Intel 1520 Deciding whether to serve HTTP objects

Appendix A Caching Solutions and Performance 117

For example, if a document was last modiﬁed 32 days ago and was sent to the

appliance two days ago, the document is considered fresh in cache for three

days after it was sent. (This assumes a factor of 10%.) So for this situation,

the document is considered fresh for one more day.

Because this method could result in lengthy freshness times for documents

that have not changed for long periods, cache administrators might want to

place an upper boundary on the freshness limit. With this boundary in place

the freshness limit becomes the smaller of the two values: the boundary or the

computed freshness limit. For information on how to conﬁgure an upper

boundary, refer to the Freshness section of the Conﬁgure: Cache page of the

Manager UI. See Freshness‚ on page 36.

Default test:

For documents that do not have Expires headers or do not have both Last-

Modified and Date headers, you can specify an absolute freshness limit in

the Freshness section of the Conﬁgure: Cache page. See Freshness‚ on

page 36.

Revalidate rules:

Revalidate rules apply speciﬁc freshness limits to speciﬁc HTTP or FTP

objects. From the command-line interface, you can set freshness limits for

objects originating from particular domains or IP addresses, objects with

URL addresses that contain speciﬁed regular expressions, and objects

requested by particular clients. See Conﬁguring caching rules‚ on page 79.

Deciding whether to serve HTTP objects

Even though a document might be fresh in the cache, clients or servers could

have constraints that prevent them from retrieving the document from the cache.

For example, a client might request that a document not come from a cache, or if

it does, the document cannot have been cached for more than 10 minutes.

The Intel NetStructure Cache Appliance bases the servability of a cached

document on Cache-Control header ﬁelds. These headers can appear in both

client requests and server responses.

The following cache-control header ﬁelds affect whether objects are served:

✔The no-cache ﬁeld, sent by clients, tells the appliance to serve no objects

directly from the cache; always revalidate. You can conﬁgure the appliance to

ignore client no-cache ﬁelds. See Cache activation‚ on page 35.

✔The max-age ﬁeld, sent by servers, is compared to the document age; if the

age is less than the max-age, the document is fresh and can be served.