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Comment by saxenaabhi | Hacker News Reader

> INSERT INTO events (account_id, type, amount, timestamp) VALUES (123, 'deposit', 100, NOW())

How would it work if they had to support intra system transfers? So one user balance should be withdrawn and another should get a deposit? That's not possible to do atomically with event sourcing right?

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Any problem with event sourcing can be solved with more events (said semi-sarcastically).

In this case it’s XTransactionStarted, XTransactionDepositConfirmed, and XTransactionCreditConfirmed or something along those lines. External interactions tend to follow that kind of pattern where it tracks success/failure in the domain events.

The command side of CQRS tends to be the services that guarantee ordered events either via the backing database or with master-slave topology.

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.

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

Create an event type for transfer with amount, source account and destination account.

Right, but then your downstream consumers have to be global and can't be partitioned based on account_id or some other partition key?

Consumers will have to read all events that concern them, yes. Maybe this type of event must end up in multiple partitions and will require some architectural shenanigans.

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