Design an ad content delivery system with A/B testing

Q: Design an ad content delivery system with A/B testing

This is a System Design interview question from Warner Bros Discovery for Software Engineer roles. View the full question and solution on PracHub.

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Question

Design an ad content delivery (ad serving) system that selects and serves ads to users, and supports A/B testing / experimentation.

Core scenario

A client (web/mobile app) requests an ad for a placement (e.g., homepage banner). The system must choose an eligible ad and return creative metadata quickly.

Requirements

Functional

Create/manage campaigns and creatives (basic campaign metadata is enough).
Targeting (at least: country/device/app placement; optionally user segments).
Select an ad to serve for each request.
Impression and click tracking.
Basic delivery controls (at least one): pacing/budget, frequency capping, deduplication.
A/B testing : run experiments that change selection/ranking logic or creatives; assign users deterministically to variants; log outcomes.

Non-functional

Very low latency for ad requests (typically tens of milliseconds).
High throughput (can assume millions of requests/day+).
High availability.
Accurate enough reporting (define what must be real-time vs eventually consistent).

Deliverables

High-level architecture and key services.
Data model/storage choices for campaigns, targeting, and events.
Serving path (request → decision → response) including caching.
Event logging pipeline and metrics.
A/B test design: assignment, exposure logging, metric computation, and guardrails.
Key trade-offs and failure modes.