CoreData + CloudKit synchronization is very unstable & unreliable

CloudKit sync is very unstable. Sometimes it just stops syncing for no reason, other times it works almost instantly.

The core issue with synchronization is that CoreData relies mostly on two things:

• silent push notifications, which are by design unreliable and can be throttled
• user interactions, I noticed that the local database is updated most likely periodically and also based on some app events like entering the foreground.

Unfortunately, there is no SDK function that allows us to force sync with CloudKit, which basically prevents us from providing some features to recover if a user encounters problems.

After thousands of tests, I finally discovered what was wrong and how to make the synchronization stable. Basically, I noticed that at some point CoreData decides that it won't synchronize data unless you deactivate and activate the application, which is crazy. It's getting even worse if we talk about extensions like the keyboard extension on iOS. The same happens on all platforms.

Therefore, knowing that I implemented a trick that happened to work perfectly. The workaround requires to periodically sending an event pretending that the app is going foreground.

macOS:

var cancellable = Set<AnyCancellable>()
// ...
Timer.publish(every: 20.0, on: RunLoop.main, in: .common)
  .autoconnect()
  .sink { _ in 
     NotificationCenter.default.post(.init(name: NSApplication.willBecomeActiveNotification)) 
  }
  .store(in: &cancellable)

iOS:

var cancellable = Set<AnyCancellable>()
// ...
Timer.publish(every: 20.0, on: RunLoop.main, in: .common)
  .autoconnect()
  .sink { _ in 
     NotificationCenter.default.post(.init(name: UIApplication.didBecomeActiveNotification)) 
  }
  .store(in: &cancellable)

After that, everything works perfectly. Pitty that the solution mostly meant for enterprise is so unstable and there is not even a single SDK function to recover from that (force sync).

Any plans to fix CoreData+CloudKit? I also created a ticket: #FB14531806.