FCM

Firebase Cloud Messaging (FCM) is a cross-platform messaging service provided by Google that enables application servers to send messages to client apps on Android, iOS, and the web. It is part of the Firebase platform and evolved from Google Cloud Messaging (GCM) to offer a unified system for delivering push notifications, data messages, and device-to-cloud communication.

How it works: Developers obtain device registration tokens from the client SDKs. Messages are sent to FCM via HTTP (REST) or, for upstream messaging, via XMPP. FCM then routes messages to the intended devices, which can be addressed individually by tokens, to device groups, or to devices subscribed to a topic. FCM handles queueing, retries, and delivery to devices even when the app is not running, providing reliable delivery with platform-specific behavior for foreground and background apps.

Message types and handling: FCM supports notification messages, which are displayed by the device’s system UI when the app is in the background, and data messages, which are delivered to the app for custom processing. Messages can be a combination of both. The Firebase SDKs for Android, iOS, and the web help receive and process incoming messages.

Security and management: Access to FCM requires a Firebase project and appropriate credentials (server keys or OAuth 2.0 tokens). Registration tokens can change or be invalidated, so apps should refresh tokens regularly. There is no charge for using FCM; it is provided as part of Firebase’s free tier with Google infrastructure support.