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Do you mean inter-system? Intra-system would mean on the same db, so a simple transaction would do.

For inter-system consistency, you’d probably need a reconciliation mechanism or some kind of 2 phase commit

I mean a bank account sending money to another bank account. That would be 2 events(one withdraw and one deposit)?

But if I'm downstream consumer consuming the event log and computing the state from that, if for some reasons I receives only first event the state computed would be invalid and not represent the real state of accounts?

Nobody relies on atomic transactions to model money transfers in the real world. You can read up on "clearing" and "settlement" processes to get an idea.

It could be one event with two database rows inserted, CQRS doesn’t have to map 1:1 to db entries. But in general, these kinds of systems are more complex than 1 or 2 writes, so relying purely on transactions isn’t very common

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