Check out https://cacm.acm.org/magazines/2015/4/184701-how-amazon-web-... ("How Amazon Web Services Uses Formal Methods") and https://d1.awsstatic.com/Security/pdfs/One_Click_Formal_Meth... ("One-Click Formal Methods") for more info.

The folks involved gave a neat talk about the verification techniques and tools they used as part of AWS Pi Week recently: https://www.twitch.tv/videos/951537246?t=1h10m10s