How easy is it to put a lock on a file while writing on it? What happens in an environment where you have multiple frontend servers that need to write to the sa

How easy is it to put a lock on a file while writing on it? What happens in an environment where you have multiple frontend servers that need to write to the same file? Are you going to have like one server that does the reading/writing from a file and have like a protocol with which other servers can send commands? Great, you've just invented a half-assed DBMS.