Concurrency problems:

Locking is intended to eradicate three forms of inconsistency due to concurrency.

Lost Updates- If transaction T1 updates a record before updated by transaction T2 then undoing T2 will also undo the update of T1 (that is if transaction T1 updates record R from 100 to 101 as well as then transaction T2 updates A from 101 to 151 then backing out T1 will set A back to the original value of 100 losing the update of T2.) This is described a Write ->Write dependency.

Dirty Read: If transaction T1 updates a record that is read by T2 afterwards if T1 aborts T2 will have read a record that never existed. (that is T1 updates R to 100,000,000, T2 reads this value T1 then aborts and the record returns to the value 100.) This is described a Write ->Read dependency.

Un-repeatable Read: If transaction T1 reads a record that is afterwards altered and committed by T2 and if T1 re-reads the record then T1 will see two different committed values for the sane record. Such a dependency is described a Read ->Write dependency.

If there were no concurrency afterwards none of these anomalous cases will arise.

Note that the order in which reads take place does not affect concurrency.

In particular reads commute that is why we don’t care about Read -> Read dependencies

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