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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