ACID properties with an example

Model answer. The transfer is two updates: debit A, credit B.

• Atomicity: both updates happen or neither does; if the credit fails, the debit is rolled back.
• Consistency: the transaction moves the database from one valid state to another (e.g. balances never go negative if that is a rule; total money is unchanged).
• Isolation: concurrent transactions do not see each other's half-done work; another transfer reading A meanwhile sees either the old or the new balance, depending on the isolation level.
• Durability: once committed, the transfer survives a crash (write-ahead log flushed to disk).

Follow-ups: isolation levels (read committed, repeatable read, serializable) and anomalies such as dirty reads and lost updates.