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Atomicity is a feature of databases systems dictating where a transaction must be all-or-nothing. That is, the transaction must either fully happen, or not happen at all. It must not complete partially.
Atomicity is part of the ACID model (Atomicity, Consistency, Isolation, Durability), which is a set of principles used to guarantee the reliability of database transactions. Atomicity is usually achieved by complex mechanisms such as journaling or logging, or via operating-system calls.
The definition of what constitutes an atomic transaction is decided by its context or the environment in which it being implemented. For example, in an online airline-booking system, a booking may consist of 2 separate actions that together form a transaction -- paying for the seat, and reserving the seat for the customer who’s just paid. Business logic dictates that these two, though distinct and separate actions, must occur together. If one happens without the other, problems can occur. For example, the system may reserve the same seat for two separate customers.
It is essential that a database system that claims to offer atomicity be able to do so even in the face of failure in power supply or the underlying operating system or application that uses the database.