Can you explain the concept of Propagation in the context of transactions?

In the context of transactions in Java (for example, when using the Spring Framework), the concept of Propagation defines how methods annotated with @Transactional interact with existing transactions.

Main types of Propagation:

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

Example:

@Transactional(propagation = Propagation.REQUIRED)
public void methodA() {
    // code
    methodB();
}

@Transactional(propagation = Propagation.REQUIRES_NEW)
public void methodB() {
    // code that runs in a separate transaction
}

Thus, Propagation allows flexible management of transaction behavior when calling methods, ensuring data integrity and consistency.