<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
negative	B-Error_Name
cache	I-Error_Name
is	O
a	O
cache	B-General_Concept
that	O
also	O
stores	O
"	O
negative	O
"	O
responses	O
,	O
i.e.	O
</s>
<s>
Usually	O
negative	B-Error_Name
cache	I-Error_Name
is	O
a	O
design	O
choice	O
,	O
but	O
it	O
can	O
also	O
be	O
a	O
software	B-Error_Name
bug	I-Error_Name
.	O
</s>
<s>
Consider	O
a	O
web	B-Application
browser	I-Application
which	O
attempts	O
to	O
load	O
a	O
page	O
while	O
the	O
network	O
is	O
unavailable	O
.	O
</s>
<s>
The	O
browser	B-Application
will	O
receive	O
an	O
error	O
code	O
indicating	O
the	O
problem	O
,	O
and	O
may	O
display	O
this	O
error	O
message	O
to	O
the	O
user	O
in	O
place	O
of	O
the	O
requested	O
page	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
incorrect	O
for	O
the	O
browser	B-Application
to	O
place	O
the	O
error	O
message	O
in	O
the	O
page	O
cache	B-General_Concept
,	O
as	O
this	O
would	O
lead	O
it	O
to	O
display	O
the	O
error	O
again	O
when	O
the	O
user	O
tries	O
to	O
load	O
the	O
same	O
page	O
-	O
even	O
after	O
the	O
network	O
is	O
back	O
up	O
.	O
</s>
<s>
The	O
error	O
message	O
must	O
not	O
be	O
cached	O
under	O
the	O
page	O
's	O
URL	O
;	O
until	O
the	O
browser	B-Application
is	O
able	O
to	O
successfully	O
load	O
the	O
page	O
,	O
whenever	O
the	O
user	O
tries	O
to	O
load	O
the	O
page	O
,	O
the	O
browser	B-Application
must	O
make	O
a	O
new	O
attempt	O
.	O
</s>
<s>
A	O
frustrating	O
aspect	O
of	O
negative	B-Error_Name
caches	I-Error_Name
is	O
that	O
the	O
user	O
may	O
put	O
a	O
great	O
effort	O
into	O
troubleshooting	O
the	O
problem	O
,	O
and	O
then	O
after	O
determining	O
and	O
removing	O
the	O
root	O
cause	O
,	O
the	O
error	O
still	O
does	O
not	O
vanish	O
.	O
</s>
<s>
For	O
instance	O
,	O
DNS	B-Protocol
requires	O
that	O
caching	B-General_Concept
nameservers	O
remember	O
negative	O
responses	O
as	O
well	O
as	O
positive	O
ones	O
.	O
</s>
<s>
The	O
negative	O
response	O
may	O
be	O
perceived	O
as	O
a	O
failure	O
at	O
the	O
application	O
level	O
;	O
however	O
,	O
to	O
the	O
nameserver	O
caching	B-General_Concept
it	O
,	O
it	O
is	O
not	O
a	O
failure	O
.	O
</s>
<s>
The	O
cache	B-General_Concept
times	O
for	O
negative	O
and	O
positive	O
caching	B-General_Concept
may	O
be	O
tuned	O
independently	O
.	O
</s>
<s>
A	O
negative	B-Error_Name
cache	I-Error_Name
is	O
normally	O
only	O
desired	O
if	O
failure	O
is	O
very	O
expensive	O
and	O
the	O
error	O
condition	O
raises	O
automatically	O
without	O
user	O
's	O
action	O
.	O
</s>
<s>
When	O
a	O
failure	O
is	O
cached	O
,	O
the	O
program	O
should	O
provide	O
a	O
clear	O
indication	O
of	O
what	O
must	O
be	O
done	O
to	O
clear	O
the	O
cache	B-General_Concept
,	O
in	O
addition	O
to	O
a	O
description	O
of	O
the	O
cause	O
of	O
the	O
error	O
.	O
</s>
<s>
In	O
such	O
conditions	O
a	O
negative	B-Error_Name
cache	I-Error_Name
is	O
an	O
example	O
of	O
a	O
design	O
anti-pattern	O
.	O
</s>
<s>
Negative	B-Error_Name
cache	I-Error_Name
still	O
may	O
recover	O
if	O
the	O
cached	O
records	O
expires	O
.	O
</s>
