This specification defines an API to manage usage and availability of local storage resources, and defines a means by which a user agent (UA) may grant Web applications permission to use more local space, temporarily or persistently, via various different storage APIs.

This document is a proposal that is being made available for public review in order to solicit feedback, particularly from implementors, with a goal of potential cross-browser implementation and standardization.

Introduction

Today we have a variety of storage APIs that can store inherently complex or large data in order to satisfy offline data requirements of Web applications. Examples of these APIs include: Application Cache [[OFFLINE-WEBAPPS]], FileSystem API [[FILE-SYSTEM]][[NEW-FILE-SYSTEM]], Indexed Database [[INDEXEDDB]] and Web SQL Database [[WEB-SQL]].

These APIs may require larger local space than the conventional cookie storage or Web Storage [[WEBSTORAGE]], but they do not provide a means by which a Web application can query and manage how much data is currently stored and how much more can be stored.

This specification defines an API to query and manage usage and availability of a user's local storage. The storage space granted by the API is intended to be shared by different storage APIs, therefore a user and UA only need to manage single upper limit for all storage per logical application unit, e.g. per origin or per browser (which is implementation specific).

User agents that use ECMAScript to implement the APIs defined in this specification MUST implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [[WEBIDL]] as this specification uses that specification and terminology.

Terminology

The terms event handler, event handler event type, fires a simple event are defined in [[HTML5]].

The Event, EventTarget, EventListener interfaces are defined in [[DOM4]].

The DOMError and DOMException interfaces are defined in [[DOM4]].

The Promise interface is currently defined and discussed in the WHATWG DOM specification.

The terms service worker and scope url, and the types ServiceWorkerGlobalScope, ExtendableEvent, and ExtendableEventInit are defined in [[!SERVICE-WORKERS]].

Storage Types

Temporary vs Persistent

A Web application can request temporary or persistent local storage space depending on its purpose.

Temporary type of storage is especially useful if an application wants to cache data locally to improve its performance, but can fall back to fetching or recreating the same data at a data loss.

Conversely, persistent type of storage is useful if an application wants to store critical offline data that is necessary to be functional, or wants to manage local data amount and lifetime on its own policy rather than relying on the UA's default eviction policy for temporary storage.

Examples

Suppose there is a photo editing application. This application manages user photo data using Indexed Database [[INDEXEDDB]], stores photo images using Filesystem API [[FILE-SYSTEM]] [[NEW-FILE-SYSTEM]] and optionally utilizes Application Cache [[OFFLINE-WEBAPPS]] to make it work offline.

The application needs to query how much data it can store in the temporary storage to determine its initial cache size.

          // Query current usage and availability in Temporary storage:
          navigator.storageQuota.queryInfo("temporary").then(
            function(storageInfo) {
              // Continue to initialize local cache using the obtained
              // usage and remaining space (quota - usage) information.
              initializeCache(storageInfo.usage,
                              storageInfo.quota - storageInfo.usage);
            });
          

Similarly, the application needs to request additional persistent storage to support offline mode when it is enabled by the user.

          // A function which is to be called when 'offline-mode' is enabled
          // by the user.
          function onOfflineModeEnabled(amountOfSpaceNeeded) {
            // First check how much we can use in the Persistent storage.
            navigator.storageQuota.queryInfo("persistent").then(
              function (storageInfo) {
                var availableSpace = storageInfo.quota - storageInfo.usage;
                if (availableSpace >= amountOfSpaceNeeded) {
                  // We're fine; just continue with the returned storage info.
                  return storageInfo;
                }
                return navigator.storageQuota.requestPersistentQuota(
                    amountOfSpaceNeeded + storageInfo.usage);
              }
            ).then(
              function (storageInfo) {
                // Prepare for offline mode using the current available
                // storage space.
                prepareForOfflineMode(storageInfo.quota - storageInfo.usage);
              }, function (error) {
                // Handle error.
              }
            );
          }
        

Quota Management API

StorageType enum

temporary
Indicates temporary storage type.
persistent
Indicates persistent storage type.

StorageInfo dictionary

unsigned long long usage

The total amount of data (in bytes) stored by the application for a given storage type. Depending on how the UA calculates data usage the returned value may differ from the exact real-time usage of the user's physical local storage.

unsigned long long quota

The current upper limit of the storage space that can be used by the application for a given storage type. This includes the storage area that is already used by the application, so storageInfo.usage needs to be subtracted from storageInfo.quota to get the remaining available storage space.

For temporary storage this value may reflect the actual storage space available on the user's local device and may change from time to time. For persistent storage this value must return the consistent quota size that is granted to the application by requestPersistentQuota. If the application does not have the associated persistent quota yet the UA may return a UA-specific default quota value (which could be 0).

StorageQuota interface

The StorageQuota interface provides means to query and request storage usage and quota information. The API provided by the interface is asynchronous since querying or allocating space in a user's local storage may require blocking I/O operations, e.g. examining the local disk status or making changes in a local database.

