One of my friends was asked this question in interview. Given two different databases one on which only write is done and the other on which only read is done. How does one maintain synchronization between the two given a specified time lag. I would like to add that these databases have same schema.

Eg: R is my Read database and W is my Write database. I want to ensure that read database should be the replica of Write database say a minute earlier. 

What I am keen here is on the techniques(concepts) this is done rather than the implementation. Lets say the RDBMS is SQL Server.