Description

Google Cloud Messaging (GCM) is a free service that enables users to send messages between servers and client apps. This includes downstream messages from servers to client apps, and upstream messages from client apps to servers. For example, a lightweight downstream message could inform a client app that there is new data to be fetched from the server, as in the case of a “new email” notification. For use cases such as instant messaging, a GCM message can transfer up to 4kb of payload to the client app. The GCM service handles all aspects of queuing of messages and delivery to and from the target client app. A GCM implementation includes a Google connection server, an app server in your environment that interacts with the connection server via HTTP or XMPP protocol, and a client app. Google GCM Connection Servers take messages from your app server and send them to a client app. On your app server, you implement either the http and xmpp protocol to communicate with the GCM connection server(s). App servers send messages to a GCM connection server; the connection server enqueues and stores the message, and then sends it to the client app. The Client App is a GCM-enabled client app. To receive GCM messages, this app must register with GCM and get a unique identifier called a registration token. If you are using the XMPP connection server, the client app can send “upstream” messages back to the app server.