How do @Transactional Isolation and Propagation Affect Your Spring Application?

Understanding @Transactional: Exploring Isolation and Propagation

The @Transactional annotation in Spring plays a crucial role in managing transactions within your application. It defines two essential parameters: isolation and propagation.

Propagation

Propagation determines how a transaction interacts with existing transactions. Key options include:

• REQUIRED: The code executes within an existing transaction, creating a new one only if none exists.
• REQUIRES_NEW: A new transaction is always created, suspending an existing one if present.

The default value is REQUIRED, which is suitable in most situations.

Isolation

Isolation defines the data visibility rules between transactions. Several levels are available:

• ISOLATION_READ_UNCOMMITTED: Allows "dirty reads" where changes made in uncommitted transactions are visible.
• ISOLATION_READ_COMMITTED: Prevents dirty reads but may allow "non-repeatable reads," where values from multiple reads differ.
• ISOLATION_REPEATABLE_READ: Ensures consistency when reading the same row twice, preventing non-repeatable reads.
• ISOLATION_SERIALIZABLE: Serializes transactions, executing them one after the other to guarantee atomicity.

The optimal isolation level depends on your application's specific needs.

When to Adjust Default Values

Consider modifying the default values when:

• You want to prevent specific data access issues, such as dirty reads.
• You need to enforce absolute consistency for data retrievals.
• You encounter performance issues due to high transaction contention.

Example: Isolation Levels and Dirty Reads

A dirty read occurs when thread 1 writes a value (x) and thread 2 reads (x) before it's committed. If thread 1 rolls back its changes, thread 2 now holds an incorrect value.

To prevent dirty reads, you can set isolation to ISOLATION_READ_COMMITTED or ISOLATION_REPEATABLE_READ. This ensures that thread 2 only reads committed values or consistent snapshots.

Code Example: Transaction Propagation

Consider the following code snippet:

<code class="java">@Transactional(propagation=Propagation.REQUIRES_NEW)
public void provideService() {
    // Code that requires a new transaction
}</code>

With propagation set to REQUIRES_NEW, a new transaction is always created when entering provideService() and committed upon leaving, regardless of the surrounding transaction context.

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