We would like to add a maintenance page to our front-end which should appear when the back-end is currently unavailable (e.g. stopped or deploying). When the application is not running, the following message is displayed together with a 404 status code:
404 Not Found: Requested route ('name.scapp.io') does not exist.
Additionally, there is header present, when the application is stopped (and only then):
X-Cf-Routererror: unknown_route
Is this header reliably added if the application is not running? If this is the case, I can use this flag to display a maintenance page.
By the way: Wouldn't it make more sense to provide a 5xx status code if the application is not started/crashed, i.e. differ between stopped applications and wrong request routes? Catching a 503 error would be much easier, as it does not interfere with our business logic (404 is used inside the application).
Another option is to use a wildcard route.
An application mapped to a wildcard route acts as a fallback app for route requests if the requested route does not exist.
Thus you can map a wildcard route to a static app that displays a maintenance page. Then if your app mapped to a specific route is down or unavailable the maintenance page will get displayed instead of the 404.
In regards to your question...
By the way: Wouldn't it make more sense to provide a 5xx status code if the application is not started/crashed, i.e. differ between stopped applications and wrong request routes? Catching a 503 error would be much easier, as it does not interfere with our business logic (404 is used inside the application).
The GoRouter maintains a list of routes for mapping incoming requests to applications. If your application is down then there is no route in the routing table, that's why you end up with a 404. If you think about it from the perspective of the GoRouter, it makes sense. There's no route, so it returns a 404 Not Found. For a 503 to make sense, the GoRouter would have to know about the app and know it's down or not responding.
I suppose you might be able to achieve that behavior if you used a wildcard route above, but instead of displaying a maintenance page just have it return an HTTP 503.
Hope that helps!