Google Cloud Messaging Server

Introduction

Engage your users across Android, iOS and Chrome.
Send data from your server to your users' devices, and receive messages from devices on the same connection. The GCM service handles all aspects of queueing of messages and delivery to client applications running on target devices, and it is completely free.

Full description
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Versatile Messaging Targets.
Distribute messages to your client app in any of three ways — to single devices, to groups of devices, or to devices subscribed to topics.

Downstream Messaging.
For purposes such as alerting users, chat messaging or kicking off background processing before the user opens the client app, GCM provides a reliable and battery-efficient connection between your server and devices.