Caching headers are easy to get wrong. Public resources may be cached in one layer, while the server configuration prevents an otherwise cacheable resource from being stored where you expect.
Controlling caching without a service like Peakhour usually means working directly with Cache-Control headers. This header has a wide range of fields and directives. Each one tells downstream clients (e.g., proxies, shared caches, and end browsers) how to handle caching for a particular resource.
Getting this right can be complicated, and it can become brittle when web server configuration also affects the response.
Enter CDN-Cache-Control
CDN-Cache-Control is a recently proposed header specified in https://datatracker.ietf.org/doc/html/draft-cdn-control-header-01
The draft RFC states:
This specification defines a convention for HTTP response header
fields that allow directives controlling caching to be targeted at
specific caches or classes of caches. It also defines one such
header field, targeted at [Content Delivery Network](/learning/cdn/) (CDN) caches.
What does this mean in practice? Imagine a Magento application where pages should be cacheable by a shared cache but not by a browser. A Cache-Control header could look like:
Cache-Control: max-age=0, s-max-age=600
This tells browser caches not to reuse the response, while shared caches may cache it for 600 seconds.
What if the application does not want every shared cache to store the resource, because the application already sends purges to the cache when content changes?
Cache-Control does not give you that separation. The header above enables any shared cache to store the resource for the specified time. With CDN-Cache-Control, the application could instead send:
Cache-Control: max-age=0, s-max-age=60
CDN-Cache-Control: max-age=3600
This tells the browser not to cache the page, allows a shared cache to keep the page for 60s, and allows the CDN to keep it for one hour.
Targeted CDN-Cache-Control
What if you need to target a specific CDN only? Use the provider's targeted CDN-Cache-Control header.
For example, Peakhour-CDN-Cache-Control instructs Peakhour only to cache the resource. Other CDNs in the request path will not honour this header.
Header format
Examples
The following header fields instruct a CDN cache to consider the response fresh for 600 seconds, other shared caches for 120 seconds, and any remaining caches for 60 seconds:
Cache-Control: max-age=60, s-maxage=120
CDN-Cache-Control: max-age=600
These header fields instruct a CDN cache to consider the response fresh for 600 seconds, while all other caches are prevented from storing it:
Cache-Control: no-store
CDN-Cache-Control: max-age=600
Because CDN-Cache-Control is not present, this header field prevent all caches from storing the response:
Cache-Control: no-store
Whereas these prevent all caches except CDN caches from storing the response:
Cache-Control: no-store
CDN-Cache-Control: none
(note that 'none' is not a registered cache directive; it is used here to avoid sending a header field with an empty value, because such a header might not be preserved in all cases).
