Remote Association

caution

Remote association is a product of combining Jimmer with microservice architecture. It will be introduced in detail in Spring Cloud and Remote Association.

The purpose of this article is not to systematically explain remote associations, just to introduce the mapping configuration required for remote associations.

@Entity

The @Entity annotation has an optional parameter microServiceName whose default value is "".

Association properties involve two entity types. Since associations must point from the source to the target (self association properties are special, the source and target overlap):

• If the microServiceName of the source entity and target entity of the association property are equal, then this association is a local association (All entities in the examples before this article use the default microServiceName, so they are local associations).

• If the microServiceName of the source entity and target entity of the association property are not equal, then this association is a remote association.

For example:

• Owning side (required): Book.authors

  Java

  Book.java

  @Entity(microServiceName = "book-service")
  public interface Book {
  
      @ManyToMany
      List<Author> authors();
  
      ...Omit other code...
  }

  Kotlin

  Book.kt

  @Entity(microServiceName = "book-service") 
  interface Book {
  
      @ManyToMany
      val authors: List<Author>
  
      ...Omit other code...
  }

• Inverse side (optional): Author.authors

  Java

  Author.java

  @Entity(microServiceName = "author-service")
  public interface Author {
  
      @ManyToMany(mappedBy = "authors") 
      List<Book> books();
  
      ...Omit other code...
  }

  Kotlin

  Author.kt

  @Entity(microServiceName = "author-service")
  interface Author {
  
      @ManyToMany(mappedBy = "authors")
      val books: List<Book>
  
      ...Omit other code...
  }

Here, the microservice name of Book is book-service, and the microservice name of Author is author-service. The two are not equal, so Book.authors and Author.books are remote associations.

@MappedSuperclass

As a super type, MappedSuperclass has two usages:

• Can contain association properties, but must belong to the same microservice

  Java

  BookServiceCommonEntity.java

  @MappedSuperclass(microServiceName = "book-service")
  public interface BookServiceCommonEntity {
  
      LocalDateTime createdTime();
  
      @ManyToOne  
      User createdBy();
  
      LocalDateTime modifiedTime();
  
      @ManyToOne
      User modifiedBy();
  }

  Kotlin

  BookServiceCommonEntity.kt

  @MappedSuperclass(microServiceName = "book-service") 
  interface BookServiceCommonEntity {
  
      val createdTime: LocalDateTime
  
      @ManyToOne
      val createdBy: User
  
      val modifiedTime: LocalDateTime
  
      @ManyToOne
      val modifiedBy: User
  }

  Here BookServiceCommonEntity contains association properties createdBy and modifiedBy, but only Entity or other MappedSuperclass belonging to the microservice "book-service" can inherit it.

• Can span any microservices, but must not contain association properties

  Java

  CommonEntity.java

  @MappedSuperclass(acrossMicroServices = true)
  public interface CommonEntity {
  
      LocalDateTime createdTime();
  
      LocalDateTime modifiedTime(); 
  }

  Kotlin

  CommonEntity.kt

  @MappedSuperclass(acrossMicroServices = true)  
  interface CommonEntity {
  
      val createdTime: LocalDateTime
  
      val modifiedTime: LocalDateTime
  }

  Here CommonEntity can span any microservices. Any Entity or other MappedSuperclass under any microservice can inherit it, but it itself does not contain any association properties.