[mrkeen]

You're getting hung up on the "double" entries. That's mostly for reducing accidental and deliberate (fraud) calculation errors if you have humans crunching the numbers. The two entries can be in the same row if you like.

As for the inter-system transfers, It's not possible in the general case. You might not have a network cable between the two systems. And if you do, you run into CAP, two-generals, etc.

ACID is out of the question because the two parties don't share a DB. And if they did, some tech lead would turn down the acidity level for performance reasons.

The best you can do is intelligently interpret the information that has been given to you. That's all ledgers are: a list of things that some system knows. 'UPDATE CustomerBalance...' (CRUD) is not a fact, but 'Customer paid...' is a fact, and that's all event-sourcing is.