What do isolation, readOnly, and timeout mean on @Transactional?

I verify the effective settings against the real database and driver because names do not guarantee identical behavior across vendors. Constraints, atomic conditional updates, optimistic versions, and explicit locks often protect an invariant more precisely than globally raising isolation. Under load I measure lock waits, deadlocks or serialization failures, pool occupancy, retries, and tail latency. A timeout produces a failure path that still needs rollback and outcome handling; it does not make an operation safely retryable by itself.