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domain-services.md

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Domain Services Best Practices & Conventions

This document offers best practices for implementing Domain Service classes in your modules and applications based on Domain-Driven-Design principles.

Ensure you've read the Domain Services document first.

Domain Services

  • Do define domain services in the domain layer.
  • Do not create interfaces for the domain services unless you have a good reason to (like mock and test different implementations).
  • Do name your domain service with Manager suffix.

For the example of a domain service:

public class IssueManager : DomainService
{
	//...
}

Domain Service Methods

  • Do not define GET methods. GET methods do not change the state of an entity. Hence, use the repository directly in the Application Service instead of Domain Service method.

  • Do define methods that only mutates data; changes the state of an entity or an aggregate root.

  • Do not define methods with generic names (like UpdateIssueAsync).

  • Do define methods with self explanatory names (like AssignToAsync) that implements the specific domain logic.

  • Do accept valid domain objects as parameters.

public async Task AssignToAsync(Issue issue, IdentityUser user)
{
    //...
}

  • Do throw BusinessException or custom business exception if a validation fails.

    • Do use domain error codes with unique code-namespace for exception localization.
public async Task AssignToAsync(Issue issue, IdentityUser user)
{
    var openIssueCount = await _issueRepository.GetCountAsync(
            i => i.AssignedUserId == user.Id && !i.IsClosed
        );

        if (openIssueCount >= 3)
        {
            throw new BusinessException("IssueTracking:ConcurrentOpenIssueLimit");
        }

        issue.AssignedUserId = user.Id;
}
  • Do not return DTO. Return only domain objects when you need.
  • Do not involve authenticated user logic. Instead, define extra parameter and send the related data of CurrentUser from the Application Service layer.

See Also