How do iPhone and Android phones receive push notifications?

How do we receive push notifications on our phones or computers?

Usually we can use the messaging solution Firebase to support push notifications. The following figure shows how Firebase Cloud Messaging (FCM) works.

FCM is a cross-platform messaging solution that can reliably write, send, queue, and route push notifications. It provides a unified API between message senders (application servers) and receivers (client apps). Application developers can use this solution to improve user retention.

Step 1 - 2

When the client application starts for the first time, the client application sends credentials to FCM, including Sender ID, API Key, and App ID. FCM generates a registration token (Registration Token, so the registration token is also called instance ID) for the client application instance. This token must be included in the notification.

Step 3

The client application sends the registration token to the application server. The application server caches the token for subsequent communications. Over time, the application server will have too many tokens to maintain, so the recommended practice is to store tokens with timestamps and delete expired tokens from time to time.

Step 4

There are two ways to send a message. One is to write the message directly in the console GUI (step 4.1), and the other is to send the message from the application server (step 4.2).

Step 5

FCM receives the message and puts it into a storage queue if the device is not online.

Step 6

FCM forwards the information to the platform-level transport layer. The transport layer handles platform-specific configuration.

Step 7

Routes information to the target device. Notifications can be displayed based on the configuration sent by the application server.