Web Locks API

1. Introduction

This section is non-normative.

A lock request is made by script for a particular resource name and mode . A scheduling algorithm looks at the state of current and previous requests, and eventually grants a lock request. A lock is a granted request; it has a resource name and mode . It is represented as an object returned to script. As long as the lock is held it may prevent other lock requests from being granted (depending on the name and mode). A lock can be released by script, at which point it may allow other lock requests to be granted.

The API provides optional functionality that may be used as needed, including:

returning values from the asynchronous task,
shared and exclusive lock modes,
conditional acquisition,
diagnostics to query the state of locks, and
an escape hatch to protect against deadlocks.

Cooperative coordination takes place within the scope of same-origin agents ; this may span multiple agent clusters .

1.1. Usage Overview

The API is used as follows:

The lock is requested.
Work is done while holding the lock in an asynchronous task.
The lock is automatically released when the task completes.

A basic example of the API usage is as follows:

navigator.locks.request('my_resource', async lock => {
   // The lock has been acquired.
   await do_something();
   await do_somethng_else();
   // Now the lock will be released.
});

Within an asynchronous function, the request itself can be awaited:

// Before requesting the lock.
await navigator.locks.request('my_resource', async lock => {
   // The lock has been acquired.
   await do_something();
   // Now the lock will be released.
});
// After the lock has been released

1.2. Motivating Use Cases

A web-based document editor stores state in memory for fast access and persists changes (as a series of records) to a storage API such as the Indexed Database API for resiliency and offline use, and to a server for cross-device use. When the same document is opened for editing in two tabs the work must be coordinated across tabs, such as allowing only one tab to make changes to or synchronize the document at a time. This requires the tabs to coordinate on which will be actively making changes (and synchronizing the in-memory state with the storage API), knowing when the active tab goes away (navigated, closed, crashed) so that another tab can become active.

In a data synchronization service, a "primary tab" is designated. This tab is the only one that should be performing some operations (e.g. network sync, cleaning up queued data, etc). It holds a lock and never releases it. Other tabs can attempt to acquire the lock, and such attempts will be queued. If the "primary tab" crashes or is closed then one of the other tabs will get the lock and become the new primary.

The Indexed Database API defines a transaction model allowing shared read and exclusive write access across multiple named storage partitions within an origin. Exposing this concept as a primitive allows any Web Platform activity to be scheduled based on resource availability, for example allowing transactions to be composed for other storage types (such as Caches [Service-Workers] ), across storage types, even across non-storage APIs (e.g. network fetches).

2. Concepts

For the purposes of this specification:

Separate user profiles within a browser are considered separate user agents.
Every private mode browsing session is considered a separate user agent.

A user agent has a lock task queue which is the result of starting a new parallel queue .

The task source for steps enqueued below is the web locks tasks source .

2.1. Resources Names

A resource name is a JavaScript string chosen by the web application to represent an abstract resource.

A resource name has no external meaning beyond the scheduling algorithm, but is global across browsing contexts within an origin . Web applications are free to use any resource naming scheme.

To mimic transaction locking over named stores within a named database in [IndexedDB-2] , a script might compose resource names as:

encodeURIComponent(db_name) + '/' + encodeURIComponent(store_name)

Resource names starting with U+002D HYPHEN-MINUS (-) are reserved; requesting these will cause an exception.

2.2. Lock Managers

A user agent has a lock manager for each origin , which encapsulates the state of all locks and lock requests for that origin.

Note: Pages and workers ( agents ) on a single origin opened in the same user agent share a lock manager even if they are in unrelated browsing contexts .

To obtain a lock manager , given an environment settings object environment, run these steps:

Let origin be environment ’s origin .
If origin is an opaque origin , then return failure.
Return origin ’s associated lock manager .

2.3. Modes and Scheduling

A mode is either " exclusive " or " shared ". Modes can be used to model the common readers-writer lock pattern. If an " exclusive " lock is held, then no other locks with that name can be granted. If a " shared " lock is held, other " shared " locks with that name can be granted — but not any " exclusive " locks. The default mode in the API is " exclusive ".

