<s>
Cache	B-General_Concept
invalidation	I-General_Concept
is	O
a	O
process	O
in	O
a	O
computer	O
system	O
whereby	O
entries	O
in	O
a	O
cache	B-General_Concept
are	O
replaced	O
or	O
removed	O
.	O
</s>
<s>
It	O
can	O
be	O
done	O
explicitly	O
,	O
as	O
part	O
of	O
a	O
cache	B-General_Concept
coherence	I-General_Concept
protocol	O
.	O
</s>
<s>
Cache	B-General_Concept
invalidation	I-General_Concept
can	O
be	O
used	O
to	O
push	B-General_Concept
new	O
content	O
to	O
a	O
client	B-Protocol
.	O
</s>
<s>
Invalidation	O
is	O
carried	O
out	O
by	O
changing	O
the	O
application	O
data	O
,	O
which	O
in	O
turn	O
marks	O
the	O
information	O
received	O
by	O
the	O
client	B-Protocol
as	O
out-of-date	O
.	O
</s>
<s>
After	O
the	O
cache	B-General_Concept
is	O
invalidated	O
,	O
if	O
the	O
client	B-Protocol
requests	O
the	O
cache	B-General_Concept
,	O
they	O
are	O
delivered	O
a	O
new	O
version	O
.	O
</s>
<s>
There	O
are	O
three	O
specific	O
methods	O
to	O
invalidate	O
a	O
cache	B-General_Concept
,	O
but	O
not	O
all	O
caching	B-General_Concept
proxies	O
support	O
these	O
methods	O
.	O
</s>
<s>
Removes	O
content	O
from	O
caching	B-General_Concept
proxy	O
immediately	O
.	O
</s>
<s>
When	O
the	O
client	B-Protocol
requests	O
the	O
data	O
again	O
,	O
it	O
is	O
fetched	B-General_Concept
from	O
the	O
application	O
and	O
stored	O
in	O
the	O
caching	B-General_Concept
proxy	O
.	O
</s>
<s>
The	O
content	O
previously	O
stored	O
in	O
the	O
cache	B-General_Concept
is	O
replaced	O
with	O
a	O
new	O
version	O
from	O
the	O
application	O
.	O
</s>
<s>
Client	B-Protocol
requests	O
are	O
then	O
checked	O
against	O
this	O
blacklist	O
,	O
and	O
if	O
a	O
request	O
matches	O
,	O
new	O
content	O
is	O
fetched	B-General_Concept
from	O
the	O
application	O
,	O
returned	O
to	O
the	O
client	B-Protocol
,	O
and	O
added	O
to	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
This	O
method	O
,	O
unlike	O
purge	O
,	O
does	O
not	O
immediately	O
remove	O
cached	O
content	O
from	O
the	O
caching	B-General_Concept
proxy	O
.	O
</s>
<s>
Instead	O
,	O
the	O
cached	O
content	O
is	O
updated	O
after	O
a	O
client	B-Protocol
requests	O
that	O
specific	O
information	O
.	O
</s>
<s>
There	O
are	O
a	O
few	O
alternatives	O
to	O
cache	B-General_Concept
invalidation	I-General_Concept
that	O
still	O
deliver	O
updated	O
content	O
to	O
the	O
client	B-Protocol
.	O
</s>
<s>
One	O
alternative	O
is	O
to	O
expire	O
the	O
cached	O
content	O
quickly	O
by	O
reducing	O
the	O
time-to-live	B-Protocol
(	O
TTL	O
)	O
to	O
a	O
very	O
low	O
value	O
.	O
</s>
<s>
A	O
third	O
option	O
is	O
to	O
not	O
cache	B-General_Concept
volatile	O
content	O
requested	O
by	O
the	O
client	B-Protocol
.	O
</s>
<s>
Cache	B-General_Concept
invalidation	I-General_Concept
must	O
be	O
carried	O
out	O
through	O
a	O
caching	B-General_Concept
proxy	O
;	O
these	O
requests	O
can	O
impact	O
performance	O
of	O
the	O
caching	B-General_Concept
proxy	O
,	O
causing	O
information	O
to	O
be	O
transferred	O
at	O
a	O
slower	O
rate	O
to	O
clients	O
.	O
</s>
