[waiwai933]

You're both partially right. Some regions have random mapping for AZs; all regions since 2012 have static AZ mapping. https://docs.aws.amazon.com/global-infrastructure/latest/reg...

[nevon]

Oh wow. Thanks for telling me this. I didn't know that this was different for different regions. I just checked some of my accounts, and indeed the mapping is stable between accounts for for example Frankfurt, but not Sydney.