VaultPay is a wallet microservice I built on top of AuthShield. This is the first technical post in the series if you want the origin story, that's in the previous post. Previous parts: Part 1 is here: I Built AuthShield and Immediately Knew It Wasn't Enough How I learned that good error handling and actual data consistency are two completely different things When I started designing the transfer flow for VaultPay, my first instinct was simple. Validate the request, check the balance, move the money, log it. Linear steps, each one after the other. That instinct was wrong. Not in a subtle way. In a "this would silently destroy data in production" kind of way. Here's the scenario that changed how I thought about it. Imagine a transfer of ₹500. The sender's balance gets deducted. Then network hiccup, database timeout, process crash, anything - the credit to the receiver never happens. The money is gone from one wallet and hasn't appeared in the other. No error the user can act on.…