Cacheable
A cacheable response is an HTTP response that can be cached, that is stored to be retrieved and used later, saving a new request to the server. Not all HTTP responses can be cached, these are the following constraints for an HTTP response to be cached:
- The method used in the request is itself cacheable, that is either a
GET
or aHEAD
method. A response to aPOST
orPATCH
request can also be cached if freshness is indicated and theContent-Location
header is set, but this is rarely implemented. For example, Firefox does not support it (Firefox bug 109553). Other methods, likePUT
orDELETE
are not cacheable and their result cannot be cached. - The status code of the response is known by the application caching, and it is considered cacheable. The following status code are cacheable:
200
,203
,204
,206
,300
,301
,404
,405
,410
,414
, and501
. - There are specific headers in the response, like
Cache-Control
, that prevents caching.
Note that some non-cacheable requests/responses to a specific URI may invalidate previously cached responses on the same URI. For example, a PUT
to pageX.html will invalidate all cached GET
or HEAD
requests to the same URI.
When both, the method of the request and the status of the response, are cacheable, the response to the request can be cached:
http
GET /pageX.html HTTP/1.1
(…)
200 OK
(…)
A PUT
request cannot be cached. Moreover, it invalidates cached data for request to the same URI done via HEAD
or GET
:
http
PUT /pageX.html HTTP/1.1
(…)
200 OK
(…)
A specific Cache-Control
header in the response can prevent caching:
http
GET /pageX.html HTTP/1.1
(…)
200 OK
Cache-Control: no-cache
(…)