Conflict serializability; Isolation; Locking protocol; Pessimistic scheduler
A locked transaction is a transaction which, in addition to read and write actions, contains lock and unlock operations to the data items. Lock and unlock operations enable a database system to control the order of read and write actions of a concurrent set of transactions. A locking policy is a set of rules which restrict the possible ways to introduce lock and unlock operations into a transaction. A locking policy is safe, if, whenever all the transactions conform to the policy, any history of the transactions is guaranteed to be serializable. Two-Phase Locking is a safe locking policy which is based on the simple rule saying a transaction is not allowed to further lock a data item once it has already unlocked some data item.
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