Katkat
Back

Design a Donation System for a Three-Day Fundraising Campaign

DoorDashSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSystem Design

Last updated

Design a donation system for a fundraising campaign that runs for exactly three days, where donors choose a charity, pay through a payment processor and see running totals. It tests payment idempotency and reconciliation, handling a time-boxed traffic burst, avoiding write hotspots on totals, and fraud protection.

Design a system that collects donations during a fundraising campaign that runs for three days. Donors pick a charity, choose an amount and pay. The system records every donation and reports how much has been raised.

The prompt fixes only the domain and the three-day duration. Establish the remaining requirements with the interviewer before designing.

Constraints and Clarifications

  • The campaign has a fixed start and a fixed end, three days apart.
  • Assume that card payments go through a third-party payment processor, and that the system never stores card numbers.

Clarifying Questions

  • Who donates: signed-in users of an existing app, anonymous web visitors, or both?
  • How many charities take part, and can one donation be split between several of them?
  • Is a live running total shown to donors? If so, how fresh must it be?
  • What peak traffic should the design handle, for example right after the campaign is promoted?
  • Are there minimum or maximum amounts, matching gifts, refunds or tax receipts?
  • What happens to a payment that is still being confirmed at the moment the campaign closes?

Hints

Hint 1

Money moves through someone else: Think about which part of the donation flow you control and which part a payment processor controls. Then ask how the two stay in agreement when a request or a callback is lost.

Hint 2

The calendar is a requirement: Ask what a hard three-day window implies: capacity planning before the start, donations still in flight at the end, and work that happens only after the campaign closes.

Follow-up questions

  • How would you add a sponsor who matches donations up to a cap?
  • The payment processor is down for an hour on day two. What do donors see, and how do you recover?
  • How would you show a live leaderboard of charities?
  • After the campaign, how do you prove that the reported total matches the money actually received?