilling	O
to	O
pay	O
,	O
as	O
at	O
the	O
time	O
C++	B-Language
was	O
more	O
widely	O
used	O
and	O
the	O
effort	O
to	O
shift	O
codebases	O
to	O
an	O
entirely	O
new	O
language	O
and	O
paradigm	O
was	O
considered	O
too	O
onerous	O
.	O
</s>
<s>
PDO	O
never	O
saw	O
much	O
use	O
,	O
and	O
NeXT	O
's	O
emphasis	O
shifted	O
to	O
its	O
new	O
WebObjects	B-Language
framework	O
in	O
1995	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
OS	O
X	O
platform	O
,	O
there	O
is	O
GNUstep	B-Application
,	O
which	O
has	O
its	O
own	O
implementation	O
of	O
Distributed	B-Operating_System
Objects	I-Operating_System
.	O
</s>
