Understanding HTTP caching

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The best request is the request that doesn't have to communicate with the server

Every browser comes with an implementation of HTTP cache. Browsers are instructed on what strategies to use for caching from the response sent from the server. A user requests a resource from a server and the server responds with a response with response headers

The most common response headers are Status, cache-control, length and e-tag

Validating with Etags

If a response has expired as stated in the cache control header, if the original response had an etag, the etag is sent back to the server using the If-None-Match request header to see if the browser should get an updated version of the response. If not, then the response is returned with a 304 Not Modified header and the browser does not download the response, saving bandwidth. This is all done automatically, the only thing that needs to be done is the validation of the e-tag on the server

Each resource can define it's own caching policy using the cache-control header which describes who can cache the response, under what conditions, and for how long

Caching Policies

No-Cache - No cache means check to see if there are any changes in the resource first (using the e-tag)

No-store - The server directs the client browser to never store this resource. For example if there is sensitive information in the reponse

Private cache policies mean that it's intended for a user's browser and not an intermediary. If a resource is private it can be cached by a browser but not a CDN

max-age - States how long in seconds can the resource be used until it has to validate using an e-tag

Cache Busting

Cache busting is changing the URL of the resource that is already cached, forcing the user to use the new resource.

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Understanding HTTP caching

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