@b9g/broadcastchannel-redis

Redis pub/sub backend for Shovel's BroadcastChannel implementation. Enables cross-process message delivery using Redis PUBLISH/SUBSCRIBE.

Features

• Cross-process BroadcastChannel relay via Redis
• Automatic echo prevention (messages don't loop back to sender)
• Two Redis connections created automatically (Redis requires separate connections for pub and sub)
• Drop-in backend -- no changes to application BroadcastChannel code

Installation

npm install @b9g/broadcastchannel-redis redis

Requires @b9g/platform as a peer dependency.

Usage

import {RedisPubSubBackend} from "@b9g/broadcastchannel-redis";
import {setBroadcastChannelBackend} from "@b9g/platform/runtime";

setBroadcastChannelBackend(new RedisPubSubBackend({
  url: "redis://localhost:6379",
}));

Once configured, any BroadcastChannel.postMessage() call publishes to Redis, and messages from other processes are delivered to local BroadcastChannel instances automatically.

In Shovel Config

Configure in shovel.json to enable cross-process BroadcastChannel in production:

{
  "broadcastChannel": {
    "module": "@b9g/broadcastchannel-redis",
    "export": "RedisPubSubBackend",
    "url": "redis://localhost:6379"
  }
}

API

new RedisPubSubBackend(options?)

| Option | Type | Default | Description | |--------|------|---------|-------------| | url | string? | Redis default | Redis connection URL (e.g. "redis://localhost:6379") |

Creates two Redis client connections (publisher + subscriber) and connects them immediately.

Methods

Implements the BroadcastChannelBackend interface:

• publish(channelName, data) -- Publishes to Redis channel shovel:bc:{channelName} with sender ID
• subscribe(channelName, callback) -- Subscribes to Redis channel, filters out own messages, returns unsubscribe function
• dispose() -- Gracefully closes both Redis connections

Echo Prevention

Each backend instance generates a random UUID on creation. Published messages include this sender ID in the payload. When a message is received via subscription, the sender ID is checked -- messages from the same instance are silently dropped. This prevents duplicate delivery when a process both publishes and subscribes to the same channel.

How It Works

Process A                    Redis                    Process B
─────────                    ─────                    ─────────
bc.postMessage("hi")
  → publish("shovel:bc:chat", {data:"hi", sender:"A"})
                              → PUBLISH
                              → subscriber B receives
                                                      → sender !== "B" ✓
                                                      → deliverBroadcastMessage("chat", "hi")
                              → subscriber A receives
  → sender === "A" ✗ (skip)

License

MIT