Appian Records Locking Mechanism

please have a look at this article 

community.appian.com/.../data-locking-strategies

To prevent multiple user access and edit, add a new column in the table, isLocked. When a user opens the form through the process model, pertaining to that record, set isLocked=true, also, and just before the end node of the process, mark isLocked=false. Next, when a user access the same form, check first if isLocked=true, if it is, disallow the user, else, repeat the previous logic.

This is one of the ways to implement locking at DB level, there are other methods to do this as well.