Not every command is able to completely execute in a single ACID transaction. A very common example that pops up quite often as an argument for transactions is the money transfer. It is often believed that an atomic and consistent transaction is absolutely required to transfer money from one account to another. Well, it's not. On the contrary, it is quite impossible to do. What if money is transferred from an account on Bank A, to another account on Bank B? Does Bank A acquire a lock in Bank B's database? If the transfer is in progress, is it strange that Bank A has deducted the amount, but Bank B hasn't deposited it yet? Not really, it's "underway". On the other hand, if something goes wrong while depositing the money on Bank B's account, Bank A's customer would want his money back. So we do expect some form of consistency, ultimately.