Apple announced iCloud service part of the features built in with iOS5. iCloud sync automatically the content across all your devices, you can take a photo on your iPhone and immediately share it to your friends using iPad, or store anything, your photo, music, calendar notes, mail, messages on iCloud. It is completely free to use.
Apple allows iOS and Mac Developer Program members utilized this great feature iCloud integrated into their apps by providing iCloud Storage API. Utilizing iCloud Storage API the developers can store the user documents on iCloud, or storing key-value data for small amount information. Developers need not worry about the background operation of iCloud service, as iCloud service resolved all the possible conflicts may happened. iCloud also provides auto notification of file changes to your apps.
Your apps may be a document-based application. You sure familiar with document-based application existed on both desktop and mobile application such as word processors, spreadsheets and drawing program. In iOS5, you can easily implement a document-based application by subclassing UIDocument class, and providing any details of your document structure, while iCloud technology takes care of the background reading and writing of data asynchronously.
Your apps can store key-value data to share small amount of information, you may store integer, string, or date data in the key-value pair. The key-value data is stored in iCloud and shared across all user devices and all the apps. Any changes of data from one apps will automatically reflected in changes into other apps that using that data. Therefore, you may configure your bundles of apps to share certain key-value data to store some configuration or else.