Additional properties may influence scheduling, such as timeouts, fairness, and so on.

2.4. Locks

A lock represents exclusive access to a shared resource.

A lock has an agent which is an agent .

A lock has a clientId which is an opaque string.

A lock has a manager which is a lock manager .

A lock has a name which is a resource name .

A lock has a mode which is one of " exclusive " or " shared ".

A lock has a waiting promise which is a Promise.

A lock has a released promise which is a Promise.

There are two promises associated with a lock’s lifecycle:

A promise provided either implicitly or explicitly by the callback when the lock is granted which determines how long the lock is held. When this promise settles, the lock is released. This is known as the lock’s waiting promise .
A promise returned by LockManager 's request() method that settles when the lock is released or the request is aborted. This is known as the lock’s released promise .

const p1 = navigator.locks.request('resource', lock => {
  const p2 = new Promise(r => {
    // Logic to use lock and resolve promise...
  });
  return p2;
});

In the above example, p1 is the released promise and p2 is the waiting promise . Note that in most code the callback would be implemented as an async function and the returned promise would be implicit, as in the following example:

const p1 = navigator.locks.request('resource', async lock => {
  // Logic to use lock...
});

The waiting promise is not named in the above code, but is still present as the return value from the anonymous async callback. Further note that if the callback is not async and returns a non-promise, the return value is wrapped in a promise that is immediately resolved; the lock will be released in an upcoming microtask, and the released promise will also resolve in a subsequent microtask.

Each lock manager has a held lock set which is a set of locks .

When lock lock ’s waiting promise settles (fulfills or rejects), enqueue the following steps on the lock task queue :

Release the lock lock.
Resolve lock ’s released promise with lock ’s waiting promise .

2.5. Lock Requests

A lock request represents a pending request for a lock .

A lock request is a struct with items agent , clientId , manager , name , mode , callback , promise , and signal .

A lock request queue is a queue of lock requests .

Each lock manager has a lock request queue map , which is a map of resource names to lock request queues .

To get the lock request queue from lock request queue map queueMap from resource name name, run these steps:

If queueMap [ name ] does not exist , set queueMap [ name ] to a new empty lock request queue .
Return queueMap [ name ].

A lock request request is said to be grantable if the following steps return true:

Let manager be request ’s manager .
Let queueMap be manager ’s lock request queue map .
Let name be request ’s name .
Let queue be the result of getting the lock request queue from queueMap for name.
Let held be manager ’s held lock set
Let mode be request ’s mode
If queue is not empty and request is not the first item in queue, then return false.
If mode is " exclusive ", then return true if no lock in held has name equal to name, and false otherwise.
Otherwise, mode is " shared "; return true if no lock in held has mode " exclusive " and has name equal to name, and false otherwise.

2.6. Agent Integration

When an agent terminates, enqueue the following steps on the lock task queue :

Identify a normative reference for terminates .

For each lock request request with agent equal to the terminating agent:
1. Abort the request request.
For each lock lock with agent equal to the terminating agent:
1. Release the lock lock.

3. API

3.1. Navigator Mixins

[SecureContext]
interface mixin NavigatorLocks {
  readonly attribute LockManager locks;
};
Navigator includes NavigatorLocks;
WorkerNavigator includes NavigatorLocks;

Each environment settings object has a LockManager object.

Navigator/locks

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The locks getter’s steps are to return this 's relevant settings object 's LockManager object.

3.2. `LockManager` class

LockManager

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[SecureContext, Exposed=(Window,Worker)]
interface LockManager {
  Promise<any> request(DOMString name,
                       LockGrantedCallback callback);
  Promise<any> request(DOMString name,
                       LockOptions options,
                       LockGrantedCallback callback);
  Promise<LockManagerSnapshot> query();
};
callback LockGrantedCallback = Promise<any> (Lock? lock);
enum LockMode { "shared", "exclusive" };
dictionary LockOptions {
  LockMode mode = "exclusive";
  boolean ifAvailable = false;
  boolean steal = false;
  AbortSignal signal;
};
dictionary LockManagerSnapshot {
  sequence<LockInfo> held;
  sequence<LockInfo> pending;
};
dictionary LockInfo {
  DOMString name;
  LockMode mode;
  DOMString clientId;
};

A LockManager instance allows script to make lock requests and query the state of the lock manager .

