I am doing a POC to implement a system like this:
User: has credit
A server will call a http request on user's behalf, when the response status returns 200 -> mark it as completed and subtract user's credit
The HTTP request could be a very critical operation (like placing an order on an exchange)
Very simple system but things are getting tricky in the real world when everything is unreliable
Approach:
Opening a transaction and locking user's row first -> subtract the credit -> send the HTTP request
- If the response returns an error, send it again, which is fine here because the 3rd party server never processed it
- If the 3rd-party server returns 200, commit the transaction. However, there is a latency between receiving the response status and committing a transaction. Imagine the latency is 50 ns, if the systems crash within this time, my server will never know the HTTP request was successful. Hence it will retry (meaning the 2nd request)
Am I facing Two Generals' Problem here? I have been thinking a lot and there is no way I can guarantee it
I am curious how can banking systems in the world can deal with this. How can they guarantee the client can never lose a penny?