Save data of many users to database concurrently on a single document

I am using realtimeboard.com where you can create a board and draw and write many things. It is also in a collaborative mode where you can invite as many users to join the board and can contribute, I was just wondering how these updated data is getting saved from different users. Example : suppose user A draws a rectangle and at the same time user B draws a triangle on the same board, so how the data of the board is getting updated and overridden to the because B does not have what A has written and A does not have what B has written.