alt Hacker NewsI agree with Roy Fielding on this:
> It is a bad design trade-off to send a bunch of header fields on every request just to tell the server all of the possible variations of preference held by the user, particularly when there is a very small chance that any of those dimensions are applicable to the target resource. It has been a bad design trade-off ever since the very brief period in 1993-94 when folks didn't know which image format would be usable on all UAs and there was no CSS or javascript to allow for client-side adaptation.
> ...The caching impact of proactive negotiation is far worse than the one extra round trip per site for reactive negotiation, and even that round-trip isn't necessary in formats that support client-side adaptation.
On the caching impact, Simon Willison wrote:
> ...you can’t deploy an application that uses content negotiation via the Accept header behind the Cloudflare CDN — for example serving JSON or HTML for the same URL depending on the incoming Accept header. If you do, Cloudflare may serve cached JSON to an HTML client or vice-versa.
Note: I posted this in another comment with links to those two quotes which I couldn't copy easily now - will add later.
Replies
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.
Conceptually, I’m not sure I agree. There’s elegance in clients saying "I want this resource, and I’d like to get your markdown version of it. If you don’t have one, I’ll also take HTML." And if you couple that with optional "file extensions" at the end of the url to force a specific format (say, /foo for automatic negotiation, and /foo.html, /foo.json, or /foo.md for the respective media type,) you have a very easy to use API that adapts to the client; not the other way around.
I take the point that it makes caching harder, but I don’t think that should overrule ergonomics concerns.
If CloudFlare isn't honouring vary: accept that's a pretty serious bug
>On the caching impact, Simon Willison wrote:
Wrong: https://developers.cloudflare.com/cache/concepts/vary/