Fully agree on this, plus (a very important plus) test that severing down an AZ doesn't bring the services on the good AZ down too. And test this frequently. I

Fully agree on this, plus (a very important plus) test that severing down an AZ doesn't bring the services on the good AZ down too. And test this frequently. I would be very, very surprised if the companies mentioned, in particular banks, weren't running on multiple AZs, but I wouldn't be surprised if the scenario of severing down an AZ was not tested.