App Backup Best Practices in iOS

You do not have to prepare your app in any way for backup and restore operations. Devices with an active iCloud account have their app data backed up to iCloud at appropriate times. And for devices that are plugged into a computer, iTunes performs an incremental backup of the app’s data files. However, iCloud and iTunes do not back up the contents of the following directories:

• Application_Home/AppName.app
• Application_Home/Library/Caches
• Application_Home/tmp

To prevent the syncing process from taking a long time, be selective about where you place files inside your app’s home directory. Apps that store large files can slow down the process of backing up to iTunes or iCloud. These apps can also consume a large amount of a user's available storage, which may encourage the user to delete the app or disable backup of that app's data to iCloud. With this in mind, you should store app data according to the following guidelines:

• Critical data should be stored in the /Documents directory. Critical data is any data that cannot be recreated by your app, such as user documents and other user-generated content.
• Support files include files your application downloads or generates and that your application can recreate as needed. The location for storing your application’s support files depends on the current iOS version.
  
  • In iOS 5.1 and later, store support files in the /Library/Application Support directory and add the NSURLIsExcludedFromBackupKey attribute to the corresponding NSURL object using the setResourceValue:forKey:error: method. (If you are using Core Foundation, add the kCFURLIsExcludedFromBackupKey key to your CFURLRef object using the CFURLSetResourcePropertyForKey function.) Applying this attribute prevents the files from being backed up to iTunes or iCloud. If you have a large number of support files, you may store them in a custom subdirectory and apply the extended attribute to just the directory.
  • In iOS 5.0 and earlier, store support files in the /Library/Caches directory to prevent them from being backed up. If you are targeting iOS 5.0.1, see How do I prevent files from being backed up to iCloud and iTunes? for information about how to exclude files from backups.
• Cached data should be stored in the /Library/Caches directory. Examples of files you should put in the Caches directory include (but are not limited to) database cache files and downloadable content, such as that used by magazine, newspaper, and map apps. Your app should be able to gracefully handle situations where cached data is deleted by the system to free up disk space.
• Temporary data should be stored in the /tmp directory. Temporary data comprises any data that you do not need to persist for an extended period of time. Remember to delete those files when you are done with them so that they do not continue to consume space on the user's device.

Although iTunes backs up the app bundle itself, it does not do this during every sync operation. Apps purchased directly from a device are backed up when that device is next synced with iTunes. Apps are not backed up during subsequent sync operations, though, unless the app bundle itself has changed.