Monday, 2 April 2018

What is the result of interleaved transactions in database transactions

Result of interleaved transactions in database transactions, Lost Update problem


Question:
Consider the given schedule S1 with transactions T1 and T2; if the value of X at the beginning of the transactions is 100, what will be the value of X at the end of the transactions? Also, find the problem with the given schedule.
Transaction T1
Transaction T2
READ X
X:=X - 20


WRITE X


READ X
X:=X - 30

WRITE X

Solution:
The value of X at the end of the transactions is 70. This value is wrong. From the given schedule, it is very clear that both transactions are reading the initial value of X as 100, both calculates the new value to be written, and both writes the new value of X. here, T1 finishes transaction first and writes it’s new X value 80. Then, T2 overwrites the value written by T1. Hence, we will see only one result which is written later.


This problem is called Lost Update problem.

Lost update problem – it happens when two transactions access the same resource in an interleaved fashion (i.e., few instructions from T1 and few from T2 and so on) and one of them overwrites the update made by the first.

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