How to receive feedback from server

When an update is happening on the server side for a specific user then the server will notify to that user by pushing a notification.

Let two client A and B are friend. Both have registered to the server. Both have a message field in the da开发者_开发技巧tabase for storing message receiving from others.

B has sent a message to A's message field. Now server's task is to check A's message field. If there is a recent message for A, then the server will notify A by sending this message. How can i solve this. Any suggestion will appreciate..

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Google has a push notification system for android called C2DM

Keep in mind this system is merely designed to sign A that there is a new message, not to actually push the complete message to the device. After receiving the notification the device should contact the server to fetch the message.