<s>
Edge	B-Language
Side	I-Language
Includes	I-Language
(	O
ESI	O
)	O
is	O
a	O
small	O
markup	O
language	O
for	O
edge	B-Device
level	O
dynamic	O
web	O
content	O
assembly	O
.	O
</s>
<s>
It	O
is	O
an	O
application	O
of	O
edge	B-Device
computing	I-Device
.	O
</s>
<s>
This	O
creates	O
a	O
problem	O
for	O
caching	B-Protocol
systems	O
.	O
</s>
<s>
To	O
overcome	O
this	O
problem	O
a	O
group	O
of	O
companies	O
(	O
Akamai	B-Application
,	O
Art	O
Technology	O
Group	O
,	O
BEA	O
Systems	O
,	O
Circadence	O
Corporation	O
,	O
Digital	O
Island	O
,	O
Inc.	O
,	O
Interwoven	O
,	O
Inc.	O
,	O
Open	O
Market	O
,	O
whose	O
ESI-related	O
technology	O
is	O
now	O
owned	O
by	O
FatWire	O
Software	O
,	O
Oracle	B-Application
Corporation	I-Application
and	O
Vignette	O
Corporation	O
)	O
developed	O
the	O
ESI	O
specification	O
and	O
submitted	O
it	O
to	O
the	O
W3C	O
for	O
approval	O
.	O
</s>
<s>
ESI	O
is	O
implemented	O
by	O
some	O
content	B-Operating_System
delivery	I-Operating_System
networks	I-Operating_System
,	O
such	O
as	O
Akamai	B-Application
,	O
and	O
by	O
some	O
caching	B-Protocol
proxy	B-Application
servers	I-Application
such	O
as	O
Varnish	B-Language
,	O
Squid	B-Protocol
and	O
Mongrel	B-Application
ESI	O
,	O
although	O
many	O
do	O
not	O
implement	O
the	O
complete	O
specification	O
.	O
</s>
<s>
Akamai	B-Application
also	O
adds	O
additional	O
features	O
to	O
the	O
version	O
they	O
support	O
.	O
</s>
<s>
The	O
XML	B-Protocol
based	O
ESI	O
tags	O
indicate	O
to	O
the	O
edge-side	O
processing	O
agent	O
the	O
action	O
that	O
needs	O
to	O
be	O
taken	O
to	O
complete	O
the	O
page	O
's	O
assembly	O
.	O
</s>
<s>
The	O
ESI	O
system	O
is	O
usually	O
a	O
caching	B-Protocol
proxy	B-Application
server	I-Application
so	O
it	O
may	O
have	O
a	O
local	O
copy	O
of	O
these	O
files	O
which	O
it	O
can	O
insert	O
without	O
going	O
back	O
to	O
the	O
server	O
.	O
</s>
<s>
Alternatively	O
the	O
whole	O
page	O
with	O
the	O
ESI	O
tags	O
may	O
be	O
cached	O
,	O
and	O
only	O
the	O
ESI	O
requests	O
may	O
be	O
made	O
to	O
the	O
origin	B-Application
server	I-Application
.	O
</s>
<s>
This	O
allows	O
different	O
caching	B-Protocol
times	O
for	O
different	O
parts	O
of	O
the	O
page	O
,	O
or	O
different	O
degrees	O
of	O
personalisation	O
.	O
</s>
<s>
variables	O
which	O
can	O
be	O
set	O
from	O
cookies	B-Application
or	O
HTTP	B-Protocol
headers	I-Protocol
and	O
then	O
used	O
in	O
other	O
ESI	O
statements	O
or	O
written	O
into	O
markup	O
;	O
</s>
<s>
conditions	O
so	O
that	O
different	O
markup	O
can	O
be	O
used	O
based	O
on	O
variables	O
,	O
for	O
example	O
if	O
a	O
cookie	B-Application
is	O
set	O
or	O
not	O
;	O
</s>
<s>
error	O
handling	O
,	O
so	O
that	O
a	O
failover	B-General_Concept
can	O
be	O
used	O
if	O
an	O
origin	B-Application
server	I-Application
is	O
unavailable	O
.	O
</s>
<s>
to	O
do	O
assembly	O
on	O
the	O
origin	B-Application
server	I-Application
using	O
Server	O
Side	O
Includes	O
and	O
many	O
types	O
of	O
server-side	B-Language
scripting	I-Language
.	O
</s>
<s>
to	O
do	O
assembly	O
in	O
the	O
browser	B-Application
using	O
web	B-Language
components	I-Language
.	O
</s>
