When a device reconnects, it presents its last acknowledged cursor and fetches missing messages. Push notifications can alert offline users but should avoid leaking sensitive content when privacy settings require generic previews. Deduplicate by message ID because reconnect and replay can repeat delivery.

Apply rate limits, membership checks, attachment scanning, abuse reporting, and retention controls. End-to-end encryption requires careful key lifecycle and device enrollment; it also limits server-side content inspection and search. Make these trade-offs explicit rather than treating encryption as a single toggle.

Scale gateways horizontally while planning connection counts, file descriptors, heartbeat traffic, memory per connection, and uneven room sizes. Monitor connection churn, delivery lag, reconnect success, message persistence latency, and per-partition skew.