What is ACID? What is a constraint in ACID terminology?

ACID is a set of properties of transactions in databases that guarantee correctness and reliability of operations:

• Atomicity: the transaction is completed in full or not at all.
• Consistency: after the transaction, the database remains in a correct state, satisfying all constraints.
• Isolation: concurrent transactions do not affect each other, resulting in the same outcome as if they were executed sequentially.
• Durability: after a transaction is committed, its changes are saved even in case of failures.

The term constraint in the context of ACID refers to rules that ensure data consistency. These can be:

• Integrity constraints (e.g., uniqueness, foreign keys).
• Business rules implemented at the database level.

A constraint guarantees that data does not violate specified rules, and a transaction that violates a constraint will be rolled back, maintaining the consistency of the database.