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Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

Database Transactions Deep Dive: ACID, Isolation Levels, Savepoints

A transaction is a unit of work that the database executes atomically. Transactions are the bedrock of data integrity in relational databases. Understanding them deeply separates competent engineers from great ones.

ACID in Practice

ACID stands for Atomicity, Consistency, Isolation, Durability. Each property maps to concrete database behavior.

Atomicity : All operations within a transaction succeed or none do. In PostgreSQL, atomicity is guaranteed by the write-ahead log (WAL). If the server crashes mid-transaction, the WAL replay applies only committed transactions and discards partial ones.

BEGIN;

UPDATE accounts SET balance = balance - 100 WHERE id = 1;

UPDATE accounts SET balance = balance + 100 WHERE id = 2;

COMMIT; -- Both succeed, or neither does

Consistency : A transaction brings the database from one valid state to another. Constraints, triggers, and foreign keys enforce consistency rules automatically.

ALTER TABLE accounts ADD CONSTRAINT positive_balance

CHECK (balance >= 0);

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\-- Any transaction that would violate this constraint is rolled back

Isolation : Concurrent transactions should not interfere. PostgreSQL implements four isolation levels to control how much concurrency is allowed.

Durability : Once COMMIT returns, the data is safe. PostgreSQL writes transaction commit records to the WAL and forces a fsync() before returning success.

Isolation Levels

SQL standard defines four isolation levels. PostgreSQL implements three (it does not expose the dirty-read behavior of READ UNCOMMITTED; it maps it to READ COMMITTED).

Read Committed (Default)

Each statement in the transaction sees a snapshot of rows committed before the statement began. Two SELECT statements in the same transaction can see different data.

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\-- Session 1

BEGIN;

SELECT balance FROM accounts WHERE id = 1; -- returns 1000

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\-- Session 2 concurrently:

UPDATE accounts SET balance = 500 WHERE id = 1; COMMIT;

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\-- Session 1 continues:

SELECT balance FROM accounts WHERE id = 1; -- now returns 500!

COMMIT;

Repeatable Read

The transaction sees a snapshot taken when the first query or data-modification statement executes. Subsequent reads return the same data, even if concurrent transactions commit changes.

BEGIN ISOLATION LEVEL REPEATABLE READ;

SELECT balance FROM accounts WHERE id = 1; -- returns 1000

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\-- Session 2 updates and commits

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\-- Session 1:

SELECT balance FROM accounts WHERE i

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