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Next: <a rel="next" accesskey="n" href="Memory-cache.html#Memory-cache">Memory cache</a>,
Previous: <a rel="previous" accesskey="p" href="Caching.html#Caching">Caching</a>,
Up: <a rel="up" accesskey="u" href="Caching.html#Caching">Caching</a>
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<h3 class="section">4.1 Cache transparency and validation</h3>
<p><a name="index-transparent-cache-182"></a><a name="index-cache-transparency-183"></a><a name="index-out_002dof_002ddate-instances-184"></a><a name="index-validation-185"></a><a name="index-revalidation-186"></a><a name="index-expire-187"></a><a name="index-stale-188"></a><a name="index-fresh-189"></a>
If resources on a server change, it is possible for a cached instance
to become out-of date. Ideally, a cache would be perfectly
<dfn>transparent</dfn>, meaning that it never serves an out-of-date
instance; in a universe with a finite speed of signal propagation,
however, this ideal is impossible to achieve.
<p>If a caching proxy decides that a cached instance is new enough to
likely still be valid, it will directly serve the instance to the
client; we then say that the cache decided that the instance is
<dfn>fresh</dfn>. When an instance is <dfn>stale</dfn> (not fresh), the cache
will check with the upstream server whether the resource has changed;
we say that the cached instance is being <dfn>revalidated</dfn>.
<p>In HTTP/1.1, responsibility for revalidation is shared between the
client, the server and the proxy itself. The client can override
revalidation policy by using the ‘<samp><span class="samp">Cache-Control</span></samp>’
header<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>; for example,
some user-agents will request end-to-end revalidation in this way when
the user shift-clicks on <em>reload</em>. The server may choose to
specify revalidation policy by using the ‘<samp><span class="samp">Expires</span></samp>’ and
‘<samp><span class="samp">Cache-Control</span></samp>’ headers. As to the proxy, it needs to choose a
revalidation policy for instances with neither server- nor client-side
cache control information. Of course, nothing (except the HTTP/1.1
spec, but that is easily ignored) prevents a proxy from overriding the
client's and server's cache control directives.
<ul class="menu">
<li><a accesskey="1" href="Tuning-validation.html#Tuning-validation">Tuning validation</a>: Tuning Polipo's validation behaviour.
<li><a accesskey="2" href="Tweaking-validation.html#Tweaking-validation">Tweaking validation</a>: Further tweaking of validation.
</ul>
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<hr>
<h4>Fußnoten</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> Or the obsolete ‘<samp><span class="samp">Pragma</span></samp>’ header.</p>
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