<s>
In	O
distributed	B-Architecture
computing	I-Architecture
,	O
code	B-Protocol
on	I-Protocol
demand	I-Protocol
is	O
any	O
technology	O
that	O
sends	O
executable	O
software	O
code	O
from	O
a	O
server	O
computer	O
to	O
a	O
client	O
computer	O
upon	O
request	O
from	O
the	O
client	O
's	O
software	O
.	O
</s>
<s>
Some	O
well-known	O
examples	O
of	O
the	O
code	B-Protocol
on	I-Protocol
demand	I-Protocol
paradigm	O
on	O
the	O
web	O
are	O
Java	B-Application
applets	I-Application
,	O
Adobe	O
's	O
ActionScript	B-Operating_System
language	O
for	O
the	O
Flash	B-Operating_System
Player	I-Operating_System
,	O
and	O
JavaScript	B-Language
.	O
</s>
<s>
The	O
program	O
code	O
lies	O
inactive	O
on	O
a	O
web	B-Application
server	I-Application
until	O
a	O
user	O
(	O
client	O
)	O
requests	O
a	O
web	O
page	O
that	O
contains	O
a	O
link	O
to	O
the	O
code	O
using	O
the	O
client	O
's	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
Upon	O
this	O
request	O
,	O
the	O
web	O
page	O
and	O
the	O
program	O
are	O
transported	O
to	O
the	O
user	O
's	O
machine	O
using	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
When	O
the	O
page	O
is	O
displayed	O
,	O
the	O
code	O
is	O
started	O
in	O
the	O
browser	B-Application
and	O
executes	O
locally	O
,	O
inside	O
the	O
user	O
's	O
computer	O
until	O
it	O
is	O
stopped	O
(	O
e.g.	O
,	O
by	O
the	O
user	O
leaving	O
the	O
web	O
page	O
)	O
.	O
</s>
<s>
Code	B-Protocol
on	I-Protocol
demand	I-Protocol
is	O
a	O
specific	O
use	O
of	O
mobile	B-Operating_System
code	I-Operating_System
,	O
within	O
the	O
field	O
of	O
code	B-Operating_System
mobility	I-Operating_System
.	O
</s>
<s>
Code	B-Protocol
on	I-Protocol
demand	I-Protocol
(	O
COD	O
)	O
is	O
the	O
only	O
optional	O
constraint	O
in	O
REST	O
.	O
</s>
<s>
For	O
instance	O
,	O
with	O
code	B-Protocol
on	I-Protocol
demand	I-Protocol
,	O
a	O
client	O
can	O
download	O
a	O
Javascript	B-Language
,	O
Java	B-Application
applet	I-Application
or	O
even	O
a	O
Flash	O
application	O
in	O
order	O
to	O
encrypt	O
communication	O
so	O
servers	O
are	O
not	O
aware	O
of	O
any	O
encryption	O
routines	O
/	O
keys	O
used	O
in	O
this	O
process	O
.	O
</s>
