Auto Backup for Apps

Since Android 6.0 (API 23), Android has offered the Auto Backup for Apps feature as a way for developers to quickly add backup functionality to their apps. Auto Backup preserves app data by uploading it to the user's Google Drive account, where it is protected by the user's Google account credentials. The amount of data is limited to 25MB per user of your app and there is no charge for storing backup data.

Files that are backed up

By default, Auto Backup includes files in most of the directories that are assigned to your app by the system:

• Shared preferences files.
• Files in the directory returned by getFilesDir().
• Files in the directory returned by getDatabasePath(String), which also includes files created with the SQLiteOpenHelper class.
• Files in directories created with getDir(String, int).
• Files on external storage in the directory returned by getExternalFilesDir(String).

Auto Backup excludes files in directories returned by getCacheDir(), getCodeCacheDir(), or getNoBackupFilesDir(). The files saved in these locations are only needed temporarily, or are intentionally excluded from backup operations.

You can configure your app to include and exclude particular files. For more information, see the Include and exclude files section.

Backup location

Backup data is stored in a private folder in the user's Google Drive account, limited to 25MB per app. The saved data does not count towards the user's personal Google Drive quota. Only the most recent backup is stored. When a backup is made, the previous backup (if one exists) is deleted.

Users can see a list of apps that have been backed up in the Google Drive app under Settings -> Auto Backup for apps -> Manage backup. The backup data cannot be read by the user or other applications on the device.

Backups from each device-setup-lifetime are stored in separate datasets as shown in the following examples:

• If the user owns two devices, then a backup dataset exists for each device.
• If the user factory resets a device and then sets up the device with the same account, the backup is stored in a new dataset. Obsolete datasets are automatically deleted after a period of inactivity.

Caution: If the amount of data is over 25MB, the system fires the onQuotaExceeded(long, long) callback and does not backup data to the cloud. The system periodically checks whether the amount of data later falls under the 25MB threshold and continues Auto Backup when it does.

Backup schedule

Backups occur automatically when all of the following conditions are met:

• The user has enabled backup on the device in Settings > Backup & Reset.
• At least 24 hours have elapsed since the last backup.
• The device is idle and charging.
• The device is connected to a Wi-Fi network. If the device is never connected to a wifi network, then Auto Backup never occurs.

In practice, these conditions occur roughly every night. To conserve network bandwidth, upload takes place only if app data has changed.

During Auto Backup, the system shuts down the app to make sure it is no longer writing to the file system. By default, the backup system ignores apps that are running in the foreground because users would notice their apps being shut down. You can override the default behavior by setting the backupInForeground attribute to true.

To simplify testing, Android includes tools that let you manually initiate a backup of your app. For more information, see Testing Backup and Restore.

Restore schedule

Data is restored whenever the app is installed, either from the Play store, during device setup (when the system installs previously installed apps), or from running adb install. The restore operation occurs after the APK is installed, but before the app is available to be launched by the user.

During the initial device setup wizard, the user is shown a list of available backup datasets and is asked which one to restore the data from. Whichever backup dataset is selected becomes the ancestral dataset for the device. The device can restore from either its own backups or the ancestral dataset. The device prioritize its own backup if backups from both sources are available. If the user didn't go through the device setup wizard, then the device can restore only from its own backups.

To simplify testing, Android includes tools that let you manually initiate a restore of your app. For more information, see Testing Backup and Restore.

Enabling and disabling backup

Apps that target Android 6.0 (API level 23) or higher automatically participate in Auto Backup. This is because the android:allowBackup attribute, which enables/disables backup, defaults to true if omitted. To avoid confusion, we recommend you explicitly set the attribute in the <application> element of your AndroidManifest.xml. For example:

<application ...
    android:allowBackup="true">
</app>

To disable Auto Backup, set android:allowBackup to false. You may want to disable backups when your app can recreate its state through some other mechanism or when your app deals with sensitive information that should not be backed up.

Including and excluding files

By default, the system backs up almost all app data. For more information, see Files that are backed up. This section shows you how to define custom XML rules to control what gets backed up.

