<s>
Post/Redirect/Get	B-Application
(	O
PRG	O
)	O
is	O
a	O
web	O
development	O
design	O
pattern	O
that	O
lets	O
the	O
page	O
shown	O
after	O
a	O
form	B-General_Concept
submission	O
be	O
reloaded	O
,	O
shared	O
,	O
or	O
bookmarked	O
without	O
ill	O
effects	O
,	O
such	O
as	O
submitting	O
the	O
form	B-General_Concept
another	O
time	O
.	O
</s>
<s>
When	O
a	O
web	B-General_Concept
form	I-General_Concept
is	O
submitted	O
to	O
a	O
server	O
through	O
an	O
HTTP	B-Protocol
POST	I-Protocol
request	I-Protocol
,	O
attempts	O
to	O
refresh	O
the	O
server	O
response	O
can	O
cause	O
the	O
contents	O
of	O
the	O
original	O
POST	B-Protocol
to	O
be	O
resubmitted	O
,	O
possibly	O
causing	O
undesired	O
results	O
,	O
such	O
as	O
a	O
duplicate	O
web	O
purchase	O
.	O
</s>
<s>
Some	O
browsers	O
mitigate	O
this	O
risk	O
by	O
warning	O
the	O
user	O
that	O
they	O
are	O
about	O
to	O
re-issue	O
a	O
POST	B-Protocol
request	I-Protocol
.	O
</s>
<s>
To	O
avoid	O
this	O
problem	O
,	O
many	O
web	O
developers	O
use	O
the	O
PRG	O
pattern	O
—	O
instead	O
of	O
responding	O
with	O
content	O
,	O
the	O
server	O
responds	O
to	O
a	O
POST	B-Protocol
request	I-Protocol
by	O
redirecting	O
the	O
client	O
to	O
another	O
location	O
.	O
</s>
<s>
The	O
HTTP	B-Protocol
1.1	I-Protocol
specification	O
introduced	O
the	O
HTTP	B-Protocol
303	I-Protocol
(	O
"	O
See	B-Protocol
other	I-Protocol
"	O
)	O
response	O
code	O
to	O
ensure	O
that	O
in	O
this	O
situation	O
,	O
browsers	O
can	O
safely	O
refresh	O
the	O
server	O
response	O
without	O
causing	O
the	O
initial	O
POST	B-Protocol
request	I-Protocol
to	O
be	O
resubmitted	O
.	O
</s>
<s>
The	O
PRG	O
pattern	O
cannot	O
address	O
every	O
scenario	O
of	O
duplicate	O
form	B-General_Concept
submission	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
web	O
user	O
refreshes	O
before	O
the	O
initial	O
submission	O
completes	O
,	O
possibly	O
because	O
of	O
server	O
lag	O
,	O
a	O
duplicate	O
POST	B-Protocol
occurs	O
in	O
certain	O
user	O
agents	O
.	O
</s>
<s>
User	O
agents	O
(	O
such	O
as	O
browsers	O
)	O
store	O
only	O
the	O
URL	B-Protocol
of	O
an	O
HTTP	B-Protocol
request	O
as	O
a	O
bookmark	O
.	O
</s>
<s>
Because	O
of	O
this	O
,	O
a	O
response	O
based	O
on	O
the	O
body	O
of	O
the	O
HTTP	B-Protocol
POST	I-Protocol
request	I-Protocol
cannot	O
be	O
bookmarked	O
.	O
</s>
<s>
With	O
the	O
PRG	O
pattern	O
,	O
the	O
URI	B-Protocol
of	O
the	O
request	O
can	O
safely	O
be	O
bookmarked	O
.	O
</s>
