<s>
Live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
(	O
also	O
abbreviated	O
as	O
live	B-Operating_System
object	I-Operating_System
)	O
refers	O
to	O
a	O
running	O
instance	O
of	O
a	O
distributed	B-Architecture
multi-party	O
(	O
or	O
peer-to-peer	O
)	O
protocol	O
,	O
viewed	O
from	O
the	O
object-oriented	B-Language
perspective	O
,	O
as	O
an	O
entity	O
that	O
has	O
a	O
distinct	O
identity	B-Application
,	O
may	O
encapsulate	B-Application
internal	O
state	B-Application
and	O
threads	B-Operating_System
of	I-Operating_System
execution	I-Operating_System
,	O
and	O
that	O
exhibits	O
a	O
well-defined	O
externally	O
visible	O
behavior	O
.	O
</s>
<s>
The	O
key	O
programming	O
language	O
concepts	O
,	O
as	O
applied	O
to	O
live	B-Operating_System
distributed	I-Operating_System
objects	I-Operating_System
,	O
are	O
defined	O
as	O
follows	O
:	O
</s>
<s>
Identity	B-Application
.	O
</s>
<s>
The	O
identity	B-Application
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
is	O
determined	O
by	O
the	O
same	O
factors	O
that	O
differentiate	O
between	O
instances	O
of	O
the	O
same	O
distributed	B-Architecture
protocol	O
.	O
</s>
<s>
The	O
object	O
consists	O
of	O
a	O
group	O
of	O
software	O
components	O
physically	O
executing	O
on	O
some	O
set	O
of	O
physical	O
machines	O
and	O
engaged	O
in	O
mutual	O
communication	O
,	O
each	O
executing	O
the	O
distributed	B-Architecture
protocol	O
code	O
with	O
the	O
same	O
set	O
of	O
essential	O
parameters	O
,	O
such	O
as	O
the	O
name	O
of	O
a	O
multicast	O
group	O
,	O
the	O
identifier	O
of	O
a	O
publish-subscribe	B-Operating_System
topic	O
,	O
the	O
identity	B-Application
of	O
a	O
membership	O
service	O
,	O
etc	O
.	O
</s>
<s>
Thus	O
,	O
for	O
example	O
,	O
publish-subscribe	B-Operating_System
channels	O
and	O
multicast	O
groups	O
are	O
examples	O
of	O
live	B-Operating_System
distributed	I-Operating_System
objects	I-Operating_System
:	O
for	O
each	O
channel	O
or	O
group	O
,	O
there	O
exists	O
a	O
single	O
instance	O
of	O
a	O
distributed	B-Architecture
protocol	O
running	O
among	O
all	O
computers	O
sending	O
,	O
forwarding	O
,	O
or	O
receiving	O
the	O
data	O
published	O
in	O
the	O
channel	O
or	O
multicast	O
within	O
the	O
group	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
object	O
's	O
identity	B-Application
is	O
determined	O
by	O
the	O
identifier	O
of	O
the	O
channel	O
or	O
group	O
,	O
qualified	O
with	O
the	O
identity	B-Application
of	O
the	O
distributed	B-Architecture
system	I-Architecture
that	O
provides	O
,	O
controls	O
,	O
and	O
manages	O
the	O
given	O
channel	O
or	O
group	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
multicast	O
,	O
the	O
identity	B-Application
of	O
the	O
system	O
might	O
be	O
determined	O
,	O
for	O
example	O
,	O
by	O
the	O
address	O
of	O
the	O
membership	O
service	O
(	O
the	O
entity	O
that	O
manages	O
the	O
membership	O
of	O
the	O
multicast	O
group	O
)	O
.	O
</s>
<s>
The	O
proxy	O
