リージョンとアベイラビリティーゾーン
https://docs.aws.amazon.com/ja_jp/AmazonRDS/latest/UserGuide/Concepts.RegionsAndAvailabilityZones.html
Amazon Relational Database Service (Amazon RDS) とは
https://docs.aws.amazon.com/ja_jp/AmazonRDS/latest/UserGuide/Welcome.html