Sweet Baby

All caches have a set of rules that they use to determine when to serve a representation from the cache, if it’s available. Some of these rules are set in the protocols (HTTP 1.0 and 1.1), and some are set by the administrator of the cache (either the user of the browser cache, or the proxy administrator).

Generally speaking, these are the most common rules that are followed (don’t worry if you don’t understand the details, it will be explained below):

If the response’s headers tell the cache not to keep it, it won’t.

If the request is authenticated or secure (i.e., HTTPS), it won’t be cached.

A cached representation is considered fresh (that is, able to be sent to a client without checking with the origin server) if:

It has an expiry time or other age-controlling header set, and is still within the fresh period, or

If the cache has seen the representation recently, and it was modified relatively long ago.

Fresh representations are served directly from the cache, without checking with the origin server.

If a representation is stale, the origin server will be asked to validate it, or tell the cache whether the copy that it has is still good.

Under certain circumstances — for example, when it’s disconnected from a network — a cache can serve stale responses without checking with the origin server.

If no validator (an ETag or Last-Modified header) is present on a response, and it doesn’t have any explicit freshness information, it will usually — but not always — be considered uncacheable.

Together, freshness and validation are the most important ways that a cache works with content. A fresh representation will be available instantly from the cache, while a validated representation will avoid sending the entire representation over again if it hasn’t changed.