What is a Repository?

In Domain-Driven Design (DDD), a Repository is a design pattern that provides an abstraction for accessing and manipulating domain objects. Its primary role is to handle the persistence and retrieval of these objects, allowing the domain model to focus on business logic rather than data access concerns.

Here's a brief overview of the Repository concept in DDD:

1. Abstraction: Repositories abstract away the details of the data storage mechanism, whether it's a database, an external service, or in-memory storage. This separation of concerns helps keep the domain model clean and focused on the business logic.

2. Collection-Like Interface: Repositories typically provide a collection-like interface for accessing domain objects. This means you can add, remove, and query domain objects using methods similar to those found in a collection, such as `add()`, `remove()`, and `findById()`.

3. Querying: Repositories may also offer methods for querying domain objects based on various criteria. This helps encapsulate query logic and keeps it separate from the domain model.

4. Persistence Ignorance: By using repositories, domain objects can remain "persistence ignorant," meaning they don’t need to know or care about how they're stored or retrieved. This makes the domain model more flexible and easier to work with.

5. Unit of Work Integration: Repositories often work in conjunction with the Unit of Work pattern, which manages transactions and coordinates the writing out of changes to the database.

Example:

Imagine you have a `Customer` domain model in your application. You would define a `CustomerRepository` interface with methods like:

- `Customer findById(String id)`

- `List<Customer> findAll()`

- `void save(Customer customer)`

- `void delete(Customer customer)`

The concrete implementation of `CustomerRepository` would handle the details of querying and persisting `Customer` objects, while the rest of your application interacts with the repository without worrying about these details.

In summary, the Repository pattern helps maintain a clear separation between the domain logic and data access concerns, facilitating a more organized and maintainable codebase.