Detailed Explanation of MySQL Database Transaction Isolation Levels: From Read Uncommitted to Serializable

# MySQL Database Transaction Isolation Levels Explained: From Read Uncommitted to Serializable

# 1. Transaction Basics**

A transaction is a group of operations in a database that either all succeed or all fail together. The purpose of a transaction is to ensure the integrity and consistency of the database data. A transaction consists of the following four properties:

- **Atomicity:** All operations within a transaction either fully succeed or fully fail.
- **Consistency:** After a transaction is completed, the database must be in a consistent state, meaning all business rules are satisfied.
- **Isolation:** Transactions are isolated from other concurrent transactions, meaning one transaction's modifications to the database will not affect others.
- **Durability:** Once a transaction is committed, modifications to the database are permanently saved, even in the event of system failures.

# 2. Transaction Isolation Levels

Transaction isolation levels are mechanisms employed by Database Management Systems (DBMS) to ensure data consistency when transactions are executed concurrently. Different isolation levels provide various guarantees of data consistency, ranging from the lowest level, Read Uncommitted, to the highest level, Serializable.

### 2.1 Read Uncommitted

**Definition:**

Read Uncommitted allows a transaction to read data that has been written by other uncommitted transactions.

**Characteristics:**

***Lowest data consistency:** Transactions may read inconsistent or incomplete data.
***Highest transaction concurrency:** Transactions are not blocked by other transactions, which increases concurrency.

**Example:**

-- Transaction A
BEGIN TRANSACTION;
UPDATE accounts SET balance = balance + 100 WHERE id = 1;
-- Transaction B
SELECT balance FROM accounts WHERE id = 1;
COMMIT;
-- Transaction A
COMMIT;

In this example, Transaction B may read updates from Transaction A that have not yet been committed, leading to inconsistent data being read.

### 2.2 Read Committed

**Definition:**

Read Committed allows a transaction to read data that has been written by other committed transactions.

**Characteristics:**

***Higher data consistency than Read Uncommitted:** Transactions will not read inconsistent or incomplete data.
***Slightly lower concurrency than Read Uncommitted:** Transactions may be blocked by other transactions, reducing concurrency.

**Example:**

-- Transaction A
BEGIN TRANSACTION;
UPDATE accounts SET balance = balance + 100 WHERE id = 1;
-- Transaction B
SELECT balance FROM accounts WHERE id = 1;
-- Transaction A
COMMIT;

In this example, Transaction B can only read data after Transaction A has committed, ensuring consistent data is read.

### 2.3 Repeatable Read

**Definition:**

Repeatable Read guarantees that the data read during a transaction's execution will not be modified by other transactions.

**Characteristics:**

***Higher data consistency than Read Committed:** A transaction always reads the same data, even if other transactions modify the data.
***Lower concurrency than Read Committed:** Transactions may be blocked by other transactions, further reducing concurrency.

**Example:**

-- Transaction A
BEGIN TRANSACTION;
SELECT balance FROM accounts WHERE id = 1;
-- Transaction B
UPDATE accounts SET balance = balance + 100 WHERE id = 1;
-- Transaction A
SELECT balance FROM accounts WHERE id = 1;
COMMIT;
-- Transaction B
COMMIT;

In this example, Transaction A reads the data twice, and Transaction B modifies the data in between. However, Transaction A still reads the data from the first read because the Repeatable Read level ensures data consistency.

### 2.4 Serializable

**Definition:**

Serializable is the highest isolation level, ensuring that the execution of a transaction is as if no other transactions are concurrently executing.

**Characteristics:**

***Highest data consistency:** Transactions are not affected by other transactions, ensuring data consistency and integrity.
***Lowest concurrency:** Transactions may be heavily blocked by other transactions, significantly reducing concurrency.

**Example:**

-- Transaction A
BEGIN TRANSACTION;
UPDATE accounts SET balance = balance + 100 WHERE id = 1;
-- Transaction B
UPDATE accounts SET balance = balance - 50 WHERE id = 1;
-- Transaction A
COMMIT;
-- Transaction B
COMMIT;

In this example, Transaction A and Transaction B both modify the same row. Due to the Serializable level, Transaction A will block Transaction B until Transaction A commits. This ensures sequential execution of transactions, thereby ensuring data consistency.

# 3.1 Examples of Isolation Levels

**Read Uncommitted**

***Example:**

-- Transaction A
BEGIN TRANSACTION;
SELECT * FROM accounts WHERE account_id = 1;
UPDATE accounts SET balance = balance - 100 WHERE account_id = 1;
--