Merge pull request #595 from isabiq/master

CQRS pattern
2017-07-29 20:45:55 +03:00 · 2017-07-29 20:45:55 +03:00 · 0982f00a61
commit 0982f00a61
parent ac721c695c 871df4f918
21 changed files with 1142 additions and 0 deletions
--- a/cqrs/README.md
+++ b/cqrs/README.md
@ -0,0 +1,29 @@
+---
+layout: pattern
+title: CQRS
+folder: cqrs
+permalink: /patterns/cqrs/
+pumlid: 7SPR4a0m3030gt00pR_RH6I8QQFouFgC_TfHb6gkd5Q7FQBx363ub4rYpoMTZKuDrYXqDX37HIuuyCPfPPTDfuuHREhGqBy0NUR0GNzAMYizMtq1
+categories: Architectural
+tags:
+ - Java
+ - Difficulty-Intermediate
+---
+
+## Intent
+CQRS Command Query Responsibility Segregation - Separate the query side from the command side.
+
+![alt text](./etc/cqrs.png "CQRS")
+
+## Applicability
+Use the CQRS pattern when
+
+* you want to scale the queries and commands independently.
+* you want to use different data models for queries and commands. Useful when dealing with complex domains.
+* you want to use architectures like event sourcing or task based UI.
+
+## Credits
+
+* [Greg Young - CQRS, Task Based UIs, Event Sourcing agh!](http://codebetter.com/gregyoung/2010/02/16/cqrs-task-based-uis-event-sourcing-agh/)
+* [Martin Fowler - CQRS](https://martinfowler.com/bliki/CQRS.html)
+* [Oliver Wolf - CQRS for Great Good](https://www.youtube.com/watch?v=Ge53swja9Dw)
--- a/cqrs/etc/cqrs.png
+++ b/cqrs/etc/cqrs.png
--- a/cqrs/etc/cqrs.ucls
+++ b/cqrs/etc/cqrs.ucls
@ -0,0 +1,115 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.2.0" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
+  associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
+  <interface id="1" language="java" name="com.iluwatar.cqrs.commandes.ICommandService" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/commandes/ICommandService.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="291" y="-49"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </interface>  
+  <class id="2" language="java" name="com.iluwatar.cqrs.commandes.CommandServiceImpl" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/commandes/CommandServiceImpl.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="263" width="256" x="170" y="87"/>    
+    <display autosize="false" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <interface id="3" language="java" name="com.iluwatar.cqrs.queries.IQueryService" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/queries/IQueryService.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="182" width="248" x="176" y="428"/>    
+    <display autosize="false" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </interface>  
+  <class id="4" language="java" name="com.iluwatar.cqrs.queries.QueryServiceImpl" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/queries/QueryServiceImpl.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="258" width="253" x="169" y="665"/>    
+    <display autosize="false" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="5" language="java" name="com.iluwatar.cqrs.domain.model.Book" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/domain/model/Book.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="326" width="158" x="778" y="-93"/>    
+    <display autosize="false" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="6" language="java" name="com.iluwatar.cqrs.dto.Book" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/dto/Book.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="219" width="150" x="541" y="607"/>    
+    <display autosize="false" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="7" language="java" name="com.iluwatar.cqrs.domain.model.Author" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/domain/model/Author.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="608" y="70"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="8" language="java" name="com.iluwatar.cqrs.dto.Author" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/dto/Author.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="834" y="719"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="9" language="java" name="com.iluwatar.cqrs.util.HibernateUtil" project="cqrs" 
+    file="/cqrs/src/main/java/com/iluwatar/cqrs/util/HibernateUtil.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="662" y="412"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <dependency id="10">    
+    <end type="SOURCE" refId="4"/>    
+    <end type="TARGET" refId="9"/>  
+  </dependency>  
+  <dependency id="11">    
+    <end type="SOURCE" refId="2"/>    
+    <end type="TARGET" refId="9"/>  
+  </dependency>  
+  <realization id="12">    
+    <end type="SOURCE" refId="4"/>    
+    <end type="TARGET" refId="3"/>  
+  </realization>  
+  <association id="13">    
+    <end type="SOURCE" refId="5" navigable="false">      
+      <attribute id="14" name="author"/>      
+      <multiplicity id="15" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="7" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <realization id="16">    
