Can you explain the purpose and use of the REQUIRED key in the context of propagation?

In the context of transaction management in Java (for example, using the Spring Framework), propagation determines how a method should behave concerning an existing transaction.

The "REQUIRED" key means that the method should run within the current transaction if it exists. If a transaction is already active, the method joins it. If there is no transaction, a new one is created.

This behavior is convenient for ensuring data integrity when multiple methods are called sequentially and should be part of the same transaction.

Example of usage in Spring:

@Transactional(propagation = Propagation.REQUIRED)
public void updateData() {
    // code that will be executed within a transaction
}

Thus, REQUIRED guarantees that the method always executes within a transaction, creating a new one if necessary or using the existing one.