We want to make a bank server. The bank server models the behaviour of a real bank. You can deposit money with the bank, query the status of your account and take money out of your account.

Banks, being suspicious by nature do not let you take out more money than you have in your account.

Banks are worried about security and like to offer round-the-clock services. If one of their computer fails they still like to serve their customers. In the event of a failure they like everything to work as if the failure had not occurred. In particular even of a failure they do not want you to remove more money from you account than you have deposited ...