transactional-patterns

# Transactional Patterns ## Basic Rules - `@Transactional` belongs on **service methods**, never controllers or repositories - Default propagation is `REQUIRED` — joins existing transaction or creates one - Always use on methods that write to the DB or coordinate multiple writes - `@Transactional(readOnly = true)` on all read-only service methods — enables optimizations ```java @Service @RequiredArgsConstructor @Transactional(readOnly = true) // default for all methods in this service public class OrderService { @Transactional // overrides readOnly for writes public Order createOrder(CreateOrderRequest request) { inventoryService.reserve(request.items()); // participates in same TX return orderRepository.save(Order.from(request)); } public Optional<Order> findById(UUID id) { return orderRepository.findById(id); // readOnly = true inherited } } ``` ## Propagation | Propagation | Behavior | |-------------|----------| | `REQUIRED` (default) | Join existing TX or create new | | `REQUIRES_NEW` | Always create new TX, suspend existing | | `SUPPORTS` | Join if exists, proceed without TX if not | | `NOT_SUPPORTED` | Always run without TX | | `MANDATORY` | Must have existing TX, throw if not | | `NEVER` | Must NOT have TX, throw if one exists | ```java // REQUIRES_NEW — for audit logging that must survive rollback @Transactional(propagation = Propagation.REQUIRES_NEW) public void logAuditEvent(AuditEvent event) { auditRepository....