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Explainer for proposed web platform Shared Storage API

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Shared Storage API Explainer

Authors: Alex Turner, Camillia Smith Barnes, Josh Karlin, Yao Xiao

Introduction

In order to prevent cross-site user tracking, browsers are partitioning all forms of storage (cookies, localStorage, caches, etc) by top-frame site. But, there are many legitimate use cases currently relying on unpartitioned storage that will vanish without the help of new web APIs. We’ve seen a number of APIs proposed to fill in these gaps (e.g., Conversion Measurement API, Private Click Measurement, Storage Access, Private State Tokens, TURTLEDOVE, FLoC) and some remain (including cross-origin A/B experiments and user measurement). We propose a general-purpose storage API that can help to serve as common infrastructure for privacy preserving cross-site use cases.

Shared Storage is a key/value store that is partitioned by calling origin (but not top-frame site). The keys and values are strings. While it's possible to write to Shared Storage from nearly anywhere (including response headers!), it is only possible to read from Shared Storage in tightly controlled environments, such as a JavaScript worklet environment which is provided by Shared Storage. These worklets have no capability of communicating with the outside world. They have no network communication and no postMessage. The only way data can leave these worklets, is via privacy-preserving APIs.

Specification

See the draft specification.

APIs built on top of Shared Storage

This document only describes the core shared storage framework and infrastructure to store cross-site data privately and to read that data from within a secure worklet environment. APIs that use shared storage's data to produce some output are linked to below.

Private Aggregation

The Private Aggregation API allows for aggregated histograms to be sent based on data read from shared storage. The histograms are differentially private.

Select URL

The selectURL API allows for content selection based on cross-site data. It takes 8 possible URLs as input and sends them to a worklet which selects from a small list of URLs. The chosen URL is stored in a fenced frame config as an opaque form that can only be read by a fenced frame; the embedder does not learn this information.

Demonstration

You can try out Shared Storage along with some APIs built for it using Chrome 104+.

Example 1: Writing an experiment id to Shared Storage from a document

Since Shared Storage is meant for writing from anywhere, but reading is tightly constrained, it's not actually possible to know what you might have written to your storage from other sites. Is this the first time you've seen this user? Who knows! As such, Shared Storage provides some useful functions beyond just set to write to keys only if they're not already present, and to append to a value rather than overwrite it.

For example, let's say that you wanted to add a user to an experiment group, with a random assignment. But you want that group assignment to be sticky for the user across all of the sites that they visit that your third-party script is on. You may not know if you've ever written this key from this site before, but you certainly don't know if you've set it from another site. To solve this issue, utilize the ignoreIfPresent option.

try {
    sharedStorage.set('group', Math.floor(Math.random() * 1000), { ignoreIfPresent: true });
} catch (error) {
    // Error handling
}

And Shared Storage will only write the value if the key is not already present.

Example 2: Writing to Shared Storage via a worklet

In the event that ignoreIfPresent is not sufficient, and you need to read your existing Shared Storage data before adding new data, consider passing the information that you want to record to a worklet, and letting the worklet read the existing data and perform the write. Like so:

try {
    const worklet = sharedStorage.createWorklet('https://site.example/writerWorklet.js');
    worklet.run('write', {data: {group: Math.floor(Math.random() * 1000)}});
} catch (error) {
    // Error handling
}

And your writerWorklet.js script would look like this: writerWorklet.js

class Writer {
  async run(data) {
    const existingGroup = sharedStorage.get('group');
    if (!existingGroup) {
        cibst newGroup = data['group'];
        sharedStorage.set('group', newGroup);
    }
  }
}
register('write', Writer);

Example 3: Writing to Shared Storage with response headers

It may be faster and more convenient to write to Shared Storage directly from response headers than from JavaScript. This is encouraged in cases where data is coming from a server anyway as it's faster and less intensive than JavaScript methods if you're writing to an origin other than the current document's origin.