or	O
a	O
replica	O
of	O
a	O
live	B-Operating_System
object	I-Operating_System
is	O
one	O
of	O
the	O
software	O
component	O
instances	O
involved	O
in	O
executing	O
the	O
live	B-Operating_System
object	I-Operating_System
's	O
distributed	B-Architecture
protocol	O
.	O
</s>
<s>
The	O
object	O
can	O
thus	O
be	O
alternatively	O
defined	O
as	O
a	O
group	O
of	O
proxies	O
engaged	O
in	O
communication	O
,	O
jointly	O
maintaining	O
some	O
distributed	B-Architecture
state	B-Application
,	O
and	O
coordinating	O
their	O
operations	O
.	O
</s>
<s>
In	O
this	O
sense	O
,	O
the	O
concept	O
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
proxy	O
generalizes	O
the	O
notion	O
of	O
a	O
RPC	B-Operating_System
,	O
RMI	B-Language
,	O
or	O
.NET	B-General_Concept
remoting	I-General_Concept
client-side	O
proxy	O
stub	O
.	O
</s>
<s>
The	O
behavior	O
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
is	O
characterized	O
by	O
the	O
set	O
of	O
possible	O
patterns	O
of	O
external	O
interactions	O
that	O
its	O
proxies	O
can	O
engage	O
in	O
with	O
their	O
local	O
runtime	O
environments	O
.	O
</s>
<s>
State	B-Application
.	O
</s>
<s>
The	O
state	B-Application
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
is	O
defined	O
as	O
the	O
sum	O
of	O
all	O
internal	O
,	O
local	O
states	O
of	O
its	O
proxies	O
.	O
</s>
<s>
By	O
definition	O
,	O
it	O
is	O
distributed	B-Architecture
and	O
replicated	O
.	O
</s>
<s>
The	O
different	O
replicas	O
of	O
the	O
object	O
's	O
state	B-Application
may	O
be	O
strongly	O
or	O
only	O
weakly	O
consistent	O
,	O
depending	O
on	O
the	O
protocol	O
semantics	O
:	O
an	O
instance	O
of	O
a	O
consensus	B-Operating_System
protocol	O
will	O
have	O
the	O
state	B-Application
of	O
its	O
replicas	O
strongly	O
consistent	O
,	O
whereas	O
an	O
instance	O
of	O
a	O
leader	B-Operating_System
election	I-Operating_System
protocol	O
will	O
have	O
a	O
weakly	O
consistent	O
state	B-Application
.	O
</s>
<s>
In	O
this	O
sense	O
,	O
the	O
term	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
generalizes	O
the	O
concept	O
of	O
a	O
replicated	O
object	O
;	O
the	O
latter	O
is	O
a	O
specific	O
type	O
of	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
that	O
uses	O
a	O
protocol	O
such	O
as	O
Paxos	O
,	O
virtual	O
synchrony	O
,	O
or	O
state	B-Application
machine	O
replication	O
to	O
achieve	O
strong	O
consistency	O
between	O
the	O
internal	O
states	O
of	O
its	O
replicas	O
.	O
</s>
<s>
The	O
state	B-Application
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
should	O
be	O
understood	O
as	O
a	O
dynamic	O
notion	O
:	O
as	O
a	O
point	O
(	O
or	O
consistent	O
cut	O
)	O
in	O
a	O
stream	O
of	O
values	O
,	O
rather	O
than	O
as	O
a	O
particular	O
value	O
located	O
in	O
a	O
given	O
place	O
at	O
a	O
given	O
time	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
externally	O
visible	O
state	B-Application
of	O
a	O
leader	B-Operating_System
election	I-Operating_System
object	O
