How to setup 503 page if backend app is down

[ankur_atri]

I am looking for a way to set up 503 page which triggers automatically in case backend app is down.
I am not able to find useful detail online.
I looked at worker solution but that requires manual trigger. I would like to set something which is automatically detecting backend app functionality and if it throws 503, user must be presented custom page.

[fritex]

@ankur_atri

in case backend app is down.

May I ask how would that result? What would be the trigger?
With some 5xx error from the origin, or network interruption?

The closest what I could find out so far yet, using Cloudflare Workers here:

Can a Worker return 503 if the origin is not reachable? (fetch error handling)

Hi @user6251, Your worker as written will only return your custom 503 error if await fetch() throws an exception. However, an unreachable backend will not necessarily cause await fetch() to thrown an exception. Instead, fetch() itself will likely return a Response with status = 502 and statusText = "Bad Gateway, or some similar error. In this case, your worker as written will return that response directly to the client, so that’s what the client ends up seeing. If you would like to handle thes…

@ankur_atri

I would like to set something which is automatically detecting backend app functionality and if it throws 503, user must be presented custom page.

You modify the Worker code and redirect to a custom URL:

Worker Fetch, status 500, 501, 503 and 505

Then this is something you are looking for: if response.status equals to 500 or 501 or 503 or 505, then return fetch("https://another.site")

Or you can modify the Worker code to display custom HTML code, if that so:

• Return small HTML page · Cloudflare Workers docs