[SameObject] readonly attribute frozen array<StorageType> supportedTypes
List of all storage types supported by the UA.
Promise queryInfo(StorageType type)

This method queries the storage info of the given storage type. This returns StorageInfo that has the current data usage and available quota information for the application. When queryInfo method is called, the UA must run the following steps:

  1. Let storageType be the first argument.
  2. Let p be a new promise.
  3. Run the following substeps asynchronously:
    1. If the storageType is not supported by the user agent (i.e. not in the supportedTypes array), reject p with a "NotSupportedError" exception and abort these steps.
    2. Let usage be the total amount of data stored by the application in the storage of storageType.
    3. Let quota be the upper limit of the usage that can be stored by the application for storageType.
    4. Resolve p with a new StorageInfo object, created with the usage and quota values.
  4. Return p.

Promise requestPersistentQuota([Clamp] unsigned long long newQuota)

Requests a new quota in persistent storage for the requesting application. It is not guaranteed that the requested amount of space is granted just by calling this API, and the UA may return a new StorageInfo object (without rejecting the request) with a smaller quota value than requested. Calling this API may trigger user prompting to request explicit user permission to proceed.

When requestPersistentQuota method is called, the UA must run the following steps:
  1. Let newQuota be the first argument.
  2. Let p be a new promise.
  3. Run the following substeps asynchronously:
    1. Let usage be the total amount of data stored by the application in persistent storage.
    2. Let quota be the current quota size that is already granted to the application for persistent storage. If this application does not have associated persistent quota yet, initialize quota with a UA-specific default quota value (which could be 0).
      1. If quota is equal to or greater than newQuota, let grantedQuota be the same value as the current quota.
      2. Else:
        1. Check how much amount of storage the UA can allocate for the application. This step may optionally prompt the user in a UA-specific manner for permission to allow the application to store more data.
        2. Let grantedQuota be the new quota size associated to the application. The grantedQuota may be smaller than the requested newQuota, but is guaranteed to be equal to or greater than the old quota.
    3. Resolve p with a new StorageInfo object, created with usage and grantedQuota.
  4. Return p.

Accessing StorageQuota interface

readonly attribute StorageQuota storageQuota
Returns the StorageQuota interface.

StorageEvent interface

An event object implementing this interface is passed to onstoragechange event handler when storage information is updated.

readonly attribute unsigned long long usage
The total amount of data (in bytes) stored by the application for a given storage type.
readonly attribute unsigned long long quota
The current upper limit of the storage space that can be used by the application for a given storage type.

Events are constructed as defined in constructing events in [[DOM4]].

unsigned long long usage = 0
unsigned long long quota = 0

StorageWatcher interface

StorageWatcher interface is to watch real time storage changes. This fires storagechange event every time a storage status change is detected by the UA, or about every rate millisecond(s), whichever is least frequent.

If rate is not given in the constructor, the UA fires storagechange event whenever it detects the usage or quota changes in the backend, but no more frequent than 50ms.

Regardless of the rate value, the UA must fire one storagechange event with the current storage status immediately (but asynchronously) after a watcher is constructed, unless close() is called before the first storagechange event is dispatched.

void close()
Explicitly closes the watcher. Once close() is called the UA never fires events on this instance.
readonly attribute StorageType type
Returns the storage type type which this watcher is constructed with and is monitoring changes on.
readonly attribute unsigned long rate
Returns the rate value which this watcher is constructed with.
attribute EventListener onstoragechange
The event handler for the storagechange event.

Restrictions

The space queried and granted by StorageQuota have the following properties:

Storage Pressure Events

Extensions to the ServiceWorkerGlobalScope interface

The Service Worker specification [[SERVICE-WORKERS]] defines a ServiceWorkerGlobalScope interface, which this specification extends.

attribute EventHandler onbeforeevicted
The onbeforeevicted attribute is an event handler whose corresponding event handler type is beforeevicted.
attribute EventHandler onevicted
The onevicted attribute is an event handler whose corresponding event handler type is evicted.

This event is going to be dispatched when the Quota Management API detects that there is a storage pressure. If the service worker that received this event did not give back enough space the UA may evict its content entirely.

BeforeEvictEvent interface

readonly attribute unsigned long long requestedBytes
The number of bytes that is requested to free up.

EvictedEvent interface

This event is only going to be dispatched if there are foreground pages that are otherwise keep the service worker's script and registrations alive. It is thrown to let the service worker communicate to clients that the app has been removed and is unlikely to continue to work. The clients can show appropriate UI in response.

Quota handling in storage APIs

Storage APIs except for Web Storage, i.e. Application Cache, File System API, Indexed Database and Web SQL Database, should respect the quota management API and must have following properties:

Indexed Database [[INDEXEDDB]] is expected to have temporary and persistent storage types in its next version, and when that happens the UA should store data for temporary database in temporary storage and for persistent database in persistent storage, respectively.

Acknowledgements

Many thanks to Robin Berjon for making our lives so much easier with his cool tool.