hierarchy. ·If a parent transaction is aborted, however, the effect of all its completed subtransactionsis undone. ·Consequently, if the root transaction is aborted all commited subtransactionsare undone. 21. Two Phase Locking (2PL), Deadlocks 2PL ·The most popular concurrency control technique. Used in: RDBMSs(Oracle, Ingres, Sybase, DB/2, etc.) ODBMSs (O2, ObjectStore, Versant, etc.) Transaction Monitors (CICS, etc) ·Concurrent processes acquire locks on shared resources from lock manager. ·The principal component that implements 2PL is a lock manager from which concurrent processes or threads acquire locks on every shared resource they access. ·Lock manager grants lock if request does not conflict with already granted locks. ·Two phase locking guarantees serialisability. Deadlock ·2PL may lead to processes waiting for each other to release locks. ·These situations are called deadlocks
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