Understanding the Differences Between Repository and Data Access Object (DAO)

Repository and Data Access Object (DAO) play crucial roles in software development and data handling. However, their purposes and contexts differ, especially when we consider how they relate to the business logic of an application. Let’s explore the key differences between these concepts, where they originate, and when you should choose one.

About Repository and Data Access Object (DAO) Patterns

The Repository pattern originates from Domain-Driven Design (DDD), as described by Eric Evans in his book, "Domain-Driven Design: Tackling Complexity in the Heart of Software." Repositories are not just about managing data; they encapsulate business logic, ensuring that operations adhere to the Ubiquitous Language of the domain.

On the other hand, the Data Access Object (DAO) pattern comes from early enterprise application design, and its formalization can be found in the book "Core J2EE Patterns: Best Practices and Design Strategies" by Deepak Alur, John Crupi, and Dan Malks, published in 2001. DAO abstracts and encapsulates all access to a data source, separating persistence logic from business logic. This allows changes to the underlying data source without affecting the rest of the application, which is especially useful in enterprise contexts where systems might switch databases or data storage mechanisms over time.

The primary distinction between Repository and DAO lies in the semantics of the API they expose:

• DAO: DAO is data-centric, providing database operations such as insert, update, delete, and find. These methods directly map to database actions, focusing on how data is stored and retrieved.
• Repository: This is domain-driven and aligns with the* business logic*. It represents a collection of aggregate roots or entities. Instead of database operations, repositories offer operations that reflect the domain language. For instance, in a hotel system, a Repository might provide checkIn, checkout, or checkReservation, abstracting away the actual data operations.

Illustration

Let’s illustrate this difference with a simple example in Java. We’ll model a Room entity and treat the Guest as a Value Object:

public class Room {

    private int number;
    private boolean available;
    private boolean cleaned;
    private Guest guest;

    // Getters and setters omitted for brevity
}

public record Guest(String name, String document) {
}

Now, let’s look at how the DAO and Repository would handle operations for this entity:

In the case of a DAO, the method terminology reflects database operations, like insert, update, or delete:

public interface RoomDAO {

    void insert(Room room);

    void update(Room room);

    void delete(Room room);

    Optional<Room> findById(int number);
}

The Repository, on the other hand, defines operations that align with the business domain. The methods relate to actions that make sense in the context of managing a hotel:

public interface Hotel {

    void checkIn(Room room);

    void checkOut(Room room);

    Optional<Room> checkReservation(int number);
}

In this sample code, consider the Hotel as a collection of Rooms. Check-in represents the process of assigning a guest to a room and updating its status accordingly, while checkout involves freeing the room and marking it as available once the guest departs.

It’s important to understand that this example focuses solely on the basic aspects of room management related to check-in and checkout. This simplified approach is intended to clarify the concept and provide a solid foundation for grasping more complex scenarios in the future. In a real-world situation, these processes would likely involve additional services, such as payment processing, which would expand the concept beyond a simple repository and into a service layer. However, to keep this example focused and easy to understand, the complexities of those integrations are beyond the scope of this article. This aims to reassure the reader.

• Use DAO when your application primarily performs database operations without complex business logic. For example, if you’re working on a simple CRUD app or your data source is the focus, DAO fits naturally.
• Use Repository when your application has complex domain logic that needs to be encapsulated. This is especially relevant when following Domain-Driven Design principles, where the Repository is an abstraction that ensures business operations are consistent with domain rules.

They understand the distinction between DAO and Repository when building scalable and maintainable applications. While both patterns handle data, DAOs are more focused on the persistence layer, and Repositories encapsulate business logic, abstracting away how the data is stored. Choosing the correct pattern for your scenario ensures that your application remains flexible, robust, and easy to maintain.

The sample Java code provided reveals that Repositories operate at the domain level, interacting with entities and business operations, while DAOs focus on the underlying data storage and retrieval. This conceptual difference is important as it shapes the structure and maintainability of your code in the long run.

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