CloudKit (iCloud sync — iOS 26 era)

Apple's hosted backend for syncing app data across a user's devices (and, via shares, between users). No server code; you model records and CloudKit handles storage, auth (the iCloud account), and delivery.

• Fintech caution: the public database is world-readable and the record data is not end-to-end encrypted at rest under your key. Never put balances, PANs, transactions, or PII in it. For sensitive fields, either keep them out of CloudKit or store a CryptoKit-sealed blob whose key lives in the Keychain (see swift-security) — CloudKit syncs opaque bytes fine.
• When not to use CloudKit: you need server-side business logic, non-Apple clients, or a queryable ledger of record. CloudKit is a per-user sync cache, not a system of record.
• Modern default: for a hand-rolled model use CKSyncEngine (iOS 17+). For a SwiftData/Core Data model, turn on the built-in mirroring instead of touching CKRecord directly.

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