The goal of this document is to show implications for tosca model template to move from Eclipse-Link to Hibernate. Primary focus is to show the database compatibility and eventually schema changes where is necessary.
In all these tests I am using some strategies to make sure that tables and fields name generated by Hibernate will be the same as Eclipse-Link.

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Definitions

• ElementCollection: Specifies a collection of instances of a basic type or embeddable class. Must be specified if the collection is to be mapped by means of a collection table.
• OneToMany: Specifies a many-valued association with one-to-many multiplicity. If the collection is defined using generics to specify the element type, the associated target entity type need not be specified; otherwise the target entity class must be specified. If the relationship is bidirectional, the mappedBy element must be used to specify the relationship field or property of the entity that is the owner of the relationship. The OneToMany annotation may be used within an embeddable class contained within an entity class to specify a relationship to a collection of entities. If the relationship is bidirectional, the mappedBy element must be used to specify the relationship field or property of the entity that is the owner of the relationship. When the collection is a java.util.Map, the cascade element and the orphanRemoval element apply to the map value.

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Control Loop runtime uses persistence.xml file: is the deployment descriptor file for persistence using JPA. It specifies the persistence units and declares the managed persistence classes, the object/relation mapping, and the database connection details.

Hibernate

SpringBoot auto-configuration can automatically scan entity classes. In Control Loop Runtime we can use @EntityScan annotation because entity classes are not placed in the main application package or its sub-packages. In this situation, we need declare the package or list of packages in the main configuration class within @EntityScan annotation.

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In the most of the cases, in tosca model template there is compatibility in field types. Only two types (java.util.Date and Boolean) are not compatible between Eclipse-Link and Hibernate.

Note about precision in DATETIME (https://mariadb.com/kb/en/datetime/): MariaDB can store microseconds with a precision between 0 and 6. If no microsecond precision is specified, then 0 is used by default.

@Embeddable
@Data
public class ExampleKey implements Serializable {

    private static final long serialVersionUID = 1L;

    @Column(name = "name", length = 120)
    private String name;

    @Column(name = "version", length = 20)
    private String version;
}

@Entity
@Table(name = "Example")
@Data
public class JpaExample implements Serializable {

    private static final long serialVersionUID = 1L;

    @EmbeddedId
    @VerifyKey
    @NotNull
    private ExampleKey key;

    @Column
    private Boolean primed;

    @Column(name = "timeStamp", precision = 3)
    @Temporal(TemporalType.TIMESTAMP)
    @NotNull
    private Date timeStamp;
}

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ElementCollection should be used with basic type or embeddable class, this example shows side effects using it with entity.

@Entity
@Table(name = "ExampleObjMapEc")
@Data
public class JpaExampleObjMapEc implements Serializable {

    private static final long serialVersionUID = 1L;

    @EmbeddedId
    @VerifyKey
    @NotNull
    private ExampleKey key;

    @ElementCollection
    @Lob
    private Map<@NotNull String, @NotNull JpaExample> examples;
}

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ElementCollection should be used with basic type or embeddable class, this example shows side effects using it with entity.

@Entity
@Table(name = "ExampleObjListEc")
@Data
public class JpaExampleObjListEc implements Serializable {

    private static final long serialVersionUID = 1L;

    @EmbeddedId
    @VerifyKey
    @NotNull
    private ExampleKey key;

    @ElementCollection
    @Lob
    private List<@NotNull JpaExample> examples;
}

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MariaDB [controlloop]> select * from ExampleObjMap;
+---------------+---------+
| name          | version |
+---------------+---------+
| ExampleObjMap | 1.0.0   |
+---------------+---------+

MariaDB [controlloop]> select * from Example;
+----------+---------+--------+-----------+
| name     | version | primed | timeStamp |
+----------+---------+--------+-----------+
| example1 | 1.0.0   | NULL   | NULL      |
| example2 | 1.0.0   | NULL   | NULL      |
+----------+---------+--------+-----------+

MariaDB [controlloop]> select * from ExampleObjMap_Example;
+---------------+----------------+------------+---------------+--------------+
| parent_name   | parent_version | child_name | child_version | examples_KEY |
+---------------+----------------+------------+---------------+--------------+
| ExampleObjMap | 1.0.0          | example1   | 1.0.0         | MyKey1       |
| ExampleObjMap | 1.0.0          | example2   | 1.0.0         | MyKey2       |
+---------------+----------------+------------+---------------+--------------+

Multi templates

Right now using Spring repositories it is possible to create more than one service template, as service templates in policy-models/tosca. It works in both Eclipse-Link and Hibernate.

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• Move to Hibernate with EclipseLink tables is possible.
• The presence of longblob types used to store whole objects is an issues, it is readable only by Java language or by any other language compatible with Java binary code. I am not sure if a db-migrator can handle that.
• Document databases store all information for a given object in a single instance in the database, and every stored object can be different from every other. So, using MongoDB/Cassandra will solve all issues. JpaRepository and MongoRepository extend CrudRepository and expose the capabilities of the underlying persistence technology in addition to the rather generic persistence technology-agnostic interfaces like e.g. CrudRepository.