Add article on App Services vs Domain Services

docs/en/Community-Articles/2025-08-25-App-Services-vs-Domain-Services/POST.md

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+# App Services vs Domain Services: Deep Dive into Two Core Service Types in ABP Framework
+
+In ABP's layered architecture, we frequently encounter two types of services that appear similar but serve distinctly different purposes: Application Services and Domain Services. Understanding the differences between them is crucial for building clear and maintainable enterprise applications.
+
+## Architectural Positioning
+
+In ABP's layered architecture:
+
+- **Application Services** reside in the application layer and are responsible for coordinating use case execution
+- **Domain Services** reside in the domain layer and are responsible for implementing core business logic
+
+This layered design follows Domain-Driven Design (DDD) principles, ensuring clear separation of business logic and system maintainability.
+
+## Application Services: Use Case Orchestrators
+
+### Core Responsibilities
+
+Application Services are stateless services primarily used to implement application use cases. They act as a bridge between the UI layer and domain layer, responsible for:
+
+- **Use Case Orchestration**: Organizing and coordinating multiple domain objects to complete specific business use cases
+- **Data Transformation**: Handling conversion between DTOs and domain objects
+- **Authorization**: Checking user permissions and access control
+- **Transaction Management**: Ensuring operation atomicity
+
+### Design Principles
+
+1. **DTO Boundaries**: Application service methods should only accept and return DTOs, never directly expose domain entities
+2. **Use Case Oriented**: Each method should correspond to a clear user use case
+3. **Thin Layer Design**: Avoid implementing complex business logic in application services
+
+### Typical Execution Flow
+
+A standard application service method typically follows this pattern:
+
+```csharp
+public async Task<BookDto> CreateBookAsync(CreateBookDto input)
+{
+    // 1. Validate permissions
+    await CheckCreatePermissionAsync();
+    
+    // 2. Get or validate related data
+    var author = await _authorRepository.GetAsync(input.AuthorId);
+    
+    // 3. Call domain service to execute business logic
+    var book = await _bookManager.CreateAsync(input.Title, author, input.Price);
+    
+    // 4. Persist changes
+    await _bookRepository.InsertAsync(book);
+    
+    // 5. Return DTO
+    return ObjectMapper.Map<Book, BookDto>(book);
+}
+```
+
+### Integration Services: Specialized Application Services
+
+It's worth mentioning that ABP also provides a special type of application service—Integration Services. They are used for:
+
+- **Inter-service Communication**: Service-to-service calls in microservice architectures
+- **Module Integration**: Inter-module communication in large monolithic applications
+- **Internal APIs**: Internal interfaces not exposed to the public
+
+Integration services are marked with the `[IntegrationService]` attribute and use the `/integration-api` route prefix by default, making it easy to implement access control at the gateway layer.
+
+We have a community article dedicated to integration services: [Integration Services Explained — What they are, when to use them, and how they behave](https://abp.io/community/articles/integration-services-explained-what-they-are-when-to-use-lienmsy8)
+
+## Domain Services: Guardians of Business Logic
+
+### Core Responsibilities
+
+Domain Services implement core business logic and are particularly suitable for:
+
+- **Cross-Aggregate Operations**: Business logic that needs to operate on multiple aggregate roots
+- **Complex Business Rules**: Complex logic that doesn't fit within a single entity
+- **External Dependencies**: Business operations that need to access repositories or external services
+
+### Design Principles
+
+1. **Domain Object Interaction**: Method parameters and return values should be domain objects (entities, value objects)
+2. **Business Logic Focus**: Focus on implementing pure business rules
+3. **Stateless Design**: Maintain the stateless nature of services
+
+### Implementation Example
+
+```csharp
+public class IssueManager : DomainService
+{
+    private readonly IRepository<Issue, Guid> _issueRepository;
+    
+    public async Task AssignAsync(Issue issue, AppUser user)
+    {
+        // Business rule: Check user's unfinished task count
+        var openIssueCount = await _issueRepository.GetCountAsync(
+            i => i.AssignedUserId == user.Id && !i.IsClosed);
+            
+        if (openIssueCount >= 3)
+        {
+            throw new BusinessException("IssueTracking:ConcurrentOpenIssueLimit");
+        }
+        
+        // Execute assignment logic
+        issue.AssignedUserId = user.Id;
+        issue.AssignedDate = Clock.Now;
+    }
+}
+```
+
+## Key Differences Comparison
+
+| Dimension | Application Services | Domain Services |
+|-----------|---------------------|-----------------|
+| **Layer Position** | Application Layer | Domain Layer |
+| **Primary Responsibility** | Use Case Orchestration | Business Logic Implementation |
+| **Data Interaction** | DTOs | Domain Objects |
+| **Callers** | UI Layer/Clients | Application Services/Other Domain Services |
+| **Authorization** | Responsible for permission checks | No authorization logic |
+| **Transaction Management** | Manages transaction boundaries | Participates in transactions but doesn't manage |
+| **Naming Convention** | `*AppService` | `*Manager` or `*Service` |
+
+## Collaboration Patterns in Practice
+
+In real-world development, these two types of services typically work together:
+
+```csharp
+// Application Service
+public class BookAppService : ApplicationService
+{
+    private readonly BookManager _bookManager;
+    private readonly IRepository<Book> _bookRepository;
+    
+    public async Task<BookDto> UpdatePriceAsync(Guid id, decimal newPrice)
+    {
+        // Application service handles: permission checks, data retrieval, DTO conversion
+        await CheckUpdatePermissionAsync();
+        
+        var book = await _bookRepository.GetAsync(id);
+        
+        // Delegate to domain service for business logic
+        await _bookManager.ChangePriceAsync(book, newPrice);
+        
+        await _bookRepository.UpdateAsync(book);
+        
+        return ObjectMapper.Map<Book, BookDto>(book);
+    }
+}
+
+// Domain Service
+public class BookManager : DomainService
+{
+    public async Task ChangePriceAsync(Book book, decimal newPrice)
+    {
+        // Domain service focuses on business rules
+        if (newPrice <= 0)
+        {
+            throw new BusinessException("Book:InvalidPrice");
+        }
+        
+        if (book.IsDiscounted && newPrice > book.OriginalPrice)
+        {
+            throw new BusinessException("Book:DiscountedPriceCannotExceedOriginal");
+        }
+        
+        book.ChangePrice(newPrice);
+    }
+}
+```
+
+## Best Practice Recommendations
+
+### Application Services
+- Create a corresponding application service for each aggregate root
+- Use clear naming conventions (e.g., `IBookAppService`)
+- Implement standard CRUD operation methods (`GetAsync`, `CreateAsync`, `UpdateAsync`, `DeleteAsync`)
+- Avoid inter-application service calls
+
+### Domain Services
+- Use the `Manager` suffix for naming (e.g., `BookManager`)
+- Only define state-changing methods, avoid query methods
+- Throw business exceptions with clear error codes
+- Keep methods pure, avoid involving user context
+
+## Summary
+
+Application Services and Domain Services each have their distinct roles in the ABP framework: the former serves as use case orchestrators, handling external interactions and data transformation; the latter serves as implementers of business logic, ensuring proper execution of core rules. Correctly understanding and applying these two service patterns is key to building high-quality ABP applications.
+
+Through clear separation of responsibilities, we can not only build more maintainable code but also flexibly switch between monolithic and microservice architectures—this is precisely the elegance of ABP framework design.
+
+## References
+
+- [Application Services](https://abp.io/docs/en/app-services)
+- [Integration Services](https://abp.io/docs/en/integration-services)
+- [Domain Services](https://abp.io/docs/en/domain-services)