FCM Architectural Overview

FCM relies on the following set of components that build, transport, and receive messages:

Tooling to compose or build message requests. The Notifications composer provides a GUI-based option for creating notification requests. For full automation and support for all message types, you must build message requests in a trusted server environment that supports the Firebase Admin SDK or the FCM server protocols. This environment could be Cloud Functions for Firebase, Google App Engine, or your own app server.
The FCM backend, which (among other functions) accepts message requests, performs fanout of messages via topics, and generates message metadata such as the message ID.
A platform-level transport layer, which routes the message to the targeted device, handles message delivery, and applies platform-specific configuration where appropriate. This transport layer includes:
- Android transport layer (ATL) for Android devices with Google Play services
- Apple Push Notification service (APNs) for iOS devices
- Web push protocol for web apps
  
  Note: Platform-level transport layers are outside the FCM service. FCM messages routed to a platform-level transport layer may be subject terms specific to that platform rather than FCM’s terms of service. Android message routing via ATL falls under the Google APIs terms of service.
The FCM SDK on the user’s device, where the notification is displayed or the message is handled according to the app’s foreground/background state and any relevant application logic.

Lifecycle flow

Register devices to receive messages from FCM. An instance of a client app registers to receive messages, obtaining a registration token that uniquely identifies the app instance.
Send and receive downstream messages.
- Send a message. The app server sends messages to the client app:
  1. The message is composed, either in the Notifications composer or trusted environment, and a message request is sent to the FCM backend.
  2. The FCM backend receives the message request, generates a message ID and other metadata, and sends it to the platform specific transport layer.
  3. When the device is online, the message is sent via the platform-specific transport layer to the device.
  4. On the device, the client app receives the message or notification.