Your biggest biller goes dark. Do you take the money?

transcript

The interviewer asks: your biggest biller just stopped answering. Users are still tapping Pay. Do you take their money?

It's the 1st. The largest electricity board carries 30 percent of payments. It goes silent through BillHub. For 40 minutes. Meanwhile, 19,999 other billers are perfectly fine.

Trap one: a single breaker for all of BillHub. The board's timeouts trip it. Now every biller is blocked. All 20,000. So the candidate puts a breaker on each biller. Only the board's breaker opens. The rest never notice.

Then the move that wins the round. Check the breaker before debiting. Breaker open? Don't take the money at all. Closed? Debit, then send it on to BillHub as normal.

The user reads: this biller isn't responding. We'll remind you in 30 minutes. Or they can schedule the payment, with their consent.

The interviewer pushes back. You're turning money away. The candidate doesn't argue. He does the maths.

Evening peak: 30 percent of 74 a second. About 22 a second. Forty minutes is 2,400 seconds. Roughly 53,000 payments, deliberately not debited.

Each one would be money taken that couldn't be delivered. That means refunds, and angry users. He takes the trade. Never take money you can't deliver.

Full breakdown in the lecture.