ACID Transaction Properties

The ACID guarantees

ACID is an acronym for the four properties that a DBMS guarantees for transactions:

Property	Meaning
Atomicity	All changes are performed as a single operation, or none are
Consistency	A transaction transforms the database from one consistent state to another
Isolation	Concurrent transactions do not see each other’s intermediate state
Durability	Once committed, changes survive system failures

Atomicity

All changes to data are performed as if they are a single operation. All the changes are performed, or none of them are.

Example: in a funds transfer, if a debit is made from one account, the corresponding credit must be made to the other account. If the system fails after the debit but before the credit, atomicity ensures that the debit is rolled back. Neither half-effect survives.

Consistency

Every transaction applied to a consistent database leaves it in a consistent state. The database has integrity constraints (declared in the schema), and a valid transaction must preserve them.

Example: conservation of money. The total value of funds held over all accounts remains constant. A transfer moves money but does not create or destroy it. If the database starts with $\\pounds 1000$ across all accounts, it still has $\\pounds 1000$ after any valid transaction.

Isolation

The intermediate state of a transaction is invisible to other transactions. As a result, transactions that run concurrently appear to be serialised. Each transaction behaves as if it ran alone on the database.

Example: another concurrent transaction sees the transferred funds in one account or the other, but not in both, nor in neither. It never observes the moment when money has left the source account but not yet arrived at the destination.

Durability

After a transaction successfully completes (commits), changes to data persist and are not undone, even in the event of a system failure. The DBMS typically achieves this through write-ahead logging: changes are written to a persistent log before being applied to the database files.

Implementation note

Implementing ACID transactions is lectured in Part IB Concurrent and Distributed Systems. The mechanisms (locking, logging, two-phase commit) are not part of this course.

Summary

Property	Key guarantee
Atomicity	All or nothing
Consistency	Valid state preserved
Isolation	Non-interference between concurrent transactions
Durability	Survives crashes