In this type of locking protocol, the transaction should acquire a lock after it releases one of its locks. Two phase locking basic 2pl each object has associated with it a lock. In the second part, the transaction acquires all the locks. Two phase locking has two phases, one is growing, where all the locks are being acquired by the transaction. Two phase locking a transaction is said to follow two phase locking protocol if locking and unlocking can be done in two phases. Twophase locking tutorial to learn twophase locking in simple, easy and step by step way with syntax, examples and notes. Ae3b33osd lesson 11 page 3 silberschatz, korth, sudarshan s.
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. To claim an exclusive write lock, a transaction must first acquire a shared read lock and then upgrade it to an exclusive lock. This locking protocol divides the execution phase of a transaction into three different parts. An appropriate lock must be acquired before a txn accesses the object. Existing locks may be released but no new locks can be acquired. Twophase locking and its special cases twophase locking. According to the twophase locking protocol, a transaction handles its locks in two distinct, consecutive phases during the transactions execution. After acquiring all the locks in the first phase, the transaction continues to execute normally. Two phase locking 2pl the two phase locking protocol divides the execution phase of the transaction into three parts. Two locks, pl i x and ql j y, conflict if xy and i j.
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