Project Architecture Setup

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Architectural Style?

Define and document the architectural style (e.g., layered, hexagonal, onion, clean). Outline how this style will support scalability, maintainability, modularity, testability and the domain-driven approach. Define Project Layers and Boundaries

Specify layers and boundaries (e.g., domain, application, infrastructure). Create a high-level diagram to illustrate the architecture.

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https://www.youtube.com/watch?v=JubdZIdLQ4M https://ccd-akademie.de/en/clean-architecture-vs-onion-architecture-vs-hexagonale-architektur/ https://jeffreypalermo.com/2008/07/the-onion-architecture-part-1/ https://en.wikipedia.org/wiki/Hexagonal_architecture_(software) https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html

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Initial Approach: N-Tier Architecture

• Structure: N-Tier (or Layered) Architecture consists of a series of stacked layers—typically User Interface (UI), Business Logic, and Data Access. Each layer depends on the one below it.

• Dependency Flow: The UI layer depends on the Business Logic layer, which in turn depends on the Data Access layer. The Data Access layer has no dependencies on other layers.
• Cons:
  • Tight Coupling: Dependencies flow in a single direction, leading each layer to directly depend on the layer beneath it. This makes it difficult to change or replace a single layer without affecting others.
  • Limited Flexibility and Testability: Since the core logic is not isolated from infrastructure concerns, testing becomes challenging, especially in isolation.
  • Hard to Adapt: Integrating new technologies or changing data sources requires adjustments throughout the dependent layers, impacting the entire application stack.

• Shifting Towards a Decoupled and Adaptable Architecture: The main idea is to invert the dependency between business logic and data access. The business logic can make calls to the data access layer, but it shouldn't need to know the specifics of what it's calling. The business logic can define an interface, which will be implemented in the data access layer. This interface is called a "port," and its implementation within the data access layer is called an "adapter."

Evolution to Hexagonal Architecture (Ports and Adapters)

• Introduction of Ports and Adapters: Hexagonal Architecture, also known as Ports and Adapters, was introduced to improve the flexibility and testability of the N-Tier architecture by inverting dependencies between layers.
• Structure: The application core defines "ports" (interfaces), and the infrastructure layer provides "adapters" (implementations) for these ports.

• Dependency Flow: Dependencies now point inward. The core (business logic) does not know the specifics of external systems, only the interface it communicates through.

• Benefits:

  • Core Isolation: The core logic remains pure and untainted by external details, making it easier to test and adapt.
  • Adaptability: Different adapters can be added around the core without changing core logic, supporting integration with various external systems, such as UIs, APIs, databases, or third-party services.
  • Isolation of Business Logic: Core logic is protected from external dependencies.
  • Use of Ports and Adapters: Ports are defined in the core; adapters implement them for external interaction.
  • Independence from Delivery Mechanisms: Delivery methods (UI, CLI) do not affect core logic.
  • Testability Through Decoupling: Decoupling enables isolated and simplified testing.
  • Separation of Concerns: The core handles business logic, while adapters manage external concerns.

• Limitations: While Hexagonal Architecture improved flexibility, it left open questions about layering within the core itself. For applications needing deeper business logic structuring, additional layers could enhance organization.

Next Development: Onion Architecture

• Further Layering within the Core: Onion Architecture builds upon Hexagonal principles, further organizing the core into multiple internal layers with dependencies directed inward.
• Structure:
  • Inner Domain Model (Core): Holds central business logic, often designed using Domain-Driven Design (DDD) principles.
  • Domain Services and Data Abstractions: Surrounding the domain model are layers for domain services (logic spanning multiple domain objects) and data abstractions (e.g., IRepository or IUnitOfWork interfaces).
  • Outer Layers (Service Classes, Commands, and Queries): These orchestrate interactions between domain objects and can be organized using patterns like CQRS.

