Push notifications play a pivotal role in enhancing user engagement and retention in mobile apps by facilitating direct communication between apps and users via the operating system's push notification service, such as Apple Push Notification Service (APNS) for iOS and Firebase Cloud Messaging (FCM) for Android. To initiate push notifications, apps must obtain user consent, which involves generating a unique device token that is used to register the device with the app server. This token allows the server to send notifications to the device through the respective push notification service. The services handle the routing and delivery of notifications, considering device availability and network conditions, while developers manage the creation and management of notification payloads and device tokens on their servers. Ensuring that payloads conform to size limits and securing device tokens are critical for maintaining effective notification systems. Services like Knock further simplify this process by managing connections and allowing for unified messaging across multiple platforms without the need for developers to handle different payload structures or maintain stateful connections with push notification services.