How to send Push Notifications from server to devices.

What is Push Notification ? Push notification is a feature that allows an app to notify user of new messages or events, even when the user is not actively using your application. Like on Android devices, how a whatsapp message will make a sound and icon appear in the status bar. What is Apple Push Notification service(APNs) ? "Apple Push Notification service" is the service for implementing the push notification feature for IOS devices. How it works ? Each device establishes encrypted IP connection with the service and receives notifications over this persistent connection. If a notification for an application