Design a donations service with 3-day rolling totals

Q: Design a donations service with 3-day rolling totals

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.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a medium difficulty System Design question, commonly asked during Onsite rounds at DoorDash.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at DoorDash during technical interviews.

Question

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

Requirements

Core features

Ingest donation events:
- Each donation has: donation_id (unique), user_id , campaign_id , amount , currency , created_at (event time).
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

Design a donations service with 3-day rolling totals

Quick Overview

Requirements

Core features

Correctness and scale expectations

Non-functional

Deliverables

Solution

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