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Handling Duplicate Upstream Order Records in Dasher Pay Calculation

DoorDashSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenSystem Design

Last updated

Explain how a Dasher pay calculation driven by upstream order records stays correct when those records contain duplicates. Tests deciding when two records describe the same fact, making processing safe against replayed records, handling conflicting and late data or corrections after payout, and preventing double payouts.

A delivery platform pays its Dashers from order activity. During every minute, a Dasher earns a base rate multiplied by the number of orders they have in progress, and the pay is doubled inside designated double-pay windows. The calculation learns when each order was accepted and when it was delivered from records produced by an upstream system.

The upstream data can contain duplicates. How do you handle them so that every Dasher is paid correctly?

Constraints and Clarifications

  • This came up as a system-design follow-up to coding the pay calculation; you do not need to write the calculation again.
  • Not given: the volume of records, whether pay is computed continuously or once per pay period, and whether records carry a unique ID. Ask, or state your assumptions.

Clarifying Questions

  • What does a duplicate look like: the same record delivered twice with identical content, or two records for the same order and action with different timestamps?
  • Does each record carry a unique ID assigned by the producer, or only the order, the action and a timestamp?
  • Is pay computed incrementally as records arrive, or recomputed for a pay period from all of its records?
  • Can records arrive late or out of order, and can an order's data be corrected after the Dasher has been paid?

Hints

Hint 1

What should a repeat change?: Picture the same delivery record being processed twice. Decide what must be identical after the second pass, and what key lets you recognize the record as something you have already seen.

Hint 2

Not every repeat is a copy: Two records about the same order can disagree. Decide whether they describe the same fact, and how you choose between them.

Follow-up questions

  • A corrected delivery time arrives after the Dasher has already been paid for that period. What happens?
  • Your deduplication data grows without bound. How do you limit it, and what risk does that create?
  • The service that sends payouts times out and retries. How do you make sure a Dasher is not paid twice?