Intel 1520 ユーザーズマニュアル

Intel NetStructure Cache Appliance Administrator’s Guide

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:

Some documents come with 

Expires

 headers or 

max-age

 headers 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. 

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:

In the above formula, the 

Date

 header provides the date the object was sent 

to the appliance and the 

Last-Modified

 header provides the date the object 

was last modified on the origin server. 

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