
1990s on-line backup
I know of an on-line system where the redundant system is also on-line, monitoring the transactions and mirroring the first.
The first system can utterly fail, and the redundant system will actually complete the in-progress transactions, transparent to the users.
It's hand-coded. Been in place for decades.
Intended to have no single point of failures (except 'The Internet' and the user's end).