Although caching can reduce user-perceived response times, it introduces a new problem--the copy of an object residing
in the cache may be stale. The object housed in the Web server may have been modified since the copy was cached at the
client. HTTP has a mechanism that allows a cache to verify that its objects are up to date. This mechanism is
called the coditional GET. An HTTP request message isa so-called conditional GET message if (1) the
request message uses the GET method and (2) the request message includes an If-Modified-Since: header line.
The cache forwards the object to the requesting browser but also caches the object locally. The cache also stores
the last-modified date along withthe object. Third, one week later, another browser request the same object via the
cache, and the object is still in the cache. Since this object may have been modified at the Web server in the past
week, the cache performs an up-to-date check by issuing a conditional GET.
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