iOS Data Storage: Classify Your Data Before You Choose a Database

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Learn how to design iOS storage by data type, using UserDefaults, Keychain, databases, Application Support, Caches, and sync metadata where each fits best.
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An iOS app rarely stores one kind of data. A travel planner stores six: preferences, secrets, domain entities, user files, disposable cache, and sync metadata. Five of the six get placed without naming a persistence framework at all, which makes the SwiftData versus Core Data question the last one to ask rather than the first. ▎ ▎ Along the way: which accessibility class a Keychain token needs, why the ThisDeviceOnly classes are not about keeping secrets out of backups, which file protection class an attachment needs, when the system may delete your cache (never while the app is running), and what to do when the store refuses to open.