NOTE
Spring Transaction Propagation
REQUIRED, REQUIRES_NEW, NESTED and other propagation modes as policies for existing versus new transaction contexts.
This is a historical learning note and may contain outdated or incomplete understanding.
Transaction propagation defines what a transactional method does when a transaction already exists.
REQUIRED joins an existing transaction or creates one. REQUIRES_NEW suspends the current one and creates an independent transaction, consuming another resource/connection as required. NESTED typically relies on savepoint-style capabilities and is not identical to an independent transaction.
Other modes express mandatory/no-transaction/support semantics.
Propagation is local transaction-context policy; it does not create a distributed transaction across unrelated services/datastores.