3.2.1. The `request()` method

LockManager/request

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promise = navigator . locks . request ( name, callback )

promise = navigator . locks . request ( name, options, callback )

The request() method is called to request a lock.

The name (initial argument) is a resource name string.

The callback (final argument) is a callback function invoked with the Lock when granted. This is specified by script, and is usually an async function. The lock is held until the callback function completes. If a non-async callback function is passed in, then it is automatically wrapped in a promise that resolves immediately, so the lock is only held for the duration of the synchronous callback.

The returned promise resolves (or rejects) with the result of the callback after the lock is released, or rejects if the request is aborted.

Example:

try {
  const result = await navigator.locks.request('resource', async lock => {
    // The lock is held here.
    await do_something();
    await do_something_else();
    return "ok";
    // The lock will be released now.
  });
  // |result| has the return value of the callback.
} catch (ex) {
  // if the callback threw, it will be caught here.
}

The lock will be released when the callback exits for any reason — either when the code returns, or if it throws.

An options dictionary can be specified as a second argument; the callback argument is always last.

options . mode

The mode option can be " exclusive " (the default if not specified) or " shared ". Multiple tabs/workers can hold a lock for the same resource in " shared " mode, but only one tab/worker can hold a lock for the resource in " exclusive " mode.

The most common use for this is to allow multiple readers to access a resource simultaneously but prevent changes. Once reader locks are released a single exclusive writer can acquire the lock to make changes, followed by another exclusive writer or more shared readers.

await navigator.locks.request('resource', {mode: 'shared'}, async lock => {
  // Lock is held here. Other contexts might also hold the lock in shared mode,
  // but no other contexts will hold the lock in exclusive mode.
});

options . ifAvailable: If the ifAvailable option is true, then the lock is only granted if it can be without additional waiting. Note that this is still not synchronous ; in many user agents this will require cross-process communication to see if the lock can be granted. If the lock cannot be granted, the callback is invoked with null. (Since this is expected, the request is not rejected.)

await navigator.locks.request('resource', {ifAvailable: true}, async lock => {
  if (!lock) {
    // Didn’t get it. Maybe take appropriate action.
    return;
  }
  // Lock is held here.
});

options . signal: The signal option can be set to an AbortSignal. This allows aborting a lock request, for example if the request is not granted in a timely manner:

const controller = new AbortController();
setTimeout(() => controller.abort(), 200); // Wait at most 200ms.
try {
  await navigator.locks.request(
    'resource', {signal: controller.signal}, async lock => {
      // Lock is held here.
  });
  // Done with lock here.
} catch (ex) {
  // |ex| will be a DOMException with error name "AbortError" if timer fired.
}

If an abort is signalled before the lock is granted, then the request promise will reject with an AbortError. Once the lock has been granted, the signal is ignored.

options . steal

If the steal option is true, then any held locks for the resource will be released (and the released promise of such locks will resolve with AbortError ), and the request will be granted, preempting any queued requests for it.

If a web application detects an unrecoverable state — for example, some coordination point like a Service Worker determines that a tab holding a lock is no longer responding — then it can "steal" a lock using this option.

Use the



steal

option with caution. When used, code previously holding a lock will now be executing without guarantees that it is the sole context with access to the resource. Similarly, the code that used the option has no guarantees that other contexts will not still be executing as if they have access to the abstract resource. It is intended for use by web applications that need to attempt recovery in the face of application and/or user-agent defects, where behavior is already unpredictable.

