Katkat
Back

Design a donations service with 3-day rolling totals

DoorDashSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSystem Design

Last updated

This question evaluates a candidate's competency in designing scalable, low-latency streaming data systems, covering event-time windowing, rolling-window aggregation, idempotency for duplicate events, late/out-of-order handling, and auditability.

Design a service that ingests donations and serves rolling 3-day donation totals.

Requirements

Core features

  1. Ingest donation events:
  • Each donation has: donation_id (unique), user_id, campaign_id, amount, currency, created_at (event time).
  1. Query APIs:
  • GET /campaigns/{id}/total?window=72h → total donated to that campaign in the last 72 hours (rolling window).
  • GET /leaderboard?window=72h&k=100 → top-K campaigns by donated amount in the last 72 hours.

Correctness and scale expectations

  • Must handle duplicate submissions (retries) safely (idempotency).
  • Donations may arrive late/out of order (e.g., mobile offline, retries).
  • Support high write throughput (assume 5k–50k donations/sec) and high read QPS for totals/leaderboard.
  • Windowing is based on event time (created_at), not ingestion time.

Non-functional

  • Low latency reads (p95 < 200ms for totals and leaderboard).
  • Strong auditability (ability to reconcile totals from raw events).

Deliverables

  • High-level architecture
  • Data model and storage choices
  • Windowed aggregation approach
  • Handling idempotency, late events, and consistency
  • Scaling, caching, and failure modes