How are push notifications implemented and delivered to users on mobile devices and web applications?

Push notifications are implemented through special services that allow the server to send messages to users' devices.

For mobile devices (iOS and Android), platform services are usually used:

• Apple Push Notification Service (APNs) for iOS
• Firebase Cloud Messaging (FCM) for Android

The process looks like this:

1. The app registers with the notification service and receives a unique device token.
2. This token is sent to the app server.
3. When a notification needs to be sent, the server creates a message and sends it via APNs or FCM, specifying the recipient's token.
4. The service delivers the notification to the device, where the app or system displays it to the user.

For web applications, the Web Push API is used, where the browser registers with the push service and receives an endpoint to which the server sends notifications.

An example of a simplified server process (Python):

import requests

def send_push(token, message):
    headers = {'Authorization': 'key=YOUR_SERVER_KEY'}
    data = {
        'to': token,
        'notification': {'title': 'New message', 'body': message}
    }
    response = requests.post('https://fcm.googleapis.com/fcm/send', headers=headers, json=data)
    return response.status_code

Thus, push notifications provide asynchronous delivery of messages from the server to the client.