Yeah, and in my experience especially for anything that is extremely time-critical + has the potential to generate huge in/direct losses. For example at the en

Yeah, and in my experience especially for anything that is extremely time-critical + has the potential to generate huge in/direct losses. For example at the end of each day a bank has to generate the final balances of the accounts that it has with other banks and then it has to check for irregularities and then finally set those balances just right (e.g. enough to cover payments made by its customers but without leaving there millions) => if it doesn't then it might not be able to use the money that is "parked" there by mistake, or it might have a huge position with a risky bank, or if the balance is too low other banks might decide not to perform the final credit into the target accounts involved in the customers' payments (which would then generate a lot of complaints + lower the bank's reputation with its long-term consequences), etc... . (the opposite happens as well - https://www.investopedia.com/ask/answers/051815/what-difference-between-nostro-and-vostro-account.asp https://www.investopedia.com/ask/answers/051815/what-differe... ) Nowadays even accounting (coupled with risk management) has become time-critical because of the many regulations - a 1-day delay filing the numbers with regulators and/or central banks can result in huge fines and loss of reputation (if the news about the problem becomes public it could happen that the news services try to highlight it increasing the focus on that negative publicity). In our case the detailed/low-level data (e.g. client X bought N shares of some company, client Y retrieved N$ from the ATM, etc) is all processed by the mainframe, which works fine 99.99999% of the time => then anything that comes afterwards which involves analysis/aggregation/reporting/etc of that data happens in other non-mainframe apps (they're all very different - from tiny to huge apps with distributed databases) and in general any such app has usually some kind of problem that results in a delay at least once every quarter due to the SW itself or even the HW => it's usually not a big problem (usually some hours are lost but there is a buffer) but if the central SW (running on the mainframe) would have the same problem then there would be a chain-reaction on all the other apps that need that data (and then their specific problems would be on top of that, and then there would be a hotspot of needed 100% CPU/RAM/network resources as they would all run at the same time as fast as possible which could cause further delays, and so on).