Syncing Core Data across multiple devices using iCloud

Question

There has been a lot of discussion lately about the issues with iCloud and Core Data and how Apple's APIs are currently broken in iOS 5 and possibly iOS 6.

Is it possible, given the current state of Apple's Core Data API, to reliably sync across multiple devices using iCloud?

If so, how would you do this? If not, please recommend an alternative approach.

Answer 1

This blog post will lead you to a chain of recent articles about the travails of developers attempting this approach.

From my own understanding and experience, I believe it is doable, but don't buy into the idea that you will get anything "for free". Depending on your data model, you may be better off syncing your whole persistent store as a document rather than using the documented core data / iCloud approach.

You may have better luck if you're already comfortable with Core Data. Just be sure you think through how to handle several important cases.

One is what to do if the user signs out of their iCloud account. When this happens, the local ubiquitous persistent store is deleted. If it makes sense for the user to still have access to their data, you'll need to manage a copy in local storage, and then manage resynchronizing when they sign back in.

Another is that changes can apparently be quite slow to propagate by default, so you may want to consider an alternative mechanism, such as the key value store, to quickly propagate sufficient information to avoid a bad user experience.

Conflict management is perhaps the most challenging (depending on your model). While the framework provides a mechanism to inform you of conflicts, you are on your own for providing a mechanism to resolve them, and there are reports that the conflict notifications may not be reliable (see linked articles), which seems strongly linked to the lag in updating.

In short, if you go into this understanding that the actual support is pretty bare bones and that you'll need to code very defensively, you may have a chance. There aren't any good recipes out there, so if you do make it work, please come back and tell us what works!

Answer 2

It depends on what you want to do. There are two types of Core Data-iCloud integration, as described here: http://developer.apple.com/library/ios/#releasenotes/DataManagement/RN-iCloudCoreData/_index.html

There are broadly speaking two types of Core Data-based application that integrate with iCloud:

Library-style applications, where the application usually has a single persistent store, and data from the store is used throughout the application. Examples of this style of application are Music and Photos.

Document-based applications, where different documents may be opened at different times during the lifetime of the application. Examples of this style of application are Keynote and Numbers.

If you're using the library-type, this article is the first of a series that goes into a lot of the problems that will come up: http://mentalfaculty.tumblr.com/post/23163747823/under-the-sheets-with-icloud-and-core-data-the-basics.

You can also check out sessions 218 (for document-based) or 227 (for library-style) of this year's wwdc.