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How do you design a repository interface for an aggregate root?

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Domain-Driven Design (DDD) is a software development approach that focuses on the core domain and its business logic. One of the key concepts in DDD is the aggregate, which is a cluster of related entities that form a consistency boundary. An aggregate root is the entity that controls the access and integrity of the aggregate. To persist and retrieve aggregates from a data source, you need a repository interface that defines the operations and contracts for working with the aggregate root. In this article, you will learn how to design a repository interface for an aggregate root following some best practices and principles.

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1 What is a repository interface?

A repository interface is an abstraction that hides the details of how the aggregate root is stored and retrieved from a data source. It provides a collection-like interface that allows you to add, update, remove, and query aggregate roots. A repository interface should not expose the internal structure or state of the aggregate root, but only its identity and behavior. A repository interface should also adhere to the principle of least astonishment, meaning that it should behave in a predictable and consistent way.

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John C.

Logical Wireframer, Solution Architect Consultant, Distributed Design & DDD Evangelist, Eventstormer, Speaker, Trainer, Drummer
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I believe it is more accurate to state that the repository does the work of retrieving, storing and removing aggregates and the aggregate root itself defines the contracts for working with the aggregate.

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James Hickey

Microsoft MVP | Principal Software Engineer & Architect | Author | Speaker
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A repository is an abstraction on top of your data storage. They should be used when interacting with aggregates - which are useful on the "write-side" of your domain logic. "Read-side" or query logic should not be a part of a repository (those are reports, queries, projections, etc.) Repositories should only do a few things: - "Give me the aggregate having id X" - "Save/store this new aggregate having id X" - "Delete aggregate having id X"

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2 Why do you need a repository interface?

A repository interface is useful for several reasons. First, it decouples the domain layer from the infrastructure layer, making the code more maintainable and testable. You can change the data source or the persistence mechanism without affecting the domain logic or the clients of the repository. Second, it encapsulates the data access logic, making it easier to implement caching, concurrency, transactions, and other cross-cutting concerns. Third, it simplifies the domain model, making it more expressive and cohesive. You can focus on the business rules and the invariants of the aggregate root, rather than the technical details of how it is stored and retrieved.

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James Hickey

Microsoft MVP | Principal Software Engineer & Architect | Author | Speaker
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A repository is typically called/used inside a domain command handler. Using an interface just allows you to mock or use fakes for unit testing your command handlers or domain services.

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3 How do you design a repository interface?

Designing a repository interface for an aggregate root requires following certain guidelines and steps. Firstly, identify the aggregate root and its identity, which should be used as the primary key for repository operations. Secondly, define the repository contract which should be consistent with the domain and ubiquitous language. Lastly, implement the repository implementation by using a DAO or ORM to interact with the data source and map it to and from the aggregate root. Additionally, handle any exceptions, errors, or failures that occur during data access operations.

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4 What are some best practices for designing a repository interface?

Designing a repository interface for an aggregate root is not a simple task, and requires following some best practices and principles to ensure its effectiveness and reliability. Firstly, the single responsibility principle (SRP) should be followed, meaning that a class should only have one reason to change. A repository interface should only be responsible for persisting and retrieving aggregate roots, not performing validation, authorization, or logging. Secondly, the interface segregation principle (ISP) should be followed, which states that clients should not be forced to depend on methods that they do not use. Therefore, the repository interface should be small and focused, with only the methods needed by the clients. Lastly, the dependency inversion principle (DIP) should be followed so that high-level modules do not depend on low-level modules. The repository interface is an abstraction that allows the domain layer to depend on the infrastructure layer without knowing its details. The domain layer should only rely on the repository contract and not on the implementation; this should be injected using dependency injection (DI) or inversion of control (IoC) techniques.

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John C.

Logical Wireframer, Solution Architect Consultant, Distributed Design & DDD Evangelist, Eventstormer, Speaker, Trainer, Drummer
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Technically, the repository should retrieve and store the whole of an aggregate by the root identity, not retrieve and store just the aggregate root alone.

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5 How do you test a repository interface?

Testing a repository interface is essential to guarantee it functions as expected and meets the requirements. There are various tests you can perform on a repository interface, such as unit tests, integration tests, and end-to-end tests. To test a repository interface, you can use test doubles to isolate the interface from its dependencies and simulate different scenarios. Additionally, test data can be used to check that the repository interface can persist and retrieve aggregate roots correctly. Lastly, test frameworks can be used to automate and simplify the testing process. Examples of these tools include xUnit, Moq, FluentAssertions, xBehave, and SpecFlow.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you design a repository interface for an aggregate root?

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1 What is a repository interface?

2 Why do you need a repository interface?

3 How do you design a repository interface?

4 What are some best practices for designing a repository interface?

5 How do you test a repository interface?

6 Here’s what else to consider

Domain-Driven Design (DDD)

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