> you can't accept a withdrawal if the user doesn't have enough funds. You can. ATMs around the country aren't doing account-level locking. If you overdraw,

> you can't accept a withdrawal if the user doesn't have enough funds. You can. ATMs around the country aren't doing account-level locking. If you overdraw, you overdraw. If you store the ledger operations (reliably! no half-writing them and then throwing an exception!), then your system knows what happened. Having a ledger which you're confident about is more important than trying to use exceptions to prevent 'bad' ledger messages being written.