Strong consistency can stop at the region boundary

A database can offer strongly consistent reads and ACID transactions and still offer neither across regions. DynamoDB global tables are a clear case: both guarantees hold only inside the region that served the request.

A strongly consistent read returns the newest value in its own region. If the item changed more recently elsewhere, the read does not see it, and replication between regions can take several seconds. A transaction is atomic where it commits; its updates then travel by ordinary replication, so another region can briefly see some of them without the rest.

Global tables also accept writes in every region. When the same item is updated in two regions at once, the last writer wins and the other update is silently discarded, which is a reconciliation policy chosen on the application's behalf.

For a failover plan, the consequence is that a guarantee on the configuration page may describe one region only. Moving users to another region moves them onto data that is only as current as its lag, and a user who has just written may not see it there: Read-your-writes breaks quietly.