Modern chat systems (Slack, Discord, WhatsApp, Microsoft Teams) sit at the difficult intersection of stateful network connections, intense write concurrency, and strict ordering requirements. When hundreds of millions of users exchange messages across flaky cellular networks, servers cannot rely on standard stateless HTTP request-response paradigms.
This module deconstructs the distributed architecture of real-time communication: how to terminate millions of persistent TCP/TLS sockets at edge gateway tiers, route messages across broker clusters with sub-100ms latency, track user online/offline status with minimal battery and network overhead, and reconcile state when users reconnect after hours offline. You will also analyze per-conversation causal sequencing (Snowflake IDs and monotonic channel counters), pub/sub session routing registries, and bounded heartbeat lease debouncing to prevent presence fan-out storms across large group channels.