AutoPay goes 10×. Why more servers won't help.

transcript

The interviewer drops big news. AutoPay is about to grow ten times. The candidate's answer: more servers won't save you.

Today's worst day: 1.2 million payments. Two BillHub calls each. BillHub allows 300 calls a second. AutoPay gets 200. 2.4 million over 200 a second. About 3 point 3 hours.

Today it drains in 3 point 3 hours. Comfortable. At 10 times: 24 million calls. About 33 hours. A day only has 24. AutoPay misses the due date.

Our side scales fine. More workers. Shard payments when needed. But the ceiling is BillHub's 300 a second. That limit is outside the company. No server you add changes it.

So the first fix is a conversation. Negotiate a higher limit, with these numbers. Then cut calls. Fetch the bill a day early. Bills don't change overnight. Then spread AutoPay over the two days before the due date.

What was being tested? Redoing the maths before answering. The bottleneck wasn't a server. It was a partner's rate limit. When the limit isn't yours, the fix starts with a conversation.

Full breakdown in the lecture.