storage.sync

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 storage.sync.set() are rejected with an error. An extension can use 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 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 storage.StorageArea type:

storage.sync.get()

Retrieves one or more items from the storage area.

storage.sync.getBytesInUse()

Gets the amount of storage space (in bytes) used for one or more items in the storage area.

storage.sync.getKeys()

Retrieves the keys of all items in the storage area.

storage.sync.set()

Stores one or more items in the storage area. If the item exists, its value is updated.

storage.sync.remove()

Removes one or more items from the storage area.

storage.sync.clear()

Removes all items from the storage area.

Events

The sync object implements the events defined on the storage.StorageArea type:

storage.sync.onChanged

Fires when one or more items in the storage area change.

Example extensions

• favourite-colour

Browser compatibility