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 Jump to heading#

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

let channel = new BroadcastChannel(channelName);

Parameters Jump to heading#

The return type of the constructor is a BroadcastChannel instance.

Properties Jump to heading#

Methods Jump to heading#

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 Jump to heading#

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.