Domain Services

Introduction

In a Domain Driven Design (DDD) solution, the core business logic is generally implemented in aggregates (entities) and the Domain Services. Creating a Domain Service is especially needed when;

• You implement a core domain logic that depends on some services (like repositories or other external services).
• The logic you need to implement is related to more than one aggregate/entity, so it doesn't properly fit in any of the aggregates.

ABP Domain Service Infrastructure

Domain Services are simple, stateless classes. While you don't have to derive from any service or interface, ABP Framework provides some useful base classes and conventions.

DomainService & IDomainService

Either derive a Domain Service from the DomainService base class or directly implement the IDomainService interface.

Example: Create a Domain Service deriving from the DomainService base class.

using Volo.Abp.Domain.Services;

namespace MyProject.Issues
{
    public class IssueManager : DomainService
    {
        
    }
}

When you do that;

• ABP Framework automatically registers the class to the Dependency Injection system with a Transient lifetime.
• You can directly use some common services as base properties, without needing to manually inject (e.g. ILogger and IGuidGenerator).

It is suggested to name a Domain Service with a Manager or Service suffix. We typically use the Manager suffix as used in the sample above.

Example: Implement the domain logic of assigning an Issue to a User

public class IssueManager : DomainService
{
    private readonly IRepository<Issue, Guid> _issueRepository;

    public IssueManager(IRepository<Issue, Guid> issueRepository)
    {
        _issueRepository = issueRepository;
    }
    
    public async Task AssignAsync(Issue issue, AppUser user)
    {
        var currentIssueCount = await _issueRepository
            .CountAsync(i => i.AssignedUserId == user.Id);
        
        //Implementing a core business validation
        if (currentIssueCount >= 3)
        {
            throw new IssueAssignmentException(user.UserName);
        }

        issue.AssignedUserId = user.Id;
    }    
}

Issue is an aggregate root defined as shown below:

public class Issue : AggregateRoot<Guid>
{
    public Guid? AssignedUserId { get; internal set; }
    
    //...
}

• Making the setter internal ensures that it can not directly set in the upper layers and forces to always use the IssueManager to assign an Issue to a User.

Using a Domain Service

A Domain Service is typically used in an application service.

Example: Use the IssueManager to assign an Issue to a User

using System;
using System.Threading.Tasks;
using MyProject.Users;
using Volo.Abp.Application.Services;
using Volo.Abp.Domain.Repositories;

namespace MyProject.Issues
{
    public class IssueAppService : ApplicationService, IIssueAppService
    {
        private readonly IssueManager _issueManager;
        private readonly IRepository<AppUser, Guid> _userRepository;
        private readonly IRepository<Issue, Guid> _issueRepository;

        public IssueAppService(
            IssueManager issueManager,
            IRepository<AppUser, Guid> userRepository,
            IRepository<Issue, Guid> issueRepository)
        {
            _issueManager = issueManager;
            _userRepository = userRepository;
            _issueRepository = issueRepository;
        }

        public async Task AssignAsync(Guid id, Guid userId)
        {
            var issue = await _issueRepository.GetAsync(id);
            var user = await _userRepository.GetAsync(userId);

            await _issueManager.AssignAsync(issue, user);
            await _issueRepository.UpdateAsync(issue);
        }
    }
}

Since the IssueAppService is in the Application Layer, it can't directly assign an issue to a user. So, it uses the IssueManager.

Application Services vs Domain Services

While both of Application Services and Domain Services implement the business rules, there are fundamental logical and formal differences;

• Application Services implement the use cases of the application (user interactions in a typical web application), while Domain Services implement the core, use case independent domain logic.
• Application Services get/return Data Transfer Objects, Domain Service methods typically get and return the domain objects (entities, value objects).
• Domain services are typically used by the Application Services or other Domain Services, while Application Services are used by the Presentation Layer or Client Applications.

Lifetime

Lifetime of Domain Services are transient and they are automatically registered to the dependency injection system.