Android Cloud to Device Messaging (C2DM) is a push notification service that helps developers send data from servers to their applications on Android devices which launched together with Android 2.2 by Google.[1][2] The service provides a simple, lightweight mechanism that servers can use to tell mobile applications to contact the server directly, to fetch updated application or user data. The C2DM service handles all aspects of queueing of messages and delivery to the target application running on the target device.
Applications on an Android device don’t need to be running to receive messages. The system will wake up the application via Intent broadcast when the message arrives, as long as the application is set up with the proper broadcast receiver and permissions. C2DM does not provide any built-in user interface or other handling for message data. Instead, it simply passes raw message data received straight to the application, which has full control of how to handle it. For example, the application might post a notification, display a custom user interface, or silently sync data. It requires devices running Android 2.2 or higher that also have the Market application installed. It uses an existing connection for Google services. This requires users to set up their Google account on their mobile devices. Each notification message size is limit to 1024 bytes, and Google limits the number of messages a sender sends in aggregate, and the number of messages a sender sends to a specific device.
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