Chromium Docs

Pending Beacon API

This document describes the status of the current implementation of the Pending Beacon API (a.k.a. PendingBeacon API) in Chrome, and how to enable it manually.

Starting from version 107, Chrome experimentally supports the Pending Beacon API, which allows website authors to specify one or more beacons (HTTP requests) that should be sent reliably when the page is being unloaded.

See the public explainer to learn more about how it works.

Note that this API is not enabled by default. Instead, Chrome plans to run A/B testing to evaluate its impact. But Chrome also provides some ways to fully opt-in to the API for web developers who what to try the features.

What’s supported

Chrome supports all the JavaScript APIs described in the explainer, specifically:

What’s not supported

The following features are not yet supported in Chrome:

  • When calling setData(data) on a PendingPostBeacon, the data payload cannot be
    • A complex body, i.e. for a data of FormData type, it can only have single entry/part.
    • A streaming body, i.e. a data cannot be a ReadableStream.
  • Crash recovery related behaviors and privacy requirements: not yet supported. Chrome currently doesn't store any PendingBeacon on disk.
  • Delete pending beacons for a site if a user clears site data: not supported yet, as crash recovery from disk is not yet supported.
  • Beacon requests are not yet observable in Chrome DevTools.

The following features work differently than the one described in explainer:

  • Beacons must not leak navigation history to the network provider that it should not know: supported by forcing to send out all beacons on navigating away from a document.
  • The beacon destination URL should be modifiable: only PendingGetBeacon can update its URL via setURL() method.
  • Beacon Sending behavior: the Chrome implementation currently queues all created instances of Pending*Beacon for sending. But in the explainer, it specifies that a PendingPostBeacon is only queued if it has non-undefined and non-null data (described in setData() method).
  • Beacons must be sent over HTTPS: current implementation doesn't enforce HTTPS, which means if web developer creates a Pending*Beacon with HTTP URL property, it will still work.
  • Beacons max TTL is bound by Chrome's back/forward cache TTL, which is currently 10 minutes.

Activation

The API can be enabled by a command line flag, or via Origin Trial.

Using command line flag

Passing the --enable-features=PendingBeaconAPI command line flag to Chrome enables PendingBeacon API support.

Using Origin Trial

Trial for Pending Beacon.

You can opt any page on your origin into PendingBeacon API Origin Trial by requesting a token for your origin via the above link. Include the token in your page so that Chrome can recognize your page is opted in.

The simplest way is to include the following line in your page:

<meta http-equiv="origin-trial" content="**your token**">

Or you can also include the token in your HTTP request.

NOTE: Even with the Origin Trial token, NOT all of visits to your page will have the API enabled. It is because as mentioned above, Chrome plan to do A/B testing on the API.

Verifying the API is working

Added the following line to a web page, and load the page into a Chrome tab.

<script>
new PendingGetBeacon('/test');
</script>

Close the tab, and you should be able to observe a request sent to /test on your web server that hosts the page.

Related Links