+    <end type="SOURCE" refId="2"/>    
+    <end type="TARGET" refId="1"/>  
+  </realization>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>
--- a/cqrs/etc/cqrs.urm.puml
+++ b/cqrs/etc/cqrs.urm.puml
@ -0,0 +1,124 @@
+@startuml
+package com.iluwatar.cqrs.util {
+  class HibernateUtil {
+    - LOGGER : Logger {static}
+    - SESSIONFACTORY : SessionFactory {static}
+    + HibernateUtil()
+    - buildSessionFactory() : SessionFactory {static}
+    + getSessionFactory() : SessionFactory {static}
+  }
+}
+package com.iluwatar.cqrs.app {
+  class App {
+    + App()
+    + main(args : String[]) {static}
+  }
+}
+package com.iluwatar.cqrs.dto {
+  class Author {
+    - email : String
+    - name : String
+    - username : String
+    + Author()
+    + Author(name : String, email : String, username : String)
+    + equals(obj : Object) : boolean
+    + getEmail() : String
+    + getName() : String
+    + getUsername() : String
+    + hashCode() : int
+    + toString() : String
+  }
+  class Book {
+    - price : double
+    - title : String
+    + Book()
+    + Book(title : String, price : double)
+    + equals(obj : Object) : boolean
+    + getPrice() : double
+    + getTitle() : String
+    + hashCode() : int
+    + toString() : String
+  }
+}
+package com.iluwatar.cqrs.commandes {
+  class CommandServiceImpl {
+    - sessionFactory : SessionFactory
+    + CommandServiceImpl()
+    + authorCreated(username : String, name : String, email : String)
+    + authorEmailUpdated(username : String, email : String)
+    + authorNameUpdated(username : String, name : String)
+    + authorUsernameUpdated(oldUsername : String, newUsername : String)
+    + bookAddedToAuthor(title : String, price : double, username : String)
+    + bookPriceUpdated(title : String, price : double)
+    + bookTitleUpdated(oldTitle : String, newTitle : String)
+    - getAuthorByUsername(username : String) : Author
+    - getBookByTitle(title : String) : Book
+  }
+  interface ICommandService {
+    + authorCreated(String, String, String) {abstract}
+    + authorEmailUpdated(String, String) {abstract}
+    + authorNameUpdated(String, String) {abstract}
+    + authorUsernameUpdated(String, String) {abstract}
+    + bookAddedToAuthor(String, double, String) {abstract}
+    + bookPriceUpdated(String, double) {abstract}
+    + bookTitleUpdated(String, String) {abstract}
+  }
+}
+package com.iluwatar.cqrs.queries {
+  interface IQueryService {
+    + getAuthorBooks(String) : List<Book> {abstract}
+    + getAuthorBooksCount(String) : BigInteger {abstract}
+    + getAuthorByUsername(String) : Author {abstract}
+    + getAuthorsCount() : BigInteger {abstract}
+    + getBook(String) : Book {abstract}
+  }
+  class QueryServiceImpl {
+    - sessionFactory : SessionFactory
+    + QueryServiceImpl()
+    + getAuthorBooks(username : String) : List<Book>
+    + getAuthorBooksCount(username : String) : BigInteger
+    + getAuthorByUsername(username : String) : Author
+    + getAuthorsCount() : BigInteger
+    + getBook(title : String) : Book
+  }
+}
+package com.iluwatar.cqrs.domain.model {
+  class Author {
+    - email : String
+    - id : long
+    - name : String
+    - username : String
+    # Author()
+    + Author(username : String, name : String, email : String)
+    + getEmail() : String
+    + getId() : long
+    + getName() : String
+    + getUsername() : String
+    + setEmail(email : String)
+    + setId(id : long)
+    + setName(name : String)
+    + setUsername(username : String)
+    + toString() : String
+  }
+  class Book {
+    - author : Author
+    - id : long
+    - price : double
+    - title : String
+    # Book()
+    + Book(title : String, price : double, author : Author)
+    + getAuthor() : Author
+    + getId() : long
+    + getPrice() : double
+    + getTitle() : String
+    + setAuthor(author : Author)
+    + setId(id : long)
+    + setPrice(price : double)
+    + setTitle(title : String)
+    + toString() : String
+  }
+}
+Book -->  "-author" Author
+CommandServiceImpl ..|> ICommandService 
+QueryServiceImpl ..|> IQueryService 
+@enduml
--- a/cqrs/pom.xml
+++ b/cqrs/pom.xml
@ -0,0 +1,42 @@
+<?xml version="1.0"?>
+<!-- The MIT License Copyright (c) 2014-2016 Ilkka Seppälä Permission is 
+	hereby granted, free of charge, to any person obtaining a copy of this software 
+	and associated documentation files (the "Software"), to deal in the Software 
+	without restriction, including without limitation the rights to use, copy, 
+	modify, merge, publish, distribute, sublicense, and/or sell copies of the 
+	Software, and to permit persons to whom the Software is furnished to do so, 
+	subject to the following conditions: The above copyright notice and this 
+	permission notice shall be included in all copies or substantial portions 
+	of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY 
+	KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 