1. In AndroidManifest.xml, add the android:fullBackupContent attribute to the <application> element. This attribute points to an XML file that contains backup rules. For example:

   <application ...
       android:fullBackupContent="@xml/my_backup_rules">
     </app>
   

2. Create an XML file called my_backup_rules.xml in the res/xml/ directory. Inside the file, add rules with the <include> and <exclude> elements. The following sample backs up all shared preferences except device.xml:

   <?xml version="1.0" encoding="utf-8"?>
   <full-backup-content>
       <include domain="sharedpref" path="."/>
       <exclude domain="sharedpref" path="device.xml"/>
   </full-backup-content>
   

XML Config Syntax

The XML syntax for the configuration file is shown below:

<full-backup-content>
    <include domain=["file" | "database" | "sharedpref" | "external" | "root"]
    path="string" />
    <exclude domain=["file" | "database" | "sharedpref" | "external" | "root"]
    path="string" />
</full-backup-content>

Inside the <full-backup-content> tag, you can define <include> and <exclude> elements:

• <include> - Specifies a file or folder to backup. By default, Auto Backup includes almost all app files. If you specify an <include> element, the system no longer includes any files by default and backs up only the files specified. To include multiple files, use multiple <include> elements.

  note: Files in directories returned by getCacheDir(), getCodeCacheDir(), or getNoBackupFilesDir() are always excluded even if you try to include them.

• <exclude> - Specifies a file or folder to exclude during backup. Here are some files that are typically excluded from backup:
  • Files that have device specific identifiers, either issued by a server or generated on the device. For example, Google Cloud Messaging (GCM) needs to generate a registration token every time a user installs your app on a new device. If the old registration token is restored, the app may behave unexpectedly.
  • Account credentials or other sensitive information. Consider asking the user to reauthenticate the first time they launch a restored app rather than allowing for storage of such information in the backup.
  • Files related to app debugging, such as instant run files. To exclude instant run files, add the rule <exclude domain="file" path="instant-run"/>
  • Large files that cause the app to exceed the 25MB backup quota.

Note: If your configuration file specifies both elements, then the backup contains everything captured by the <include> elements minus the resources named in the <exclude> elements. In other words, <exclude> takes precedence.

Each element must include the following two attributes:

• domain - specifies the location of resource. Valid values for this attribute include the following:
  • root - the directory on the filesystem where all private files belonging to this app are stored.
  • file - directories returned by getFilesDir().
  • database - directories returned by getDatabasePath(). Databases created with SQLiteOpenHelper are stored here.
  • sharedpref - the directory where SharedPreferences are stored.
  • external the directory returned by getExternalFilesDir()

    Note: You cannot backup files outside of these locations.

• path: Specifies a file or folder to include in or exclude from backup. Note that:
  • This attribute does not support wildcard or regex syntax.
  • You can use . to reference the current directory, however, you cannot reference the parent directory .. for security reasons.
  • If you specify a directory, then the rule applies to all files in the directory and recursive sub-directories.

Implementing BackupAgent

Apps that implement Auto Backup do not need to implement BackupAgent. However, you can optionally implement a custom BackupAgent. Typically, there are two reasons for doing this:

• You want to receive notification of backup events such as, onRestoreFinished() or onQuotaExceeded(long, long). These callback methods are executed even if the app is not running.
• You can't easily express the set of files you want to backup with XML rules. In these rare cases, you can implement a BackupAgent that overrides onFullBackup(FullBackupDataOutput) to store what you want. To retain the system's default implementation, call the corresponding method on the superclass with super.onFullBackup().

If you implement a BackupAgent, by default the system expects your app to perform key/value backup and restore. To use the file-based Auto Backup instead, set the android:fullBackupOnly attribute to true in your app's manifest.

During auto backup and restore operations, the system launches the app in a restricted mode to both prevent the app from accessing files that could cause conflicts and let the app execute callback methods in its BackupAgent. In this restricted mode, the app's main activity is not automatically launched, its Content Providers are not initialized, and the base-class Application is instantiated instead of any subclass declared in the app's manifest.

Caution: To avoid errors, make sure that the parts of your app that execute in the restricted mode (mostly your BackupAgent) do not access content providers in the same app or attempt to cast the Application object. If you cannot avoid those patterns, then consider implementing Key/Value backup or disabling backup entirely.

Note: Your BackupAgent must implement the abstract methods onBackup() and onRestore(). Those methods are used for Key/Value Backup. So if you are not using Key/Value Backup, implement those methods and leave them blank.

For more information, see Extending BackupAgent.