storage.sync

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Represents the sync storage area. Items in sync storage are synced by the browser. The data is then available on all instances of the browser the user is logged into (for example, when using a Mozilla account on desktop versions of Firefox or a Google account on Chrome) across different devices.

For desktop Firefox, a user must have Add-ons selected in the “Sync” section in "about:preferences". Firefox for Android does not synchronize data with the user’s account. See Firefox bug 1625257.

The implementation of storage.sync in Firefox relies on the Add-on ID. If you use storage.sync, you must set an ID for your extension using the browser_specific_settings manifest.json key.

The main use case of this API is to store preferences about your extension and allow the user to sync them to different profiles.

Storage quotas for sync data

The browser enforces limits on the amount of data each extension is allowed to store in the sync area:

If an extension attempts to store items that exceed these limits, calls to {{WebExtAPIRef("storage.StorageArea.set()", "storage.sync.set()")}}  are rejected with an error. An extension can use {{WebExtAPIRef("storage.StorageArea.getBytesInUse()", "storage.sync.getBytesInUse()")}}  to find out how much of its quota is in use.

Synchronization process

In Firefox, extension data is synced every 10 minutes or whenever the user selects Sync Now (in Settings > Sync or from the Mozilla account icon). When the browser performs a sync, for each key stored, it:

• compares the value on the server with the value at the last sync; if they are different, the value from the server is written to the key in the browser’s sync storage.
• compares the browser’s sync storage values with the value on the server; if they are different, writes the browser’s key value to the server.

This means that, for each key, a change on the server takes precedence over a change in the browser’s sync storage.

This mechanism is generally OK for data such as user preferences or other global settings changed by the user.

However, a key’s value can be updated on one browser and synchronized then updated on a second browser before the second browser is synchronized, resulting in the local update being overwritten during sync. This mechanism is, therefore, not ideal for data aggregated across devices, such as a count of page views or how many times an option is used. To handle such cases, use {{WebExtAPIRef("storage.StorageArea.onChanged", "storage.sync.onChanged")}}  to listen for sync updates from the server (for example, a count of page views on another browser instance). Then adjust the value locally to take the remote value into account (for example, update the total views based on the remote count and new local count).

Methods

The sync object implements the methods defined on the {{WebExtAPIRef("storage.StorageArea")}}  type:

• {{WebExtAPIRef("storage.StorageArea.get()", "storage.sync.get()")}} 
  • : Retrieves one or more items from the storage area.
• {{WebExtAPIRef("storage.StorageArea.getBytesInUse()", "storage.sync.getBytesInUse()")}} 
  • : Gets the amount of storage space (in bytes) used for one or more items in the storage area.
• {{WebExtAPIRef("storage.StorageArea.set()", "storage.sync.set()")}} 
  • : Stores one or more items in the storage area. If the item exists, its value is updated.
• {{WebExtAPIRef("storage.StorageArea.remove()", "storage.sync.remove()")}} 
  • : Removes one or more items from the storage area.
• {{WebExtAPIRef("storage.StorageArea.clear()", "storage.sync.clear()")}} 
  • : Removes all items from the storage area.

Events

The sync object implements the events defined on the {{WebExtAPIRef("storage.StorageArea")}}  type:

• {{WebExtAPIRef("storage.StorageArea.onChanged", "storage.sync.onChanged")}} 
  • : Fires when one or more items in the storage area change.

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Browser compatibility

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[!NOTE] This API is based on Chromium’s chrome.storage API. This documentation is derived from storage.json in the Chromium code.