Everything about Unrepeatable Reads totally explained
In
computer science, in the field of
databases,
Read-Write Conflict, also known as
unrepeatable reads, is a computational anomaly associated with interleaved execution of transactions.
Given a schedule D
»
In this example, T1 has read the original value of A, and is waiting for T2 to finish. T2 also reads the original value of A, overwrites A, and commits.
However, when T1 reads to A, it discovers two different versions of A, and T1 would be forced to
abort, because T1 wouldn't know what to do. This is an unrepeatable read.
Here is a real-life example. Suppose two people Ben and Kathryn went to Ticketmaster and wanted to order season tickets to see the
Toronto Raptors. There's only one season ticket left. Ben signs on to Ticketmaster first and finds one left, but it's $600! Ben has to think about it for a while. Kathryn signs on and finds one ticket left, orders it instantly because she's $600 to spend. Kathryn logs off. Finally Ben agrees to buy a ticket...... but to his surprise there are no more tickets!! Ben is angry and sues Ticketmaster.
This could never occur in a serial schedule.
Strict two-phase locking (Strict 2PL) prevents this conflict.
Further Information
Get more info on 'Unrepeatable Reads'.
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