32 CHAPTER 1: WEB CACHING CONCEPTS AND DEPLOYMENTCurrent and ExpiredContentContent stored in the cache can either be current (also known as fresh) orexpired (also known as stale). If it is current, the content is up to date andthe 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 If it is expired,the content is out of date and the Webcache connects to the origin Webserver and retrieves the content.The Webcache determines if content is expired or current differentlydepending on the protocol involved: HTTP — Web documents support optional author-specified expirationdates. The Webcache adheres to these expiration dates; otherwise ituses advanced heuristics to pick an expiration date based on howfrequently the document is changing. In addition, documents can berevalidated, where the Webcache checks with the origin server to findout if a document is still current. FTP — FTP documents stay in the cache for 72 hours.Revalidating HTTPContentIf HTTP content in the cache is expired, the Webcache revalidates it. Arevalidation is a query to the origin server that asks if the content isunchanged. The result of a revalidation can be: The content is still current; the Webcache resets its limit and serves thecontent. A current copy of the content is available; the Webcache caches thecurrent content, replacing the expired copy, and serves the content tothe user simultaneously. The content no longer exists on the origin server; the Webcache doesnot serve the cached copy. The origin server does not respond to the revalidation query. TheWebcache serves the expired content.Deployment ModesOverviewTo operate successfully the Webcache must be able to intercept andcontrol client machine requests for Web content before those requestsreach the WAN. You must either explicitly configure the client machinesto send their requests directly to the Webcache, or use additionalnetwork devices to transparently redirect the requests. You must choose amethod of directing Web requests from client machines in your networkto the Webcache before you install the Webcache in your network.dua1611-5aaa04.book Page 32 Friday, November 29, 2002 8:56 PM
