A chat system needs identity, conversation membership, message persistence, delivery, ordering, presence, and synchronization across devices. WebSockets provide a long-lived bidirectional connection, but the server must handle reconnects, heartbeats, backpressure, and connection distribution across instances.

Persist a message before acknowledging durable acceptance. Assign a conversation-scoped sequence or stable message ID to support pagination and deduplication. Global order across all conversations is usually unnecessary; per-conversation ordering is more meaningful and easier to scale.

Presence is approximate and ephemeral: a disconnected client may remain marked online until a heartbeat timeout. Separate transient presence from durable messages. For offline delivery, use a durable store or queue and synchronize from the last acknowledged cursor after reconnect. End-to-end encryption changes server-side search, moderation, recovery, and multi-device key-management trade-offs.