Inside Facebook’s video delivery system

This post details the unification of Facebook's video delivery system, consolidating separate infrastructure for Watch and Reels into a single, streamlined system. It describes the three main components: Ranking (RecSys) for content recommendation, Server (WWW) for brokering and feature logic, and Mobile (FB4A/FBiOS) for optimized user experience including client-side ranking. The unification process involved significant technical challenges in merging data models, architectures, and features across billions of users and tens of thousands of lines of code, while maintaining engagement and performance metrics. It also covers the migration of Watch users to a Reels surface and the unification of ranking layers to handle mixed inventory of short-form and long-form video. Key principles for video delivery include prioritizing fresh content, letting ranking decide order, vending content only when needed, deterministic fetching, and delivering content based on clear user signals. The lifecycle of a video feed network request is explained, from mobile client triggers (prefetch, head load, tail load) to server-side processing through GraphQL and the generalized video delivery stack.