Interview 11 June 2026 Zoom System Design Interview: Routing Live Video to Millions With an SFU Zoom is a real-time video conferencing system, and the whole design turns on one decision: how do you get every participant's camera and microphone to everyone else in a meeting without melting a server or a laptop. A full mesh where each client sends directly to every other client scales as N squared and dies past a handful of people. An MCU that decodes, mixes, and re-encodes one combined stream per participant is CPU-brutal and adds latency. Zoom, like most modern systems, uses a Selective Forwarding Unit: each client uploads one stream, the media router forwards the right copies down to each receiver, and no decoding or mixing happens on the server. Clients help by sending simulcast, meaning several quality layers of the same camera at once, so the router can hand a weak connection a low bitrate copy and a strong one the full resolution. Media rides UDP because a late packet is worse than a lost one, and the app leans on jitter buffers, forward error correction, and adaptive bitrate to survive real networks. Signaling, region selection to the nearest data center, large-meeting fanout, and cloud recording all sit around that media core.
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