Testing in Production? Whatttt!

We have all done it, atleast in some point in time of our career. But it was a awkward when we think about testing in production in a financial system, As it involves a lot of side effects to the existing data, credit, debit, available balance, allowed credit balance.

If we are releasing a new feature how do we know that its working always. If you get bombarded with calls from clients or see a number of texts from clients just after feature release/ feeling anxious afterwards.

Think end to end for entire lifecycle of code.