Discord: seventy thousand online, and a voice channel you can walk into

How a real-time community platform at Discord's scale works: fan-out that grows with the square of a server's online members; one ordered Elixir process per server, with passive sessions and relays of up to 15,000 sessions doing the fan-out, so a million-online server costs about 67 sends a message; websocket sessions that outlive their sockets and resume from a sequence number behind bounded buffers; presence combined per user and sent only to the member-list rows a client is watching; messages partitioned by channel and 10-day bucket, protected by request coalescing, and moved from 177 Cassandra nodes to 72 ScyllaDB nodes in days, not months; drop-in voice placed on one host per call and joined over a websocket, UDP and an MLS group; per-route rate limits, slowmode and AutoMod for raids; and the 25 March 2026 outage, where recreated calls flooded two single-process supervisors and the fix was to cut demand before adding supply.