Discussion
Consider the following transaction involving two bank accounts x and y.
read(x); x := x - 50; write(x); read(y); y := y + 50; write(y)
The constraint that the sum of the accounts x and y should remain constant is that of
Answer: B
The constraint is that of Consistency.
Consistency means a transaction takes the database from one valid state to another, preserving its integrity constraints. Keeping the sum of x and y unchanged after moving 50 from one account to the other is exactly such a constraint.
Atomicity ensures both writes happen or neither does, isolation keeps concurrent transactions from seeing each other's intermediate states, and durability keeps committed results after a crash. None of them is the rule about the sum itself.
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