Comment by chuckadams

9 hours ago

If you do content negotiation, then it’s imperative to send “Vary: accept” in your response. CF and all other CDNs will automatically do the right thing when they see that header.

Content negotiation still has its uses, but most of the time you’re better off using different endpoints.

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chuckadams

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stillpointlab  3 hours ago

Many CDNs do not support arbitrary Vary headers. Cloudfront, for example asks you to create a "Cache and Origin Request Policies" that gives you the ability to choose what headers are part of the request that gets sent to your origin. These will get added to the cache key, but it is a static list based on the request to the origin and not the response.

Akamai is another case where Vary is harmful, from their docs [1]:

    > As the content in response may be different for the same URL,  
    > Akamai  edge servers don't cache responses that include the Vary header, 
    > even if the content is cacheable by definition. The only exception is 
    > the case where the Vary header's value is Accept-Encoding and the
    > Content-Encoding header's value is br or gzip – edge servers cache 
    > such responses, applying the caching rules you defined in your property.

Cloudflare's docs do seem to indicate they support the Vary header as does Fastly. But one should read the docs of their CDN to find out the behavior. Do not assume Vary is supported.

https://techdocs.akamai.com/property-mgr/docs/rm-vary-header