When one SQL Server get failure ,How other sqlserver should Automatically Connect to the Application while 1st Sql server get recoverd?

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C#

ASP.NET

SQL-Server

Actually in my current Application i call the Connection string by using a XML file and i used Merge replication for maintain two databases in two different servers. what i want is if the connection to main database(publisher) fails i want to automatically switch my connection string to other one(subscriber DB )with out minimum delay and after Main DB recovered from the issue( ex :server failure) then it should again connected to the Application and subscriber DB should synchronized with publisher DB. if some one Knows the proper Switching Mechanism for achieving this task. Thanks in advance .