• Dependency Flow: Dependencies are strictly inwards; even domain services and data abstractions depend only on the core domain model.
• Benefits:
  • Enhanced Organization: Onion Architecture’s inner layers better organize the application, making it ideal for complex systems with multi-layered business logic.
  • Testability and Isolation: The domain model remains isolated and unaffected by infrastructure, making the architecture ideal for applications that need a domain-centric structure.
• Limitations: Onion Architecture doesn’t provide specific guidance on how many layers to use, and there’s room for more flexible naming and layer structuring based on application needs.

Unified Concept: Clean Architecture

• Comprehensive and Flexible Approach: Clean Architecture builds on Hexagonal and Onion principles, further refining the structure by introducing clear terminology and allowing teams to customize the number and purpose of layers.
• Structure:
  • Core Entities (Enterprise Business Rules): Represents core business objects that change infrequently.
  • Application Layer (Application-Specific Business Rules): Contains use cases and related interfaces for command and query models.
  • Interface Adapters: This layer includes elements like controllers (handling requests), presenters (preparing responses), and gateways (interfacing with external data sources, typically through repositories).
  • Frameworks and Drivers Layer: The outermost layer for specific frameworks, databases, UI elements, and other external drivers.

• Dependency Flow: All dependencies point inward, with the innermost layers entirely insulated from external concerns.
• Benefits:
  • Enhanced Modularity and Maintainability: Clean Architecture uses adaptable layers, allowing for better alignment with application requirements.
  • Consistency and Flexibility: It introduces more abstract terms like controllers, presenters, and gateways to generalize the architecture and make it more applicable across different types of applications.
• Limitations: Clean Architecture can introduce complexity due to the variety of layers and terminologies, and it requires a disciplined approach to maintain consistency.

[khalid-dev-0001]

Considering the requirements for scalability, maintainability, modularity, and testability, the application should be structured into distinct layers, each responsible for a specific concern. To ensure intrinsic testability, the design must adhere to the Dependency Rule. All three architectural styles aim to achieve a clear separation of concerns while respecting this rule, making each a viable choice for our system. However, Clean Architecture aligns well with Domain-Driven Design (DDD) by using familiar terminology, such as Entities and Value Objects, and offers a more descriptive structure. For these reasons, Clean Architecture is our preferred choice.

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Here’s a comprehensive guide to implementing Domain-Driven Design (DDD) based on Eric Evans' principles :

1. Understand the Domain and Build a Ubiquitous Language

• Domain Knowledge: Focus on the problem the software is solving rather than on technical aspects. Developers should immerse themselves in the domain, learning the intricacies and complexities directly from domain experts.
• Ubiquitous Language: Create a shared language that everyone on the team can use consistently. This language, developed collaboratively with domain experts, clarifies domain concepts, improves communication, and evolves along with the model. Using this language consistently in discussions, documentation, and code ensures alignment and reduces misunderstandings.

2. Develop a Model-Driven Design

• Model as Core Design Driver: Make the model the heart of both software design and team understanding. This connection ensures that domain insights translate directly into the software.
• Iterative Refinement: Start with a basic model and refine it iteratively, gradually capturing a deeper understanding of the domain. Each iteration should move the model closer to accurately reflecting core business processes.

3. Use Strategic Design Patterns

• Bounded Context: Define boundaries within which each model applies. This isolates the domain into manageable parts, each with its own language and rules. Using Bounded Contexts minimizes complexity and reduces conflicts.
• Core Domain: Identify and focus on the part of the system that delivers the most value to the business. The Core Domain represents the essential area where the team should invest time and resources to gain competitive advantage.

4. Implement Building Blocks for Model Representation

• Entities and Value Objects: Distinguish entities (objects with a unique identity and lifecycle) from value objects (objects defined solely by their attributes). This division simplifies modeling by allowing objects to be managed based on their purpose and properties.
• Aggregates: Organize related entities and value objects into aggregates, with a root entity controlling access and ensuring internal consistency. Aggregates help manage relationships and rules, maintaining model integrity and reducing complexity.
• Repositories and Factories: Use repositories to handle the persistence and retrieval of aggregates, focusing the Domain layer on behavior rather than data storage. Factories manage complex object creation, ensuring consistency and adherence to domain rules.

