<s>
Xgrid	B-Operating_System
is	O
a	O
proprietary	B-Application
program	I-Application
and	O
distributed	B-Architecture
computing	I-Architecture
protocol	O
developed	O
by	O
the	O
Advanced	O
Computation	O
Group	O
subdivision	O
of	O
Apple	O
Inc	O
that	O
allows	O
networked	B-Architecture
computers	O
to	O
contribute	O
to	O
a	O
single	O
task	B-General_Concept
.	O
</s>
<s>
It	O
provides	O
network	B-Architecture
administrators	O
a	O
method	O
of	O
creating	O
a	O
computing	B-Architecture
cluster	I-Architecture
,	O
which	O
allows	O
them	O
to	O
exploit	O
previously	O
unused	O
computational	O
power	O
for	O
calculations	O
that	O
can	O
be	O
divided	O
easily	O
into	O
smaller	O
operations	O
,	O
such	O
as	O
Mandelbrot	B-Algorithm
maps	I-Algorithm
.	O
</s>
<s>
The	O
setup	O
of	O
an	O
Xgrid	B-Operating_System
cluster	B-Architecture
can	O
be	O
achieved	O
at	O
next	O
to	O
no	O
cost	O
,	O
as	O
Xgrid	B-Operating_System
client	O
is	O
pre-installed	O
on	O
all	O
computers	O
running	O
Mac	B-Application
OS	I-Application
X	I-Application
10.4	I-Application
to	O
Mac	B-Application
OS	I-Application
X	I-Application
10.7	I-Application
.	O
</s>
<s>
The	O
Xgrid	B-Operating_System
client	O
was	O
not	O
included	O
in	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
10.8	I-Operating_System
.	O
</s>
<s>
The	O
Xgrid	B-Operating_System
controller	B-Architecture
,	O
the	O
job	B-General_Concept
scheduler	I-General_Concept
of	O
the	O
Xgrid	B-Operating_System
operation	O
,	O
is	O
also	O
included	O
within	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
Server	I-Operating_System
and	O
as	O
a	O
free	O
download	O
from	O
Apple	O
.	O
</s>
<s>
Apple	O
has	O
kept	O
the	O
command-line	B-Application
job	B-General_Concept
control	O
mechanism	O
minimalist	O
while	O
providing	O
an	O
API	B-General_Concept
to	O
develop	O
more	O
sophisticated	O
tools	O
built	O
around	O
it	O
.	O
</s>
<s>
This	O
communication	O
protocol	O
interfaces	O
with	O
the	O
BEEP	B-Protocol
infrastructure	I-Protocol
,	O
a	O
network	B-Architecture
application	O
protocol	O
framework	B-Architecture
.	O
</s>
<s>
Computers	O
discovered	O
by	O
the	O
Xgrid	B-Operating_System
system	O
,	O
that	O
is	O
computers	O
with	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
's	O
Xgrid	B-Operating_System
service	O
enabled	O
,	O
are	O
automatically	O
added	O
to	O
the	O
list	O
of	O
available	O
computers	O
to	O
use	O
for	O
processing	O
tasks	O
.	O
</s>
<s>
When	O
the	O
initiating	O
computer	O
sends	O
the	O
complete	O
instructions	O
,	O
or	O
job	B-General_Concept
,	O
for	O
processing	O
to	O
the	O
controller	B-Architecture
,	O
the	O
controller	B-Architecture
splits	O
the	O
task	B-General_Concept
up	O
into	O
these	O
small	O
instruction	B-Language
packets	I-Language
,	O
known	O
as	O
tasks	O
.	O
</s>
<s>
The	O
design	O
of	O
the	O
Xgrid	B-Operating_System
system	O
consists	O
of	O
these	O
small	O
packets	O
being	O
