Given that I am a bit confused with CQRS I would like to understand it further in the following scenario.
I have an Actor that charge Users' credit card. To do so it contact a bank external service that does the operation, get a confirmation result. I would like to know how can I apply this with CQRS.
The information that needs to be written here is that a specific user has been charge a certain amount. So the event generated is Charged (UserID, Card, Amount). Something like that.
The problem is that all the examples I have seen especially with AKKA, would only generate the event after a Command is validated, such that it is persisted in a journal, and used to update the state of the actor. The Journal could then be red on the other side, such that to create a Reading view here.
Also usually, in those examples, the update state function has a logic that somewhat execute the command, because the command correspond straightforwardly to a state update at the end of the day. This is the typical BasketShoping example: CreateOrder, AddLineItem. All Of this Command, are directly translated in Event, that correspond to a specific code of the Update state function.
However in this example, one needs to actually contact an external service, charge the user and then generate an event. Contacting the external service can't be done in the update state, or after reading the journal. It would not make sense.
How is that done, and where, and when exactly, in the spirit of CQRS?
I can think of 2 ways of doing this.
First is a simple way. The command is DoCharge(UserId, Card, Amount). Upon reception of this command, you call the external payment service. If this has been successfully completed, you generate an event, Charged(UserId, Card, Amount, TransactionId) and store it in the journal.
Now, of course, it's not completely safe way, because your Actor can crash after it has sent the request to payment service, but before it has received and persisted the confirmation of the successful completion. Then you risk of charging the user twice. To overcome this risk, you have to make your payment operation idempotent. Here's how to do it. This example is based on the classic "RESTify Day trader" article. I'll summarize it here.
You need to split the payment operation in 2 phases. In first one, payment service creates a transaction token. It just identifies the transaction, and no financial operations are performed yet. Upon the creation, the identifier is received by your service and persisted in the journal.
In next phase you perform a payment associated with the identifier from phase one. If your actor now fails in the middle, while operation is performed successfully on the payment service side, the transaction token will already be marked as processed by the payment service, and it won't let you charge the customer twice. Now, if you restart the failed Actor, and it tries to run the payment associated with the existing transaction token, the payment service should return result like "Already executed" or such. Of course, at the end you also persist the result of the operation in the journal.