I've built generative tests for an entire betting exchange. We run the database in-memory, throw generated events in one end and make assertions about the state

I've built generative tests for an entire betting exchange. We run the database in-memory, throw generated events in one end and make assertions about the state when processing is finished. The tests range from very specific, narrow pseudo-unit-tests to coarse properties like 'if we process 1000 random orders from 80 different customers in 3 different currencies, the total amount of money in the system remains constant'. I can't imagine reaching the same level of stability in the core exchange if we had to write all the test cases by hand.