Deliver events through a shared channel

How to do it

A client is connected to one server, but the change that concerns it can happen on any server. Publish each event to a shared channel, such as Redis or Valkey pub/sub, NATS or PostgreSQL LISTEN and NOTIFY. Every server subscribes and forwards events to its own clients. Publish only after the change is committed, through an outbox, so clients never see a change that was rolled back. Plain pub/sub drops messages while a server is down, so keep the event IDs from reconnect and resume to fill the gap.

Examples

JavaScript example

import { createClient } from 'redis';

const subscriber = createClient({ url: process.env.REDIS_URL });
await subscriber.connect();
await subscriber.subscribe('shop-events', raw => {
  const event = JSON.parse(raw);
  for (const socket of socketsByShop.get(event.shopId) ?? []) socket.send(raw);
});

Python example

import json
import redis.asyncio as redis

async def forward_events():
    pubsub = redis.from_url(REDIS_URL).pubsub()
    await pubsub.subscribe("shop-events")
    async for message in pubsub.listen():
        if message["type"] != "message":
            continue
        event = json.loads(message["data"])
        for socket in sockets_by_shop.get(event["shopId"], []):
            await socket.send(message["data"])

How to check

Run two server instances. Connect a test client to the first and change its order through the second. The client receives the event within 2 seconds.

Sources

• Redis pub/sub delivery semantics

Related rules

• Send side effects through an outbox in Data
• Reconnect with backoff and resume from the last event in API design