In Domain-Driven Design (DDD), a repository is a mechanism used to manage collections of domain objects. It is an abstraction over the persistence layer that provides a way to store and retrieve domain objects while hiding the details of how they are persisted. The repository pattern enables the domain layer to remain agnostic of the persistence mechanism and allows the use of different persistence technologies without affecting the domain model.

A repository typically provides methods to perform CRUD (Create, Read, Update, and Delete) operations on domain objects, as well as methods to query and retrieve specific objects based on certain criteria. The repository can also encapsulate other concerns related to persistence, such as transaction management, caching, and batching.

Repositories are typically defined as interfaces within the domain layer and are implemented by infrastructure or persistence layer classes that handle the actual data storage and retrieval.

Example

In Java, a repository is typically implemented as an interface that defines the operations for accessing and persisting domain objects. Here’s an example:

public interface OrderRepository {
    Order findById(OrderId id);
    List<Order> findAll();
    void save(Order order);
    void delete(Order order);
}

n this example, the OrderRepository interface defines four methods for accessing and persisting Order objects:

• findById(OrderId id) – finds an Order object by its unique identifier (OrderId is a value object)
• findAll() – returns a list of all Order objects
• save(Order order) – saves a new Order object or updates an existing one
• delete(Order order) – deletes an existing Order object

The implementation of this repository interface can vary depending on the specific data storage mechanism being used (e.g. relational database, NoSQL database, etc.). Here’s an example implementation using a simple in-memory data structure:

public class InMemoryOrderRepository implements OrderRepository {
    private Map<OrderId, Order> orders = new HashMap<>();

    @Override
    public Order findById(OrderId id) {
        return orders.get(id);
    }

    @Override
    public List<Order> findAll() {
        return new ArrayList<>(orders.values());
    }

    @Override
    public void save(Order order) {
        orders.put(order.getId(), order);
    }

    @Override
    public void delete(Order order) {
        orders.remove(order.getId());
    }
}

In this implementation, the Order objects are stored in a Map data structure, where the keys are the unique identifiers of the orders. The four methods defined by the OrderRepository interface are implemented to perform the appropriate operations on the Map.

Using a repository provides a layer of abstraction between the domain model and the underlying data storage mechanism, which can make it easier to switch out the data storage mechanism in the future. Additionally, repositories can provide a convenient API for working with domain objects that abstracts away the details of how the data is stored and retrieved.