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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
| name | type | description |
|---|---|---|
| channelName | string | The name for the underlying broadcast channel connection. |
The return type of the constructor is a BroadcastChannel instance.
Properties Jump to heading
| 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 Jump to heading
| 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 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 "jsr:@hono/hono"; // 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.