How does the Always Online feature decide when your website is unavailable

We’ve noticed a few times that our web app is triggering the Always Online “Retry for a live version” button.

The DNS is hosted with Cloudflare and using the “Proxied” Proxy status.
The Caching configuration has Always Online enabled.

My question is, what methods/techniques are used by the Cloudflare proxy function to determine that the website is offline, and therefore serve up the cached version of the website?

Is there a way to look in any logs to see why this was triggered?

Thanks in advance.

May I suggest finding more useful information at the below articles:

Thanks @fritex … unfortunately each one of these pages mentions “if your server goes down”, but doesn’t actually describe how it determines when the server is down… which is what i’m interested in.

How does Always Online determine that your “server is down”?

The first article says it’s due to 520–527 problems (those are Cloudflare’s descriptions of the server issue), but not 404, 503, or 500.

Thanks @sdayman, I feel like i’m getting closer. So does this sound right…

Cloudflare (for the purpose of Always Online) isn’t maintaining a status via healthcheck of our servers, but is taking a per-request view. That is, Cloudflare will receive a request from a client, and try to forward that request on to the origin. If there are issues with that request to the origin server (520 Web Server Returned an Unknown Error, 521 Web Server Is Down, 522 Connection Timed Out, 523 Origin Is Unreachable, 524 A Timeout Occurred, 525 SSL Handshake Failed, 526 Invalid SSL Certificate, 527 Railgun Error), then Always Online will return the cached pages for the response to that request.

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