Section 6: Following One Comment Around the World - High-Level Architecture

Executive Summary: This section draws the whole system on one whiteboard: a write path that is an ordinary, durable pipeline, and a delivery path built as a push tree from one owner per video out to 1,000 gateways. We defend each component with the alternative it beat, then follow Maya's comment hop by hop from São Paulo to Tomás's screen in Lisbon against a ~435 ms latency budget. We trace a viewer joining, and sort the design into durable and soft state.

The interviewer slides the marker across: "Draw it." Section 3 already told you where to spend the ink. Writes are 50K comments/s, a normal workload. Deliveries are 150M/s, a 3,000× fan-out. So the board splits in two: a write path that must be durable and boring, and a delivery path that does nearly all the work.

The Whole System on One Whiteboard

Off the diagram sits the Settings Service (slow mode, blocked words, pinned comment, followers-only). The Comment Write Service consults it on every post, and changes reach viewers as settings events through the same pipeline.