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"I'm beginning to feel like a Rap God"

DataMapper in rap for domain model

Rapper is a Reactive Data Mapper library for Java, which provides an implementation of a Data Mapper for a domain entity. This implementations follows the principles stated by Martin Fowler in [Fowler, 2002] about Object-Relational Behavioral and Data Source Architectural Patterns, but applied to a reactive approach. Thus the Rapper API is asynchronous and the data mapper effects are strongly typed in Promises [Friedman and Wise, 1976].

The Rapper implementation use the following design patterns:


  • MapperRegistry is the class to aim for when we want to manipulate the data. This static class supplies DataMapper instances for data manipulation.

  • Each UnitOfWork will have an Identity Map, which holds the recents objects read/altered from the DB.

  • The objects in the Identity Map are immutable, to change the data, a new immutable object will be created to replace the one in the map. The replacement will only happen if the commit is successful.

  • A UnitOfWork will have a transaction per commit, this means, when a commit is done on a UnitOfWork, a new transaction will be opened as soon a connection to the DB is needed.

  • There will be a connection pool to establish links with the DB. This allows the re-use of connections already made, avoiding the creation of multiple connections.

  • The Identity Map implements a LRU cache. They have a limit of what they can hold in memory, and when it reaches the limit, they will delete the least-read elements.

  • A DomainObject may contain Function<UnitOfWork, CompletableFuture<DomainObject>> as a reference to another table. The field holding it, must be annotated with @ColumnName, in which is passed name as the name of the column(s) in the representing table where the ID of the external DomainObject takes place.

  • A DomainObject may also contain a Function<UnitOfWork, CompletableFuture<List<DomainObject>>> as a reference to another table. The field holding it, must be annotated with @ColumnName, in which is passed foreignName as the name of the column(s) in the referenced table where the ID of the DomainObject takes place, the table in case of N-N relation and the externalName which is the name of the column(s) in the referenced table where the ID of the external DomainObject takes place. externalName is only needed when table is given.

  • Each DomainObject may have a version field (an auto incremented field on insertions and updates). This field must be annotated with @Version and it will be used to successful synchronise the in-memory data and DB as it will only allow writes in the DB, if the DomainObject's version that is being written exists on it.


  • You must create an environment variable to connect to the DB. The environment variable must have the following format:




  • All domain objects must implement interface DomainObject.

  • The domain object classes must have the same name as respective DB table, same for the fields/rows.

  • The field corresponding to the primary key must have the annotation @Id and if it's auto-generated by the DB, isIdentity() must return true. If it is a composed key it is needed to create a class that extends EmbeddedIdClass. This class must contain the keys and respective names, the domain object must have a field of that class and mark it with the annotation @EmbeddedId.

  • The class that extends EmbeddedIdClass must call its super on the contructor, passing the values of the ids

  • A DomainObject and the field annotated with @EmbeddedId must have a 0 arguments constructor.


An example illustrating the correct use of DomainObject interface and the annotations.

public class Book implements DomainObject<Long> {

    @Id(isIdentity = true)
    private long id;
    private String name;
    private long version;

    @ColumnName(foreignName = "bookId", table = "BookAuthor", externalName = "authorId")
    private Function<UnitOfWork, CompletableFuture<List<Author>>> authors;

    public Book(long id, String name, long version, Function<UnitOfWork, CompletableFuture<List<Author>>> authors) { = id; = name;
        this.version = version;
        this.authors = authors;

    public Book() { }

    public String getName() {
        return name;

    public Function<UnitOfWork, CompletableFuture<List<Author>>> getAuthors() {
        return authors;

    public Long getIdentityKey() {
        return id;

    public long getVersion() {
        return version;

The table Book has an N-N relation with table Author, being BookAuthor the table that holds both table's primary keys. This table has 2 columns, being bookId the id of the Book and authorId the id of the Author. As such Book class has a CompletableFuture<List<Author>> with the references to the book's authors.
The primary key of the table Book is an auto incremented value, so we mark it with the annotation @Id and say it's an identity value


  • Although the API is asynchronous, JDBC driver is blocking and thus this internal implementation is blocking too.

  • Because of the use of vertx-jdbc-client library, we do not support fields of type short


Async DataMapper for JDBC




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