8.7 Chat Presence, Offline Delivery, and Ordering

Messages in conversation c42 already travel a durable path. The server checks a deduplication marker, assigns a sequence number with INCR, stores the message, and only then publishes a signal over Pub/Sub. A per-user hash, presence:{u42}, records which chat server holds each device, with a TTL on every device field.

This lesson covers the rest of the system. It builds presence and typing indicators on the ephemeral path, delivers messages to offline users through Redis Streams, keeps order within each conversation, and lists how each component fails.