# System Design Interview: Ad Serving System (Google Ads / Meta Ads)

Source: https://www.techinterview.org/post/3233460537/system-design-ad-serving/
Updated: 2026-04-17 · techinterview.org

Ad serving is at the core of Google and Meta businesses and is one of the highest-stakes systems in the world — a 10ms increase in ad serving latency measurably reduces revenue. Designing an ad system involves real-time bidding, targeting, ranking, click fraud detection, and a billing system — all under strict latency budgets.

## Two Models: Direct Ads vs Real-Time Bidding

| Model | How It Works | Example |
| --- | --- | --- |
| Direct / Reserved | Advertiser buys specific inventory at a fixed price (e.g., homepage banner for $10K/day) | Brand campaigns, sponsorships |
| Real-Time Bidding (RTB) | Ad slot auctioned in real time (

## Frequently Asked Questions

### How does real-time bidding (RTB) work in digital advertising?

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