Design a donations service with 3-day rolling totals

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).
2. 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