Run A/B Testing

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

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.

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

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 fencedFrameConfig = await window.sharedStorage.selectURL(
      { url: `https://your-server.example/content/default-content.html` },
      { url: `https://your-server.example/content/experiment-content-a.html` }
      resolveToConfig: true

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



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);

Use cases

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.

Content selection

Select and display different content on different websites in fenced frames based on information collected in Shared Storage. The output gate for these use cases is URL selection.

  • Creative rotation: Store data, such as creative ID, view counts, and user interaction, to determine which creative users' see across different sites.
  • A/B testing: You can assign a user to an experiment group, then store that group in Shared Storage to be accessed cross-site.
  • Custom user experiences: Share custom content and calls-to-action based on a user's registration status or other user states

Generate summary reports

Collect information with Shared Storage and generated a noisy, aggregated summary report. The output gate for these use cases is the Private Aggregation API.

  • Unique reach measurement: Many content producers and advertisers want to know how many unique people saw their content. Use Shared Storage to record the first time a user saw your ad, embedded video, or publication, and prevent duplicative counting of that same user on different sites. You can then use the Private Aggregation API to output a summary report for your 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.

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.