[hcarvalhoalves]

Here's how you can do this. You have 3 accounts: A, B and in-flight.

1. Debit A, Credit in-flight.

2. Credit B, Debit in-flight.

If 1. fails, nothing happened and everything is consistent.

If 2. fails, you know (because you have money left on in-flight), and you can retry later, or refund A.

This way at no point your total balance decreases, so everything is always consistent.

[withinboredom]

It can fail at your commas in 1&2, then you are just as broke as everyone else.

This isn't an easy-to-solve problem when it comes to distributed computing.

[hcarvalhoalves]

It should be an atomic transaction, double-entry style, so it can’t fail between commas.

The important thing is not having money go missing.

[withinboredom]

The only way that is possible is if the tables exist on the same database server. Otherwise, you are right back at distributed transaction problems.

[hcarvalhoalves]

That's the whole point of double-entry. You don't split the entries.

[withinboredom]

Then this is off-topic af. We are talking about distributed systems here.