Simperium is a simple way for developers to move data as it changes, instantly and automatically. This is the iOS / OSX library. You can browse the documentation or try a tutorial.
You can sign up for a hosted version of Simperium. There are Simperium libraries for other languages too.
The easiest way to add Simperium is to download the latest release. Unzip the source code somewhere convenient.
Then, drag and drop Simperium.xcodeproj into your application's project, and add Simperium.framework in your target's Build Phase tab (under Link Binary with Libraries). You'll still need to add some dependencies.
Everything works pretty much the same on OSX. Some changes are noted in the online documentation.
The master branch always has the latest stable release, and is tagged. Simperium is used by hundreds of thousands of people across many different apps and devices, and is considered production-ready.
The develop branch has an ongoing development build (not intended for production use).
Simperium. Everything is accessed from a Simperium instance. This class can be safely instantiated more than once in the same app (e.g. for unit testing).
Object. Simperium does a lot of diffing in order to send only data that changes. Any object class that conforms to the SPDiffable protocol can be supported. SPManagedObject is for Core Data, and SPObject is a container for raw JSON (not yet supported). SPGhost is an object's opinion of its own state on the server (the name "ghost" was borrowed from the Tribes Networking Model).
Diffing. An SPDiffer can perform diffs on any SPDiffable object. Each differ adheres to an SPSchema. The schema stores a list of members/properties (of type SPMember) for an object of a particular type. Each subclass of SPMember corresponds to a data type, and knows how to diff itself. In the future these will be parameterized for custom diffing, conflict resolution, validation, etc.
System. An SPBucket provides access to a synchronized bucket of objects of a particular type. The SPBucket has an SPDiffer to perform diffs, an SPStorageProvider for locally reading and writing data, an SPChangeProcessor for processing incoming and outgoing changes, and an SPIndexProcessor for processing indexes retrieved from the server. The processors run in their own threads.
Storage. An SPStorageProvider defines an interface for local reading and writing of objects. In particular it defines a threadSafeStorage method that returns a thread safe instance. SPCoreDataProvider is currently the only fully functional storage provider.
Authentication. An SPAuthenticator handles all authentication with Simperium, and can be customized or overridden as necessary. There are companion classes for iOS and OSX that provide a code-driven UI for signing in and signing up (SPAuthenticationViewController and SPAuthenticationWindowController).
Networking. An SPNetworkProvider defines an interface for remote reading and writing of objects in an SPBucket. The network provider sends local data and receives remote data in the background, passing it through threaded processors as necessary. Although there is an HTTP provider, the WebSockets provider is intended to become the default (but is still under development).
User. Basic access to a user's data. In the future this will hold custom properties and presence information.
Helpers. Exporter, keychain, etc.
The Simperium iOS library is available for free and commercial use under the MIT license.