Core Data with CloudKit: The Basics

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At WWDC 2019, Apple introduced a significant update to Core Data by introducing the NSPersistentCloudKitContainer. This means that with Core Data with CloudKit, users can seamlessly access data in their applications across all their Apple devices without writing extensive code.

Core Data provides powerful object graph management capabilities for developing applications with structured data. CloudKit allows users to access their data on each device where they are logged into their iCloud account, also offering a consistently available backup service. Core Data with CloudKit combines the advantages of local persistence with cloud backup and network distribution.

In 2020 and 2021, Apple continued to enhance Core Data with CloudKit, adding public and shared database synchronization to its initial support for private database synchronization.

I will introduce the usage, debugging techniques, console settings of Core Data with CloudKit, and try to delve deeper into its synchronization mechanisms through several blog posts.

Limitations of Core Data with CloudKit

  • Apple Ecosystem Only

    Unlike other cross-platform solutions, Core Data with CloudKit can only operate within the Apple ecosystem and is only available to users in this ecosystem.

  • High Testing Threshold

    Access to the CloudKit service and the development team’s CKContainer requires an Apple Developer Program account during development. Additionally, the performance on simulators is not as reliable as on actual devices.

Advantages of Core Data with CloudKit

  • Almost Free

    Developers generally do not have to pay extra for network services. Private databases are stored in the user’s personal iCloud space, and the capacity of public databases automatically increases with the number of app users, up to 1 PB of storage, 10 TB of database storage, and 200 TB of daily traffic. It’s almost free, considering Apple takes a 15-30% cut of app revenues.

  • Secure

    Apple ensures data security through various technical means such as sandbox containers, database segregation, encrypted fields, and authentication. Additionally, given Apple’s long-standing image as a defender of user privacy, using Core Data with CloudKit can increase users’ trust in your application.

    In fact, it was after seeing this feature at WWDC 2019 that I was inspired to develop Health Notes — ensuring data privacy while keeping data for a long time.

  • High Integration, Good User Perception

    Authentication and distribution are seamless. Users can enjoy all the features without any extra login.

Core Data

Core Data was introduced in 2005, with its predecessor EOF gaining considerable user recognition as early as 1994. After years of evolution, Core Data has become quite mature. As an object graph and persistence framework, almost every tutorial will tell you not to treat it as a database or an ORM.

Core Data’s features include, but are not limited to, managing serialized versions, object life cycle management, object graph management, SQL isolation, change handling, data persistence, memory optimization, and data querying.

Core Data offers many features, but it is not very beginner-friendly and has a steep learning curve. In recent years, Apple has addressed this issue by adding PersistentContainer to greatly reduce the difficulty of creating Stacks; the emergence of SwiftUI and Core Data Template has made it easier for beginners to use its powerful features in projects.

CloudKit

For several years after Apple launched iCloud, developers were unable to integrate their applications with iCloud. This problem was resolved in 2014 with the introduction of the CloudKit framework.

CloudKit is a collective service of databases, file storage, and user authentication systems, providing a mobile data interface between applications and iCloud containers. Users can access data saved in iCloud on multiple devices.

The data types and internal logic of CloudKit differ greatly from Core Data, and compromises or processing are required to convert data objects between the two. In fact, as soon as CloudKit was released, developers strongly hoped for a convenient conversion between the two. Before the introduction of Core Data with CloudKit, third-party developers had already provided solutions to synchronize Core Data or other data objects (like realm) to CloudKit, most of which are still supported today.

Relying on the previously introduced Persistent History Tracking feature, Apple finally provided its own solution, Core Data with CloudKit, in 2019.

Core Data Objects vs. CloudKit Objects

Both frameworks have their own fundamental object types that are not directly interchangeable. Here’s a brief introduction and comparison of some basic object types mentioned in this article:

  • NSPersistentContainer vs. CKContainer

    NSPersistentContainer handles the managed object model (NSManagedObjectModel), creating and managing the persistence coordinator (NSPersistentStoreCoordinator) and managed object context (NSManagedObjectContext). Developers create its instance through code.

    CKContainer is similar to an app’s sandbox logic, where you can store various resources like structured data and files. Each app using CloudKit should have its own CKContainer (though one app can correspond to multiple CKContainers and vice versa). Developers usually don’t create new CKContainers directly in the code but rather through the iCloud console or in the Xcode Target’s Signing & Capabilities.

  • NSPersistentStore vs. CKDatabase/CKRecordZone

    NSPersistentStore is the abstract base class for all Core Data persistence storage, supporting four types of persistence (SQLite, Binary, XML, and In-Memory). Multiple NSPersistentStore instances (possibly of different types) can be held in an NSPersistentContainer by declaring multiple NSPersistentStoreDescriptions. NSPersistentStore doesn’t have a user authentication concept but can be set to read-only or read-write modes. Due to the Persistent History Tracking requirement of Core Data with CloudKit, only SQLite type NSPersistentStores can be synchronized, where each instance on the device points to a SQLite database file.

    In CloudKit, there is only one type of structured data storage, but it’s distinguished in two dimensions.

    From a user authentication perspective, CKDatabase provides three forms of databases: private, public, and shared. Only the app’s user (who has logged into their iCloud account) can access their private database, which is stored in the user’s personal iCloud space, and no one else can operate on its data. Data stored in the public database can be accessed by any authorized app, even if the app’s user hasn’t logged into an iCloud account. App users can share some data with other users of the same app, which is placed in the shared database, where the sharer can set read-write permissions for other users.

    Data in CKDatabase is also not scattered; it’s placed in designated RecordZones. We can create any number of Zones in the private database (public and shared databases only support the default Zone). When a CKContainer is created, each type of database will automatically generate a CKRecordZone named _defaultZone.

