Intel 1520 manual Revalidating objects, Http object freshness tests, Expires header test

HTTP Web documents support optional author-specified expiration dates. The appliance adheres to these expiration dates; otherwise it picks an expiration date based on how frequently the document is changing and on administrator-chosen freshness guidelines. In addition, documents can be revalidated, checking with the server if a document is still fresh.

Revalidating objects

If an HTTP object is stale, the Intel NetStructure Cache Appliance revalidates the object. A revalidation is a query to the origin server that asks if the object is unchanged. The result of a revalidation could be:

✔The object is still fresh; the appliance resets its freshness limit and serves the object.

✔A new copy of the object is available; the appliance caches the new object, replacing the stale copy, and serves the object to the user simultaneously.

✔The object no longer exists on the origin server; the appliance does not serve the cached copy.

✔The origin server does not respond to the revalidation query. The appliance serves the stale object along with a 111 Revalidation Failed warning.

HTTP object freshness tests

Here’s how the Intel NetStructure Cache Appliance determines an HTTP document’s freshness:

Expires header test:

Some documents come with Expires headers or max-ageheaders that explicitly define how long the document can be cached. A simple comparison of the current time with the expiration time determines whether or not the document is fresh.

Last-Modified / Date header test:

If no expiration information exists, the appliance can use the Last- Modified and Date headers to estimate a freshness limit. The Last- Modified header indicates how long ago a document was modified. If a document was last modified two years ago, it is unlikely to suddenly change, so the appliance can cache it safely for a while. But if the document just changed five minutes ago, it might be quite volatile, and the appliance should not cache it very long. The appliance stores an object for some percentage of the time (F) that elapsed since the object last changed. The percentage is 10% by default:

freshness limit = F * (Date - Last-Modified)

In the above formula, the Date header provides the date the object was sent to the appliance and the Last-Modifiedheader provides the date the object was last modified on the origin server.

116Intel NetStructure Cache Appliance Administrator’s Guide