<s>
Perlbal	B-Language
is	O
a	O
Perl-based	O
reverse	B-General_Concept
proxy	I-General_Concept
load	B-Application
balancer	I-Application
and	O
web	B-Application
server	I-Application
.	O
</s>
<s>
Perlbal	B-Language
is	O
maintained	O
by	O
a	O
group	O
connected	O
to	O
Danga	O
Interactive	O
.	O
</s>
<s>
Like	O
Perl	B-Language
,	O
Perlbal	B-Language
is	O
distributed	O
under	O
both	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
and	O
the	O
Artistic	B-License
License	I-License
and	O
is	O
thus	O
free	B-Application
software	I-Application
.	O
</s>
<s>
Perlbal	B-Language
also	O
features	O
a	O
so-called	O
"	O
re-proxy	O
"	O
mechanism	O
.	O
</s>
<s>
This	O
allows	O
a	O
backend	O
webapp	O
to	O
send	O
back	O
a	O
small	O
response	O
to	O
Perlbal	B-Language
which	O
acts	O
as	O
an	O
"	O
internal	O
redirect	O
"	O
and	O
causes	O
it	O
to	O
get	O
the	O
response	O
data	O
from	O
elsewhere	O
.	O
</s>
<s>
This	O
is	O
often	O
used	O
so	O
that	O
expensive	O
application	O
server	O
processes	O
can	O
defer	O
to	O
much	O
simpler	O
,	O
faster	O
HTTP	B-Application
servers	I-Application
to	O
serve	O
static	O
files	O
,	O
freeing	O
up	O
the	O
application	O
server	O
for	O
other	O
computing	O
work	O
.	O
</s>
<s>
This	O
is	O
used	O
on	O
LiveJournal	B-Application
to	O
provide	O
predefined	O
page	O
templates	O
whose	O
colors	O
,	O
including	O
those	O
"	O
baked	O
"	O
into	O
images	O
,	O
can	O
be	O
customized	O
by	O
the	O
user	O
.	O
</s>
<s>
Doing	O
this	O
relatively	O
simple	O
filtering	O
task	O
at	O
the	O
load	B-Application
balancer	I-Application
layer	O
avoids	O
the	O
overhead	O
of	O
passing	O
this	O
request	O
back	O
to	O
the	O
full	O
application	O
servers	O
.	O
</s>
