> I guess they are caching account balances and limits in some in-memory DB. This is a common design choice for payment processing systems. They are a payments

> I guess they are caching account balances and limits in some in-memory DB. This is a common design choice for payment processing systems. They are a payments processor, not an issuer. That means they are on the merchant's side of the transaction, not the purchaser's. Their role in authorization is simply routing. The issuer is responsible for performing the authorization. The only balance they need to track is how much money they owe the merchant.