# Backend Expertise

## Hexagonal Architecture (Ports and Adapters)

Dependency direction flows from outer to inner layers. Reverse dependencies are prohibited.

```
adapter (external) → application (use cases) → domain (business logic)
```

Directory structure:

```
{domain-name}/
├── domain/                  # Domain layer (framework-independent)
│   ├── model/
│   │   └── aggregate/       # Aggregate roots, value objects
│   └── service/             # Domain services
├── application/             # Application layer (use cases)
│   ├── usecase/             # Orchestration
│   └── query/               # Query handlers
├── adapter/                 # Adapter layer (external connections)
│   ├── inbound/             # Input adapters
│   │   └── rest/            # REST Controller, Request/Response DTOs
│   └── outbound/            # Output adapters
│       └── persistence/     # Entity, Repository implementations
└── api/                     # Public interface (referenceable by other domains)
    └── events/              # Domain events
```

Layer responsibilities:

| Layer | Responsibility | May Depend On | Must Not Depend On |
|-------|---------------|---------------|-------------------|
| domain | Business logic, invariants | Standard library only | Frameworks, DB, external APIs |
| application | Use case orchestration | domain | Concrete adapter implementations |
| adapter/inbound | HTTP request handling, DTO conversion | application, domain | outbound adapter |
| adapter/outbound | DB persistence, external API calls | domain (interfaces) | application |

```kotlin
// CORRECT - Domain layer is framework-independent
data class Order(val orderId: String, val status: OrderStatus) {
    fun confirm(confirmedBy: String): OrderConfirmedEvent {
        require(status == OrderStatus.PENDING)
        return OrderConfirmedEvent(orderId, confirmedBy)
    }
}

// WRONG - Spring annotations in domain layer
@Entity
data class Order(
    @Id val orderId: String,
    @Enumerated(EnumType.STRING) val status: OrderStatus
) {
    fun confirm(confirmedBy: String) { ... }
}
```

| Criteria | Judgment |
|----------|----------|
| Framework dependencies in domain layer (@Entity, @Component, etc.) | REJECT |
| Controller directly referencing Repository | REJECT. Must go through UseCase layer |
| Outward dependencies from domain layer (DB, HTTP, etc.) | REJECT |
| Direct dependencies between adapters (inbound → outbound) | REJECT |

## API Layer Design (Controller)

Keep Controllers thin. Their only job: receive request → delegate to UseCase → return response.

```kotlin
// CORRECT - Thin Controller
@RestController
@RequestMapping("/api/orders")
class OrdersController(
    private val placeOrderUseCase: PlaceOrderUseCase,
    private val queryGateway: QueryGateway
) {
    // Command: state change
    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    fun post(@Valid @RequestBody request: OrderPostRequest): OrderPostResponse {
        val output = placeOrderUseCase.execute(request.toInput())
        return OrderPostResponse(output.orderId)
    }

    // Query: read
    @GetMapping("/{id}")
    fun get(@PathVariable id: String): ResponseEntity<OrderGetResponse> {
        val detail = queryGateway.query(FindOrderQuery(id), OrderDetail::class.java).join()
            ?: return ResponseEntity.notFound().build()
        return ResponseEntity.ok(OrderGetResponse.from(detail))
    }
}

// WRONG - Business logic in Controller
@PostMapping
fun post(@RequestBody request: OrderPostRequest): ResponseEntity<Any> {
    // Validation, stock check, calculation... should NOT be in Controller
    val stock = inventoryRepository.findByProductId(request.productId)
    if (stock.quantity < request.quantity) {
        return ResponseEntity.badRequest().body("Insufficient stock")
    }
    val total = request.quantity * request.unitPrice * 1.1  // Tax calculation
    orderRepository.save(OrderEntity(...))
    return ResponseEntity.ok(...)
}
```