would	O
be	O
defined	O
as	O
the	O
identity	B-Application
of	O
the	O
currently	O
elected	O
leader	O
.	O
</s>
<s>
The	O
identity	B-Application
is	O
not	O
stored	O
at	O
any	O
particular	O
location	O
;	O
rather	O
,	O
it	O
materializes	O
as	O
a	O
stream	O
of	O
messages	O
of	O
the	O
form	O
elected(x )	O
concurrently	O
produced	O
by	O
the	O
proxies	O
involved	O
in	O
executing	O
this	O
protocol	O
,	O
and	O
concurrently	O
consumed	O
by	O
instances	O
of	O
the	O
application	O
using	O
this	O
protocol	O
,	O
on	O
different	O
machines	O
distributed	B-Architecture
across	O
the	O
network	O
.	O
</s>
<s>
The	O
interface	O
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
is	O
defined	O
by	O
the	O
types	O
of	O
interfaces	O
exposed	O
by	O
its	O
proxies	O
;	O
these	O
may	O
include	O
event	O
channels	O
and	O
various	O
types	O
of	O
graphical	O
user	O
interfaces	O
.	O
</s>
<s>
Interfaces	O
exposed	O
by	O
the	O
proxies	O
are	O
referred	O
to	O
as	O
the	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
's	O
endpoints	O
.	O
</s>
<s>
To	O
say	O
that	O
a	O
live	B-Operating_System
object	I-Operating_System
exposes	O
a	O
certain	O
endpoint	O
means	O
that	O
each	O
of	O
its	O
proxies	O
exposes	O
an	O
instance	O
of	O
this	O
endpoint	O
to	O
its	O
local	O
environment	O
,	O
and	O
each	O
of	O
the	O
endpoint	O
instances	O
carries	O
events	O
of	O
the	O
same	O
types	O
(	O
or	O
binds	O
to	O
the	O
same	O
type	O
of	O
a	O
graphical	O
display	O
)	O
.	O
</s>
<s>
The	O
reference	O
to	O
a	O
live	B-Operating_System
object	I-Operating_System
is	O
a	O
complete	O
set	O
of	O
serialized	B-Application
,	O
portable	O
instructions	O
for	O
constructing	O
its	O
proxy	O
.	O
</s>
<s>
To	O
dereference	O
a	O
reference	O
means	O
to	O
locally	O
parse	O
and	O
follow	O
these	O
instructions	O
on	O
a	O
particular	O
computer	O
,	O
to	O
produce	O
a	O
running	O
proxy	O
of	O
the	O
live	B-Operating_System
object	I-Operating_System
.	O
</s>
<s>
Defined	O
this	O
way	O
,	O
a	O
live	B-Operating_System
object	I-Operating_System
reference	O
plays	O
the	O
same	O
role	O
as	O
a	O
Java	B-Language
reference	O
,	O
a	O
C/C	O
++	O
pointer	O
,	O
or	O
a	O
web	O
service	O
's	O
WSDL	O
description	O
;	O
it	O
contains	O
a	O
complete	O
information	O
sufficient	O
to	O
locate	O
the	O
given	O
object	O
and	O
interact	O
with	O
it	O
.	O
</s>
<s>
Since	O
live	B-Operating_System
distributed	I-Operating_System
objects	I-Operating_System
may	O
not	O
reside	O
in	O
any	O
particular	O
place	O
(	O
but	O
rather	O
span	O
across	O
a	O
dynamically	O
changing	O
set	O
of	O
computers	O
)	O
,	O
the	O
information	O
contained	O
in	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
's	O
reference	O
cannot	O
be	O
limited	O
to	O
just	O
an	O
address	O
.	O
</s>
<s>
If	O
the	O
object	O
is	O
identified	O
by	O
some	O
sort	O
of	O
a	O
globally	O
unique	O
identifier	O
(	O
as	O
might	O
be	O
the	O
case	O
for	O
publish-subscribe	B-Operating_System
topics	O
or	O
