Cache-Control headers

Even though an object might be fresh in the cache, clients or servers might have constraints that prevent them from retrieving the object from the cache. For example, a client might request that a object not come from a cache, or if it does, it cannot have been cached for more than 10 minutes.

Content Gateway bases the servability of a cached object on Cache-Control headers. Cache-Control headers can appear in both client requests and server responses.

The following Cache-Control headers affect whether objects are served from the cache:

The no-cache header, sent by clients, tells the proxy to serve no objects directly from the cache; always obtain the object from the origin server. You can configure the proxy to ignore client no-cache headers (see Configuring the proxy to ignore client no-cache headers).

The max-age header, sent by servers, is compared to the object age; if the age is less than max-age, the object is fresh and can be served.

The min-fresh header, sent by clients, is an acceptable freshness tolerance. The client wants the object to be at least this fresh. If a cached object does not remain fresh at least this long in the future, it is revalidated.

The max-stale header, sent by clients, permits the proxy to serve stale objects provided they are not too old. Some browsers might be willing to take slightly old objects in exchange for improved performance, especially during periods of poor Internet availability.

The proxy applies Cache-Control servability criteria after HTTP freshness criteria. For example, an object might be considered fresh, but if its age is greater than its max-age, it is not served.