CloudKit and iCloud Sync

Sync data across devices using CloudKit, iCloud key-value storage, and iCloud Drive. Covers container setup, record CRUD, queries, subscriptions, CKSyncEngine, SwiftData integration, conflict resolution, and error handling. Targets iOS 26+ with Swift 6.2; older availability noted where relevant.

Container and Database Setup
CKRecord CRUD
CKQuery
CKSubscription
CKSyncEngine (iOS 17+)
SwiftData + CloudKit
NSUbiquitousKeyValueStore
iCloud Drive File Sync
Account Status and Error Handling
Conflict Resolution
Common Mistakes
Review Checklist
References

Container and Database Setup

Enable iCloud + CloudKit in Signing & Capabilities. A container provides three databases:

Database	Scope	Requires iCloud	Storage Quota
Public	All users	Read: No, Write: Yes	App quota
Private	Current user	Yes	User quota
Shared	Shared records	Yes	Owner quota

swift

import CloudKit

let container = CKContainer.default()
// Or named: CKContainer(identifier: "iCloud.com.example.app")

let publicDB  = container.publicCloudDatabase
let privateDB = container.privateCloudDatabase
let sharedDB  = container.sharedCloudDatabase

CKRecord CRUD

Records are key-value pairs. Max 1 MB per record (excluding CKAsset data).

swift

// CREATE
let record = CKRecord(recordType: "Note")
record["title"] = "Meeting Notes" as CKRecordValue
record["body"] = "Discussed Q3 roadmap" as CKRecordValue
record["createdAt"] = Date() as CKRecordValue
record["tags"] = ["work", "planning"] as CKRecordValue
let saved = try await privateDB.save(record)

// FETCH by ID
let recordID = CKRecord.ID(recordName: "unique-id-123")
let fetched = try await privateDB.record(for: recordID)

// UPDATE -- fetch first, modify, then save
fetched["title"] = "Updated Title" as CKRecordValue
let updated = try await privateDB.save(fetched)

// DELETE
try await privateDB.deleteRecord(withID: recordID)

Custom Record Zones (Private/Shared Only)

Custom zones support atomic commits, change tracking, and sharing.

swift

let zoneID = CKRecordZone.ID(zoneName: "NotesZone")
let zone = CKRecordZone(zoneID: zoneID)
try await privateDB.save(zone)

let recordID = CKRecord.ID(recordName: UUID().uuidString, zoneID: zoneID)
let record = CKRecord(recordType: "Note", recordID: recordID)

CKQuery

Query records with NSPredicate. Supported:

==

!=

<=

>=

BEGINSWITH

CONTAINS

IN

AND

NOT

BETWEEN

distanceToLocation:fromLocation:

swift

let predicate = NSPredicate(format: "title BEGINSWITH %@", "Meeting")
let query = CKQuery(recordType: "Note", predicate: predicate)
query.sortDescriptors = [NSSortDescriptor(key: "createdAt", ascending: false)]

let (results, _) = try await privateDB.records(matching: query)
for (_, result) in results {
    let record = try result.get()
    print(record["title"] as? String ?? "")
}

// Fetch all records of a type
let allQuery = CKQuery(recordType: "Note", predicate: NSPredicate(value: true))

// Full-text search across string fields
let searchQuery = CKQuery(
    recordType: "Note",
    predicate: NSPredicate(format: "self CONTAINS %@", "roadmap")
)

// Compound predicate
let compound = NSCompoundPredicate(andPredicateWithSubpredicates: [
    NSPredicate(format: "createdAt > %@", cutoffDate as NSDate),
    NSPredicate(format: "tags CONTAINS %@", "work")
])

CKSubscription

Subscriptions trigger push notifications when records change server-side. CloudKit auto-enables APNs -- no explicit push entitlement needed.

swift

// Query subscription -- fires when matching records change
let subscription = CKQuerySubscription(
    recordType: "Note",
    predicate: NSPredicate(format: "tags CONTAINS %@", "urgent"),
    subscriptionID: "urgent-notes",
    options: [.firesOnRecordCreation, .firesOnRecordUpdate]
)
let notifInfo = CKSubscription.NotificationInfo()
notifInfo.shouldSendContentAvailable = true  // silent push
subscription.notificationInfo = notifInfo
try await privateDB.save(subscription)