The request( name , callback ) and request( name , options , callback ) method steps are:

If options was not passed, then let options be a new LockOptions dictionary with default members.
Let environment be this 's relevant settings object .
If environment ’s responsible document is not fully active , then return a promise rejected with a " InvalidStateError " DOMException.
Let manager be the result of obtaining a lock manager given environment. If that returned failure, then return a promise rejected with a " SecurityError " DOMException.
If name starts with U+002D HYPHEN-MINUS (-), then return a promise rejected with a " NotSupportedError " DOMException.
If both options [" steal "] and options [" ifAvailable "] are true, then return a promise rejected with a " NotSupportedError " DOMException.
If options [" steal "] is true and options [" mode "] is not " exclusive ", then return a promise rejected with a " NotSupportedError " DOMException.
If options [" signal "] exists , and either of options [" steal "] or options [" ifAvailable "] is true, then return a promise rejected with a " NotSupportedError " DOMException.
If options [" signal "] exists and is aborted , then return a promise rejected with ~~an " AbortError " {{DOMException}.~~ signal ’s abort reason .
Let promise be a new promise .
Request a lock with promise, the current agent , environment ’s id , manager, callback, name, options [" mode "], options [" ifAvailable "], options [" steal "], and options [" signal "].
Return promise.

3.2.2. The `query()` method

LockManager/query

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state = await navigator . locks . query ()

The query() method can be used to produce a snapshot of the lock manager state for an origin, which allows a web application to introspect its usage of locks, for logging or debugging purposes.

The returned promise resolves to state, a plain-old-data structure (i.e. JSON-like data) with this form:

{
  held: [
    { name: "resource1", mode: "exclusive",
      clientId: "8b1e730c-7405-47db-9265-6ee7c73ac153" },
    { name: "resource2", mode: "shared",
      clientId: "8b1e730c-7405-47db-9265-6ee7c73ac153" },
    { name: "resource2", mode: "shared",
      clientId: "fad203a5-1f31-472b-a7f7-a3236a1f6d3b" },
  ],
  pending: [
    { name: "resource1", mode: "exclusive",
      clientId: "fad203a5-1f31-472b-a7f7-a3236a1f6d3b" },
    { name: "resource1", mode: "exclusive",
      clientId: "d341a5d0-1d8d-4224-be10-704d1ef92a15" },
  ]
}

The clientId field corresponds to a unique context (frame or worker), and is the same value returned by Client 's id attribute.

The query() method steps are:

Let environment be this 's relevant settings object .
If environment ’s responsible document is not fully active , then return a promise rejected with a " InvalidStateError " DOMException.
Let manager be the result of obtaining a lock manager given environment. If that returned failure, then return a promise rejected with a " SecurityError " DOMException.
Let promise be a new promise .
Enqueue the steps to snapshot the lock state for manager with promise to the lock task queue .
Return promise.

3.3. `Lock` class

Lock

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[SecureContext, Exposed=(Window,Worker)]
interface Lock {
  readonly attribute DOMString name;
  readonly attribute LockMode mode;
};

A Lock object has an associated lock .

Lock/name

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The name getter’s steps are to return the associated lock 's name .

Lock/mode

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The mode getter’s steps are to return the associated lock 's mode .

4. Algorithms

4.1. Request a lock

To request a lock with promise, agent, clientId, manager, callback, name, mode, ifAvailable, steal, and signal:

Let request be a new lock request ( agent, clientId, manager, name, mode, callback, promise, signal ).
If signal is present, then add the algorithm signal to abort the request request with signal to signal.
Enqueue the following steps to the lock task queue :
1. Let queueMap be manager ’s lock request queue map .
2. Let queue be the result of getting the lock request queue from queueMap for name.
3. Let held be manager ’s held lock set .
4. If steal is true, then run these steps:
  1. For each lock of held:
    1. If lock ’s name is name, then run these steps:
      1. Remove lock from held.
      2. Reject lock ’s released promise with an " AbortError " DOMException.
  2. Prepend request in queue.
5. Otherwise, run these steps:
  1. If ifAvailable is true and request is not grantable , then enqueue the following steps on callback ’s relevant settings object 's responsible event loop :
    1. Let r be the result of invoking callback with null as the only argument.
    2. Resolve promise with r and abort these steps.
  2. Enqueue request in queue.
6. Process the lock request queue queue.
Return request.