transferred	O
to	O
all	O
the	O
Xgrid-enabled	O
computers	O
on	O
the	O
network	B-Architecture
.	O
</s>
<s>
These	O
computers	O
,	O
or	O
nodes	O
,	O
execute	B-General_Concept
the	O
instructions	O
provided	O
by	O
the	O
controller	B-Architecture
and	O
then	O
return	O
the	O
results	O
.	O
</s>
<s>
The	O
controller	B-Architecture
assembles	O
the	O
individual	O
task	B-General_Concept
results	O
into	O
the	O
whole	O
job	B-General_Concept
results	O
and	O
returns	O
them	O
to	O
the	O
initiating	O
computer	O
.	O
</s>
<s>
Apple	O
modeled	O
the	O
design	O
of	O
Xgrid	B-Operating_System
on	O
the	O
Zilla	O
program	O
,	O
distributed	O
with	O
NeXT	O
's	O
OpenStep	B-Operating_System
operating	O
system	O
application	O
programming	O
interface	O
(	O
API	B-General_Concept
)	O
,	O
which	O
Apple	O
owned	O
the	O
rights	O
to	O
.	O
</s>
<s>
The	O
company	O
also	O
opted	O
to	O
provide	O
the	O
client	O
version	O
of	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
with	O
only	O
command-line	B-Application
functions	O
and	O
little	O
flexibility	O
,	O
while	O
giving	O
the	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
Server	I-Operating_System
version	O
of	O
Xgrid	B-Operating_System
a	O
GUI	B-Application
control	O
panel	O
and	O
a	O
full	O
set	O
of	O
features	O
.	O
</s>
<s>
Xgrid	B-Operating_System
's	O
original	O
concept	O
can	O
be	O
traced	O
back	O
to	O
Zilla.app	O
,	O
found	O
in	O
the	O
OPENSTEP	B-Operating_System
operating	O
system	O
,	O
created	O
by	O
NeXT	O
in	O
the	O
late	O
1980s	O
.	O
</s>
<s>
Zilla	O
was	O
the	O
first	O
distributed	B-Architecture
computing	I-Architecture
program	O
released	O
on	O
an	O
end-user	O
operating	O
system	O
and	O
which	O
used	O
the	O
idle	O
screen-saver	O
motif	O
,	O
a	O
design	O
feature	O
since	O
found	O
in	O
widely	O
used	O
projects	O
such	O
as	O
Seti	B-Application
@Home	I-Application
and	O
Distributed.net	O
.	O
</s>
<s>
Apple	O
acquired	O
Zilla	O
,	O
along	O
with	O
the	O
rest	O
of	O
NeXT	O
,	O
in	O
1997	O
and	O
later	O
used	O
Zilla	O
as	O
inspiration	O
for	O
Xgrid	B-Operating_System
.	O
</s>
<s>
The	O
first	O
beta	O
version	O
of	O
Xgrid	B-Operating_System
was	O
released	O
in	O
January	O
2004	O
.	O
</s>
<s>
Several	O
organizations	O
have	O
adopted	O
Xgrid	B-Operating_System
in	O
large	O
international	O
computing	O
networks	O
.	O
</s>
<s>
One	O
example	O
of	O
an	O
Xgrid	B-Operating_System
cluster	B-Architecture
is	O
MacResearch	O
's	O
OpenMacGrid	O
,	O
where	O
scientists	O
can	O
request	O
access	O
to	O
large	O
amounts	O
of	O
processing	O
power	O
to	O
run	O
tasks	O
related	O
to	O
their	O
research	O
.	O
</s>
<s>
Another	O
was	O
the	O
now	O
defunct	O
Xgrid	B-Operating_System
@Stanford	O
project	O
,	O
which	O
used	O
a	O
range	O
of	O
computers	O
on	O
the	O
Stanford	O
University	O
campus	O
and	O
around	O
the	O
world	O
to	O
perform	O
biochemical	O
research	O
.	O
</s>
<s>
In	O
a	O
pre-release	O
promotional	O
piece	O
