How about if someone just forgets the document open on their browser? Or their network connection is down when they close the browser window? You eventually hav

How about if someone just forgets the document open on their browser? Or their network connection is down when they close the browser window? You eventually have to somehow deal with documents that are left locked, no matter what (google CAP theorem.) On a general level, locking is a subpar solution for distributed systems. You should try to build support for automatic resolution of (most) edit conflicts instead, and forego locking altogether. If that's not possible, try to make the locks smart, e.g. so they auto expire unless periodically renewed.