A/B Testing

Use a Shared Storage worklet to run A/B testing.

Published on Updated on

The Shared Storage API is a Privacy Sandbox proposal for general purpose, cross-site storage, which supports many possible use cases. One such example is A/B testing, which is available to test in Chrome 104.0.5086.0 and later.

With URL selection, you can assign a user to an experiment group, then store that group in Shared Storage to be accessed in a cross-site environment.

Try A/B testing

To experiment with A/B testing with Shared Storage, confirm you're using Chrome 104.0.5086.0 or later. Then enable the Privacy Sandbox Ads APIs experiment flag at chrome://flags/#privacy-sandbox-ads-apis.

Set Privacy Sandbox Ads APIs experiment to enabled to use these APIs

You can also enable Shared Storage with the --enable-features=PrivacySandboxAdsAPIsOverride,OverridePrivacySandboxSettingsLocalTesting,SharedStorageAPI,FencedFrames flag in the command line.

Experiment with code samples

The following code samples were created to demonstrate how the API may be used for the given use cases. These are not meant to be used in production.

To see if an experiment has the desired effect, you can run A/B testing across multiple sites. As an advertiser or a content producer, you can choose to render different content or ads based on what group the user is assigned to. The group assignment is saved in shared storage, but cannot be exfiltrated.

In this example:

  • ab-testing.js should be embedded in a frame, which maps a control and two experiment contents. The script calls the shared storage worklet for the experiment.
  • ab-testing-worklet.js is the shared storage worklet that returns which group the user is assigned to, determining which ad is shown.


// Randomly assigns a user to a group 0 or 1
function getExperimentGroup() {
return Math.round(Math.random());

async function injectContent() {
// Register the Shared Storage worklet
await window.sharedStorage.worklet.addModule('ab-testing-worklet.js');

// Assign user to a random group (0 or 1) and store it in Shared Storage
window.sharedStorage.set('ab-testing-group', getExperimentGroup(), {
ignoreIfPresent: true,

// Run the URL selection operation
const opaqueURL = await window.sharedStorage.selectURL(
{ url: `https://your-server.example/content/default-content.html` },
{ url: `https://your-server.example/content/experiment-content-a.html` }

// Render the chosen URL into a fenced frame
document.getElementById('content-slot').src = opaqueURL;



class SelectURLOperation {
async run(urls, data) {
// Read the user's experiment group from Shared Storage
const experimentGroup = await this.sharedStorage.get('ab-testing-group');

// Return the corresponding URL (first or second item in the array)
return urls.indexOf(experimentGroup);

register('ab-testing', SelectURLOperation);

Other use cases

Explore other Shared Storage use cases and code samples:

Generate reports with Private Aggregation

  • Unique reach measurement: Many content producers and advertisers often want to know how many unique people saw their content. You can use Shared Storage to report on the first time a user saw your ad, embedded video, publication, and prevent duplicative counting of that same user on a different site, giving you an aggregated noisy report of your approximate unique reach.
  • Demographics measurement: Content producers often want to understand the demographics of their audience. You can use Shared Storage to record user demographic data in a context where you have it, such as your first-party site, and use aggregated reporting to report on it across many other sites, such as embedded content.
  • K+ frequency measurement: Sometimes described as "effective frequency," there is often a minimum number views before a user will recognize or recall certain content (often in the context of advertisement views). You can use Shared Storage to build reports of unique users that have seen a piece of content at least K number of times.

URL selection

  • Frequency control: run a worklet script to select a URL from a provided list, based on the stored data, and then render that URL in a fenced frame. This has many possible uses, such as selecting new content when a frequency cap is reached.
  • A/B testing: You can assign a user to an experiment group, then store that group in Shared Storage to be accessed cross-site.
  • Creative rotation: You can store the creative rotation mode, and other metadata, to rotate the creatives across different sites.
  • Known customer for payment provider: You can store whether the user has registered on your site into shared storage, then render a different element based on that stored status.

These are only some of the possible use cases for Shared Storage. We'll continue to add examples as we receive feedback and discover new use cases.

Engage and share feedback

The Shared Storage proposal is under active discussion and subject to change in the future. If you try this API and have feedback, we'd love to hear it.

Updated on Improve article

We serve cookies on this site to analyze traffic, remember your preferences, and optimize your experience.