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<h3 class="section">1.5 Caching partial instances</h3>
<p><a name="index-partial-instance-15"></a><a name="index-range-request-16"></a>
A partial instance is an instance that is being cached but only part
of which is available in the local cache. There are three ways in
which partial instances can arise: client applications requesting only
part of an instance (Adobe's Acrobat Reader plugin is famous for
that), a server dropping a connection mid-transfer (because it is
short on resources, or, surprisingly often, because it is buggy), a
client dropping a connection (usually because the user pressed
<em>stop</em>).
<p>When an instance is requested that is only partially cached, it is
possible to request just the missing data by using a feature of HTTP
known as a <dfn>range</dfn> request. While support for range requests is
optional, most servers honour them in case of static data (data that
are stored on disk, rather then being generated on the fly e.g. by a
CGI script).
<p>Caching partial instances has a number of positive effects. Obviously,
it reduces the amount of data transmitted as the available data
needn't be fetched again. Because it prevents partial data from being
discarded, it makes it reasonable for a proxy to unconditionally abort
a download when requested by the user, and therefore reduces network
traffic.
<p>Polipo caches arbitrary partial instances in its in-memory cache. It
will only store the initial segment of a partial instance (from its
beginning up to its first hole) in its on-disk cache, though. In
either case, it will attempt to use range requests to fetch the
missing data.
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