HTTP caching is a standard for caching of resources downloaded from the network. By default, all browsers have an implementation of an HTTP cache. This feature provides faster load-times and better server performance.
The caching details of each resource is provided in the response headers returned by the server as a part of the response to a resource request. The Cache-Control header specifies whether the resource can be cached, caching duration, and whether the cache is shareable or not.
By default, Cortex returns no-cache and no-transform directives in the Cache-Control header in the GET requests. The no-cache directive specifies that the client validates each GET request to the server and the no-transform indicates that the client must not modify the resource when caching.
- private: Specifies that the resource is for a single user and not shareable.
- no-transform: specifies that the resource must not be modified when cached.
- max-age=<seconds>: Specifies for how long the cached resource is considered as up-to-date.
ETag in Cortex Responses
Cortex returns an ETag header with all resources. The ETag header is used to validate a stale resource when the client sends a request for the same resource again. A resources is considered stale when the max-age expires or when a no-cache directive is returned.
For a stale resource, the next request from the client sends the ETag value in a If-None-Match header. When the ETag value on the server and client matches, the server sends a 304 Not Modified status without a body to the client. This response indicates that the client can use the previously returned response of the resource. With this header, server need not send the complete response back and can save the bandwidth.