    Therefore, when we save data to a CloudKit database, we need to specify not only the database type (private, public, shared) but also the specific zoneID (which is unnecessary when saving to _defaultZone).

  • NSManagedObjectModel vs. Schema

    NSManagedObjectModel is the managed object model, representing the Core Data corresponding data entities. Most developers define it using Xcode’s Data Model Editor, which is saved in the xcdatamodeled file, containing entity properties, relationships, indexes, constraints, validations, configurations, etc.

    When CloudKit is enabled in an app, a Schema is created in the CKContainer. The Schema includes record types (Record Type), possible relationships between record types, indexes, and user permissions.

    Besides directly creating Schema content in the iCloud console, you can also create CKRecord in the code, letting CloudKit automatically create or update corresponding content in the Schema.

    Schema has permission settings (Security Roles), which can set different read-write permissions for world, icloud, and creator.

  • Entities vs. Record Types

    Although we often emphasize that Core Data is not a database, entities (Entities) are very similar to tables in a database. We describe objects in entities, including their names, properties, and relationships, eventually forming them into NSEntityDescription and summarizing them in NSManagedObjectModel.

    In CloudKit, data objects’ names and properties are described using Record Types.

    Entity has a lot of configurable information, but Record Types can only correspond to describing part of it. Since the two cannot correspond one-to-one, there are specific regulations to follow when designing Core Data with CloudKit data objects (which we will explore in the next article).

  • Managed Object vs. CKRecord

    Managed Objects (Managed Object) are model objects representing persistent storage records. They are instances of NSManagedObject or its subclasses and are registered in the

    managed object context (NSManagedObjectContext). In any given context, there is at most one managed object instance corresponding to a given record in persistent storage.

    In CloudKit, each record is called a CKRecord.

    We don’t need to worry about the creation process of Managed Object IDs (NSManagedObjectID), as Core Data handles everything, but for CKRecord, we usually need to explicitly set a CKRecordIdentifier for each record in the code. As the unique identifier of CKRecord, CKRecordIdentifier is used to determine the unique location of that CKRecord in the database. If the data is stored in a custom CKRecordZone, we also need to indicate it in CKRecord.ID.

  • CKSubscription

    CloudKit is a cloud service that must respond to data changes in different devices of the same iCloud account (private database) or devices using different iCloud accounts (public database).

    Developers create CKSubscription on iCloud through CloudKit. When data in CKContainer changes, the cloud server checks whether the change meets any CKSubscription’s trigger condition. If so, it sends a remote notification (Remote Notification) to the subscribed devices. This is why Xcode automatically adds Remote Notification when CloudKit functionality is added in the Xcode Target’s Signing & Capabilities.

    In practice, three subclasses of CKSubscription are used to perform different subscription tasks:

    CKQuerySubscription, which pushes a Notification when a CKRecord meets the set NSPredicate.

    CKDatabaseSubscription, which subscribes to and tracks the creation, modification, and deletion of records in the database (CKDatabase). This subscription can only be used in custom CKRecordZones in private and shared databases and will only notify the subscription creator. Future articles will show how Core Data with CloudKit uses this subscription in private databases.

    CKRecordZoneNotification, which executes when a user, or in some cases, CloudKit, modifies records in that zone (CKRecordZone), for example, when a field’s value in a record changes.

    For remote notifications pushed by the iCloud server, applications need to respond in the Application Delegate. In most cases, remote notifications can be in the form of silent notifications, and for this, developers need to enable Backgroud ModesRemote notifications in their application.

Implementation Speculation of Core Data with CloudKit

Let’s speculate on the implementation process of Core Data with CloudKit, combining the basic knowledge introduced above.

Taking private database synchronization as an example:

  • Initialization:

    1. Create CKContainer
    2. Configure Schema based on NSManagedObjectModel
    3. Create a CKRecordZone with ID com.apple.coredata.cloudkit.zone in the private database
    4. Create CKDatabaseSubscription on the private database
  • Data Export (exporting local Core Data data to the cloud):

    1. NSPersistentCloudKitContainer creates background tasks to respond to Persistent History Tracking’s NSPersistentStoreRemoteChange notifications
    2. Based on the transaction from NSPersistentStoreRemoteChange, convert Core Data operations into CloudKit operations, such as converting NSManagedObject instances into CKRecord instances for new data.
    3. Pass the converted CKRecord or other CloudKit operations to the iCloud server through CloudKit
  • Server-side:

    1. Process CloudKit operation data submitted from remote devices in order
    2. Check with the CKDatabaseSubscription created during initialization if the operation causes changes in data in the com.apple.coredata.cloudkit.zone of the private database
    3. Distribute remote notifications to all devices (same iCloud account) that created CKDatabaseSubscription
  • Data Import (synchronizing remote data locally):

    1. NSPersistentCloudKitContainer’s background tasks respond to cloud silent push
    2. Send a refresh operation request to the cloud with the last operation token
    3. The cloud returns changes in the database since the last refresh for each device based on their token
    4. Convert remote data into local data (delete, update, add, etc.)
    5. Since the view context’s automaticallyMergesChangesFromParent property is true, local data changes will automatically be reflected in the view context

The above steps omit all technical difficulties and details, only describing the general process.

Conclusion

This article briefly introduced some basic knowledge about Core Data, CloudKit, and Core Data with CloudKit. In the next article, we will explore how to use Core Data with CloudKit to implement synchronization between local and private databases.

PS: There are many articles on how to use NSPersistentContainer, but like other Core Data features, it’s not easy to use effectively. I’ve encountered numerous issues over more than two years of use. Recently, I have systematically relearned Core Data with CloudKit and organized its key points. I hope this series of articles will help more developers understand and use the Core Data with CloudKit feature.

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