Real-Time and Streaming System Design Questions

Designing client-facing, always-on live delivery systems: real-time communication transports (WebSockets, server-sent events, long-polling, WebRTC, MQTT), connection lifecycle and scaling for millions of persistent connections, presence, pub/sub fan-out to online users, chat and notifications, live feeds and tickers, real-time collaboration (CRDT vs OT, offline sync and reconciliation), and live and on-demand video delivery (ingest, transcoding, adaptive bitrate, CDN delivery, low-latency protocols, playback entitlement and content protection). Covers latency budgets, per-client ordering, delivery guarantees across disconnect and reconnect, per-client backpressure, capacity estimation for connection fleets, authentication, authorization, and revocation for real-time channels and premium content, and operational readiness (SLOs and error budgets, observability and incident response, rate limiting, safe rollout, and load and chaos testing) for live-delivery platforms. Data-pipeline stream processing (Kafka or Flink jobs, windowed aggregation, exactly-once pipelines, real-time analytics ingestion) is out of scope.