### Request/Response DTO Design

Define Request and Response as separate types. Never expose domain models directly via API.

```kotlin
// Request: validation annotations + init block
data class OrderPostRequest(
    @field:NotBlank val customerId: String,
    @field:NotNull val items: List<OrderItemRequest>
) {
    init {
        require(items.isNotEmpty()) { "Order must contain at least one item" }
    }

    fun toInput() = PlaceOrderInput(customerId = customerId, items = items.map { it.toItem() })
}

// Response: factory method from() for conversion
data class OrderGetResponse(
    val orderId: String,
    val status: String,
    val customerName: String
) {
    companion object {
        fun from(detail: OrderDetail) = OrderGetResponse(
            orderId = detail.orderId,
            status = detail.status.name,
            customerName = detail.customerName
        )
    }
}
```

| Criteria | Judgment |
|----------|----------|
| Returning domain model directly as response | REJECT |
| Business logic in Request DTO | REJECT. Only validation is allowed |
| Domain logic (calculations, etc.) in Response DTO | REJECT |
| Same type for Request and Response | REJECT |

### RESTful Action Design

Express state transitions as verb sub-resources.

```
POST   /api/orders              → Create order
GET    /api/orders/{id}         → Get order
GET    /api/orders              → List orders
POST   /api/orders/{id}/approve → Approve (state transition)
POST   /api/orders/{id}/cancel  → Cancel (state transition)
```

| Criteria | Judgment |
|----------|----------|
| PUT/PATCH for domain operations (approve, cancel, etc.) | REJECT. Use POST + verb sub-resource |
| Single endpoint branching into multiple operations | REJECT. Separate endpoints per operation |
| DELETE for soft deletion | REJECT. Use POST + explicit operation like cancel |

## Validation Strategy

Validation has different roles at each layer. Do not centralize everything in one place.

| Layer | Responsibility | Mechanism | Example |
|-------|---------------|-----------|---------|
| API layer | Structural validation | `@NotBlank`, `init` block | Required fields, types, format |
| UseCase layer | Business rule verification | Read Model queries | Duplicate checks, precondition existence |
| Domain layer | State transition invariants | `require` | "Cannot approve unless PENDING" |

```kotlin
// API layer: "Is the input structurally correct?"
data class OrderPostRequest(
    @field:NotBlank val customerId: String,
    val from: LocalDateTime,
    val to: LocalDateTime
) {
    init {
        require(!to.isBefore(from)) { "End date must be on or after start date" }
    }
}

// UseCase layer: "Is this business-wise allowed?" (Read Model reference)
fun execute(input: PlaceOrderInput) {
    customerRepository.findById(input.customerId)
        ?: throw CustomerNotFoundException("Customer does not exist")
    validateNoOverlapping(input)  // Duplicate check
    commandGateway.send(buildCommand(input))
}

// Domain layer: "Is this operation allowed in current state?"
fun confirm(confirmedBy: String): OrderConfirmedEvent {
    require(status == OrderStatus.PENDING) { "Cannot confirm in current state" }
    return OrderConfirmedEvent(orderId, confirmedBy)
}
```

| Criteria | Judgment |
|----------|----------|
| Domain state transition rules in API layer | REJECT |
| Business rule verification in Controller | REJECT. Belongs in UseCase layer |
| Structural validation (@NotBlank, etc.) in domain | REJECT. Belongs in API layer |
| UseCase-level validation inside Aggregate | REJECT. Read Model queries belong in UseCase layer |

## Error Handling

### Exception Hierarchy Design

Domain exceptions are hierarchized using sealed classes. HTTP status code mapping is done at the Controller layer.

```kotlin
// Domain exceptions: sealed class ensures exhaustiveness
sealed class OrderException(message: String) : RuntimeException(message)
class OrderNotFoundException(message: String) : OrderException(message)
class InvalidOrderStateException(message: String) : OrderException(message)
class InsufficientStockException(message: String) : OrderException(message)

// Controller layer maps to HTTP status codes
@RestControllerAdvice
class OrderExceptionHandler {
    @ExceptionHandler(OrderNotFoundException::class)
    fun handleNotFound(e: OrderNotFoundException) =
        ResponseEntity.status(HttpStatus.NOT_FOUND).body(ErrorResponse(e.message))

    @ExceptionHandler(InvalidOrderStateException::class)
    fun handleInvalidState(e: InvalidOrderStateException) =
        ResponseEntity.status(HttpStatus.CONFLICT).body(ErrorResponse(e.message))

    @ExceptionHandler(InsufficientStockException::class)
    fun handleInsufficientStock(e: InsufficientStockException) =
        ResponseEntity.status(HttpStatus.UNPROCESSABLE_ENTITY).body(ErrorResponse(e.message))
}
```

| Criteria | Judgment |
|----------|----------|
| HTTP status codes in domain exceptions | REJECT. Domain must not know about HTTP |
| Throwing generic Exception or RuntimeException | REJECT. Use specific exception types |
| Empty try-catch blocks | REJECT |
| Controller swallowing exceptions and returning 200 | REJECT |

