The problem

A payroll product stores salaries for 2 000 companies. In March, an employee at one of them finds that her salary dropped by 300 euros, and nobody in HR admits to the change.

Her company asks the vendor: who changed this field, when, and from which account? They also ask how the vendor can show that nobody edited the answer afterwards, including the vendor's own database admins.

The contract with each customer says the vendor keeps that history for 7 years.