Firebase Cloud Messaging:

Firebase Cloud Messaging (FCM) is a cross-platform messaging solution that lets you reliably send messages at no cost.

Using FCM, you can notify a client app that new email or other data is available to sync. You can send notification messages to drive user re-engagement and retention. For use cases such as instant messaging, a message can transfer a payload of up to 4000 bytes to a client app.

Key capabilities of Firebase Cloud Messaging:

Send notification messages or data messages: Send notification messages that are displayed to your user. Or send data messages and determine completely what happens in your application code.

Versatile message targeting: Distribute messages to your client app in any of 3 ways—to single devices, to groups of devices, or to devices subscribed to topics.

Send messages from client apps: Send acknowledgments, chats, and other messages from devices back to your server over FCM’s reliable and battery-efficient connection channel.