Response headers can be used on document, image, and fetch requests.

e.g.,:

<iframe src="https://site.example/iframe" sharedstoragewritable></iframe>

The document request for "https://site.example/iframe" will include a Sec-Shared-Storage-Writable: ?1 request header. Any request with this header can have a corresponding Shared-Storage-Write response header that can write, like so:

Shared-Storage-Write: set;key="group";value="4";ignore_if_present

Example 4: Counting the number of views your content has received across sites

To count the number of times the user has viewed your third-party content, consider using the append option. Like so:

e.g.,:

try {
    window.sharedStorage.append('count', '1');
} catch (error) {
    // Error handling
}

Then, sometime later in your worklet, you can get the total count:

class Counter {
  async run(data) {
    const countLog = data['count']; // e.g.,: '111111111'
    const count = countLog.length;
    // do something useful with this data (such as recording an aggregate histogram) here...
  }
}
register('count', Counter);

Goals

This API intends to support the storage and access needs for a wide array of cross-site data use cases. This prevents each API from having to create its own bespoke storage APIs.

Related work

There have been multiple privacy proposals (SPURFOWL, SWAN, Aggregated Reporting) that have a notion of write-only storage with limited output. Shared Storage allows for each of those use cases, with only one storage API which is easier for developers to learn and requires less browser code. We’d also like to acknowledge the KV Storage explainer, to which we turned for API-shape inspiration.

Proposed API surface

Outside of worklets (e.g., places where writing can happen)

The modifier methods (set, append, delete, clear, and batchUpdate) should be made generally available across most any context. That includes top-level documents, iframes, shared storage worklets, Protected Audience worklets, service workers, dedicated workers, etc.

The shared storage worklet invocation methods (addModule, createWorklet, and run) are available within document contexts.

  • window.sharedStorage.set(key, value, options)
    • Sets key’s entry to value.
    • key and value are both strings.
    • Options include:
      • ignoreIfPresent (defaults to false): if true, a key’s entry is not updated if the key already exists. The embedder is not notified which occurred.
      • withLock: acquire a lock on the designated resource before executing. See Locking for modifier methods for details.
  • window.sharedStorage.append(key, value, options)
    • Appends value to the entry for key. Equivalent to set if the key is not present.
    • Options include:
  • window.sharedStorage.delete(key, options)
    • Deletes the entry at the given key.
    • Options include:
  • window.sharedStorage.clear(options)
    • Deletes all entries.
    • Options include:
  • window.sharedStorage.batchUpdate(methods, options)
    • Execute methods in order.
    • methods is an array of method objects defining the operations to perform. Each object must be one of the following types: SharedStorageSetMethod, SharedStorageAppendMethod, SharedStorageDeleteMethod, or SharedStorageClearMethod. Each method object's constructor accepts the same parameters as the corresponding individual method (e.g., set, append, delete, clear).
    • Options include:
    • The withLock option for individual method objects within the batch is ignored; only the options.withLock value applies to the entire batch.
    • This method, with the withLock option, allows multiple modifier methods to be executed atomically, enabling use cases where a website needs to maintain consistency while updating data organized across multiple keys.
  • window.sharedStorage.worklet.addModule(url, options)
    • Loads and adds the module to the worklet (i.e. for registering operations). The handling should follow the worklet standard, unless clarified otherwise below.
    • This method can only be invoked once per worklet. This is because after the initial script loading, shared storage data (for the invoking origin) will be made accessible inside the worklet environment, which can be leaked via subsequent addModule() (e.g. via timing).
    • url's origin need not match that of the context that invoked addModule(url).
      • If url is cross-origin to the invoking context, the worklet will use the invoking context's origin as its partition origin for accessing shared storage data and for budget checking and withdrawing.
      • Also, for a cross-originurl, the CORS protocol applies.
    • Redirects are not allowed.
  • window.sharedStorage.worklet.run(name, options)
    • Runs the operation previously registered by register() with matching name. Does nothing if there’s no matching operation.
    • Returns a promise that resolves to undefined when the operation is queued:
    • Options can include:
      • data, an arbitrary serializable object passed to the worklet.
      • keepAlive (defaults to false), a boolean denoting whether the worklet should be retained after it completes work for this call.
        • If keepAlive is false or not specified, the worklet will shutdown as soon as the operation finishes and subsequent calls to it will fail.
        • To keep the worklet alive throughout multiple calls to run(), each of those calls must include keepAlive: true in the options dictionary.
  • window.sharedStorage.run(name, options)
    • The behavior is identical to window.sharedStorage.worklet.run(name, options).
  • window.sharedStorage.createWorklet(url, options)
    • Creates a new worklet, and loads and adds the module to the worklet (similar to the handling for window.sharedStorage.worklet.addModule(url, options)).
    • By default, the worklet uses the invoking context's origin as its partition origin for accessing shared storage data and for budget checking and withdrawing.
      • To instead use the worklet script origin (i.e. url's origin) as the partition origin for accessing shared storage, pass the dataOrigin option with "script-origin" as its value in the options dictionary.
      • Currently, the dataOrigin option, if used, is restricted to having either "script-origin" or "context-origin" as its value. "script-origin" designates the worklet script origin as the data partition origin; "context-origin" designates the invoking context origin as the data partition origin.
    • The object that the returned Promise resolves to has the same type with the implicitly constructed window.sharedStorage.worklet. However, for a worklet created via window.sharedStorage.createWorklet(url, options), only the invocation methods (e.g., run) are available, whereas calling addModule() will throw an error. This is to prevent leaking shared storage data via addModule(), similar to the reason why addModule() can only be invoked once on the implicitly constructed window.sharedStorage.worklet.
    • Redirects are not allowed.
    • When the module script's URL's origin is cross-origin with the worklet's creator window's origin and when dataOrigin is "script-origin", a Shared-Storage-Cross-Origin-Worklet-Allowed: ?1 response header is required.
    • The script server must carefully consider the security risks of allowing worklet creation by other origins (via Shared-Storage-Cross-Origin-Worklet-Allowed: ?1 and CORS), because this will also allow the worklet creator to run subsequent operations, and a malicious actor could poison and use up the worklet origin's budget.

