•A structure in
(volatile memory) in the DBMS which
remembers which txns have set locks on which data, in which modes
•It rejects a request
to get a new lock if a conflicting
lock is already held by a different txn
•NB: a lock does not
actually prevent access to the data,
it only prevents getting a conflicting lock
–So data
protection only comes if the right lock is requested before every access to the data