Auto Switch DB to primary back (persistent mode?) when available in a High Availability Always On settings
You don't really want to do this automatically because:


A long-running transaction could be running on DB2, and you wouldn't want to interrupt it (especially given the problems with single-threaded rollbacks)
DB1 might come back online and SQL Server might be available, but the databases might have problems (or just be starting up)
DB1 may not be in sync (if DB1 went down for a while, it might take quite some time for it to get back in sync with DB2)