Session IDs Are Not User IDs:
Understanding the Difference

✅ What Session IDs Enable

⚠️ What Requires User IDs

Session ID Implementation

// Session ID: Generated fresh, stored in memory
class SessionManager {
    private var sessionId: String? = nil
    
    func getSessionId() -> String {
        // Generate new ID if none exists (app just launched)
        if sessionId == nil {
            sessionId = UUID().uuidString
        }
        return sessionId!
    }
    
    // Called on app background/timeout
    func endSession() {
        sessionId = nil  // Gone, no persistence
    }
}

Note: The ID exists only in RAM. When the app closes or the session times out, it's gone forever.

User ID Implementation

// User ID: Persisted across sessions
class UserManager {
    private let storage = UserDefaults.standard
    
    func getUserId() -> String {
        // Try to load existing ID
        if let existing = storage.string(forKey: "user_id") {
            return existing
        }
        // Create and persist new ID
        let newId = UUID().uuidString
        storage.set(newId, forKey: "user_id")  // Persisted!
        return newId
    }
    
    // Must handle deletion requests
    func deleteUserData() {
        storage.removeObject(forKey: "user_id")
        // Also need to purge server-side data...
    }
}

Note: The ID is written to disk and survives app restarts, updates, even device transfers in some cases.