// Database subscription -- fires on any database change
let dbSub = CKDatabaseSubscription(subscriptionID: "private-db-changes")
dbSub.notificationInfo = notifInfo
try await privateDB.save(dbSub)

// Record zone subscription -- fires on changes within a zone
let zoneSub = CKRecordZoneSubscription(
    zoneID: CKRecordZone.ID(zoneName: "NotesZone"),
    subscriptionID: "notes-zone-changes"
)
zoneSub.notificationInfo = notifInfo
try await privateDB.save(zoneSub)

Handle in AppDelegate:

swift

func application(
    _ application: UIApplication,
    didReceiveRemoteNotification userInfo: [AnyHashable: Any]
) async -> UIBackgroundFetchResult {
    let notification = CKNotification(fromRemoteNotificationDictionary: userInfo)
    guard notification?.subscriptionID == "private-db-changes" else { return .noData }
    // Fetch changes using CKSyncEngine or CKFetchRecordZoneChangesOperation
    return .newData
}

CKSyncEngine (iOS 17+)

CKSyncEngine

is the recommended sync approach. It handles scheduling, transient error retries, change tokens, and push notifications automatically. Works with private and shared databases only.

swift

import CloudKit

final class SyncManager: CKSyncEngineDelegate {
    let syncEngine: CKSyncEngine

    init(container: CKContainer = .default()) {
        let config = CKSyncEngine.Configuration(
            database: container.privateCloudDatabase,
            stateSerialization: Self.loadState(),
            delegate: self
        )
        self.syncEngine = CKSyncEngine(config)
    }

    func handleEvent(_ event: CKSyncEngine.Event, syncEngine: CKSyncEngine) {
        switch event {
        case .stateUpdate(let update):
            Self.saveState(update.stateSerialization)
        case .accountChange(let change):
            handleAccountChange(change)
        case .fetchedRecordZoneChanges(let changes):
            for mod in changes.modifications { processRemoteRecord(mod.record) }
            for del in changes.deletions { processRemoteDeletion(del.recordID) }
        case .sentRecordZoneChanges(let sent):
            for saved in sent.savedRecords { markSynced(saved) }
            for fail in sent.failedRecordSaves { handleSaveFailure(fail) }
        default: break
        }
    }

    func nextRecordZoneChangeBatch(
        _ context: CKSyncEngine.SendChangesContext,
        syncEngine: CKSyncEngine
    ) -> CKSyncEngine.RecordZoneChangeBatch? {
        let pending = syncEngine.state.pendingRecordZoneChanges
        return CKSyncEngine.RecordZoneChangeBatch(
            pendingChanges: Array(pending)
        ) { recordID in self.recordToSend(for: recordID) }
    }
}

// Schedule changes
let zoneID = CKRecordZone.ID(zoneName: "NotesZone")
let recordID = CKRecord.ID(recordName: noteID, zoneID: zoneID)
syncEngine.state.add(pendingRecordZoneChanges: [.saveRecord(recordID)])

// Trigger immediate sync (pull-to-refresh)
try await syncEngine.fetchChanges()
try await syncEngine.sendChanges()

Key point: persist

stateSerialization

across launches; the engine needs it to resume from the correct change token.

SwiftData + CloudKit

ModelConfiguration

supports CloudKit sync. CloudKit models must use optional properties and avoid unique constraints.

swift

import SwiftData

@Model
class Note {
    var title: String
    var body: String?
    var createdAt: Date?
    @Attribute(.externalStorage) var imageData: Data?

    init(title: String, body: String? = nil) {
        self.title = title
        self.body = body
        self.createdAt = Date()
    }
}

let config = ModelConfiguration(
    "Notes",
    cloudKitDatabase: .private("iCloud.com.example.app")
)
let container = try ModelContainer(for: Note.self, configurations: config)

CloudKit model rules: use optionals for all non-String properties; avoid

#Unique

; keep models flat; use

@Attribute(.externalStorage)

for large data; avoid complex relationship graphs.

