UserDefaults, Keychain, files, SQLite, Core Data or SwiftData: how do you choose, and what survives deletion, reinstall and backup?

Start with what the data is. Preferences and small flags (a theme, the last tab) go to UserDefaults (a property list in the app container; fast and simple, but not for large or secret data). Secrets (passwords, tokens, keys) go to the Keychain, which is encrypted and access-controlled. Documents and blobs (a user’s photo, a downloaded file) go to files in the right directory. Structured, queryable or related records (messages, a to-do list with tags) go to a database: SQLite directly, Core Data, SwiftData or a library such as GRDB. Rebuildable data (decoded thumbnails, network responses) goes in Caches and must tolerate being deleted. The directory-by-directory rules and safe writes are covered in the Foundation topic; the question here is the container.

Then ask what survives what. Delete the app: the app container (Documents, Library, UserDefaults, SQLite files) is removed, but Keychain items usually remain and reappear on reinstall (which is why “is this a first launch?” based on the Keychain misleads, and why a sign-out-on-uninstall rule needs an explicit flag). Backup and restore (iCloud or an encrypted local backup): Documents and Application Support and UserDefaults are included, Caches and tmp are not, and Keychain items come back on a new device only if they are not marked …ThisDeviceOnly (such items are never restored to another device). Low storage: the system may purge Caches (and tmp). App update: all of it persists, so the schema must be migrated. Choose by lifetime: a token must survive relaunch but not leak into backups (Keychain, ThisDeviceOnly if needed); a download that can be fetched again must be in Caches or marked excluded from backup; user-created content must be backed up and must never live only in Caches. Also mind who else needs it: an extension or widget needs an app-group container or a shared keychain access group; multi-device needs a sync story from the start; and anything personal falls under the privacy rules (minimise what you keep, and delete on sign-out).