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Paying Dashers Reliably When the Payment Service Is Unavailable

DoorDashSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenSystem Design

Last updated

A system design follow-up about paying delivery drivers when the downstream payment service is unavailable. It tests reasoning about timeouts with unknown outcomes, durable records of amounts owed, retries that must never double-pay, recovery after long outages, reconciliation, and what drivers and operators see while payouts are delayed.

A delivery platform computes each Dasher's (delivery driver's) pay from their order events: pay accrues per minute in proportion to the number of orders in progress, and peak-time minutes pay double. Once an amount is computed, the platform asks a separate payment service to actually pay the Dasher.

After the pay calculation is coded, the interviewer asks: what if the payment service is unavailable? Explain what can go wrong, and design the hand-off from pay calculation to payout so that every Dasher is eventually paid exactly the right amount, once, even while the payment service is down or unreliable.

Clarifying Questions

  • When are Dashers paid: per order, once a day, weekly, or on demand?
  • Does the payment service accept an idempotency key, and can a payout's status be looked up by that key?
  • How long may a payout be delayed before it becomes a problem for Dashers, and what should the Dasher app show in the meantime?
  • Is the payment service internal, or a third-party provider with its own rate limits?

Hints

Hint 1

Failure is not binary: Consider a call that times out: you do not know whether the money moved. Think about what the payment service must support for a retry to be safe.

Hint 2

Decouple the steps: Ask what must be recorded durably on your side before you call the payment service, so that nothing is lost if your own process crashes during an outage.

Follow-up questions

  • The payment service comes back after several hours with a large backlog. How do you drain it without overwhelming the service?
  • A bug in the pay calculation is found after some payouts have gone out. How do you correct underpayments and overpayments?
  • Could you switch to a second payment provider during an outage, and what new risks would that bring?