MySQL Transaction Properties and Isolation Levels

MySQL's transaction properties and isolation levels help ensure data reliability and consistency within database operations.

The Four Properties of Transactions (ACID)

1. Atomicity
   A transaction is an indivisible unit of operations. All operations within a transaction either complete successfully or are entirely rolled back. If one operation fails, all previous operations in the transaction are undone, ensuring data consistency.

2. Consistency
   Before and after a transaction, the database must be in a consistent state, adhering to all data integrity constraints (e.g., primary keys, foreign keys).

3. Isolation
   Transactions are executed in isolation from each other, meaning the operations of one transaction are not visible to other transactions until the transaction is complete. This property prevents issues like dirty reads.

4. Durability
   Once a transaction is committed, its results are permanently saved, even in the event of a system failure. This ensures that data remains intact and recoverable.

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Transaction Isolation Levels in MySQL

MySQL provides four isolation levels to control how and when changes made by one transaction become visible to other transactions. Each level offers a different balance between data integrity and system performance.

1. READ UNCOMMITTED

   • Allows transactions to read uncommitted changes from other transactions.
   • Susceptible to dirty reads, non-repeatable reads, and phantom reads.

2. READ COMMITTED

   • Only committed changes from other transactions can be read.
   • Prevents dirty reads but allows non-repeatable reads and phantom reads.

3. REPEATABLE READ (default in MySQL)

   • Ensures that all reads within a transaction are consistent, preventing dirty reads and non-repeatable reads.
   • Susceptible to phantom reads but uses gap locking to reduce this risk.

4. SERIALIZABLE

   • The strictest level, enforcing complete isolation by locking rows for each read operation.
   • Prevents dirty reads, non-repeatable reads, and phantom reads, but can lead to higher latency and locking contention.

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Common Issues Addressed by Isolation Levels

• Dirty Read: A transaction reads data that is not yet committed by another transaction.
• Non-Repeatable Read: A transaction reads the same row twice and gets different data because another transaction modified it in between reads.
• Phantom Read: A transaction reads a set of rows that match a condition but finds additional rows added by another transaction in a subsequent read.

Each isolation level mitigates specific issues and may impact database performance, so selecting the appropriate level depends on the application's consistency and performance requirements.