how to lock editing of an attribute if multiple users are trying to modify the same at the same time

how to lock editing of an attribute if multiple users are trying to modify the same at the same time.

here is an Object XYZ, Approver A & Approver B are there. Approver A will open XYZ and enter comments in a page 3 attribute & Approver B also trying to open that object and edit XYZ, in this case we would want to prevent edit for Approver B till Approver A is done. Here we can either display error message to Approver B saying someone is already editing it or lock editing for Approver B till A is done.