The use of locks, combined with the two-phase locking protocol, guarantees serializabil-ity of schedules. The order of transactions in the equivalent serial schedule is based on the
order in which the transactions lock the items they require. If a transaction needs an item
that is already locked, it may be forced to wait until the item is released. A different
approach that also guarantees serializability uses transaction timestamps to order trans-action execution for an equivalent serial schedule.