Lost Update Understanding
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.