Design Discord (Real-time Communities)

Design Discord, a real-time community platform with 200M monthly active users, organized into 'guilds' (servers) of up to 500K members each, with persistent text channels storing trillions of messages and live voice channels with sub-100ms latency. The interview centerpiece is the dual architecture: a sharded text-message store (Cassandra/ScyllaDB) with billions of messages per guild and per-channel ordering, plus a real-time voice infrastructure with regional voice servers and custom UDP transport. We cover guild sharding by Snowflake ID, the Elixir/Erlang gateway that holds millions of WebSocket connections, presence at the guild scale, and how Discord migrated from MongoDB to Cassandra to ScyllaDB as message volume crossed trillions.