how many messages a second can be handled synchronously? what happens if the message arrives twice? what happens if the data centre is blown up/goes up in flames? how did the other data centre guarantee to be in a consistent state? how did the backup system automatically take over?

These systems have to be up and running all the time and there can't be an option for mistakes!

When I say message I’m think of the equivalent of a TCP packet. So you have a bunch of error checking and ACKs flying about. But only one message to actually move money.

Single message systems already exist, most US card transactions move over a single message system.

> there can’t be an option for mistakes!

This is where you’re wrong. Mistakes happen all the time when moving money, a big part of any banks operations is cleaning up those mistakes.

and yet the rest of the world manages to do this instantly

What happens when the com lines are down during a storm? Will this single message still work?