There are two clusters .
There are two services/applications.
All public requests first go to a primary app in one cluster.
If the primary app knows the request (non 404 http response code) it responds appropriately.
If the primary app doesn't know the request (404 http response code) it redirects the request the fallback secondary app running on another cluster.
Question is, is traefik the way to go here and can this scenario be implemented with traefik?
Thanks for your interest in Traefik.
In Traefik v2.6, there is no health check/failover features. In v2.7, which is coming very soon, you will have such behaviors on a per-service basis. You can see the documentation here.
Moreover, your use-case is hard to understand. Do you want to have a fallback only if the first application is down? Or do you want to call the fallback based on an HTTP status per request? For the second use case, we do not provide any way to do this.