[natermer]

Each bank is different. Some banks have shitty IT and other ones have dynamic IT that can adjust to fit changing realities.

The reality is that you and Eric are right.

Banks depend on techniques based on double book keeping accounting to reconcile accounts at end of day. Different data about transactions are stored in different places by different organizations and they compare books to make sure that balances are correct.

You cannot depend on every transaction to be recorded perfectly. You must have the ability to compare books and reconcile accounts. This is simply how the world works.

Trying to make every perfect and depending on storing data in a central place with the assumption that it's always going to be consistent is too much of a liability. It doesn't work because the systems required by modern financial systems are incredibly complex and availability during markets is the highest priority. You ARE going to have faults and you ARE going to have problems. The ability to take hits gracefully and give yourself time later on to fix stuff after the fact must be built into your systems.