NSUbiquitousKeyValueStore

Simple key-value sync. Max 1024 keys, 1 MB total, 1 MB per value. Stores locally when iCloud is unavailable.

swift

let kvStore = NSUbiquitousKeyValueStore.default

// Write
kvStore.set("dark", forKey: "theme")
kvStore.set(14.0, forKey: "fontSize")
kvStore.set(true, forKey: "notificationsEnabled")
kvStore.synchronize()

// Read
let theme = kvStore.string(forKey: "theme") ?? "system"

// Observe external changes
NotificationCenter.default.addObserver(
    forName: NSUbiquitousKeyValueStore.didChangeExternallyNotification,
    object: kvStore, queue: .main
) { notification in
    guard let userInfo = notification.userInfo,
          let reason = userInfo[NSUbiquitousKeyValueStoreChangeReasonKey] as? Int,
          let keys = userInfo[NSUbiquitousKeyValueStoreChangedKeysKey] as? [String]
    else { return }

    switch reason {
    case NSUbiquitousKeyValueStoreServerChange:
        for key in keys { applyRemoteChange(key: key) }
    case NSUbiquitousKeyValueStoreInitialSyncChange:
        reloadAllSettings()
    case NSUbiquitousKeyValueStoreQuotaViolationChange:
        handleQuotaExceeded()
    default: break
    }
}

iCloud Drive File Sync

Use

FileManager

ubiquity APIs for document-level sync.

swift

guard let ubiquityURL = FileManager.default.url(
    forUbiquityContainerIdentifier: "iCloud.com.example.app"
) else { return }  // iCloud not available

let docsURL = ubiquityURL.appendingPathComponent("Documents")
let cloudURL = docsURL.appendingPathComponent("report.pdf")
try FileManager.default.setUbiquitous(true, itemAt: localURL, destinationURL: cloudURL)

// Monitor iCloud files
let query = NSMetadataQuery()
query.predicate = NSPredicate(format: "%K LIKE '*.pdf'", NSMetadataItemFSNameKey)
query.searchScopes = [NSMetadataQueryUbiquitousDocumentsScope]
NotificationCenter.default.addObserver(
    forName: .NSMetadataQueryDidFinishGathering, object: query, queue: .main
) { _ in
    query.disableUpdates()
    for item in query.results as? [NSMetadataItem] ?? [] {
        let name = item.value(forAttribute: NSMetadataItemFSNameKey) as? String
        let status = item.value(
            forAttribute: NSMetadataUbiquitousItemDownloadingStatusKey) as? String
    }
    query.enableUpdates()
}
query.start()

Account Status and Error Handling

Always check account status before sync. Listen for

.CKAccountChanged

swift

func checkiCloudStatus() async throws -> CKAccountStatus {
    let status = try await CKContainer.default().accountStatus()
    switch status {
    case .available: return status
    case .noAccount: throw SyncError.noiCloudAccount
    case .restricted: throw SyncError.restricted
    case .temporarilyUnavailable: throw SyncError.temporarilyUnavailable
    case .couldNotDetermine: throw SyncError.unknown
    @unknown default: throw SyncError.unknown
    }
}

CKError Handling

Error Code	Strategy
`.networkFailure` , `.networkUnavailable`	Queue for retry when network returns
`.serverRecordChanged`	Three-way merge (see Conflict Resolution)
`.requestRateLimited` , `.zoneBusy` , `.serviceUnavailable`	Retry after `retryAfterSeconds`
`.quotaExceeded`	Notify user; reduce data usage
`.notAuthenticated`	Prompt iCloud sign-in
`.partialFailure`	Inspect `partialErrorsByItemID` per item
`.changeTokenExpired`	Reset token, refetch all changes
`.userDeletedZone`	Recreate zone and re-upload data

swift

func handleCloudKitError(_ error: Error) {
    guard let ckError = error as? CKError else { return }
    switch ckError.code {
    case .networkFailure, .networkUnavailable:
        scheduleRetryWhenOnline()
    case .serverRecordChanged:
        resolveConflict(ckError)
    case .requestRateLimited, .zoneBusy, .serviceUnavailable:
        let delay = ckError.retryAfterSeconds ?? 3.0
        scheduleRetry(after: delay)
    case .quotaExceeded:
        notifyUserStorageFull()
    case .partialFailure:
        if let partial = ckError.partialErrorsByItemID {
            for (_, itemError) in partial { handleCloudKitError(itemError) }
        }
    case .changeTokenExpired:
        resetChangeToken()
    case .userDeletedZone:
        recreateZoneAndResync()
    default: logError(ckError)
    }
}