In the worklet, during sharedStorage.worklet.addModule(url, options) or sharedStorage.createWorklet(url, options)

  • register(name, operation)
    • Registers a shared storage worklet operation with the provided name.
    • operation should be a class with an async run() method.
      • For the operation to work with sharedStorage.run(), run() should take data as an argument and return nothing. Any return value is ignored.

In the worklet, during an operation

  • sharedStorage.get(key)
    • Returns a promise that resolves into the key‘s entry or an empty string if the key is not present.
  • sharedStorage.length()
    • Returns a promise that resolves into the number of keys.
  • sharedStorage.keys() and sharedStorage.entries()
    • Returns an async iterator for all the stored keys or [key, value] pairs, sorted in the underlying key order.
  • sharedStorage.set(key, value, options), sharedStorage.append(key, value, options), sharedStorage.delete(key, options), sharedStorage.clear(options), and sharedStorage.batchUpdate(methods, options)
    • Same as outside the worklet, except that the promise returned only resolves into undefined when the operation has completed.
  • sharedStorage.context
    • From inside a worklet created inside a fenced frame, returns a string of contextual information, if any, that the embedder had written to the fenced frame's FencedFrameConfig before the fenced frame's navigation.
    • If no contextual information string had been written for the given frame, returns undefined.
  • interestGroups()
    • Returns a promise that resolves into an array of StorageInterestGroup. A StorageInterestGroup is a dictionary that extends the AuctionAdInterestGroup dictionary with the following attributes:
      • unsigned long long joinCount
      • unsigned long long bidCount
      • sequence<PreviousWin> prevWinsMs
      • USVString joiningOrigin
      • double timeSinceGroupJoinedMs
      • double lifetimeRemainingMs
      • double timeSinceLastUpdateMs
      • double timeUntilNextUpdateMs
      • unsigned long long estimatedSize
        • The approximate size of the contents of this interest group, in bytes.
    • The AuctionAdInterestGroup's lifetimeMs field will remain unset. It's no longer applicable at query time and is replaced with attributes timeSinceGroupJoinedMs and lifetimeRemainingMs.
    • This API provides the Protected Audience buyer with a better picture of what's happening with their users, allowing for Private Aggregation reports.
  • navigator.locks.request(resource, callback) and navigator.locks.request(resource, options, callback)
    • Acquires a lock on resource and invokes callback with the lock held. navigator.locks returns a LockManager as it does in a Window. See the request method in Web Locks API for details.
    • Lock Scope: shared storage locks are partitioned by the shared storage data origin, and are independent of any locks obtained via navigator.locks.request in a Window or Worker context. This prevents contention between shared storage locks and other locks, ensuring that shared storage data cannot be inadvertently leaked.
  • Functions exposed by APIs built on top of Shared Storage such as the Private Aggregation API, e.g. privateAggregation.contributeToHistogram().
    • These functions construct and then send an aggregatable report for the private, secure aggregation service.
    • The report contents (e.g. key, value) are encrypted and sent after a delay. The report can only be read by the service and processed into aggregate statistics.
    • After a Shared Storage operation has been running for 5 seconds, Private Aggregation contributions are timed out. Any future contributions are ignored and contributions already made are sent in a report as if the Shared Storage operation had completed.

