10.3. Transaction serialization

It is clear that in order to achieve isolation of transactions, the DBMS should use any methods of regulating the joint execution of transactions.

A plan (method) of executing a set of transactions is called serial if the result of the joint execution of transactions is equivalent to the result of some sequential execution of the same transactions.

Transaction serialization is a mechanism for performing them according to some serial plan. The provision of such a mechanism is the main function of the DBMS component responsible for managing transactions. A system that supports transaction serialization provides real user isolation.

The main implementation problem is to choose a method for serializing a set of transactions that would not limit their parallelism too much. A trivial solution that comes to mind is the truly sequential execution of transactions. But there are situations in which operators of different transactions can be executed in any order while maintaining seriality. Examples are only read transactions, as well as transactions that do not conflict over database objects.

The following types of conflicts can exist between transactions:

• WW - Transaction 2 attempts to modify an object that was modified by Transaction 1 that did not end;
• RW - Transaction 2 tries to modify the object read by the non-terminated Transaction 1;
• WR - Transaction 2 tries to read an object modified by a non-terminated Transaction 1.

Practical methods of transaction serialization are based on accounting for these conflicts.