Design real-time ad impression and spend tracking

## Scenario You are building a backend that helps an **ad server** decide whether a candidate ad can be delivered (e.g., not over budget / within pacing). The system must track: - **Impressions (ad view counts)** - **Spend** (money spent attributed to those impressions/clicks/etc.) ## Requirements 1. **Event collection**: For each ad view, the system should record an event (at least: `ad_id`, `campaign_id`, `user_id` optional, `timestamp`, and fields needed to compute spend). 2. **Near real-time decisioning**: When a user requests an ad, the ad server evaluates up to **10,000 candidate ads** and needs current counters (impressions/spend and remaining budget) to filter/score them. 3. **Latency**: The ad server request path should be low-latency (assume single-digit to tens of milliseconds for fetching counters). 4. **Scale**: High QPS for both event ingestion and read traffic from ad servers. 5. **Correctness**: Counters should be accurate enough for budget enforcement; clarify acceptable freshness (e.g., seconds) and consistency model. ## Deliverables Design the end-to-end system: - APIs (event ingestion + read APIs for ad server) - Data model - Storage/streaming components - How you serve counters for 10k ads per request efficiently - Handling duplicates, retries, and late events - Monitoring and failure recovery