4.2. Release a lock

To release the lock lock:

Assert : these steps are running on the lock task queue .
Let manager be lock ’s manager .
Let queueMap be manager ’s lock request queue map .
Let name be lock ’s resource name .
Let queue be the result of getting the lock request queue from queueMap for name.
Remove lock from the manager ’s held lock set .
Process the lock request queue queue.

4.3. Abort a request

To abort the request request:

Assert : these steps are running on the lock task queue .
Let manager be request ’s manager .
Let name be request ’s name .
Let queueMap be manager ’s lock request queue map .
Let queue be the result of getting the lock request queue from queueMap for name.
Remove request from queue.
Process the lock request queue queue.

To signal to abort the request request with signal:

Enqueue the steps to abort the request request to the lock task queue .
Reject request ’s promise with ~~an " AbortError " DOMException .~~ signal ’s abort reason .

4.4. Process a lock request queue for a given resource name

To process the lock request queue queue:

Assert : these steps are running on the lock task queue .
For each request of queue:
1. If request is not grantable , then return.
  
  Note: Only the first item in a queue is grantable. Therefore, if something is not grantable then all the following items are automatically not grantable.
2. Remove request from queue.
3. Let agent be request ’s agent .
4. Let manager be request ’s manager .
5. Let clientId be request ’s clientId .
6. Let name be request ’s name .
7. Let mode be request ’s mode .
8. Let callback be request ’s callback .
9. Let p be request ’s promise .
10. Let signal be request ’s signal .
11. Let waiting be a new promise .
12. Let lock be a new lock with agent agent, clientId clientId, manager manager, mode mode, name name, released promise p, and waiting promise waiting.
13. Append lock to manager ’s held lock set .
14. Enqueue the following steps on callback ’s relevant settings object 's responsible event loop :
  1. If signal is present, then run these steps:
    1. If signal is aborted , then run these steps:
      1. Enqueue the following step to the lock task queue :
        
        Release the lock lock.
      2. Return.
    2. Remove the algorithm signal to abort the request request from signal.
  2. Let r be the result of invoking callback with a new Lock object associated with lock as the only argument.
  3. Resolve waiting with r.

4.5. Snapshot the lock state

To snapshot the lock state for manager with promise:

Assert : these steps are running on the lock task queue .
Let pending be a new list .
For each queue of manager ’s lock request queue map 's values :
1. For each request of queue:
  1. Append «[ "name" → request ’s name , "mode" → request ’s mode , "clientId" → request ’s clientId ]» to pending.
Let held be a new list .
For each lock of manager ’s held lock set :
1. Append «[ "name" → lock ’s name , "mode" → lock ’s mode , "clientId" → lock ’s clientId ]» to held.
Resolve promise with «[ "held" → held, "pending" → pending ]».

5. Usage Considerations

This section is non-normative.

5.1. Deadlocks

Deadlocks are a concept in concurrent computing, and deadlocks scoped to a particular lock manager can be introduced by this API.

An example of how deadlocks can be encountered through the use of this API is as follows.

Script 1:

navigator.locks.request('A', async a => {
  await navigator.locks.request('B', async b => {
    // do stuff with A and B
  });
});

Script 2:

navigator.locks.request('B', async b => {
  await navigator.locks.request('A', async a => {
    // do stuff with A and B
  });
});

If script 1 and script 2 run close to the same time, there is a chance that script 1 will hold lock A and script 2 will hold lock B and neither can make further progress - a deadlock. This will not affect the user agent as a whole, pause the tab, or affect other script in the origin, but this particular functionality will be blocked.

Preventing deadlocks requires care. One approach is to always acquire multiple locks in a strict order.

A helper function such as the following could be used to request multiple locks in a consistent order.

async function requestMultiple(resources, callback) {
  const sortedResources = [...resources];
  sortedResources.sort(); // Always request in the same order.
  async function requestNext(locks) {
    return await navigator.locks.request(sortedResources.shift(), async lock => {
      // Now holding this lock, plus all previously requested locks.
      locks.push(lock);
      // Recursively request the next lock in order if needed.
      if (sortedResources.length > 0)
        return await requestNext(locks);
      // Otherwise, run the callback.
      return await callback(locks);
      // All locks will be released when the callback returns (or throws).
    });
  }
  return await requestNext([]);
}

In practice, the use of multiple locks is rarely as straightforward — libraries and other utilities can often unintentionally obfuscate their use.

