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, there are the following constraints for an HTTP response to be cached:
- The method used in the request is itself cacheable, that is either a
HEADmethod. A response to a
POSTmethod can also be cached if freshness is indicated, but this is rarely implemented. Other methods, like
DELETEare not cacheable and their result cannot be.
- The status code of the response is known by the application caching, and it is considered cacheable. The following status code are cacheable:
- There is no 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
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:
GET /pageX.html HTTP/1.1 (…) 200 OK (…)
PUT /pageX.html HTTP/1.1 (…) 200 OK (…)
Cache-Control header in the response can prevent caching:
GET /pageX.html HTTP/1.1 (…) 200 OK Cache-Control: no-cache (…)
- Definition of cacheable in the HTTP specification.