From response headers

  • set(), append(), delete(), clear(), and batchUpdate() operations can be triggered via the HTTP response header Shared-Storage-Write.
  • This may provide a large performance improvement over creating a cross-origin iframe and writing from there, if a network request is otherwise required.
  • Shared-Storage-Write is a List Structured Header.
    • Each member of the List is a Token Item denoting the operation to be performed, with any arguments for the operation as associated Parameters. A string type argument or option (key, value, with_lock) can take the form of a Token Item or a String Item or a Byte Sequence Item. A boolean type option (ignore_if_present) can take the form of a Boolean Item.
    • The order of Items in the List is the order in which the operations will be performed.
    • Operations correspond to Items as follows:
      • set(<key>, <value>, {ignoreIfPresent: true}) ←→ set;key=<key>;value=<value>;ignore_if_present
      • set(<key>, <value>, {ignoreIfPresent: false}) ←→ set;key=<key>;value=<value>;ignore_if_present=?0
      • set(<key>, <value>, {withLock: <resource>}) ←→ set;key=<key>;value=<value>;with_lock=<resource>
      • set(<key>, <value>) ←→ set;key=<key>;value=<value>
      • append(<key>, <value>) ←→ append;key=<key>;value=<value>
      • delete(<key>) ←→ delete;key=<key>
      • clear() ←→ clear
      • batchUpdate(<methods>, {withLock: <resource>}) ←→ batchUpdate;methods=<serialized methods>;with_lock=<resource>
        • <serialized methods> is a String Item containing the List Structured Header serialization of batched methods in the same list structure. Note that nested batchUpdate() operations within a batchUpdate() are not allowed.
        • Example header value: batchUpdate;methods="set;key=\"123\";value=\"456\";ignore_if_present, append;key=abc;value=def";with_lock="report-lock". This corresponds to the JavaScript method: sharedStorage.batchUpdate([new SharedStorageSetMethod("123", "456", {ignoreIfPresent: true}), new SharedStorageAppendMethod("abc", "def")], { withLock: "report-lock" }).
    • <key> and <value> Parameters are of type String or Byte Sequence.
  • Performing operations via response headers requires a prior opt-in via a corresponding HTTP request header Sec-Shared-Storage-Writable: ?1.
  • The request header can be sent along with fetch requests via specifying an option: fetch(<url>, {sharedStorageWritable: true}).
  • The request header can alternatively be sent on document or image requests either
    • via specifying a boolean content attribute, e.g.:
      • <iframe src=[url] sharedstoragewritable></iframe>
      • <img src=[url] sharedstoragewritable>
    • or via an equivalent boolean IDL attribute, e.g.:
      • iframe.sharedStorageWritable = true
      • img.sharedStorageWritable = true.
  • Redirects will be followed, and the request header will be sent to the host server for the redirect URL.
  • The origin used for Shared Storage is that of the server that sends the Shared-Storage-Write response header(s).
    • If there are no redirects, this will be the origin of the request URL.
    • If there are redirects, the origin of the redirect URL that is accompanied by the Shared-Storage-Write response header(s) will be used.
  • The response header will only be honored if the corresponding request included the request header: Sec-Shared-Storage-Writable: ?1.
  • See example usage below.

Locking for Modifier Methods

All modifier methods (set, append, delete, clear, batchUpdate), whether invoked from JavaScript or from response headers, accept a withLock: <resource> option. This option instructs the method to acquire a lock on the designated resource before executing.

