How it works

1. The browser posts the form to POST /m/currencies/save.
2. MasterController.save() asks MasterRegistry for the definition "currencies".
3. It loads the entity (or creates a new Currency) and binds the request with ServletRequestDataBinder - only the fields listed in the definition.
4. Bean Validation runs the annotations on Currency.java (@NotBlank, @Pattern ...).
5. The definition's validator runs the rules that need the database (CurrencyService.validateCurrency: only one base currency).
6. MasterService.save() (@Transactional) calls the hooks, then repository.saveAndFlush() → Hibernate sends INSERT / UPDATE ... WHERE row_version = ?.
7. It publishes MasterChangedEvent; MasterListeners writes the audit log and clears caches - in the same transaction.
8. A SQL Server constraint error (uq_currency_code) is turned into a friendly message by DbErrors.
9. Success → redirect to the list with a flash message (Post/Redirect/Get).

src/main/java/com/study/masterdata
├─ domain/     JPA entities (BaseEntity + 30 tables)
├─ repo/       Spring Data repositories
├─ master/     the generic part
│   ├─ FieldSpec, MasterDefinition   describe a master
│   ├─ MasterRegistry                all 25 definitions
│   ├─ MasterService                 CRUD + search (Specification)
│   └─ MasterListeners               audit, caches, config history
├─ feature/    business logic per feature (UnitService ...)
├─ tool/       "Try it" panels (strategy pattern)
└─ web/        MVC controllers, REST API, view helpers
src/main/resources
├─ db/migration/V1__schema.sql      tables (Flyway)
├─ db/migration/V2__seed_data.sql   starter data
└─ templates/  Thymeleaf pages + tools/*.html

• Soft delete vs hard delete - ⏸ sets active = 0; 🗑 really deletes and fails while a foreign key still points at the row.
• Optimistic locking - open the same record in two tabs, save both: the second one is refused (@Version row_version).
• Pessimistic locking - running numbers lock the counter row (UPDLOCK) so 20 parallel requests get 20 numbers.
• Effective dating - tax rates and exchange rates are looked up "as of" a date.
• Caching - @Cacheable on lookups, parameters and config, cleared by an event listener.
• Self-reference & cascade - categories (tree) and province → district → subdistrict.
• Flyway - the schema is versioned SQL; Hibernate only validates it (ddl-auto=validate).

1. Create V3__department.sql with a table that has the 4 audit columns (copy one from V1).
2. Create Department extends BaseEntity with Bean Validation annotations and label().
3. Create DepartmentRepository extends MasterRepository<Department>.
4. Add one MasterDefinition.of("departments", ...) in MasterRegistry.
5. Restart: list, form, search, CSV, audit log and /api/masters/departments all work.