## Domain Model Design

### Immutable + require

Domain models are designed as `data class` (immutable), with invariants enforced via `init` blocks and `require`.

```kotlin
data class Order(
    val orderId: String,
    val status: OrderStatus = OrderStatus.PENDING
) {
    // Static factory method via companion object
    companion object {
        fun place(orderId: String, customerId: String): OrderPlacedEvent {
            require(customerId.isNotBlank()) { "Customer ID cannot be blank" }
            return OrderPlacedEvent(orderId, customerId)
        }
    }

    // Instance method for state transition → returns event
    fun confirm(confirmedBy: String): OrderConfirmedEvent {
        require(status == OrderStatus.PENDING) { "Cannot confirm in current state" }
        return OrderConfirmedEvent(orderId, confirmedBy, LocalDateTime.now())
    }

    // Immutable state update
    fun apply(event: OrderEvent): Order = when (event) {
        is OrderPlacedEvent -> Order(orderId = event.orderId)
        is OrderConfirmedEvent -> copy(status = OrderStatus.CONFIRMED)
        is OrderCancelledEvent -> copy(status = OrderStatus.CANCELLED)
    }
}
```

| Criteria | Judgment |
|----------|----------|
| `var` fields in domain model | REJECT. Use `copy()` for immutable updates |
| Factory without validation | REJECT. Enforce invariants with `require` |
| Domain model calling external services | REJECT. Pure functions only |
| Direct field mutation via setters | REJECT |

### Value Objects

Wrap primitive types (String, Int) with domain meaning.

```kotlin
// ID types: prevent mix-ups via type safety
data class OrderId(@get:JsonValue val value: String) {
    init { require(value.isNotBlank()) { "Order ID cannot be blank" } }
    override fun toString(): String = value
}

// Range types: enforce compound invariants
data class DateRange(val from: LocalDateTime, val to: LocalDateTime) {
    init { require(!to.isBefore(from)) { "End date must be on or after start date" } }
}

// Metadata types: ancillary information in event payloads
data class ApprovalInfo(val approvedBy: String, val approvalTime: LocalDateTime)
```

| Criteria | Judgment |
|----------|----------|
| Same-typed IDs that can be mixed up (orderId and customerId both String) | Consider wrapping in value objects |
| Same field combinations (from/to, etc.) appearing in multiple places | Extract to value object |
| Value object without init block | REJECT. Enforce invariants |

## Repository Pattern

Define interface in domain layer, implement in adapter/outbound.

```kotlin
// domain/: Interface (port)
interface OrderRepository {
    fun findById(orderId: String): Order?
    fun save(order: Order)
}

// adapter/outbound/persistence/: Implementation (adapter)
@Repository
class JpaOrderRepository(
    private val jpaRepository: OrderJpaRepository
) : OrderRepository {
    override fun findById(orderId: String): Order? {
        return jpaRepository.findById(orderId).orElse(null)?.toDomain()
    }
    override fun save(order: Order) {
        jpaRepository.save(OrderEntity.from(order))
    }
}
```

### Read Model Entity (JPA Entity)

Read Model JPA Entities are defined separately from domain models. `var` (mutable) fields are acceptable here.

```kotlin
@Entity
@Table(name = "orders")
data class OrderEntity(
    @Id val orderId: String,
    var customerId: String,
    @Enumerated(EnumType.STRING) var status: OrderStatus,
    var metadata: String? = null
)
```

| Criteria | Judgment |
|----------|----------|
| Domain model doubling as JPA Entity | REJECT. Separate them |
| Business logic in Entity | REJECT. Entity is data structure only |
| Repository implementation in domain layer | REJECT. Belongs in adapter/outbound |

