Learn how Cloudflare interprets Cache-Control headers set at your origin server.

Overview

Cache-Control headers are one way for web administrators to tell Cloudflare how to handle content from the origin. This article explains how Cloudflare makes caching decisions during the request and response phase for a resource, and the options you have for setting Cache-Control directives at your origin server. 

Request phase

In the request phase, the client request URL is matched against a list of cacheable file extensions. If the request matches an extension on this list, Cloudflare serves the resource from cache if present. If the content is stale in the Cloudflare cache, Cloudflare attempts to revalidate the content with the origin before serving the response to the client.

It is possible to modify what Cloudflare deems cacheable via the Cache Level setting in the Cloudflare Page Rule app. 

As an example, Cache Everything skips the extension check, and all content is treated as cacheable. Refer to Cloudflare's guide on how to enable Cache Everything.

Response phase

If Cloudflare deems a request cacheable, we first examine our caches in multiple network locations for content. If the resource is not present in cache, Cloudflare requests the resource from your origin web server to fill our cache. The response is then sent to the client who initiated the request.

At this point, our cache logic examines the HTTP response received from your origin server.

Based on how Cloudflare interprets request headers, the response is either deemed cacheable and written to disk (for use with the next request for the same resource) or deemed un-cacheable (resulting in the next request missing cache and repeating this flow).

Set cache-control directives

The Cache-Control header can include a number of directives. If multiple directives are passed together, each is separated by a comma.

If the directive takes an argument, it follows the directive separated by an equal sign.  For example:

max-age=86400

These directives break down into groupings as follows:

Cacheability - should this resource enter a cache?

public

The public response directive indicates that any cache may store the response, even if the response is normally non-cacheable or cacheable only within a private cache.

private

The private response directive indicates that the response message is intended for a single user (eg. a browser cache) and must not be stored by a shared cache (like Cloudflare, or a corporate proxy).

no-store

The no-store response directive indicates that any cache (ie a client or proxy cache) must not store any part of either the immediate request or response.

Expiration - how long should this resource stay in the cache?

max-age=seconds

The max-age response directive indicates that the response is to be considered stale after its age is greater than the specified number of seconds. Age is defined as the time in seconds since the asset was served from the origin server. The seconds argument is an unquoted integer.

s-maxage=seconds

The s-maxage response directive indicates that, in shared caches, the maximum age specified by this directive overrides the maximum age specified by either the max-age directive or the Expires header field.  The s-maxage directive also implies the semantics of the proxy-revalidate response directive.  Browsers ignore s-maxage.

no-cache

The no-cache response directive indicates that the response cannot be used to satisfy a subsequent request without successful validation on the origin server. This allows an origin server to prevent a cache from using it to satisfy a request without contacting it, even by caches that have been configured to send stale responses.

Revalidation - how should the cache behave when a resource has expired?

must-revalidate

The must-revalidate response directive indicates that once it has become stale, a cache (client or proxy) must not use the response to satisfy subsequent requests without successful validation on the origin server.

proxy-revalidate

The proxy-revalidate response directive has the same meaning as the must-revalidate response directive, except that it does not apply to private client caches.

stale-while-revalidate=seconds

When present in an HTTP response, the stale-while-revalidate Cache-Control extension indicates that caches may serve the response in which it appears after it becomes stale, up to the indicated number of seconds since the resource expired.

stale-if-error=seconds

The stale-if-error Cache-Control extension indicates that when an error is encountered, a cached stale response may be used to satisfy the request, regardless of other freshness information.

Other

no-transform

The no-transform response directive indicates that an intermediary (regardless of whether it implements a cache) must not transform the payload

vary

Cloudflare cache doesn't respect the vary header.  

immutable

Indicates to clients that the response body does not change over time. The resource, if unexpired, is unchanged on the server and therefore the client should not send a conditional revalidation for it (e.g. If-None-Match or If-Modified-Since) to check for updates, even when the user explicitly refreshes the page. This directive has no effect on public caches like Cloudflare, but does change browser behavior.

Examples

For more information about Cache-Control directives at origin servers, refer to the Mozilla Cache-Control documentation.