5. Align Model with Implementation

• Model-Driven Code: Ensure code closely reflects the model, with every concept in the model having a clear counterpart in the code. This alignment allows for seamless adaptation of the code to reflect changes in the domain.
• Continuous Refactoring: Regularly review and refine code to keep it aligned with the evolving model. As new insights into the domain are gained, refactoring ensures that the code structure remains consistent with these changes.

6. Use Strategic Layering and Isolation

• Layered Architecture: Structure the system with layers (e.g., Application, Domain, and Infrastructure) to separate concerns. This approach enables the Domain layer to remain focused on domain logic, while other concerns, like data storage or user interaction, are handled separately.
• Modules: Organize the domain layer into modules, grouping related concepts to keep the domain model cohesive. This modularity makes the model easier to understand, manage, and evolve.

7. Foster Collaborative Development and Hands-On Modeling

• Developer-Domain Expert Collaboration: Developers and domain experts should work closely together, engaging directly in model development. This collaboration ensures that the model remains relevant, accurate, and practical.
• Iterative Development and Feedback Loops: Short feedback loops, encouraged by iterative development, are essential to DDD. Continuous interaction between developers and domain experts allows for real-time model adjustments, ensuring that the software remains aligned with business needs.

Summary

Applying DDD requires a deep understanding of the domain, a Ubiquitous Language, the definition of bounded contexts and aggregates, and constant alignment of code with the model. It’s both a design and a collaborative development approach that emphasizes iterative refinement, strategic layering, and modular organization. Through hands-on collaboration, frequent feedback, and a focus on the Core Domain, DDD transforms complex requirements into a well-organized, business-aligned software system.

[khalid-dev-0001]

Combining Clean Architecture with Domain-Driven Design (DDD) creates a powerful, structured approach to software design. In this combination, Clean Architecture provides the overall structure with clear separation of concerns and dependency direction, while DDD enriches the Domain layer with a model that reflects the business.

Here’s how to design such an architecture with folder structures and layers:

1. Core Structure

In this architecture, we’ll have the following primary layers, each with specific responsibilities:

• Domain Layer: Represents the core business logic using DDD concepts.
• Application Layer: Coordinates application-specific tasks and use cases.
• Interface Layer: Provides entry points for users or external systems.
• Infrastructure Layer: Handles all external dependencies like databases, third-party services, and persistence.

2. Layered Folder Structure

Here’s a suggested folder structure to implement Clean Architecture principles alongside DDD practices.

src/
├── Domain
│   ├── Order (Aggregate Folder)
│   │   ├── Entities
│   │   │   └── Order.cs
│   │   ├── ValueObjects
│   │   │   └── OrderId.cs
│   │   ├── Services
│   │   │   └── OrderDomainService.cs
│   │   ├── Specifications
│   │   │   └── OrderSpecification.cs
│   │   ├── Events
│   │   │   └── OrderPlacedEvent.cs
│   │   ├── Factories
│   │   │   └── OrderFactory.cs
│   ├── User (Aggregate Folder)
│   │   ├── Entities
│   │   │   └── User.cs
│   │   ├── ValueObjects
│   │   │   └── UserId.cs
│   │   ├── Services
│   │   │   └── UserDomainService.cs
│   │   ├── Specifications
│   │   │   └── UserSpecification.cs
│   │   ├── Events
│   │   │   └── UserRegisteredEvent.cs
│   │   ├── Factories
│   │   │   └── UserFactory.cs
├── Application
│   ├── Orders (Use Case Folder)
│   │   ├── DTOs
│   │   │   └── OrderDTO.cs
│   │   ├── UseCases
│   │   │   ├── PlaceOrderUseCase.cs
│   │   │   └── CancelOrderUseCase.cs
│   │   ├── Services
│   │   │   └── OrderAppService.cs
│   │   ├── Events
│   │   │   └── OrderPlacedEventHandler.cs
│   └── Users (Use Case Folder)
│       ├── DTOs
│       │   └── UserDTO.cs
│       ├── UseCases
│       │   ├── RegisterUserUseCase.cs
│       │   └── UpdateUserProfileUseCase.cs
│       ├── Services
│       │   └── UserAppService.cs
│       ├── Events
│       │   └── UserRegisteredEventHandler.cs
├── Infrastructure
│   ├── Persistence
│   │   ├── Repositories
│   │   │   ├── OrderRepository.cs
│   │   │   └── UserRepository.cs
│   │   └── DatabaseContext.cs
│   ├── Messaging
│   │   └── EventPublisher.cs
│   ├── ExternalServices
│   │   └── PaymentGatewayService.cs
└── Interfaces
    ├── Controllers
    │   └── OrderController.cs
    └── REST
        └── UserEndpoint.cs

