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Serializability is the classical concurrency scheme. It ensures that a schedule for executing concurrent transactions is equivalent to one that executes the transactions serially in some order. It assumes that all accesses to the database are done using read and write operations. A schedule is called ``correct'' if we can find a serial schedule that is ``equivalent'' to it. Given a set of transactions T1...Tn, two schedules S1 and S2 of these transactions are equivalent if the following conditions are satisfied:

Read-Write Synchronization: If a transaction reads a value written by another transaction in one schedule, then it also does so in the other schedule.

Write-Write Synchronization: If a transaction overwrites the value of another transaction in one schedule, it also does so in the other schedule.

These two properties ensure that there can be no difference in the effects of the two schedules. As an example, consider the schedule in Figure 1.

It is equivalent to a schedule in which T2 is executed after T1.

There are several approaches to enforcing serializability.

Prasun Dewan
Tue Mar 20 13:49:53 EST 2001