In Unix record locking is the technique used to lock the portion of a file for certain amount of time to maintain consistency of the data from concurrent access to the file. On this mechanism, Mandatory locking is the technique which is used to lock the portion of the file exclusively. If once mandatory locking is enabled to the file, no other process can read or write the data to the locked portion of the file. So, to enable mandatory lock to a file, the following is the procedure.Turn ON the set group id bit and turn OFF the group execute bit for the file to be lock.So, what is the need of this process which helds on these group ids and Why I want to do this particularly on group ids to enable mandatorylocking. I saw many of the reference but all of them are tell only the rule instead of why they are implement the rule.  
Mandatory Locking in Unix
linux;lock