2.3 Core Building Blocks 核心构件

Core Building Blocks 核心构件

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.

DDD主要关注领域和应用层而忽略表现层和基础设施。它们被看作是细节，业务层不应该依赖于它们。

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.

这并不意味着表现层和基础设施层不重要。它们是非常重要的。UI框架和数据提供者有他们自己的规则和最佳实践，你需要了解和应用。然而，这些并不在DDD的考虑范畴。

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

本节介绍领域层和应用层的基本构件。

Domain Layer Building Blocks 领域层构件

• 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.

• 实体：实体是一个有自己属性（状态、数据）和实现对这些属性执行业务逻辑的方法的对象。一个实体由其唯一的标识（Id）来表示。两个具有不同ID的实体对象被认为是不同的实体。

• 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.

• 值对象：值对象是另一种领域对象，它通过其属性而不是唯一标识Id来识别。这意味着两个具有属性相同的值对象被认为是同一个对象。值对象通常被实现为不可变的，而且大多数比实体要简单得多。

• 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.

• 聚合与聚合根：聚合是由聚合根对象关联在一起的一组对象（实体和值对象）。聚合根是一个特定类型的实体，具有一些额外的职责。

• Repository (interface): 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 Layer contains the interfaces of the repositories.

• 存储库：存储库是一个类似集合的接口，被领域层和应用层用来访问数据持久化系统（数据库）。它将DBMS的复杂性从业务代码中隐藏起来。领域层包含存储库的接口。

• 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.

• 领域服务：领域服务是一个无状态的服务，它实现了领域的核心业务规则。它对于实现依赖于多个聚合（实体）类型或一些外部服务的领域逻辑很有用。

• Specification: A Specification is used to define named,reusable and combinable filters for entities and other business objects.

• 规范：规范被用来为实体和其他业务对象定义命名的、可重用性和可组合性的过滤器。

• Domain Event: A Domain Event is a way of informing other services in a loosely coupled manner, when a domain specific event occurs.

• 领域事件：领域事件是当一个领域的特定事件发生时以松散耦合的方式通知其他服务的一种方式。

Application Layer Building Blocks 应用层构件

• 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.

• 应用服务：一个应用服务是一个无状态的服务，它实现了应用程序的用例（交互场景）。一个应用服务通常获取和返回DTO。 它用于表示层。它使用并调用领域对象来实现用例。一个用例通常被认为是一个工作单元。

• 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.

• 数据传输对象：DTO是一个没有任何业务逻辑的简单对象，用于在应用层和表现层之间传输状态（数据）。

• 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.

• 工作单元：一个工作单元是一个原子性的工作，应该作为一个事务单元来完成。一个工作单元内的所有操作在成功时应被提交，在失败时应被回滚。