[felipesoc]

That's not true. I've worked for a company that makes banking software and we did tons of migrations. The company itself changed its own software from RPG in as400 to windows forms and now it's all web with servers in Java and .Net (they use a middleware language which generates in every new platform). They even made a transitional install for one branch of one of the most important banks. The branch used my former company's solution for a few years until they could use their own software, which had to be adapted for the new market.

Banks used to work on paper an did fine, software migration, although can take some years, is no problem for them.

[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.