How to periodically synchronize data in the background

Oct 10, 2022

The modern way #

Periodic background sync lets you show fresh content when a progressive web app or service worker-backed page is launched. It does this by downloading data in the background when the app or page is not being used.

Using the Periodic Background Sync API #

After the service worker is installed, use the Permissions API to query for periodic-background-sync. You can do this from either a window or a service worker context.

const status = await navigator.permissions.query({
  name: 'periodic-background-sync',
});
if (status.state === 'granted') {
  // Periodic background sync can be used.
} else {
  // Periodic background sync cannot be used.
}

Registering a periodic sync requires both a tag and a minimum synchronization interval (minInterval). The tag identifies the registered sync so that multiple syncs can be registered. In the example below, the tag name is 'content-sync' and the minInterval is one day.

navigator.serviceWorker.ready.then(async registration => {
  try {
	await registration.periodicSync.register('get-cats', { minInterval: 24 * 60 * 60 * 1000 });
    console.log(Periodic background sync registered.');
  } catch (err) {
    console.error(err.name, err.message);
  }
});

Call periodicSync.getTags() to retrieve an array of registration tags. The example below uses tag names to confirm that cache updating is active to avoid updating again.

const registration = await navigator.serviceWorker.ready;
if ('periodicSync' in registration) {
  const tags = await registration.periodicSync.getTags();
  // Only update content if sync isn't set up.
  if (!tags.includes('content-sync')) {
    updateContentOnPageLoad();
  }
} else {
  // If periodic background sync isn't supported, always update.
  updateContentOnPageLoad();
}

To respond to a periodic background sync event add a periodicsync event handler to your service worker. The event object passed to it will contain a tag parameter matching the value used during registration. For example, if a periodic background sync was registered with the name 'content-sync', then event.tag will be 'content-sync'.

self.addEventListener('periodicsync', (event) => {
  if (event.tag === 'content-sync') {
    event.waitUntil(syncContent());
  }
});

Browser compatibility #

Browser support

Chrome 80, Supported 80
Firefox, Not supported
Edge 80, Supported 80
Safari, Not supported

Source

The classic way #

Rather than updating data in the background so it's ready when the user loads the app, the classic way simply consists of updating the data on load.

Demo #

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1" />
    <link
      rel="icon"
      href=""
    />
    <link rel="manifest" href="./manifest.json" />
    <title>How to periodically synchronize data in the background</title>
    <link rel="stylesheet" href="/style.css" />
    <script src="/script.js" defer></script>
  </head>
  <body>
    <h1>How to periodically synchronize data in the background</h1>
    <p class="available">Periodic background sync can be used. Install the app first.</p>
    <p class="not-available">Periodic background sync cannot be used.</p>
    <h2>Last updated</h2>
    <p class="last-updated">Never</p>
    <h2>Registered tags</h2>
    <ul>
      <li>None yet.</li>
    </ul>
  </body>
</html>

{
  "name": "How to periodically synchronize data in the background",
  "short_name": "Periodic Background Sync",
  "start_url": "./",
  "id": "./",
  "icons": [
    {
      "src": "../assets/favicon.png",
      "type": "image/png",
      "sizes": "512x512"
    },
    {
      "src": "../assets/favicon.svg",
      "type": "image/svg+xml",
      "sizes": "any"
    }
  ],
  "display": "standalone"
}

const available = document.querySelector('.available');
const notAvailable = document.querySelector('.not-available');
const ul = document.querySelector('ul');
const lastUpdated = document.querySelector('.last-updated');

const updateContent = async () => {
  const data = await fetch(
    'https://worldtimeapi.org/api/timezone/Europe/London.json'
  ).then((response) => response.json());
  return new Date(data.unixtime * 1000);
};

const registerPeriodicBackgroundSync = async (registration) => {
  const status = await navigator.permissions.query({
    name: 'periodic-background-sync',
  });
  if (status.state === 'granted' && 'periodicSync' in registration) {
    try {
      // Register the periodic background sync.
      await registration.periodicSync.register('content-sync', {
        // An interval of one day.
        minInterval: 24 * 60 * 60 * 1000,
      });
      available.hidden = false;
      notAvailable.hidden = true;

      // List registered periodic background sync tags.
      const tags = await registration.periodicSync.getTags();
      if (tags.length) {
        ul.innerHTML = '';
      }
      tags.forEach((tag) => {
        const li = document.createElement('li');
        li.textContent = tag;
        ul.append(li);
      });

      // Update the user interface with the last periodic background sync data.
      const backgroundSyncCache = await caches.open('periodic-background-sync');
      if (backgroundSyncCache) {
        const backgroundSyncResponse =
          backgroundSyncCache.match('/last-updated');
        if (backgroundSyncResponse) {
          lastUpdated.textContent = `${await fetch('/last-updated').then(
            (response) => response.text()
          )} (periodic background-sync)`;
        }
      }

      // Listen for incoming periodic background sync messages.
      navigator.serviceWorker.addEventListener('message', async (event) => {
        if (event.data.tag === 'content-sync') {
          lastUpdated.textContent = `${await updateContent()} (periodic background sync)`;
        }
      });
    } catch (err) {
      console.error(err.name, err.message);
      available.hidden = true;
      notAvailable.hidden = false;
      lastUpdated.textContent = 'Never';
    }
  } else {
    available.hidden = true;
    notAvailable.hidden = false;
    lastUpdated.textContent = `${await updateContent()} (manual)`;
  }
};

if ('serviceWorker' in navigator) {
  window.addEventListener('load', async () => {
    const registration = await navigator.serviceWorker.register('./sw.js');
    console.log('Service worker registered for scope', registration.scope);

    await registerPeriodicBackgroundSync(registration);
  });
}

html {
  box-sizing: border-box;
  font-family: system-ui, sans-serif;
  color-scheme: dark light;
}

*,
*:before,
*:after {
  box-sizing: inherit;
}

body {
  margin: 1rem;
}

Open demo

Last updated

Oct 10, 2022 — Improve article

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