3. Layer Responsibilities

Domain Layer

• Aggregates: Group related entities and ensure consistency within the aggregate.
• Entities: Represent core objects with unique identities (e.g., User, Order).
• Value Objects: Represent domain values without identity (e.g., Money, Address).
• Domain Services: Encapsulate domain logic that doesn’t naturally belong to an entity (e.g., PricingService).
• Repositories (Interfaces Only): Define interfaces for data access (e.g., IOrderRepository). Implementations will go in the Infrastructure layer.
• Specifications: Define rules and criteria (e.g., OrderSpecification) for filtering and validation.
• Domain Events: Represent important state changes or actions within the domain (e.g., OrderPlacedEvent).

Application Layer

• Use Cases: Define application-specific operations or user actions, each with a single responsibility (e.g., PlaceOrderUseCase).
• DTOs: Data Transfer Objects for sending data across layers without exposing domain models.
• Application Services: Handle orchestration for complex use cases. These services call repositories, interact with use cases, and coordinate with other services.
• Event Handlers: Handle domain events by triggering relevant use cases or actions, enabling reactive responses.

Infrastructure Layer

• Persistence: Implements domain repository interfaces, manages database interactions, and handles ORM configurations.
• Messaging: Manages event publishing, subscribing, and message queues for domain events.
• External Services: Integrates third-party services (e.g., Payment Gateway, Email Service).
• Data Mappers: Transforms data between entities, DTOs, and persistence models to keep the Domain layer isolated from external dependencies.

Interface Layer

• Controllers: Expose application functionality via REST APIs, gRPC services, or GraphQL endpoints.
• GraphQL, REST, gRPC: Each service type organizes functionality, providing separate endpoints or services according to the communication method.

4. Dependency Flow and Boundaries

• Clean Architecture emphasizes that dependencies should point inward. The outer layers (Infrastructure and Interface) depend on the inner layers (Application and Domain).
• Dependency Inversion: The Domain layer has no dependencies on other layers. Repositories and Services are injected, and implementations exist in the Infrastructure layer.
• Bounded Contexts: If the domain contains multiple bounded contexts, a new project will be created for each one using the same template. Each bounded context will function as an isolated service that can be hosted independently.

5. Benefits of this Architecture

• Scalability and Maintainability: This structure enables isolated changes, with clear dependency management, making it easier to scale and maintain.
• Testability: The separation of layers and encapsulation of logic into use cases and services makes testing straightforward.
• Clear Ownership of Logic: The Domain layer owns all business rules, the Application layer coordinates them, and the Infrastructure layer implements external integrations.
• Modularity: Bounded contexts in the Domain layer and a layered structure reduce coupling, allowing each part of the system to evolve independently.

This architecture leverages Clean Architecture’s clear separation of concerns, while DDD practices ensure the Domain layer is rich with meaningful, business-driven logic that evolves with the business needs. The result is a flexible, modular, and maintainable system designed to accommodate complex business domains.