+	MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN 
+	NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, 
+	DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, 
+	ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER 
+	DEALINGS IN THE SOFTWARE. -->
+<project
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"
+	xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+	<modelVersion>4.0.0</modelVersion>
+	<parent>
+		<groupId>com.iluwatar</groupId>
+		<artifactId>java-design-patterns</artifactId>
+		<version>1.17.0-SNAPSHOT</version>
+	</parent>
+	<artifactId>cqrs</artifactId>
+	<dependencies>
+		<dependency>
+			<groupId>junit</groupId>
+			<artifactId>junit</artifactId>
+			<scope>test</scope>
+		</dependency>
+		<dependency>
+			<groupId>com.h2database</groupId>
+			<artifactId>h2</artifactId>
+		</dependency>
+		<dependency>
+			<groupId>org.hibernate</groupId>
+			<artifactId>hibernate-core</artifactId>
+		</dependency>
+	</dependencies>
+</project>
--- a/cqrs/src/main/java/com/iluwatar/cqrs/app/App.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/app/App.java
@ -0,0 +1,95 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014-2016 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.cqrs.app;
+
+import java.math.BigInteger;
+import java.util.List;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.iluwatar.cqrs.commandes.CommandServiceImpl;
+import com.iluwatar.cqrs.commandes.ICommandService;
+import com.iluwatar.cqrs.dto.Author;
+import com.iluwatar.cqrs.dto.Book;
+import com.iluwatar.cqrs.queries.IQueryService;
+import com.iluwatar.cqrs.queries.QueryServiceImpl;
+import com.iluwatar.cqrs.util.HibernateUtil;
+
+/**
+ * CQRS : Command Query Responsibility Segregation. A pattern used to separate query services from commands or writes
+ * services. The pattern is very simple but it has many consequences. For example, it can be used to tackle down a
+ * complex domain, or to use other architectures that were hard to implement with the classical way.
+ * 
+ * This implementation is an example of managing books and authors in a library. The persistence of books and authors is
+ * done according to the CQRS architecture. A command side that deals with a data model to persist(insert,update,delete)
+ * objects to a database. And a query side that uses native queries to get data from the database and return objects as
+ * DTOs (Data transfer Objects).
+ *
+ */
+public class App {
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Program entry point
+   * 
+   * @param args
+   *          command line args
+   */
+  public static void main(String[] args) {
+    ICommandService commands = new CommandServiceImpl();
+
+    // Create Authors and Books using CommandService
+    commands.authorCreated("eEvans", "Eric Evans", "eEvans@email.com");
+    commands.authorCreated("jBloch", "Joshua Bloch", "jBloch@email.com");
+    commands.authorCreated("mFowler", "Martin Fowler", "mFowler@email.com");
+
+    commands.bookAddedToAuthor("Domain-Driven Design", 60.08, "eEvans");
+    commands.bookAddedToAuthor("Effective Java", 40.54, "jBloch");
+    commands.bookAddedToAuthor("Java Puzzlers", 39.99, "jBloch");
+    commands.bookAddedToAuthor("Java Concurrency in Practice", 29.40, "jBloch");
+    commands.bookAddedToAuthor("Patterns of Enterprise Application Architecture", 54.01, "mFowler");
+    commands.bookAddedToAuthor("Domain Specific Languages", 48.89, "mFowler");
+    commands.authorNameUpdated("eEvans", "Eric J. Evans");
+
+    IQueryService queries = new QueryServiceImpl();
+
+    // Query the database using QueryService
+    Author nullAuthor = queries.getAuthorByUsername("username");
+    Author eEvans = queries.getAuthorByUsername("eEvans");
+    BigInteger jBlochBooksCount = queries.getAuthorBooksCount("jBloch");
+    BigInteger authorsCount = queries.getAuthorsCount();
+    Book dddBook = queries.getBook("Domain-Driven Design");
+    List<Book> jBlochBooks = queries.getAuthorBooks("jBloch");
+
+    LOGGER.info("Author username : {}", nullAuthor);
+    LOGGER.info("Author eEvans : {}", eEvans);
+    LOGGER.info("jBloch number of books : {}", jBlochBooksCount);
+    LOGGER.info("Number of authors : {}", authorsCount);
+    LOGGER.info("DDD book : {}", dddBook);
+    LOGGER.info("jBloch books : {}", jBlochBooks);
+
+    HibernateUtil.getSessionFactory().close();
+  }
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/commandes/CommandServiceImpl.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/commandes/CommandServiceImpl.java
@ -0,0 +1,123 @@
+package com.iluwatar.cqrs.commandes;
+
+import org.hibernate.Query;
+import org.hibernate.Session;
+import org.hibernate.SessionFactory;
+
+import com.iluwatar.cqrs.domain.model.Author;
+import com.iluwatar.cqrs.domain.model.Book;
+import com.iluwatar.cqrs.util.HibernateUtil;
+
+/**
+ * This class is an implementation of {@link ICommandService} interface. It uses Hibernate as an api for persistence.
