Lost Update Understanding

https://habr.com/en/company/postgrespro/blog/467437/ gives the following example of a Lost Update:


For example, two transactions are going to increase the amount on the same account by ₽100 (₽ is the currency sign for Russian rouble). The first transaction reads the current value (₽1000) and then the second transaction reads the same value. The first transaction increases the amount (this gives ₽1100) and writes this value. The second transaction acts the same way: it gets the same ₽1100 and writes this value. As a result, the customer lost ₽100

I read this a few times. But I don’t understand how the customer lost P100. Please explain.

There are two separate transactions (T1 and T2) that each add ₽100 to the customer's balance.

The intended outcome is:

• T1 reads the current balance as ₽1000, adds ₽100, and writes ₽1100

• T2 reads the current balance as ₽1100, adds ₽100, and writes ₽1200

Or the other way around (T2 then T1). The important point is that both increments of ₽100 are applied.

In the example of a lost update, something like the following occurs:

• T1 reads the current balance as ₽1000

• T2 reads the current balance as ₽1000

• T2 adds ₽100 (to the ₽1000 it read), and writes ₽1100

• T1 adds ₽100 (to the ₽1000 it read), and writes ₽1100

This way the final balance is ₽1100, not ₽1200, so the customer has lost ₽100.

StackExchange Database Administrators Q#249351, answer score: 14