The locks requested this way are partitioned by the shared storage data origin, and are independent of any locks obtained via navigator.locks.request in a Window or Worker context. Note that they share the same scope with the locks obtained via navigator.locks.request in the SharedStorageWorklet context.

Unlike navigator.locks.request, which offers various configuration options, the locks requested this way always use the default settings:

  • mode: "exclusive": The lock is never shared with other locks.
  • steal: false: The lock will not preempt other locks.
  • ifAvailable: false: If the lock is currently held by others, keep waiting and don't skip.

Example: Report on Multiple Keys

This example uses a lock to ensure that the read and delete operations inside the worklet runs atomically, preventing interference from the write operations outside the worklet.

Window context:

try {
  sharedStorage.batchUpdate([
    new SharedStorageSetMethod('key0', calculateValueFor('key0')),
    new SharedStorageSetMethod('key1', calculateValueFor('key1'))
  ], { withLock: 'report-lock' });

  await sharedStorage.worklet.addModule('report-on-multiple-keys-script.js');
  await sharedStorage.worklet.run('report-on-multiple-keys');
} catch (error) {
  // Handle error.
}

In the worklet script (report-on-multiple-keys-script.js):

class ReportOnMultipleKeysOperation {
  async run(data) {
    await navigator.locks.request("report-lock", async (lock) => {
      const value1 = await sharedStorage.get('key1');
      const value2 = await sharedStorage.get('key2');

      // Record an aggregate histogram with `value1` and `value2` here...

      await sharedStorage.delete('key1');
      await sharedStorage.delete('key2');
    });
  }
}
register('report-on-multiple-keys', ReportOnMultipleKeysOperation);

Caveat: Unexpected ordering

Modifier methods may block due to the lock, so may not execute in the order they appear in the code.

// Resolve immediately. Internally, this may block to wait for the lock to be granted.
sharedStorage.set('key0', 'value1', { withLock: 'resource0' });

// Resolve immediately. Internally, this will execute immediately.
sharedStorage.set('key0', 'value2');

Developers should be mindful of this potential ordering issue.

Recommendations for lock usage

If only a single key is involved, and the data is accessed at most once within and outside worklet, then the lock is unnecessary. This is because each access is inherently atomic. Example: A/B experiment.

If the worklet performs both read and write on the same key, then the lock is likely necessary. Example: creative selection by frequency.

If the logic involes updating data organized across multiple keys, then the lock is likely necessary. Example: Report on Multiple Keys.

Reporting embedder context

In using the Private Aggregation API to report on advertisements within fenced frames, for instance, we might report on viewability, performance, which parts of the ad the user engaged with, the fact that the ad showed up at all, and so forth. But when reporting on the ad, it might be important to tie it to some contextual information from the embedding publisher page, such as an event-level ID.

In a scenario where the input URLs for the fenced frame must be k-anonymous, e.g. if we create a FencedFrameConfig from running a Protected Audience auction, it would not be a good idea to rely on communicating the event-level ID to the fenced frame by attaching an identifier to any of the input URLs, as this would make it difficult for any input URL(s) with the attached identifier to reach the k-anonymity threshold.

Instead, before navigating the fenced frame to the auction's winning FencedFrameConfig fencedFrameConfig, we could write the event-level ID to fencedFrameConfig using fencedFrameConfig.setSharedStorageContext() as in the example below.

Subsequently, anything we've written to fencedFrameConfig through setSharedStorageContext() prior to the fenced frame's navigation to fencedFrameConfig, can be read via sharedStorage.context from inside a shared storage worklet created by the fenced frame, or created by any of its same-origin children.

In the embedder page:

// See https://github.com/WICG/turtledove/blob/main/FLEDGE.md for how to write an auction config.
const auctionConfig = { ... };

// Run a Protected Audience auction, setting the option to "resolveToConfig" to true.
auctionConfig.resolveToConfig = true;
const fencedFrameConfig = await navigator.runAdAuction(auctionConfig);

// Write to the config any desired embedder contextual information as a string.
fencedFrameConfig.setSharedStorageContext("My Event ID 123");

// Navigate the fenced frame to the config.
document.getElementById('my-fenced-frame').config = fencedFrameConfig;

In the fenced frame (my-fenced-frame):

// Save some information we want to report that's only available inside the fenced frame.
const frameInfo = { ... };