,	O
MacWorld	O
cited	O
Xgrid	B-Operating_System
among	O
the	O
Unix	O
features	O
in	O
"	O
10	O
Things	O
to	O
Know	O
about	O
TIGER	O
"	O
,	O
calling	O
it	O
"	O
handy	O
if	O
you	O
work	O
with	O
huge	O
amounts	O
of	O
experimental	O
data	O
or	O
render	O
complex	O
animations	O
"	O
.	O
</s>
<s>
After	O
Xgrid	B-Operating_System
's	O
introduction	O
in	O
2004	O
,	O
InfoWorld	O
noted	O
that	O
it	O
was	O
a	O
"	O
 '	O
preview	O
 '	O
grade	O
technology	O
"	O
which	O
would	O
directly	O
benefit	O
from	O
the	O
Xserve	O
G5	O
's	O
launch	O
later	O
that	O
year	O
.	O
</s>
<s>
InfoWorld	O
commentator	O
Ephraim	O
Schwartz	O
also	O
predicted	O
that	O
Xgrid	B-Operating_System
was	O
an	O
opening	O
move	O
in	O
Apple	O
's	O
entry	O
into	O
the	O
enterprise	O
computing	O
market	O
.	O
</s>
<s>
Apple	O
discontinued	O
Xgrid	B-Operating_System
with	O
OS	B-Operating_System
X	I-Operating_System
v10.8	I-Operating_System
(	O
Mountain	O
Lion	B-Application
)	O
,	O
along	O
with	O
dependent	O
services	O
such	O
as	O
Podcast	O
Producer	O
.	O
</s>
<s>
The	O
Xgrid	B-Operating_System
protocol	O
uses	O
the	O
BEEP	B-Protocol
network	B-Architecture
framework	B-Architecture
to	O
communicate	O
with	O
nodes	O
on	O
the	O
network	B-Architecture
.	O
</s>
<s>
Next	O
is	O
the	O
controller	B-Architecture
,	O
which	O
starts	O
and	O
segregates	O
the	O
calculation	O
.	O
</s>
<s>
The	O
Xgrid	B-Operating_System
protocol	O
provides	O
the	O
basic	O
infrastructure	O
for	O
computers	O
to	O
communicate	O
,	O
but	O
is	O
not	O
involved	O
in	O
the	O
processing	O
of	O
the	O
specified	O
calculation	O
.	O
</s>
<s>
Xgrid	B-Operating_System
is	O
targeted	O
towards	O
time-consuming	O
computations	O
that	O
can	O
be	O
easily	O
segregated	O
into	O
smaller	O
tasks	O
,	O
sometimes	O
called	O
embarrassingly	O
parallel	O
tasks	O
.	O
</s>
<s>
This	O
includes	O
Monte	B-Algorithm
Carlo	I-Algorithm
calculations	I-Algorithm
,	O
3D	O
rendering	O
and	O
Mandelbrot	B-Algorithm
maps	I-Algorithm
.	O
</s>
<s>
Within	O
the	O
Xgrid	B-Operating_System
protocol	O
,	O
three	O
types	O
of	O
messages	B-Architecture
can	O
be	O
passed	O
to	O
other	O
computers	O
on	O
the	O
same	O
cluster	B-Architecture
:	O
requests	O
,	O
notifications	B-Application
and	O
replies	O
.	O
</s>
<s>
Requests	O
must	O
be	O
responded	O
to	O
by	O
the	O
recipient	O
with	O
a	O
reply	O
,	O
notifications	B-Application
do	O
not	O
require	O
a	O
reply	O
,	O
and	O
replies	O
are	O
responses	O
to	O
sent	O
messages	B-Architecture
.	O
</s>
<s>
Each	O
message	O
is	O
encapsulated	O
in	O
a	O
BEEP	B-Protocol
message	O
(	O
BEEP	B-Protocol
MSG	O
)	O
and	O
is	O
acknowledged	O
on	O
receipt	O
by	O
an	O
empty	O
reply	O
(	O
RPY	O
)	O
.	O
</s>
<s>
Xgrid	B-Operating_System
does	O
not	O
leverage	O
BEEPs	B-Protocol
message/reply	O
infrastructure	O
.	O
</s>
<s>
Any	O
received	O
message	O
which	O
requires	O
a	O
response	O
merely	O
generates	O
an	O
independent	O
BEEP	B-Protocol
