What are Concurrency Conflicts?

concurrency

Solution

Assume you have an SQL statement, e.g., `UPDATE table SET a = a + 1 WHERE ...`, which would correspond to the following code:

read a
a = a + 1
write a

Assume that two clients A and B execute this simultaneously. The following could happen (time flows from top to bottom):

   A           B
read a     
            read a
a = a + 1
write a
            a = a + 1
            write a

What happens? `a` is incremented only once, although it should have been incremented twice. This is a classical concurrency conflict. To avoid such conflicts, databases use transactions and locks.

Problem

Original source