Mastering Database Management

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Concurrency Control Techniques

Transaction Processing in Distributed Systems

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A transaction is a logical unit of work constituted by one or more SQL statements executed by a single user. A transaction begins with the user’s first executable SQL statement and ends when it is committed or rolled back by that user.

A remote transaction contains only statements that access a single remote node. A distributed transaction contains statements that access more than one node.

Distributed SQL Statement

SELECT ename, dname FROM scott.emp e, d WHERE e.deptno = d.deptno; 

A distributed update statement modifies data on two or more nodes.

A distributed update is possible using a PL/SQL subprogram unit such as a procedure or trigger that includes two or more remote updates that access data on different nodes.

For example, the following PL/SQL program unit updates tables on the local database and the remote sales database:

    SET loc = 'NEW YORK' 
     WHERE deptno = 10; 
  UPDATE scott.emp 
    SET deptno = 11 
     WHERE deptno = 10;