message	O
containing	O
the	O
reply	O
.	O
</s>
<s>
The	O
Xgrid	B-Operating_System
messages	B-Architecture
are	O
encoded	O
as	O
dictionaries	O
of	O
key/value	O
pairs	O
which	O
are	O
converted	O
to	O
XML	B-Protocol
before	O
being	O
sent	O
across	O
the	O
BEEP	B-Protocol
network	B-Architecture
.	O
</s>
<s>
The	O
architecture	O
of	O
the	O
Xgrid	B-Operating_System
system	O
is	O
designed	O
around	O
a	O
job	B-General_Concept
based	O
system	O
;	O
the	O
controller	B-Architecture
sends	O
agents	O
jobs	O
,	O
and	O
the	O
agents	O
return	O
the	O
responses	O
.	O
</s>
<s>
The	O
actual	O
computation	O
that	O
the	O
controller	B-Architecture
executes	O
in	O
an	O
Xgrid	B-Operating_System
system	O
is	O
known	O
as	O
a	O
job	B-General_Concept
.	O
</s>
<s>
The	O
job	B-General_Concept
contains	O
all	O
the	O
files	O
required	O
to	O
complete	O
the	O
task	B-General_Concept
successfully	O
,	O
such	O
as	O
the	O
input	O
parameters	O
,	O
data	O
files	O
,	O
directories	B-Application
,	O
executables	B-Application
and/or	O
shell	B-Language
scripts	I-Language
,	O
the	O
files	O
included	O
in	O
an	O
Xgrid	B-Operating_System
job	B-General_Concept
must	O
be	O
able	O
to	O
be	O
executed	O
either	O
simultaneously	O
or	O
asynchronously	O
,	O
or	O
any	O
benefits	O
of	O
running	O
such	O
a	O
job	B-General_Concept
on	O
an	O
Xgrid	B-Operating_System
is	O
lost	O
.	O
</s>
<s>
Once	O
the	O
job	B-General_Concept
completes	O
,	O
the	O
controller	B-Architecture
can	O
be	O
set	O
to	O
notify	O
the	O
client	O
of	O
the	O
task	B-General_Concept
's	O
completion	O
or	O
failure	O
,	O
for	O
example	O
by	O
email	O
.	O
</s>
<s>
The	O
client	O
can	O
leave	O
the	O
network	B-Architecture
while	O
the	O
tasks	O
are	O
running	O
.	O
</s>
<s>
It	O
can	O
also	O
monitor	O
the	O
job	B-General_Concept
status	O
on	O
demand	O
by	O
querying	B-Library
the	O
controller	B-Architecture
,	O
although	O
it	O
cannot	O
track	O
the	O
ongoing	O
progress	O
of	O
individual	O
tasks	O
.	O
</s>
<s>
The	O
controller	B-Architecture
is	O
central	O
to	O
the	O
correct	O
function	O
of	O
an	O
Xgrid	B-Operating_System
,	O
as	O
this	O
node	B-Protocol
is	O
responsible	O
for	O
the	O
distribution	O
,	O
supervision	O
and	O
coordination	O
of	O
tasks	O
on	O
agents	O
.	O
</s>
<s>
The	O
program	O
running	O
on	O
the	O
controller	B-Architecture
can	O
assign	O
and	O
reassign	O
tasks	O
to	O
handle	O
individual	O
agent	O
failures	O
on	O
demand	O
.	O
</s>
<s>
The	O
number	O
of	O
tasks	O
assigned	O
to	O
an	O
agent	O
depend	O
on	O
two	O
factors	O
:	O
the	O
number	O
of	O
agents	O
on	O
an	O
Xgrid	B-Operating_System
and	O
the	O
number	O
of	O
processors	O
in	O
each	O
node	B-Protocol
.	O
</s>
<s>
The	O
number	O
of	O
agents	O
on	O
an	O
Xgrid	B-Operating_System
determines	O
how	O
the	O
controller	B-Architecture
will	O
assign	O
tasks	O
.	O
</s>
<s>
When	O
a	O
node	B-Protocol
with	O
more	O
than	O
one	O
processor	O
is	O
detected	O
on	O
an	O
