Can you explain the concept of propagation level in Spring Data and its role in transaction management?

In Spring Data (and in the Spring Framework in general), the propagation level determines how a transaction will be propagated when calling methods that may also be transactional.

Main propagation levels:

• REQUIRED (default): if there is an existing transaction, the method will run within it; if not, a new one will be created.
• REQUIRES_NEW: always creates a new transaction, suspending the current one if it exists.
• SUPPORTS: if a transaction exists, use it; if not, execute without a transaction.
• NOT_SUPPORTED: always execute without a transaction, suspending the current one if it exists.
• MANDATORY: the method must be called within an existing transaction; otherwise, an exception is thrown.
• NEVER: the method must not be called within a transaction; otherwise, an exception is thrown.
• NESTED: if a transaction exists, create a nested (savepoint) transaction; if not, create a new one.

The role of propagation in transaction management is to control how methods interact with transactions, ensuring correct behavior during nested calls and preventing undesirable situations, such as one transaction interrupting another or operations needing to be atomic.

Example:

@Transactional(propagation = Propagation.REQUIRES_NEW)
public void saveAuditLog() {
    // this method will always run in a new transaction,
    // regardless of whether an outer transaction exists
}