Message is used by the application server to send a message to the GCM server. See the documentation for GCM Architectural Overview for more informati

Response represents the GCM server's response to the application server's sent message. See the documentation for GCM Architectural Overview for more

Result represents the status of a processed message.

Sender abstracts the interaction between the application server and the GCM server. The developer must obtain an API key from the Google APIs Console