transaction
termprogrammingcloud
A group of operations that either all take effect or none do, leaving no visible half-finished state.
The guarantee is what lets a system be reasoned about at all: money moves between two accounts without a moment where it exists in neither. It is also expensive, and distributed systems spend most of their design effort deciding how much of it to give up - which is why the interesting question about any storage system is what it promises across machines rather than within one.
Also known as: acid