Transaction Propagation

Propagation controls what happens when one @Transactional method calls another -- REQUIRED joins or creates, REQUIRES_NEW always starts fresh, MANDATORY insists on an existing transaction, and NESTED creates a savepoint.

REQUIRES_NEW holds two connections per thread because it suspends the outer transaction's connection and checks out a new one -- if the pool is smaller than the concurrent thread count, the pool deadlocks and the application freezes.

An AB-BA deadlock is a circular wait at the row level where two transactions lock the same rows in opposite order -- the fix is consistent lock ordering, enforced by application code or by Hibernate's order_inserts and order_updates settings.

Never hold a database connection while waiting on a network call -- REQUIRES_NEW plus synchronous REST calls waste pool connections during network latency and create a distributed monolith disguised as microservices.