~/notes/consistency

Consistency

A transaction must transform the database from one consistent state to another consistent state.

Nov 17, 2025

Consistency ensures that a database transaction brings the database from one valid state to another valid state, maintaining all defined rules and constraints.

  • Integrity constraints must be satisfied before and after the transaction (Eg: Foreign keys constraints)
  • Business rules encoded in the database are enforced
  • If a transaction violates consistency rules, it’s rolled back

Example

If you transfer £200 between bank accounts, consistency ensures:

diagram
NoYes

Valid State A
Balance: £1000, £500

Start Transaction
Transfer £200

Check Constraints

All Rules
Satisfied?

ROLLBACK

Execute Transaction

Valid State B
Balance: £800, £700

NoYes

Valid State A
Balance: £1000, £500

Start Transaction
Transfer £200

Check Constraints

All Rules
Satisfied?

ROLLBACK

Execute Transaction

Valid State B
Balance: £800, £700

Both accounts exist

  • The sender has sufficient funds
  • Total money in the system remains the same
  • All constraints (positive balance, valid account types) are maintained

Consistency vs Atomicity

Atomicity handles completeness of execution, Consistency handles correctness of the result.

Further Reading

Linked References

Database Transactions note
"...defines whether the transaction's writes take effect together. Consistency describes preservation of declared invariants before and after a transaction. -..."