A data layer isolates communication with a database, file, API, cache, or other store from the application’s core behavior. Callers use a small interface instead of depending on storage details, so the backing technology can change without reshaping every caller. section 4.3.3
A Repository can serve as the data layer’s boundary. Define a generic contract for create, read, update, delete, and listing operations; then provide implementations for particular entity types and stores. An application service delegates persistence work to the repository rather than manipulating storage directly. section 4.3.3
Listing 4.26 Code for BaseRepository, our generic data access layer
from abc import ABC, abstractmethod
from typing import List, TypeVar, Generic
T = TypeVar('T')
class BaseRepository(ABC, Generic[T]):
@abstractmethod
def create(self, entity: T) -> None:
pass
@abstractmethod
def read(self, entity_id: int) -> T:
pass
@abstractmethod
def update(self, entity: T) -> None:
pass
@abstractmethod
def delete(self, entity_id: int) -> None:
pass
@abstractmethod
def get_entities(self) -> List[T]:
pass
This generic repository contract makes the data-layer boundary concrete: callers work with typed CRUD methods while implementations supply storage-specific behavior. section 4.3.3
Swap an in-memory store for persistent storage
An AssetManager can work through an InMemoryAssetRepository backed by a list, enabling an end-to-end CRUD test. The same repository boundary can later conceal a PostgreSQL-backed implementation. For ORM persistence, keep metadata-bearing ORM models outside the domain model and map them before objects return to the core. section 4.3.3 section 5.1
Keep the boundary useful by separating these responsibilities:
- Expose application-facing CRUD operations through a repository contract. section 4.3.3
- Implement the contract for each storage mechanism, such as an in-memory list or PostgreSQL-backed persistence. section 4.3.3 section 5.1
- Map persistence models to domain models when ORM metadata would otherwise leak into business objects. section 5.1