Conflict Resolution

When saving a record that changed server-side, CloudKit returns

.serverRecordChanged

with three record versions. Always merge into

serverRecord

-- it has the correct change tag.

swift

func resolveConflict(_ error: CKError) {
    guard error.code == .serverRecordChanged,
          let ancestor = error.ancestorRecord,
          let client = error.clientRecord,
          let server = error.serverRecord
    else { return }

    // Merge client changes into server record
    for key in client.changedKeys() {
        if server[key] == ancestor[key] {
            server[key] = client[key]           // Server unchanged, use client
        } else if client[key] == ancestor[key] {
            // Client unchanged, keep server (already there)
        } else {
            server[key] = mergeValues(          // Both changed, custom merge
                ancestor: ancestor[key], client: client[key], server: server[key])
        }
    }

    Task { try await CKContainer.default().privateCloudDatabase.save(server) }
}

Common Mistakes

DON'T: Perform sync operations without checking account status. DO: Check

CKContainer.accountStatus()

first; handle

.noAccount

swift

// WRONG
try await privateDB.save(record)
// CORRECT
guard try await CKContainer.default().accountStatus() == .available
else { throw SyncError.noiCloudAccount }
try await privateDB.save(record)

DON'T: Ignore

.serverRecordChanged

errors. DO: Implement three-way merge using ancestor, client, and server records.

DON'T: Store user-specific data in the public database. DO: Use private database for personal data; public only for app-wide content.

DON'T: Assume data is available immediately after save. DO: Update local cache optimistically and reconcile on fetch.

DON'T: Poll for changes on a timer. DO: Use

CKDatabaseSubscription

CKSyncEngine

for push-based sync.

swift

// WRONG
Timer.scheduledTimer(withTimeInterval: 30, repeats: true) { _ in fetchAll() }
// CORRECT
let sub = CKDatabaseSubscription(subscriptionID: "db-changes")
sub.notificationInfo = CKSubscription.NotificationInfo()
sub.notificationInfo?.shouldSendContentAvailable = true
try await privateDB.save(sub)

DON'T: Retry immediately on rate limiting. DO: Use

CKError.retryAfterSeconds

to wait the required duration.

DON'T: Merge conflict changes into

clientRecord

. DO: Always merge into

serverRecord

-- it has the correct change tag.

DON'T: Pass nil change token on every fetch. DO: Persist change tokens to disk and supply them on subsequent fetches.

Review Checklist

iCloud + CloudKit capability enabled in Signing & Capabilities
Account status checked before sync;
```
.noAccount
```
handled gracefully
Private database used for user data; public only for shared content
```
CKError.serverRecordChanged
```
handled with three-way merge into
```
serverRecord
```
Network failures queued for retry;
```
retryAfterSeconds
```
respected
```
CKDatabaseSubscription
```
or
```
CKSyncEngine
```
used for push-based sync
Change tokens persisted to disk;
```
changeTokenExpired
```
resets and refetches
```
.partialFailure
```
errors inspected per-item via
```
partialErrorsByItemID
```
```
.userDeletedZone
```
handled by recreating zone and resyncing
SwiftData CloudKit models use optionals, no
```
#Unique
```
,
```
.externalStorage
```
for large data

NSUbiquitousKeyValueStore.didChangeExternallyNotification

observed

Sensitive data uses
```
encryptedValues
```
on CKRecord (not plain fields)
```
CKSyncEngine
```
state serialization persisted across launches (iOS 17+)

References

See
```
references/cloudkit-patterns.md
```
for CKFetchRecordZoneChangesOperation incremental sync, CKShare collaboration, record zone management, CKAsset file storage, batch operations, and CloudKit Dashboard usage.
CloudKit Framework
CKContainer
CKRecord
CKQuery
CKSubscription
CKSyncEngine
CKShare
CKError
NSUbiquitousKeyValueStore
CKAsset

cloudkit-sync

NPX Install

Tags

SKILL.md Content

CloudKit and iCloud Sync

Contents

Container and Database Setup

CKRecord CRUD

Custom Record Zones (Private/Shared Only)

CKQuery

CKSubscription

CKSyncEngine (iOS 17+)

SwiftData + CloudKit

NSUbiquitousKeyValueStore

iCloud Drive File Sync

Account Status and Error Handling

CKError Handling

Conflict Resolution

Common Mistakes

Review Checklist

References