7.5 Lost Update Problem in Concurrency In DBMS | Why Concurrency Control Is Needed | Full Syllabus

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Why Concurrency Control is Needed?
To make system efficient and save time, it is required to execute more than one transaction
at the same time. But concurrency leads several problems. The three problems associated
with concurrency are as follows:
1. The Lost Update Problem
If any transaction Tj
updates any variable v at time t without knowing the value of v at time
t then this may leads to lost update problem. Consider the transactions shown in Figure 8.3.
Transaction Ti Time Tansaction Tj
— —
read(v) t
1 —
— —
— t
2 read(v)
— —
update(v) t
3 —
— —
— t
4 update(v)
—
The lost update problem.
Transactions and Concurrency Control 351
At time t1 and t2, transactions ti and tj reads variable v respectively and get some value of v. At time t3, ti updates v but, at t4, tj also updates v (old value of v) without looking
the new value of v (updated by ti
). So, updation made by ti
at t3 is lost at t4 because Tj
overwrites it.

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