Hi Walter,
They said you can assume one program can access the database, and that dose not mean one
thread only can access the database, those tow things are completely different.
When they say one program can access the data file that mean there is only one server accessing the database, and there is no other program accessing that data file in the same time, for example you can assume there is no editor program accessing that file or there is no other server accessing that file.
But still in your server program it will handle concurrent client�s (multi threads).