Transaction Boundary
The explicit scope within which state changes share one atomic commit/rollback decision and consistent transactional guarantees.
Where Atomicity Starts and Ends
A transaction boundary defines which operations commit or roll back together. Keeping that boundary explicit is essential when application code calls databases, message brokers or remote services.
A method annotation is not automatically the business boundary if work escapes to another thread, another resource manager or an external service.
Size Trade-Off
Very large transactions hold locks/resources longer and increase rollback cost. Very small transactions can expose intermediate states that the business rule considers invalid.
Distributed workflows may require Saga, Transactional Outbox or Two-Phase Commit depending on the required semantics.
The boundary should follow correctness requirements before performance tuning.