6. Security and Privacy Considerations

6.1. Lock Scope

The definition of a lock manager 's scope is important as it defines a privacy boundary. Locks can be used as an ephemeral state retention mechanism and, like storage APIs, can be used as a communication mechanism, and must be no more privileged than storage facilities. User agents that impose finer granularity on one of these services must impose it on others; for example, a user agent that exposes different storage partitions to a top-level page (first-party) and a cross-origin iframe (third-party) in the same origin for privacy reasons must similarly partition locking.

This also provides reasonable expectations for web application authors; if a lock is acquired over a storage resource, all same-origin browsing contexts must observe the same state.

6.2. Private Browsing

Every private mode browsing session is considered a separate user agent for the purposes of this API. That is, locks requested/held outside such a session have no affect on requested/held inside such a session, and vice versa. This prevents a website from determining that a session is "incognito" while also not allowing a communication mechanism between such sessions.

6.3. Implementation Risks

Implementations must ensure that locks do not span origins. Failure to do so would provide a side-channel for communication between script running in two origins, or allow one script in one origin to disrupt the behavior of another (e.g. denying service).

6.4. Checklist

The W3C TAG has developed a Self-Review Questionnaire: Security and Privacy for editors of specifications to informatively answer. Revisiting the questions here:

The specification does not deal with personally identifiable information, or high-value data.
No new state for an origin that persists across browsing sessions is introduced.
No new persistent, cross-origin state is exposed to the web.
No new data is exposed to an origin that it doesn’t currently have access to (e.g. via polling [IndexedDB-2] .)
No new script execution/loading mechanisms are enabled.
This specification does not allow an origin access to any of the following:
- The user’s location.
- Sensors on a user’s device.
- Aspects of a user’s local computing environment.
- Access to other devices.
- Any measure of control over a user agent’s native UI.
No temporary identifiers to the web are exposed to the web. All resource names are provided by the web application itself.
Behavior in first-party and third-party contexts is distinguished in a user agent if storage is distinguished. See § 6.1 Lock Scope .
Behavior in the context of a user agent’s "incognito" mode is described in § 6.2 Private Browsing .
No data is persisted to a user’s local device by this API.
This API does not allow downgrading default security characteristics.

7. Acknowledgements

Many thanks to Alex Russell, Andreas Butler, Anne van Kesteren, Boris Zbarsky, Darin Fisher, Domenic Denicola, Gus Caplan, Harald Alvestrand, Jake Archibald, Kagami Sascha Rosylight, L. David Baron, Luciano Pacheco, Marcos Caceres, Ralph Chelala, Raymond Toy, Ryan Fioravanti, and Victor Costan for helping craft this proposal.

Special thanks to Tab Atkins, Jr. for creating and maintaining Bikeshed , the specification authoring tool used to create this document, and for his general authoring advice.

Web Locks API

Abstract

Status of this document

1. Introduction

1.1. Usage Overview

1.2. Motivating Use Cases

2. Concepts

2.1. Resources Names

2.2. Lock Managers

2.3. Modes and Scheduling

2.4. Locks

2.5. Lock Requests

2.6. Agent Integration

3. API

3.1. Navigator Mixins

3.2. LockManager class

3.2.1. The request() method

3.2.2. The query() method

3.3. Lock class

4. Algorithms

4.1. Request a lock

4.2. Release a lock

4.3. Abort a request

4.4. Process a lock request queue for a given resource name

4.5. Snapshot the lock state

5. Usage Considerations

5.1. Deadlocks

6. Security and Privacy Considerations

6.1. Lock Scope

6.2. Private Browsing

6.3. Implementation Risks

6.4. Checklist

7. Acknowledgements

Conformance

Document conventions

Conformant Algorithms

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

Informative References

IDL Index

Issues Index

3.2. `LockManager` class

3.2.1. The `request()` method

3.2.2. The `query()` method

3.3. `Lock` class