Building a Payment Processor: Moving Payments to the Background

My first attempt at a payment processor was synchronous and dangerous. If anything timed out or failed mid-way, I ended up with inconsistent data or double charges. To fix it, I had to stop trying to do everything inside the HTTP request. I moved to a model where the API just records the intent to pay, and a separate worker handles the actual execution. Step 1: Idempotency (The "Don't Double Charge" Fix) The first thing I needed was an Idempotency-Key . It’s just a unique string from the client.