Workbox Broadcast Update

What is Broadcast Update?

When responding to requests with cached entries, while being fast, it comes with a tradeoff that users may end up seeing stale data.

The workbox-broadcast-update package provides a standard way of notifying Window Clients that a cached response has been updated. This is most commonly used along with the StaleWhileRevalidate strategy.

Whenever the "revalidate" step of that strategy retrieves a response from the network that differs from what was previously cached, this module will send a message (via postMessage()) to all Window Clients within scope of the current service worker.

Window Clients can listen for updates and take appropriate action, like automatically displaying a message to the user letting them know that updates are available.

How are updates determined?

Certain headers of the cached and new Response objects are compared, and if any of the headers have different values, it's considered an update.

By default, the Content-Length, ETag, and Last-Modified headers are compared.

Workbox uses header values instead of a byte-for-byte comparison of response bodies to be more efficient, in particular for potentially large responses

Warning: Because Workbox needs to be able to read the header values, opaque responses, whose headers are not accessible, will never trigger update messages.

Using Broadcast Update

The library is intended to be used along with the StaleWhileRevalidate caching strategy, since that strategy involves returning a cached response immediately, but also provides a mechanism for updating the cache asynchronously.

To broadcast updates, you just need to add a broadcastUpdate.BroadcastUpdatePlugin to your strategy options.

import {registerRoute} from 'workbox-routing';
import {StaleWhileRevalidate} from 'workbox-strategies';
import {BroadcastUpdatePlugin} from 'workbox-broadcast-update';

registerRoute(
  ({url}) => url.pathname.startsWith('/api/'),
  new StaleWhileRevalidate({
    plugins: [
      new BroadcastUpdatePlugin(),
    ],
  })
);

In your web app, you can listen for these events like so:

navigator.serviceWorker.addEventListener('message', async (event) => {
  // Optional: ensure the message came from workbox-broadcast-update
  if (event.data.meta === 'workbox-broadcast-update') {
    const {cacheName, updatedUrl} = event.data.payload;

    // Do something with cacheName and updatedUrl.
    // For example, get the cached content and update
    // the content on the page.
    const cache = await caches.open(cacheName);
    const updatedResponse = await cache.match(updatedUrl);
    const updatedText = await updatedResponse.text();
  }
});

Note: make sure to add the message event listener before the DOMContentLoaded event, as browsers will queue messages received early in the page load (before your JavaScript code has had a chance to run) up until (but not after) the DOMContentLoaded event.

Message format

When a message event listener is invoked in your web app, the event.data property will have the following format:

{
  type: 'CACHE_UPDATED',
  meta: 'workbox-broadcast-update',
  // The two payload values vary depending on the actual update:
  payload: {
    cacheName: 'the-cache-name',
    updatedURL: 'https://example.com/'
  }
}

Note: This message format adheres to the Flux standard action format, though it is not tied in any way to the Flux framework.

Customize Headers to Check

You can customize the headers to check by setting the headersToCheck property.

import {registerRoute} from 'workbox-routing';
import {StaleWhileRevalidate} from 'workbox-strategies';
import {BroadcastUpdatePlugin} from 'workbox-broadcast-update';

registerRoute(
  ({url}) => url.pathname.startsWith('/api/'),
  new StaleWhileRevalidate({
    plugins: [
      new BroadcastUpdatePlugin({
        headersToCheck: ['X-My-Custom-Header'],
      }),
    ],
  })
);

Advanced Usage

While most developers will use workbox-broadcast-update as a plugin of a particular strategy as shown above, it's possible to use the underlying logic in service worker code.

import {BroadcastCacheUpdate} from 'workbox-broadcast-update';

const broadcastUpdate = new BroadcastCacheUpdate({
  headersToCheck: ['X-My-Custom-Header'],
});

const cacheName = 'api-cache';
const request = new Request('https://example.com/api');

const cache = await caches.open(cacheName);
const oldResponse = await cache.match(request);
const newResponse = await fetch(request);

broadcastUpdate.notifyIfUpdated({
  cacheName,
  oldResponse,
  newResponse,
  request,
);