Doing HTTP Caching Right: Introducing httplib2
by Joe Gregorio | Pages: 1, 2, 3, 4

Cache-Control

While validators are used to test if a cached entry is still valid, the Cache-Control: header is used to signal how long a representation can be cached. The most fundamental of all the cache-control directives is max-age. This directive asserts that the cached response can be only max-age seconds old before being considered stale. Note that max-age can appear in both request headers and response headers, which gives both the client and server a chance to assert how old they like their responses cached. If a cached response is fresh then we can return the cached response immediately; if it's stale then we need to validate the cached response before returning it.

Let's take another look at our example response from above. Note that the Cache-Control: header is set and that a max-age of 7200 means that the entry can be cached for up to two hours.

HTTP/1.1 200 OK
Date: Fri, 30 Dec 2005 17:32:47 GMT
Server: Apache
ETag: "0192384-9023-1a929893"
Accept-Ranges: bytes
Content-Length: 33286
Vary: Accept-Encoding,User-Agent
Cache-Control: max-age=7200
Expires: Fri, 30 Dec 2005 19:30:56 GMT
Content-Type: text/xml

There are lots of directives that can be put in the Cache-Control: header, and the Cache-Control: header may appear in both requests and/or responses.

Directives Allowed in a Request

Directive	Description
no-cache	The cached response must not be used to satisfy this request.
no-store	Do not store this response in a cache.
max-age=delta-seconds	The client is willing to accept a cached reponse that is delta-seconds old without validating.
max-stale=delta-seconds	The client is willing to accept a cached response that is no more than delta-seconds stale.
min-fresh=delta-seconds	The client is willing to accept only a cached response that will still be fresh delta-seconds from now.
no-transform	The entity body must not be transformed.
only-if-cached	Return a response only if there is one in the cache. Do not validate or GET a response if no cache entry exists.

Directives Allowed in a Response

Directive	Description
public	This can be cached by any cache.
private	This can be cached only by a private cache.
no-cache	The cached response must not be used on subsequent requests without first validating it.
no-store	Do not store this response in a cache.
no-transform	The entity body must not be transformed.
must-revalidate	If the cached response is stale it must be validated before it is returned in any response. Overrides max-stale.
max-age=delta-seconds	The client is willing to accept a cached reponse that is delta-seconds old without validating.
s-maxage=delta-seconds	Just like max-age but it applies only to shared caches.
proxy-revalidate	Like must-revalidate, but only for proxies.

Let's look at some Cache-Control: header examples.

Cache-Control: private, max-age=3600

If sent by a server, this Cache-Control: header states that the response can only be cached in a private cache for one hour.

Cache-Control: public, must-revalidate, max-age=7200

The included response can be cached by a public cache and can be cached for two hours; after that the cache must revalidate the entry before returning it to a subsequent request.

Cache-Control: must-revalidate, max-age=0

This forces the client to revalidate every request, since a max-age=0 forces the cached entry to be instantly stale. See Mark Nottingham's Leveraging the Web: Caching for a nice example of how this can be applied.

Cache-Control: no-cache

This is pretty close to must-revalidate, max-age=0, except that a client could use a max-stale header on a request and get a stale response. The must-revalidate will override the max-stale property. I told you that giving both client and server some control would make things a bit complicated.

So far all of the Cache-Control: header examples we have looked at are on the response side, but they can also be added on the request too.

Cache-Control: no-cache

This forces an "end-to-end reload," where the client forces the cache to reload its cache from the origin server.

Cache-Control: min-fresh=200

Here the client asserts that it wants a response that will be fresh for at least 200 seconds.

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