Best architecture for notification service

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Mobile

Android

webservice

I need to design an android application with below specification:Connect to online web CRM ,read and write data ,work online and offline, and my problem is:

How to notify android, which data is modified without comparing android database with all the data and what architecture is good for me?

How much data can store in android databases normally, as you know the crm data could be in huge amount, so i am thinking about it's risk.

And i can't use any third party services like C2DM.

Thanks for your help friends.