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

To ensure reliable realtime event delivery, focus on three key principles: durable append, bounded replay, and idempotent application. Begin by assigning an immutable event identity that outlives the connection. For a chat room, use a combination of room ID and sequence number, with a globally unique event ID for traceability. The payload should be minimal, containing the room, sequence, event ID, type, and data.

Client-side deduplication alone is insufficient to prevent marketplaces from surviving reconnections or incident response bursts.

Implement a three-stage process: durable append stores the event and identity together, bounded replay prevents offline clients from causing unbounded scans, and idempotent application guarantees that repeated deliveries are harmless. When reconnecting, clients should provide the last applied sequence, and the server should validate the room token, check retention, and return events after that cursor. If the cursor is outdated, provide a snapshot and a new cursor.

On the consumer side, ensure the database transaction that records applied events commits with the projection update. A process crash between these two actions can lead to data loss disguised as recovery. Define an apply_once function that checks if the event has already been committed before inserting or updating the projection.

Retention policies are crucial; keep enough history for the longest supported reconnect plus some operational margin, then compact old events into snapshots. For high-volume incident feeds, consider partitioned append logs and materialized "current state" tables. Always obtain a reconnect token that includes the room (or set of rooms), the maximum readable sequence, and an expiry. The server should derive authorization from this token during replay requests, while clients supply the cursor without permission.

In incident response dashboards, token scope is critical to prevent unauthorized access to unrelated rooms. Separate publication authorization from subscription authorization to facilitate auditing and rotation. Choose a log based on the failure mode you are trying to mitigate, considering factors like message size, retention, and the number of rooms per tenant. Measure these variables in your production environment to determine the most suitable solution.