Requirements of YouTube Mobile App

A commuter descends into the subway, taps a video on their phone, and within seconds the experience begins to degrade. The signal weakens between stations, the stream drops in quality, buffers repeatedly, and eventually freezes. When connectivity returns, playback resumes at a different resolution, and the interaction the user made during the outage never registers.

This scenario highlights the core challenge of designing a mobile video streaming system: delivering a seamless viewing experience despite unpredictable network conditions and device constraints.

Desktop and web clients operate under fundamentally different assumptions. They enjoy stable wired connections, abundant processing power, and generous memory headroom. Mobile clients enjoy none of these luxuries. Consider a concrete scenario: a user switches from Wi-Fi to cellular mid-video. What should happen to the playback quality? Should prefetched high-resolution segments be discarded? Should queued interactions like comments or likes be retried immediately or batched? These questions motivate the precise, mobile-specific requirements we need to define, requirements that go well beyond traditional backend System Design.

Requirements

When a user opens YouTube on their phone, they expect instant playback, smooth scrolling through recommendations, and responsive interactions, all without worrying about battery drain or data overuse. These expectations feel simple, but meeting them requires translating core product goals into mobile-first functional and nonfunctional requirements.

Each requirement below is framed from the mobile client’s perspective. Functional requirements address what the app must do, while nonfunctional requirements address how well it must do it. Together, they set the foundation for the design considerations lesson that follows.

Functional requirements

These requirements define ...