Highlights Slack: Migrating Millions of Concurrent Websockets to Envoy Slack makes an extensive use of websocket technology for their messaging service. Historically, they used HAProxy as a load balance, however, they faced an issue with dynamic updates of a list of endpoints. They could also change config and restart a load balancer which is tricky as it has to maintain existing websocket connections. They’ve decided to switch to Envoy proxy as it allows dynamic change of the configuration.