abp/docs/en/Domain-Driven-Design-Implementation-Guide.md

Implementing Domain Driven Design

Introduction

Goals

This is an integrative guide for implementing the Domain Driven Design (DDD). The goals of this document are;

• Introduce and explain the DDD architecture, concepts, principles, patterns and building blocks.
• Explain the layered architecture & solution structure offered by the ABP Framework.
• Introduce explicit rules to implement DDD patterns and best practices by giving concrete examples.
• Show what ABP Framework provides you as the infrastructure for implementing DDD in a proper way.

Simple Code!

Playing football is very simple, but playing simple football is the hardest thing there is. — Johan Cruyff

If we take this famous quote for programming, we can say;

Writing code is very simple, but writing simple code is the hardest thing there is. — ???

In this document, we will introduce simple rules those are easy to implement.

Once your application grows, it will be hard to follow these rules. Sometimes you find breaking rules will save your time in a short term. However, the saved time in the short term will bring much more time loss in the middle and long term. Your code base becomes complicated and hard to maintain. Most of the business applications are re-written just because you can't maintain it anymore.

If you follow the rules and best practices, your code base will be simpler and easier to maintain. Your application react to changes faster.

What is the Domain Driven Design?

Domain-driven design (DDD) is an approach to software development for complex needs by connecting the implementation to an evolving model;

DDD is suitable for complex domains and large-scale applications rather than simple CRUD applications. It focuses on the core domain logic rather than the infrastructure details. It helps to build a flexible, modular and maintainable code base.

OOP & SOLID

Implementing DDD highly relies on the Object Oriented Programming (OOP) and SOLID principles. Actually, it implements and extends these principles. So, a good understanding of OOP & SOLID helps you a lot while truly implementing the DDD.

DDD Layers & Clean Architecture

There are four fundamental layers of a Domain Driven Based Solution;

Business Logic places into two layers, the Domain layer and the Application Layer, while they contains different kinds of business logic;

• Domain Layer implements the core, use-case independent business logic of the domain/system.
• Application Layer implements the use cases of the application based on the domain. A use case can be thought as a user interaction on the User Interface (UI).
• Presentation Layer contains the UI elements (pages, components) of the application.
• Infrastructure Layer supports other layer by implementing the abstractions and integrations to 3rd-party library and systems.

The same layering can be shown as the diagram below and known as the Clean Architecture, or sometimes the Onion Architecture:

In the Clean Architecture, each layer only depends on the layer directly inside it. The most independent layer is shown in the most inner circle and it is the Domain Layer.

Core Building Blocks

DDD mostly focuses on the Domain & Application Layers and ignores the Presentation and Infrastructure. They are seen as details and the business layers should not depend on them.

That doesn't mean the Presentation and Infrastructure layers are not important. They are very important. UI frameworks and database providers have their own rules and best practices that you need to know and apply. However these are not in the topics of DDD.

This section introduces the essential building blocks of the Domain & Application Layers.

Domain Layer Building Blocks

• Entity: An Entity is an object with its own properties (state, data) and methods that implements the business logic that is executed on these properties. An entity is represented by its unique identifier (Id). Two entity object with different Ids are considered as different entities.
• Value Object: A Value Object is another kind of domain object that is identified by its properties rather than a unique Id. That means two Value Objects with same properties are considered as the same object. Value objects are generally implemented as immutable and mostly are much simpler than the Entities.
• Aggregate & Aggregate Root: An Aggregate is a cluster of objects (entities and value objects) bound together by an Aggregate Root object. The Aggregate Root is a specific type of an entity with some additional responsibilities.
• Repository: A Repository is a collection-like interface that is used by the Domain and Application Layers to access to the data persistence system (the database). It hides the complexity of the DBMS from the business code.
• Domain Service: A Domain Service is a stateless service that implements core business rules of the domain. It is useful to implement domain logic that depends on multiple aggregate (entity) type or some external services.
• Specification: A Specification is used to define named, reusable and combinable filters for entities and other business objects.
• Domain Event: A Domain Event is a way of informing other services in a loosely coupled manner, when a domain specific event occurs.

Application Layer Building Blocks

• Application Service: An Application Service is a stateless service that implements use cases of the application. An application service typically gets and returns DTOs. It is used by the Presentation Layer. It uses and coordinates the domain objects to implement the use cases. A use case is typically considered as a Unit Of Work.
• Data Transfer Object (DTO): A DTO is a simple object without any business logic that is used to transfer state (data) between the Application and Presentation Layers.
• Unit of Work (UOW): A Unit of Work is an atomic work that should be done as a transaction unit. All the operations inside a UOW should be committed on success or rolled back on a failure.

Implementation: The Big Picture

Layering of a .NET Solution

TODO

Execution Flow a DDD Based Application

TODO

Common Principles

TODO