Current And Expired Content; Revalidating Http Content; Deployment Modes Overview - 3Com SUPERSTACK 3 WEBCACHE 3000 User Manual

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HAPTER EB
Current and Expired
Content
Revalidating HTTP
Content
Deployment Modes
Overview
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ACHING ONCEPTS AND
Content stored in the cache can either be current (also known as fresh) or
expired (also known as stale). If it is current, the content is up to date and
the Webcache serves it to the client machine as a cache hit. If it is expired,
the content is out of date and the Webcache connects to the origin Web
server and retrieves the content.
The Webcache determines if content is expired or current differently
depending on the protocol involved:
HTTP — Web documents support optional author-specified expiration
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dates. The Webcache adheres to these expiration dates; otherwise it
uses advanced heuristics to pick an expiration date based on how
frequently the document is changing. In addition, documents can be
revalidated, where the Webcache checks with the origin server to find
out if a document is still current.
FTP — FTP documents stay in the cache for 72 hours.
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If HTTP content in the cache is expired, the Webcache revalidates it. A
revalidation is a query to the origin server that asks if the content is
unchanged. The result of a revalidation can be:
The content is still current; the Webcache resets its limit and serves the
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content.
A current copy of the content is available; the Webcache caches the
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current content, replacing the expired copy, and serves the content to
the user simultaneously.
The content no longer exists on the origin server; the Webcache does
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not serve the cached copy.
The origin server does not respond to the revalidation query. The
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Webcache serves the expired content.
To operate successfully the Webcache must be able to intercept and
control client machine requests for Web content before those requests
reach the WAN. You must either explicitly configure the client machines
to send their requests directly to the Webcache, or use additional
network devices to transparently redirect the requests. You must choose a
method of directing Web requests from client machines in your network
to the Webcache before you install the Webcache in your network.
EPLOYMENT