Amr Samir | Database Transactions & ACID Isolation Levels

1. What Are ACID Guarantees?

Relational databases provide ACID transaction guarantees to maintain data integrity across concurrent client operations:

Atomicity: All operations in a transaction succeed, or all roll back.

Consistency: Database transitions only between valid states conforming to all schema constraints.

Isolation: Concurrent transactions execute without interfering with one another.

Durability: Committed data is permanently written to non-volatile storage (WAL).

2. Concurrency Anomalies & The 4 SQL Isolation Levels

code

+------------------+------------+----------------------+--------------+
| Isolation Level  | Dirty Read | Non-Repeatable Read  | Phantom Read |
+------------------+------------+----------------------+--------------+
| Read Uncommitted | YES        | YES                  | YES          |
| Read Committed   | NO         | YES                  | YES          |
| Repeatable Read  | NO         | NO                   | YES (No in PG|
| Serializable     | NO         | NO                   | NO           |
+------------------+------------+----------------------+--------------+

Dirty Read: Reading uncommitted changes made by another active transaction.

Non-Repeatable Read: Re-reading the same row within a transaction and observing modified values committed by another transaction.

Phantom Read: Re-running a range query and observing newly inserted rows that match the query condition.

3. How Multi-Version Concurrency Control (MVCC) Works

PostgreSQL implements MVCC so that readers never block writers, and writers never block readers:

Each row contains hidden metadata headers (xmin, xmax).

When a row is updated, PostgreSQL inserts a new version of the row with a higher transaction ID rather than overwriting in place.

Queries view a consistent snapshot corresponding to their transaction start time.

code

[ Row Version 1 (xmin: 100, xmax: 105) ] ---> Visible to transactions < 105
[ Row Version 2 (xmin: 105, xmax: 0)   ] ---> Visible to transactions >= 105

Database Transactions & Isolation Levels: Dirty Reads, Phantoms & MVCC Explained

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Database Transactions & Isolation Levels: Dirty Reads, Phantoms & MVCC Explained

1. What Are ACID Guarantees?

2. Concurrency Anomalies & The 4 SQL Isolation Levels

3. How Multi-Version Concurrency Control (MVCC) Works

4. Practical Takeaways