What does consistency mean?

Consistency is a transaction property meaning that before and after it runs, the data must be in a correct, non-contradictory state, one that follows the system's rules: constraints, schemas, validations, uniqueness, referential integrity, and business logic.

#What this guarantees

• a transaction can't save data that violates the rules (e.g. a unique index or a foreign key)
• the system moves from one consistent state to another consistent state
• the data can't be "broken" even by failures partway through the process (together with Atomicity)

#An example

There's a rule: an account balance can't go negative. If a transaction tries to withdraw more than the account holds, it won't be committed, because that would break consistency.

In SQL, this might be enforced by:

• CHECK constraints
• UNIQUE
• FOREIGN KEY
• triggers
• business-logic validators

#In one sentence

Consistency = "after a transaction, the data must stay logically correct and not violate the system's integrity rules".