How to Scale Realtime Duplicate Event Delivery: Node.js Chat Reconnects

Short answer: make the event identity durable, deduplicate at the consumer boundary, and resume from a server-issued cursor; a client-side set alone cannot make a marketplace chat room survive reconnects or an incident-response burst. The constraint is trust. A browser reconnects after a laptop sleeps, a mobile radio changes networks, or a tab is restored from the back-forward cache. It may replay its last request, lose an acknowledgement, or present an event twice. In an incident response dashboard, the same mechanics become dangerous at scale: an alert that appears twice can page two people, while a missing alert can hide the incident. I design the storage boundary first, because a pretty WebSocket demo does not answer either question. Start with an event identity that can outlive a connection Every published event needs an immutable identity scoped to the stream, not to a socket. For a marketplace chat room, I use (room_id, sequence) as the primary key and keep a globally unique event_id for tracing. The sequence is allocated by the room writer, so two reconnecting clients can compare progress without trusting wall-clock timestamps. The payload is deliberately boring. It includes the room, sequence, event ID, type, and data. A client can verify that an event belongs to the room it requested; it cannot mint a higher sequence or widen its token scope. That last rule matters more than transport choice. from dataclasses import dataclass from typing import Any @dataclass ( frozen = True ) class ChatEvent : room_id : str sequence : int event_id : str event_type : str data : dict [ str , Any ] def identity ( event : ChatEvent ) -> tuple [ str , int ]: """ The room sequence is the replay-safe identity. """ return event . room_id , event . sequence Do not use a payload hash as the only key. Two legitimate messages can have identical text, and a producer retry can produce different JSON ordering. Persist the identity and the payload together, with a uniqueness constraint,