Two phase locking a transaction is said to follow two phase locking protocol if locking and unlocking can be done in two phases. After acquiring all the locks in the first phase, the transaction continues to execute normally. To claim an exclusive write lock, a transaction must first acquire a shared read lock and then upgrade it to an exclusive lock. Given a transaction ti that does not follow two phase locking, we can find a transaction tj that uses two phase locking, and a. In this type of locking protocol, the transaction should acquire a lock after it releases one of its locks.
An appropriate lock must be acquired before a txn accesses the object. Existing locks may be released but no new locks can be acquired. In the second part, the transaction acquires all the locks. Twophase locking and its special cases twophase locking. In the first part, when the execution of the transaction starts, it seeks permission for the lock it requires. New locks on data items may be acquired but none can be released. Ae3b33osd lesson 11 page 3 silberschatz, korth, sudarshan s. According to the twophase locking protocol, a transaction handles its locks in two distinct, consecutive phases during the transactions execution. Two phase locking has two phases, one is growing, where all the locks are being acquired by the transaction. Two locks, pl i x and ql j y, conflict if xy and i j. Two phase locking basic 2pl each object has associated with it a lock. Two phase locking 2pl the two phase locking protocol divides the execution phase of the transaction into three parts.
555 1432 938 523 594 149 1456 1183 632 477 327 831 683 1038 594 1541 1429 1621 452 516 266 1390 583 1137 1283 1354 30 1624 538 667 1018 155 1318 526 1321 120 1107 1076 751 972