What are the key technical elements and considerations required to develop a push notification system for an app?

Focus on the architecture of the push notification system, including the client-side and server-side components. Discuss the protocols involved, such as APNs for iOS and FCM for Android, and how they handle message delivery. Consider scalability, reliability, and latency in your explanation, as well as user preferences for notification settings and opt-in processes. Address security concerns, such as data encryption and user privacy, and think about how to handle different types of notifications (e.g., transactional vs. promotional). Lastly, highlight the importance of analytics for tracking engagement and effectiveness of notifications.