I'm using Next.js custom server, which by default sends strong ETag header on each page GET request. For each request I also manually add Cache-Control header res.setHeader('Cache-Control', 'public, max-age=${60 * 60}, s-maxage=${60 * 60 * 24}, must-revalidate')
Also I'm using Cloudflare's loadbalancing and caching mechanism. According to this documentation https://support.cloudflare.com/hc/en-us/articles/218505467-Using-ETag-Headers-with-Cloudflare I sutisfied each requirement related to strong ETag. And the page rule was configured as following:
page rule configuration screenshot.
First strange behaviour is that on a client in headers I noticed for some reason weak ETag. Also the behaviour of caching is little strange. If I'll open a new site post, I'll be usually faced with cf-cache-status: HIT header, which means that Cloudflare already have prepared cache for this specific post. My expectation is that after page reloading in same tab, the page will be recreated form disk cache, but for some reason it makes a request, result of which is status 304 (Not Modified). BUT, if I'll copy the url and open the same post in new tab, it will be recreated from disk cache (as it should), and also after page reloading it makes request with response 304 (Not Modified).
Direct origin server request-response:
General:
Request URL: ...
Request Method: GET
Status Code: 200 OK
Remote Address: ...
Referrer Policy: strict-origin-when-cross-origin
Response headers:
cache-control: public, max-age=3600, s-maxage=86400, must-revalidate
connection: keep-alive
content-encoding: gzip
content-type: text/html; charset=utf-8
date: Thu, 28 Jan 2021 13:02:42 GMT
etag: "62e33-f1yLCVYVUzVH67UMp/2WOxQqfC8"
transfer-encoding: chunked
vary: Accept-Encoding
x-powered-by: Next.js
Request headers:
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate, br
Accept-Language: ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7,uk;q=0.6
Cache-Control: no-cache
Connection: keep-alive
Cookie: _ga=GA1.1.1009400285.1608315387; _gid=GA1.1.1795438935.1611755271; _gat=1
Host: ip_here
Pragma: no-cache
Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: same-origin
Sec-Fetch-User: ?1
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.104 Safari/537.36
Cloudflare proxy request-response:
General:
Request URL: ...
Request Method: GET
Status Code: 200
Remote Address: ...
Referrer Policy: strict-origin-when-cross-origin
Response headers:
age: 44448
cache-control: public, max-age=3600, s-maxage=86400, must-revalidate
cf-cache-status: HIT
cf-ray: 618ac665c984351f-KBP
cf-request-id: 07ea9a539d0000351f0faf7000000001
content-encoding: br
content-type: text/html; charset=utf-8
date: Thu, 28 Jan 2021 12:39:13 GMT
etag: W/"62e33-vLyavPBB7axQJnhj/OKdPyUNp+A"
expect-ct: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct"
nel: {"max_age":604800,"report_to":"cf-nel"}
report-to: {"endpoints":[{"url":"https://a.nel.cloudflare.com/report?s=4ASHxxkyb8necnQhGaz8th4W%2FcFEwzrBwYiidFGsHK5ZZJ%2BGEYWbmOlDSC2Uep2TYw8vYUm0DL4WkrdC9xsIK%2F6j0G1QF%2BK5817fQQ%3D%3D"}],"max_age":604800,"group":"cf-nel"}
server: cloudflare
vary: Accept-Encoding
x-powered-by: Next.js
Request headers:
:authority: ...
:method: GET
:path: ...
:scheme: https
accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
accept-encoding: gzip, deflate, br
accept-language: ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7,uk;q=0.6
cache-control: max-age=0
cookie: _ga=GA1.2.33365049.1592837735; __cfduid=d58947d60ae6adfce59e944d190e702ae1610627770; _gid=GA1.2.788163791.1611406644; connect.sid=s%3AfLFtBQSejs6Jf0gkdqiRhyzAnu8ImE_J.KukkQj15FMXkfcW6alO6BPUYT8SgPWRqtu2xo2QRe8U; __cflb=0H28urhoa5eNWhDb7XVCL24Hq5fwW5bc37oc693mgWx; _gat=1
sec-fetch-dest: document
sec-fetch-mode: navigate
sec-fetch-site: same-origin
sec-fetch-user: ?1
upgrade-insecure-requests: 1
user-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.104 Safari/537.36
Why is it happening? In ideal I want to recreate page from disk cache in nearest hour and after it make a revalidation request. And make Cloudflare's cache revalidation each day from origin server.
You can see in headers that ETag is different between origin server and cloudflare
question from:
https://stackoverflow.com/questions/65937898/status-200-from-disk-cache-and-304-not-modified-issue 