multicast	O
groups	O
)	O
,	O
the	O
reference	O
must	O
specify	O
how	O
this	O
identifier	O
is	O
resolved	O
,	O
by	O
recursively	O
embedding	O
a	O
reference	O
to	O
the	O
appropriate	O
name	B-Application
resolution	I-Application
object	O
.	O
</s>
<s>
The	O
type	O
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
determines	O
the	O
patterns	O
of	O
external	O
interactions	O
with	O
the	O
object	O
;	O
it	O
is	O
determined	O
by	O
the	O
types	O
of	O
endpoints	O
and	O
graphical	O
user	O
interfaces	O
exposed	O
by	O
the	O
object	O
's	O
proxies	O
,	O
and	O
the	O
patterns	O
of	O
events	O
that	O
may	O
occur	O
at	O
the	O
endpoints	O
.	O
</s>
<s>
Much	O
as	O
it	O
is	O
the	O
case	O
for	O
types	O
in	O
Java-like	O
languages	O
,	O
there	O
might	O
exist	O
many	O
very	O
different	O
implementations	O
of	O
the	O
same	O
type	O
.	O
</s>
<s>
Thus	O
,	O
for	O
example	O
,	O
behavior	O
characteristic	O
to	O
atomic	O
multicast	O
might	O
be	O
exhibited	O
by	O
instances	O
of	O
distributed	B-Architecture
protocols	O
such	O
as	O
virtual	O
synchrony	O
or	O
Paxos	O
.	O
</s>
<s>
Early	O
ideas	O
underlying	O
the	O
concept	O
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
have	O
been	O
influenced	O
by	O
a	O
rich	O
body	O
of	O
research	O
on	O
object-oriented	B-Language
environments	O
,	O
programming	O
language	O
embeddings	O
,	O
and	O
protocol	O
composition	O
frameworks	O
,	O
dating	O
back	O
at	O
least	O
to	O
the	O
actor	B-Application
model	I-Application
developed	O
in	O
the	O
early	O
1970s	O
;	O
a	O
comprehensive	O
discussion	O
of	O
the	O
relevant	O
prior	O
work	O
can	O
be	O
found	O
in	O
Krzysztof	O
Ostrowski	O
's	O
Ph.D.	O
dissertation	O
.	O
</s>
<s>
Originally	O
,	O
the	O
term	O
was	O
used	O
to	O
refer	O
to	O
the	O
types	O
of	O
dynamic	O
,	O
interactive	B-General_Concept
Web	O
content	O
that	O
is	O
not	O
hosted	O
on	O
servers	B-Application
in	O
data	B-Operating_System
centers	I-Operating_System
,	O
but	O
rather	O
stored	O
on	O
the	O
end-user	O
'	O
s	O
client	B-Protocol
computers	I-Protocol
,	O
and	O
internally	O
powered	O
by	O
instances	O
of	O
reliable	B-Protocol
multicast	O
protocols	O
.	O
</s>
<s>
The	O
word	O
live	O
expressed	O
the	O
fact	O
that	O
the	O
displayed	O
information	O
is	O
dynamic	O
,	O
interactive	B-General_Concept
,	O
and	O
represents	O
current	O
,	O
fresh	O
,	O
live	O
content	O
that	O
reflects	O
recent	O
updates	O
made	O
by	O
the	O
users	O
(	O
as	O
opposed	O
to	O
static	O
,	O
read-only	O
,	O
and	O
archival	B-Application
content	O
that	O
has	O
been	O
pre-assembled	O
)	O
.	O
</s>
<s>
The	O
word	O
distributed	B-Architecture
expressed	O
the	O
fact	O
that	O
the	O
information	O
is	O
not	O
hosted	O
,	O
stored	O
at	O
a	O
server	O
in	O
a	O
data	B-Operating_System
center	I-Operating_System
,	O
but	O
rather	O
,	O
it	O
is	O
replicated	O
among	O
the	O
end-user	O
computers	O
,	O
and	O
updated	O
in	O
a	O
peer-to-peer	O
fashion	O
through	O
a	O
stream	O
of	O
multicast	O
messages	O
