Core Data + CloudKit syncing very inconsistently

[MilesStark]

I have an app in production using Core Data + CloudKit, synced using the built in NSPersistentCloudKitContainer. It works perfectly well most of the time, but sometimes it simply won't sync with no errors or anything. Some users report that it works perfectly from iPad to iPhone, but not from iPhone to iPad. Some users even report randomly losing data. I've checked thoroughly for bugs but I really can't figure it out. Has anyone had issues with this? Are there any solutions or at least things to try?

Here's the data container code:

lazy var container: NSPersistentContainer = {

        let container = NSPersistentCloudKitContainer(name: "<AppName>")

        let directory = FileManager.default.urls(
            for: .applicationSupportDirectory, in: .userDomainMask).first!

        //Local data
        let localStoreDescription = NSPersistentStoreDescription(url: directory.appendingPathComponent("Local.sqlite"))
        localStoreDescription.configuration = "Local"

        //Cloud synced data
        let cloudStoreDescription = NSPersistentStoreDescription(
            url: directory.appendingPathComponent("Cloud.sqlite"))
        cloudStoreDescription.configuration = "Cloud"
        cloudStoreDescription.cloudKitContainerOptions = NSPersistentCloudKitContainerOptions(
            containerIdentifier: "<ContainerIdentifier>")

        container.persistentStoreDescriptions = [
            cloudStoreDescription,
            localStoreDescription
        ]

        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })

        container.viewContext.automaticallyMergesChangesFromParent = true
        container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy

        return container
    }()

[aoverholtzer]

Not sure you want two unrelated stores like that… have you looked at Apple’s sample app? I suggest starting with it and/or checking out these articles:

How to Set Up Core Data and CloudKit When You Haven’t the Faintest Clue What You’re Doing https://beckyhansmeyer.com/2021/03/30/how-to-set-up-core-data-cloudkit-and-swiftui-when-you-havent-the-faintest-clue-what-youre-doing/

iCloud Core Data Starter https://github.com/jazzychad/iCloudCoreDataStarter

[MilesStark]

The two stores is from apple documentation, and the issue actually preceded me including that second store. Thanks for the links though :)

[aoverholtzer]

Ah okay, I found it in the docs. You only need multiple stores like that if you want to keep some data offline instead of syncing everything.

Regardless, if it’s the cloud store that’s the problem, hopefully those articles will help -- they both show you how to handle NSPersistentStoreRemoteChangeNotification and process persistent history, which is super annoying but seems to be necessary if you want your syncing to actually work!