BroadcastChannel

In Deno Deploy, code is run in different data centers around the world in order to reduce latency by servicing requests at the data center nearest to the client. In the browser, the BroadcastChannel API allows different tabs with the same origin to exchange messages. In Deno Deploy, the BroadcastChannel API provides a communication mechanism between the various instances; a simple message bus that connects the various Deploy instances worldwide.

Constructor

The BroadcastChannel() constructor creates a new BroadcastChannel instance and connects to (or creates) the provided channel.

let channel = new BroadcastChannel(channelName);

Parameters

name	type	description
channelName	string	The name for the underlying broadcast channel connection.

The return type of the constructor is a BroadcastChannel instance.

Properties

name	type	description
name	string	The name of the underlying broadcast channel.
onmessage	function (or null)	The function that's executed when the channel receives a new message (MessageEvent).
onmessageerror	function (or null)	The function that's executed when the arrived message cannot be deserialized to a JavaScript data structure.

Methods

name	description
close()	Close the connection to the underlying channel. After closing, you can no longer post messages to the channel.
postMessage(message)	Post a message to the underlying channel. The message can be a string, object literal, a number or any kind of Object.

BroadcastChannel extends EventTarget, which allows you to use methods of EventTarget like addEventListener and removeEventListener on an instance of BroadcastChannel.

Example: Update an in-memory cache across instances

One use case for a message bus like the one enabled by BroadcastChannel is updating an in-memory cache of data between isolates running in different data centers across the network. In the example below, we show how you can configure a simple server that uses BroadcastChannel to synchornize state across all running instances of the server.

import { Hono } from "https://deno.land/x/hono/mod.ts";

// in-memory cache of messages
const messages = [];

// A BroadcastChannel used by all isolates
const channel = new BroadcastChannel("all_messages");

// When a new message comes in from other instances, add it
channel.onmessage = (event: MessageEvent) => {
  messages.push(event.data);
};

// Create a server to add and retrieve messages
const app = new Hono();

// Add a message to the list
app.get("/send", (c) => {
  // New messages can be added by including a "message" query param
  const message = c.req.query("message");
  if (message) {
    messages.push(message);
    channel.postMessage(message);
  }
  return c.redirect("/");
});

// Get a list of messages
app.get("/", (c) => {
  // Return the current list of messages
  return c.json(messages);
});

Deno.serve(app.fetch);

You can test this example yourself on Deno Deploy using this playground.