that	O
may	O
be	O
produced	O
directly	O
by	O
the	O
end-users	O
consuming	O
the	O
content	O
;	O
a	O
more	O
comprehensive	O
discussion	O
of	O
the	O
live	B-Operating_System
object	I-Operating_System
concept	O
in	O
the	O
context	O
of	O
Web	O
development	O
can	O
be	O
found	O
in	O
'	O
s	O
Ph.D.	O
dissertation	O
.	O
</s>
<s>
The	O
extension	O
of	O
the	O
term	O
has	O
been	O
motivated	O
by	O
the	O
need	O
to	O
model	O
live	B-Operating_System
objects	I-Operating_System
as	O
compositions	O
of	O
other	O
objects	O
;	O
in	O
this	O
sense	O
,	O
the	O
concept	O
has	O
been	O
inspired	O
by	O
Smalltalk	B-Application
,	O
which	O
pioneered	O
the	O
uniform	O
perspective	O
that	O
everything	O
is	O
an	O
object	O
,	O
and	O
Jini	B-Language
,	O
which	O
pioneered	O
the	O
idea	O
that	O
services	O
are	O
objects	O
.	O
</s>
<s>
When	O
applied	O
to	O
live	B-Operating_System
distributed	I-Operating_System
objects	I-Operating_System
,	O
the	O
perspective	O
dictates	O
that	O
their	O
constituent	O
parts	O
,	O
which	O
includes	O
instances	O
of	O
distributed	B-Architecture
multi-party	O
protocols	O
used	O
internally	O
to	O
replicate	O
state	B-Application
,	O
should	O
also	O
be	O
modeled	O
as	O
live	B-Operating_System
distributed	I-Operating_System
objects	I-Operating_System
.	O
</s>
<s>
The	O
need	O
for	O
uniformity	O
implies	O
that	O
the	O
definition	O
of	O
a	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
must	O
unify	O
concepts	O
such	O
as	O
live	O
Web	O
content	O
,	O
message	O
streams	O
,	O
and	O
instances	O
of	O
distributed	B-Architecture
multi-party	O
protocols	O
.	O
</s>
<s>
The	O
first	O
implementation	O
of	O
the	O
live	B-Operating_System
distributed	I-Operating_System
object	I-Operating_System
concept	O
,	O
as	O
defined	O
in	O
the	O
ECOOP	O
paper	O
,	O
was	O
the	O
Live	B-Operating_System
Distributed	I-Operating_System
Objects	I-Operating_System
platform	O
developed	O
by	O
at	O
Cornell	O
University	O
.	O
</s>
<s>
The	O
platform	O
provided	O
a	O
set	O
of	O
visual	O
,	O
drag	O
and	O
drop	O
tools	O
for	O
composing	O
hierarchical	O
documents	O
resembling	O
web	O
pages	O
,	O
and	O
containing	O
XML-serialized	O
live	B-Operating_System
object	I-Operating_System
references	O
.	O
</s>
<s>
Visual	O
content	O
such	O
as	O
chat	O
windows	O
,	O
shared	O
desktops	O
,	O
and	O
various	O
sorts	O
of	O
mashups	B-Application
could	O
be	O
composed	O
by	O
dragging	O
and	O
dropping	O
components	O
representing	O
user	O
interfaces	O
and	O
protocol	O
instances	O
onto	O
a	O
design	O
form	O
,	O
and	O
connecting	O
them	O
together	O
.	O
</s>
<s>
Since	O
the	O
moment	O
of	O
its	O
creation	O
,	O
a	O
number	O
of	O
extension	O
have	O
been	O
developed	O
to	O
embed	O
live	B-Operating_System
distributed	I-Operating_System
objects	I-Operating_System
in	O
Microsoft	B-Application
Office	I-Application
documents	O
,	O
and	O
to	O
support	O
various	O
types	O
of	O
hosted	O
content	O
such	O
as	O
Google	O
Maps	O
.	O
</s>
