Azure Front Door Premium and Vary response header

I'm using Azure Front Door in Premium Tier. I have caching enabled on my route (CDN).

I have some troubles, which seems to be the result of the fact that CDN ignores the Vary header.

I have made this test:

C:\Users\TomaszMadeyski>curl -H "Next-Url: 123" -sS -D - https://my.page.at.frontdoor.com -o /dev/null
HTTP/1.1 200 OK
Date: Fri, 12 Jul 2024 09:59:38 GMT
Content-Type: text/html; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
cache-control: public, max-age=3600, stale-while-revalidate=3600
vary: RSC, Next-Router-State-Tree, Next-Router-Prefetch, Next-Url
x-azure-ref: 20240712T095938Z-1688db7465dw7s2w5364ydfyf4000000062g00000001m8hq
X-Cache: TCP_HIT
x-fd-int-roxy-purgeid: 37
X-Cache-Info: L1_T2

curl: (23) Failure writing output to destination, passed 11365 returned 4294967295

C:\Users\TomaszMadeyski>curl -H "Next-Url: 1234" -sS -D - https://my.page.at.frontdoor.com -o /dev/null
HTTP/1.1 200 OK
Date: Fri, 12 Jul 2024 10:02:34 GMT
Content-Type: text/html; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
cache-control: public, max-age=3600, stale-while-revalidate=3600
vary: RSC, Next-Router-State-Tree, Next-Router-Prefetch, Next-Url
x-azure-ref: 20240712T100234Z-1688db7465djbxcc9gu32zk19800000006eg000000001ytv
X-Cache: TCP_HIT
x-fd-int-roxy-purgeid: 37

As you can see in the response I have Vary header, one of its values is Next-Url. So I would expect not to get TCP_HIT response from CDN while I specify different values of Next-Url header.

My conclusion: Vary header is ignored by Front Door's CDN.

Is this wrong conclusion? Is there a way to make Front Door's CDN to respect the value of Vary header?

Solution

Azure Front Door Premium and Vary response header

Currently, Azure Front Door does not honour the "Vary" header. it means that if any response with a "Vary" header which adds unique request header, such as Next-Url. by default, the CDN does not support cache response separately for each value of that header. Instead, it will cache the response based on the other cache control headers, such as "Cache-Control" and "Expires".

This will impact by unexpected caching behaviour if your application depends on the "Vary" header to serve different responses depends on the value of a specific request header.

Note: Any headers forward to Azure Front Door from the backend are also passed through to the client.

Refer:
Does Azure Front Door Premium support "Vary" header ? by KapilAnanth-MSFT

From the Front Door to the client