Context
Many operations perform multiple writes on a single database, all of which must be applied (or none of them) to ensure consistency. These writes should not be observable in an inconsistent state, and should not be lost as a result of a crash.
Example
A customer places an order to purchase multiple items. The server needs to store details about the order and reduce stock counts for each item. If the server crashes part way through, stock counts for some items could be reduced but not others.
Problem
How do we ensure the system does not get left in an inconsistent state when the system crashes after applying a subset of writes?
Solution
Perform all writes inside a transaction in an ACID database. Either all writes will succeed or none of them will. The transaction will be atomic and isolated, so no other transactions will see inconsistent states (depending on the isolation level).