Relational databases have provided the traditional properties of ACID: atomicity, consistency, isolation, and durability:
- Atomicity is self-explanatory and refers to the all-or-nothing nature of a set of operations.
- Consistency in ACID and consistency in the CAP theorem refer to different things. Consistency in ACID refers to the principle that the system must be left in a consistent state while processing transactions, it either reflects the state after successful completion of the transaction or must roll back to a state prior to the start of the transaction.
- Isolation refers to the interaction effects between transactions. Under what conditions is the state modified by one transaction visible to other active transactions in the system? It ranges from weak isolation levels, such as read-committed, and goes all the way to linearizable.
- Durability indicates that once a transaction has committed, the effects of the transaction remain despite events such as errors and crashes.
NoSQL databases vary widely in their support for these guarantees, with most of them not approaching the level of strong guarantees provided by relational databases (since these are hard to support in a distributed setting).