CSAI vs SSAI: How Player-Side and Server-Side Video Ad Insertion Compare
Company: Twitch
Role: Product Manager
Category: Software Engineering Fundamentals
Difficulty: medium
Interview Round: Other
Ads in streaming video can be inserted on the client or on the server: client-side ad insertion versus server-side ad insertion, the latter often called dynamic ad insertion. As a product manager, explain how each approach works, how the two differ, and what the difference means for viewers, advertisers and the business running the service. Finish with when you would choose each one.
```hint Trace one ad break
Follow a single mid-roll break through each approach and ask two questions: who requests the ad, and where does the ad get joined to the video? Then work out what that one choice means for the viewer, the advertiser and the team that runs it.
```
### Clarifying Questions
- Should the comparison cover live streams as well as on-demand video, given that a live ad break is triggered while the broadcast is running?
- Which devices matter most: web browsers, mobile apps, or TV and console apps?
- Does the ad sales team need interactive formats such as clickable overlays, or only standard video spots?
- How much revenue is currently lost to ad blockers?
### What a Strong Answer Covers
- The mechanics of each approach: who calls the ad decision server, where the ad is joined to the content, and what the player must do.
- Viewer experience at ad breaks, and how each approach behaves on low-powered devices.
- Measurement and targeting: how impressions are counted and verified, and which signals reach the ad server in each model.
- Trade-offs in ad blocking, ad formats, live-stream handling, infrastructure cost and engineering complexity.
- A reasoned recommendation by content type and device, including hybrid options.
### Follow-up Questions
- Advertisers distrust server-counted impressions because the requests come from your data centers. How would you restore their confidence?
- You are moving a TV app from client-side to server-side insertion. Which metrics tell you whether the switch was worth it?
- Server-side insertion makes ads much harder to block. What viewer-experience risks would you weigh before adopting it mainly for that reason?
Overview: A product manager interview question about streaming video ads that asks how client-side and server-side ad insertion work and how they differ. It tests understanding of ad requests, stream stitching, playback at ad breaks, measurement, ad blocking and device support, plus the judgment to pick an approach for a given product.