Oracle Database Transactions and Locking Revealed by Thomas Kyte, Darl Kuhn

By Thomas Kyte, Darl Kuhn

Oracle Database Transactions and Locking printed presents much-needed info for construction scalable, high-concurrency purposes and install them opposed to the Oracle Database. learn this brief, 150-page booklet that's tailored from specialist Oracle Database structure to realize an exceptional and exact realizing of ways locking and concurrency are handled by way of Oracle Database. additionally find out how the Oracle Database structure comprises person transactions, and the way you could write code to mesh with how Oracle Database is designed to function. stable transaction layout is a crucial aspect of highly-concurrent purposes which are run by means of hundreds of thousands, even millions of clients who're all executing transactions while. Transaction layout in flip depends an outstanding knowing of ways the underlying database platform manages of the locking of assets as a way to hinder entry conflicts and information loss that may in a different way outcome from concurrent entry to info within the database.

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You are now using one lock to lock everything and, typically, you are also locking a whole lot more data than you were before. Lock escalation is used frequently in databases that consider a lock to be a scarce resource and overhead to be avoided. ■■Note Oracle will never escalate a lock. Never. info Chapter 2 ■ Locking and Issues Oracle never escalates locks, but it does practice lock conversion or lock promotion, terms that are often confused with lock escalation. ■■Note The terms lock conversion and lock promotion are synonymous.

Even so, they can and do cause large locking issues. As a demonstration of the first point, if we have a pair of tables set up as follows, nothing untoward happens yet: EODA@ORA12CR1> create table p ( x int primary key ); Table created. EODA@ORA12CR1> create table c ( x references p ); Table created. EODA@ORA12CR1> insert into p values ( 1 ); 1 row created. EODA@ORA12CR1> insert into p values ( 2 ); 1 row created. EODA@ORA12CR1> commit; Commit complete. EODA@ORA12CR1> insert into c values ( 2 ); 1 row created.

After hashing, we take that value and use DBMS_LOCK to request that lock ID to be exclusively locked with a timeout of ZERO (this returns immediately if someone else has locked that value). If we timeout or fail for any reason, we raise ORA-00054 Resource Busy. Otherwise, we do nothing—it is OK to insert, we won’t block. Upon committing our transaction, all locks, including those allocated by this DBMS_LOCK call, will be released. Of course, if the primary key of your table is an INTEGER and you don’t expect the key to go over 1 billion, you can skip the hash and just use the number as the lock ID.

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