Kotlin Patterns for Backend Services

Entity Pattern

data class EntityName( val id: UUID, val name: String, val createdAt: Instant, val updatedAt: Instant? )

Service Pattern

@Service class EntityNameService( private val repository: EntityNameRepository, private val relatedService: RelatedService ) { @Transactional(propagation = Propagation.NEVER) fun create(request: CreateRequest): Pair<EntityResponse, Boolean> { // Check exists, validate, create // Return Pair for idempotent operations }

fun findById(id: UUID): EntityName? =
    repository.findById(id)

fun findAll(): List<EntityName> =
    repository.findAll()

}

Repository Pattern (JOOQ)

@Repository class EntityNameRepository( private val dsl: DSLContext ) { fun findById(id: UUID): EntityName? = dsl.selectFrom(ENTITY_NAME) .where(ENTITY_NAME.ID.eq(id)) .fetchOne() ?.toEntity()

fun findAll(): List<EntityName> =
    dsl.selectFrom(ENTITY_NAME)
        .fetch()
        .map { it.toEntity() }

fun save(entity: EntityName): EntityName =
    dsl.insertInto(ENTITY_NAME)
        .set(ENTITY_NAME.ID, entity.id)
        .set(ENTITY_NAME.NAME, entity.name)
        .set(ENTITY_NAME.CREATED_AT, entity.createdAt)
        .returning()
        .fetchOne()!!
        .toEntity()

private fun EntityNameRecord.toEntity() = EntityName(
    id = id,
    name = name,
    createdAt = createdAt,
    updatedAt = updatedAt
)

}

Controller Pattern

@RestController class EntityNameController( private val service: EntityNameService ) : EntityNameApi {

override fun create(request: CreateRequest): ResponseEntity<EntityResponse> {
    val (result, isNew) = service.create(request)
    return if (isNew) ResponseEntity.status(201).body(result)
    else ResponseEntity.ok(result)
}

override fun getById(id: UUID): ResponseEntity<EntityResponse> {
    val entity = service.findById(id)
        ?: throw ResourceNotFoundRestException("EntityName", id)
    return ResponseEntity.ok(entity.toResponse())
}

}

API Interface Pattern

@Tag(name = "Entity Name") interface EntityNameApi {

@Operation(summary = "Create entity")
@PostMapping("/api/v1/entities")
fun create(@RequestBody @Valid request: CreateRequest): ResponseEntity<EntityResponse>

@Operation(summary = "Get entity by ID")
@GetMapping("/api/v1/entities/{id}")
fun getById(@PathVariable id: UUID): ResponseEntity<EntityResponse>

}

DTO Pattern

data class CreateRequest( @field:NotBlank val name: String,

@field:Size(max = 255)
val description: String?

)

data class EntityResponse( val id: UUID, val name: String, val description: String?, val createdAt: Instant )

Exception Pattern

// Use typed exceptions throw ResourceNotFoundRestException("EntityName", id) throw ValidationRestException("Name cannot be empty") throw ConflictRestException("Entity already exists")

Null Safety Guidelines

• Use ?.let{} for optional operations

• Use when for exhaustive matching

• Instead of not-null assertion, use .single() or .firstOrNull()

• Return Pair<Result, Boolean> for idempotent operations