## Authentication & Authorization Placement

Authentication and authorization are cross-cutting concerns handled at the appropriate layer.

| Concern | Placement | Mechanism |
|---------|-----------|-----------|
| Authentication (who) | Filter / Interceptor layer | JWT verification, session validation |
| Authorization (permissions) | Controller layer | `@PreAuthorize("hasRole('ADMIN')")` |
| Data access control (own data only) | UseCase layer | Verified as business rule |

```kotlin
// Controller layer: role-based authorization
@PostMapping("/{id}/approve")
@PreAuthorize("hasRole('FACILITY_ADMIN')")
fun approve(@PathVariable id: String, @RequestBody request: ApproveRequest) { ... }

// UseCase layer: data access control
fun execute(input: DeleteInput, currentUserId: String) {
    val entity = repository.findById(input.id)
        ?: throw NotFoundException("Not found")
    require(entity.ownerId == currentUserId) { "Cannot operate on another user's data" }
    // ...
}
```

| Criteria | Judgment |
|----------|----------|
| Authorization logic in UseCase or domain layer | REJECT. Belongs in Controller layer |
| Data access control in Controller | REJECT. Belongs in UseCase layer |
| Authentication processing inside Controller | REJECT. Belongs in Filter/Interceptor |

## Test Strategy

### Test Pyramid

```
        ┌─────────────┐
        │   E2E Test  │  ← Few: verify full API flow
        ├─────────────┤
        │ Integration │  ← Repository, Controller integration verification
        ├─────────────┤
        │  Unit Test  │  ← Many: independent tests for domain models, UseCases
        └─────────────┘
```

### Domain Model Testing

Domain models are framework-independent, enabling pure unit tests.

```kotlin
class OrderTest {
    // Helper: build aggregate in specific state
    private fun pendingOrder(): Order {
        val event = Order.place("order-1", "customer-1")
        return Order.from(event)
    }

    @Nested
    inner class Confirm {
        @Test
        fun `can confirm from PENDING state`() {
            val order = pendingOrder()
            val event = order.confirm("admin-1")
            assertEquals("order-1", event.orderId)
        }

        @Test
        fun `cannot confirm from CONFIRMED state`() {
            val order = pendingOrder().let { it.apply(it.confirm("admin-1")) }
            assertThrows<IllegalArgumentException> {
                order.confirm("admin-2")
            }
        }
    }
}
```

Testing rules:
- Build state transitions via helper methods (each test is independent)
- Group by operation using `@Nested`
- Test both happy path and error cases (invalid state transitions)
- Verify exception types with `assertThrows`

### UseCase Testing

Test UseCases with mocks. Inject external dependencies.

```kotlin
class PlaceOrderUseCaseTest {
    private val commandGateway = mockk<CommandGateway>()
    private val customerRepository = mockk<CustomerRepository>()
    private val useCase = PlaceOrderUseCase(commandGateway, customerRepository)

    @Test
    fun `throws error when customer does not exist`() {
        every { customerRepository.findById("unknown") } returns null

        assertThrows<CustomerNotFoundException> {
            useCase.execute(PlaceOrderInput(customerId = "unknown", items = listOf(...)))
        }
    }
}
```

| Criteria | Judgment |
|----------|----------|
| Using mocks for domain model tests | REJECT. Test domain purely |
| UseCase tests connecting to real DB | REJECT. Use mocks |
| Tests requiring framework startup | REJECT for unit tests |
| Missing error case tests for state transitions | REJECT |

## Anti-Pattern Detection

REJECT when these patterns are found:

| Anti-Pattern | Problem |
|--------------|---------|
| Smart Controller | Business logic concentrated in Controller |
| Anemic Domain Model | Domain model is just a data structure with setters/getters |
| God Service | All operations concentrated in a single Service class |
| Direct Repository Access | Controller directly referencing Repository |
| Domain Leakage | Domain logic leaking into adapter layer |
| Entity Reuse | JPA Entity reused as domain model |
| Swallowed Exceptions | Empty catch blocks |
| Magic Strings | Hardcoded status strings, etc. |