// Send a report using shared storage and private aggregation.
try {
    await window.sharedStorage.worklet.addModule('report.js');
    await window.sharedStorage.run('send-report', {
    data: { info: frameInfo },
    });
} catch (error) {
    // Error handling
}

In the worklet script (report.js):

class ReportingOperation {
  async run(data) {
    // Helper functions that map the embedder context to a predetermined bucket and the
    // frame info to an appropriately-scaled value.
    // See also https://github.com/patcg-individual-drafts/private-aggregation-api#examples
    function convertEmbedderContextToBucketId(context) { ... }
    function convertFrameInfoToValue(info) { ... }

    // The user agent sends the report to the reporting endpoint of the script's
    // origin (that is, the caller of `sharedStorage.run()`) after a delay.
    privateAggregation.contributeToHistogram({
      bucket: convertEmbedderContextToBucketId(sharedStorage.context) ,
      value: convertFrameInfoToValue(data.info)
    });
  }
}
register('send-report', ReportingOperation);

Keeping a worklet alive for multiple operations

Callers may wish to run multiple worklet operations from the same context, e.g. they might select a URL and then send one or more aggregatable reports. To do so, they would need to use the keepAlive: true option when calling each of the worklet operations (except perhaps in the last call, if there was no need to extend the worklet's lifetime beyond that call).

Writing to Shared Storage via response headers

For an origin making changes to their Shared Storage data at a point when they do not need to read the data, an alternative to using the Shared Storage JavaScript API is to trigger setter and/or deleter operations via the HTTP response header Shared-Storage-Write as in the examples below.

In order to perform operations via response header, the origin must first opt-in via one of the methods below, causing the HTTP request header Sec-Shared-Storage-Writable: ?1 to be added by the user agent if the request is eligible based on permissions checks.

An origin a.example could initiate such a request in multiple ways.

On the client side, to initiate the request:

  1. fetch() option: 
    fetch("https://a.example/path/for/updates", {sharedStorageWritable: true});
  2. Content attribute option with an iframe (also possible with an img): 
     <iframe src="https://a.example/path/for/updates" sharedstoragewritable></iframe>
  3. IDL attribute option with an iframe (also possible with an img): 
    let iframe = document.getElementById("my-iframe");
iframe.sharedStorageWritable = true;
iframe.src = "https://a.example/path/for/updates";

On the server side, here is an example response header:

Shared-Storage-Write: clear, set;key="hello";value="world";ignore_if_present, append;key="good";value="bye", delete;key="hello", set;key="all";value="done"

Sending the above response header would be equivalent to making the following calls in the following order on the client side, from either the document or a worklet:

sharedStorage.clear();
sharedStorage.set("hello", "world", {ignoreIfPresent: true});
sharedStorage.append("good", "bye");
sharedStorage.delete("hello");
sharedStorage.set("all", "done");

Loading cross-origin worklet scripts

There are currently four (4) approaches to creating a worklet that loads cross-origin script. The partition origin for the worklet's shared storage data access depends on the approach.

Using the context origin as data partition origin

The first three (3) approaches use the invoking context's origin as the partition origin for shared storage data access and the invoking context's site for shared storage budget withdrawals.

  1. Call addModule() with a cross-origin script.

    In an "https://a.example" context in the embedder page:

    await sharedStorage.worklet.addModule("https://b.example/worklet.js");

    For any subsequent run() or selectURL() operation invoked on this worklet, the shared storage data for "https://a.example" (i.e. the context origin) will be used.

  2. Call createWorklet() with a cross-origin script.

    In an "https://a.example" context in the embedder page:

    const worklet = await sharedStorage.createWorklet("https://b.example/worklet.js");

    For any subsequent run() or selectURL() operation invoked on this worklet, the shared storage data for "https://a.example" (i.e. the context origin) will be used.

  3. Call createWorklet() with a cross-origin script, setting its dataOption to the invoking context's origin.

    In an "https://a.example" context in the embedder page:

    const worklet = await sharedStorage.createWorklet("https://b.example/worklet.js", {dataOrigin: "context-origin"});

    For any subsequent run() or selectURL() operation invoked on this worklet, the shared storage data for "https://a.example" (i.e. the context origin) will be used.

