When you stop the CAS service, you get an error saying Webpage not available (or something similar). When you restart the CAS service, until the CAS starts, you get a message saying CAS Service has not fully started.
Was there a sspecific use case you have in mind?
Thanks for reply, like Luke B. mentioned, is in the case when we restart the application after windows update, patch, or SP upgrades. And effectively there's a webpage saying is not available. But i would like to know if we could modify this webpage?
If you have a load balancer available as part of your environment, you can look at adding a rule to redirect users to another server hosting a maintenance page in the event that none of the CASs are available. For example, here's some documentation about fallback hosts in F5. I'd suggest that the server hosting the maintenance page should be separate to the DCRUM servers, as you'll probably be doing things like Windows Updates or service pack upgrades that require the whole DCRUM server to be restarted, not just the CAS service.