Xgrid	B-Operating_System
,	O
the	O
controller	B-Architecture
may	O
assign	O
one	O
task	B-General_Concept
per	O
processor	O
;	O
this	O
only	O
occurs	O
if	O
the	O
number	O
of	O
agents	O
on	O
the	O
network	B-Architecture
is	O
lower	O
than	O
the	O
number	O
of	O
tasks	O
the	O
controller	B-Architecture
has	O
to	O
complete	O
.	O
</s>
<s>
Protocol	O
(	O
BEEP	B-Protocol
)	O
,	O
an	O
IETF	O
standard	O
comparable	O
to	O
HTTP	B-Protocol
,	O
but	O
with	O
a	O
focus	O
on	O
two-way	O
multiplexed	O
communication	O
,	O
such	O
as	O
that	O
found	O
in	O
peer-to-peer	O
networks	O
.	O
</s>
<s>
BEEP	B-Protocol
,	O
in	O
turn	O
,	O
uses	O
XML	B-Protocol
to	O
define	O
profiles	O
for	O
communicating	O
between	O
multiple	O
agents	O
over	O
a	O
single	O
network	B-Architecture
or	O
internet	O
connection	O
.	O
</s>
<s>
While	O
it	O
is	O
possible	O
to	O
access	O
Xgrid	B-Operating_System
from	O
the	O
command	B-Application
line	I-Application
,	O
the	O
Xgrid	B-Operating_System
graphical	B-Application
user	I-Application
interface	I-Application
,	O
a	O
program	O
bundled	O
with	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
Server	I-Operating_System
and	O
,	O
as	O
of	O
March	O
2009	O
,	O
available	O
online	O
,	O
is	O
a	O
much	O
more	O
efficient	O
way	O
of	O
administering	O
an	O
Xgrid	B-Operating_System
system	O
.	O
</s>
<s>
Originally	O
,	O
the	O
Xgrid	B-Operating_System
agent	O
was	O
included	O
in	O
all	O
Mac	B-Application
OS	I-Application
X	I-Application
version	I-Application
10.4	I-Application
installations	O
but	O
the	O
GUI	B-Application
was	O
reserved	O
for	O
users	O
of	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
Server	I-Operating_System
.	O
</s>
<s>
Eventually	O
,	O
Apple	O
released	O
the	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
Server	I-Operating_System
Administration	O
Tools	O
to	O
the	O
public	O
,	O
which	O
included	O
the	O
Xgrid	B-Operating_System
administration	O
application	O
bundled	O
with	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
Server	I-Operating_System
.	O
</s>
<s>
Despite	O
the	O
lack	O
of	O
a	O
graphical	O
controller	B-Architecture
interface	O
in	O
the	O
standard	O
(	O
non-server	O
)	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
distribution	O
,	O
it	O
is	O
possible	O
to	O
set	O
up	O
an	O
Xgrid	B-Operating_System
controller	B-Architecture
via	O
the	O
command	B-Application
line	I-Application
tools	I-Application
xgridctl	O
and	O
xgrid	B-Operating_System
.	O
</s>
<s>
Once	O
the	O
Xgrid	B-Operating_System
controller	B-Architecture
daemon	B-Operating_System
is	O
running	O
,	O
administration	O
of	O
the	O
grid	O
with	O
Apple	O
's	O
Xgrid	B-Operating_System
Admin	I-Operating_System
tool	O
is	O
possible	O
.	O
</s>
<s>
Some	O
applications	O
,	O
such	O
as	O
VisualHub	O
,	O
provided	O
Xgrid	B-Operating_System
controller	B-Architecture
capability	O
through	O
their	O
user	O
interfaces	O
.	O
</s>
