Design a real-time ad impression aggregator
Company: Meta
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
Design an **ads impression aggregator** service with the following requirements:
- The system ingests a high-volume stream of **impression events** (each event at least contains `ad_id` and event timestamp; you may add fields like request_id/device_id for dedup).
- The system must provide **near real-time updates within 30 seconds**.
- It must expose **per-ad impression counts aggregated by hour** (e.g., counts for `ad_id=123` for `2026-02-23 10:00-10:59`).
- Discuss how you would store/serve these aggregates and handle late/out-of-order events.
**Follow-up:** How do you handle **very high write throughput** to the database/storage layer?
Quick Answer: This question evaluates a candidate's ability in scalable real-time system design, covering competencies such as stream processing, event aggregation, time-windowed metrics, handling late and out-of-order events, and designing for very high write throughput.