Using the worklet script origin as data partition origin

The fourth approach uses the worklet script's origin as the partition origin for shared storage data access and the worklet script's site for shared storage budget withdrawals.

  1. Call createWorklet() with a cross-origin script, setting its dataOption to the worklet script's origin.

    In an "https://a.example" context in the embedder page:

    const worklet = await sharedStorage.createWorklet("https://b.example/worklet.js", {dataOrigin: "script-origin"});

    For any subsequent run() or selectURL() operation invoked on this worklet, the shared storage data for "https://b.example" (i.e. the worklet script origin) will be used.

Error handling

Note that the shared storage APIs may throw for several possible reasons. The following list of situations is not exhaustive, but, for example, the APIs may throw if the site invoking the API is not enrolled and/or attested, if the user has disabled shared storage in site settings, if the "shared-storage" or "shared-storage-select-url" permissions policy denies access, or if one of its arguments is invalid.

We recommend handling exceptions. This can be done by wrapping async..await calls to shared storage JS methods in try...catch blocks, or by following calls that are not awaited with .catch:

try {
  await window.sharedStorage.worklet.addModule('worklet.js');
} catch (error) {
  // Handle error.
}
window.sharedStorage.worklet.addModule('worklet.js')
  .catch((error) => {
  // Handle error.
});

Worklets can outlive the associated document

After a document dies, the corresponding worklet (if running an operation) will continue to be kept alive for a maximum of two seconds to allow the pending operation(s) to execute. This gives more confidence that any end-of-page operations (e.g. reporting) are able to finish.

Permissions Policy

Shared storage methods can be disallowed by the "shared-storage" policy-controlled feature. Its default allowlist is * (i.e. every origin). APIs built on top of Shared Storage have their own specific permission policies, so it is possible to allow reading and writing of Shared Storage while disabling particular APIs.

Permissions Policy inside the shared storage worklet

The permissions policy inside the shared storage worklet will inherit the permissions policy of the associated document.

Data Retention Policy

Each key is cleared after thirty days of last write (set or append call). If ignoreIfPresent is true, the last write time is updated.

Data Storage Limits

Shared Storage is not subject to the quota manager, as that would leak information across sites. Therefore we limit the per-origin total key and value bytes to 5MB.

Privacy

Shared Storage takes the following protective measures to prevent its stored data from being read by means other than via approved APIs (e.g., via side channels):

  • Concealed Operation Time and Errors: When writing data or running worklet operations from the Window scope, the method returns immediately and will not expose errors that might arise from reading shared storage data.

  • Disabled Storage Access before Loading Finishes: Access to Shared Storage is disabled until a module script finishes loading. This prevents websites from using timing attacks to learn about the data stored in Shared Storage.

  • Isolated Locks: Locks requested for Shared Storage are completely separate from locks requested from the Window scope. This prevents information leakage through lock contention.

Privacy-Preserving APIs

The APIs that can read data from Shared Storage have their own privacy documentation.

Enrollment and Attestation

Use of Shared Storage requires enrollment and attestation via the Privacy Sandbox enrollment attestation model.

For each method in the Shared Storage API surface, a check will be performed to determine whether the calling site is enrolled and attested. In the case where the site is not enrolled and attested, the promise returned by the method is rejected.

Possibilities for extension

Interactions between worklets

Communication between worklets is not possible in the initial design. However, adding support for this would enable multiple origins to flexibly share information without needing a dedicated origin for that sharing. Relatedly, allowing a worklet to create other worklets might be useful.

Registering event handlers

We could support event handlers in future iterations. For example, a handler could run a previously registered operation when a given key is modified (e.g. when an entry is updated via a set or append call):

sharedStorage.addEventListener(
  'key' /* event_type */,
  'operation-to-run' /* operation_name */,
  { key: 'example-key', actions: ['set', 'append'] } /* options */);

Acknowledgements

Many thanks for valuable feedback and advice from:

Victor Costan, Christian Dullweber, Charlie Harrison, Jeff Kaufman, Rowan Merewood, Marijn Kruisselbrink, Nasko Oskov, Evgeny Skvortsov, Michael Tomaine, David Turner, David Van Cleve, Zheng Wei, Mike West.

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Explainer for proposed web platform Shared Storage API

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