Synchronization In Sql Server

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SQL-Server

I have come across a new task where in i have to upgrade my application such that when the entries are made in any of the databases located at remote site, the data must be updated at the main server. The detail problem is like we want to design the system such that the user is unaware about the connectivity of the database he just has to use the application for his work. so I want to implement such design where the data entered is always updated on the server database. The ways I thought for is the online and offline connection. Offline connection is important as there is no guarantee of having the Internet connection on every site at every time so even if there is no internet connection the user can use application and can then connect to the server at a scheduled time for updating the data at the server. I thought of restoring the database, but this can't be done everyday as the size of database will be increasing every moment. So I need to find out ways of implementing such design where the user is free from the updating purpose, the data automatically gets updated at scheduled time(in offline mode) and keeps on updating continuously when it is connected(in online mode).
For example the emails, Train tickets reservation, etc.

So can any one give any insight on this please?
Thanks in advance