4 ms·

I wish they talked more about maintaining consistency. How exactly does the reconciling of their records with their financial partner’s records work? Is it an a

by rohan_ 3y ago

I wish they talked more about maintaining consistency. How exactly does the reconciling of their records with their financial partner’s records work? Is it an asynchronous job that’s run on a schedule? Also I’m sure they have dual-write consistency problems, how exactly do they solve that? 2PC? Event sourcing? Outbox?

jpollock 3y ago
Bank reconciliation is done using files. They will arrive randomly, typically on a multi-day lag. Then the payments are matched against the requests that were sent to the bank. The amounts _can_ and _will_ differ. I understand it gets... interesting.
Brystephor 3y ago
In general, reconciliation is done asynchronously. You send a payment out. The processor some number of days later sends out a file containing information about that payment. You validate that the payment the processor recognized is the same as the one you sent out. That's a high level summary. Also regarding dual write consistency problems: they mention Google cloud. If they're using NoSQL such as Data store/firestore, they have distributed transactions. Another possibility is they must trust their payment clients to use idempotency keys correctly. If you get two requests with the same idempotency keys, then the requests are considered to be the same and therefore it wouldn't matter which one is processed first. If they're different requests with the same idempotency keys, they may have some validations. I believe DynamoDB also has conditional writes that can essentially be "if entity X does not exist: write entity X". Then there is a race condition for different requests with identical idempotency keys, but if that does happen, it'd result in an error and someone would need to look into it and figure out why that was allowed to happen.

4 ms·

I wish they talked more about maintaining consistency. How exactly does the reconciling of their records with their financial partner’s records work? Is it an a

by rohan_ 3y ago

jpollock 3y ago
Bank reconciliation is done using files. They will arrive randomly, typically on a multi-day lag. Then the payments are matched against the requests that were sent to the bank. The amounts _can_ and _will_ differ. I understand it gets... interesting.
Brystephor 3y ago
In general, reconciliation is done asynchronously. You send a payment out. The processor some number of days later sends out a file containing information about that payment. You validate that the payment the processor recognized is the same as the one you sent out. That's a high level summary. Also regarding dual write consistency problems: they mention Google cloud. If they're using NoSQL such as Data store/firestore, they have distributed transactions. Another possibility is they must trust their payment clients to use idempotency keys correctly. If you get two requests with the same idempotency keys, then the requests are considered to be the same and therefore it wouldn't matter which one is processed first. If they're different requests with the same idempotency keys, they may have some validations. I believe DynamoDB also has conditional writes that can essentially be "if entity X does not exist: write entity X". Then there is a race condition for different requests with identical idempotency keys, but if that does happen, it'd result in an error and someone would need to look into it and figure out why that was allowed to happen.