+ *
+ */
+public class CommandServiceImpl implements ICommandService {
+
+  private SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
+
+  private Author getAuthorByUsername(String username) {
+    Author author = null;
+    try (Session session = sessionFactory.openSession()) {
+      Query query = session.createQuery("from Author where username=:username");
+      query.setParameter("username", username);
+      author = (Author) query.uniqueResult();
+    }
+    if (author == null) {
+      HibernateUtil.getSessionFactory().close();
+      throw new NullPointerException("Author " + username + " doesn't exist!");
+    }
+    return author;
+  }
+
+  private Book getBookByTitle(String title) {
+    Book book = null;
+    try (Session session = sessionFactory.openSession()) {
+      Query query = session.createQuery("from Book where title=:title");
+      query.setParameter("title", title);
+      book = (Book) query.uniqueResult();
+    }
+    if (book == null) {
+      HibernateUtil.getSessionFactory().close();
+      throw new NullPointerException("Book " + title + " doesn't exist!");
+    }
+    return book;
+  }
+
+  @Override
+  public void authorCreated(String username, String name, String email) {
+    Author author = new Author(username, name, email);
+    try (Session session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.save(author);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void bookAddedToAuthor(String title, double price, String username) {
+    Author author = getAuthorByUsername(username);
+    Book book = new Book(title, price, author);
+    try (Session session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.save(book);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void authorNameUpdated(String username, String name) {
+    Author author = getAuthorByUsername(username);
+    author.setName(name);
+    try (Session session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(author);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void authorUsernameUpdated(String oldUsername, String newUsername) {
+    Author author = getAuthorByUsername(oldUsername);
+    author.setUsername(newUsername);
+    try (Session session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(author);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void authorEmailUpdated(String username, String email) {
+    Author author = getAuthorByUsername(username);
+    author.setEmail(email);
+    try (Session session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(author);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void bookTitleUpdated(String oldTitle, String newTitle) {
+    Book book = getBookByTitle(oldTitle);
+    book.setTitle(newTitle);
+    try (Session session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(book);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void bookPriceUpdated(String title, double price) {
+    Book book = getBookByTitle(title);
+    book.setPrice(price);
+    try (Session session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(book);
+      session.getTransaction().commit();
+    }
+  }
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/commandes/ICommandService.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/commandes/ICommandService.java
@ -0,0 +1,23 @@
+package com.iluwatar.cqrs.commandes;
+
+/**
+ * This interface represents the commands of the CQRS pattern
+ *
+ */
+public interface ICommandService {
+
+  void authorCreated(String username, String name, String email);
+
+  void bookAddedToAuthor(String title, double price, String username);
+
+  void authorNameUpdated(String username, String name);
+
+  void authorUsernameUpdated(String oldUsername, String newUsername);
+
+  void authorEmailUpdated(String username, String email);
+
+  void bookTitleUpdated(String oldTitle, String newTitle);
+
+  void bookPriceUpdated(String title, double price);
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/domain/model/Author.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/domain/model/Author.java
@ -0,0 +1,78 @@
+package com.iluwatar.cqrs.domain.model;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+
+/**
+ * This is an Author entity. It is used by Hibernate for persistence.
+ *
+ */
+@Entity
+public class Author {
+  @Id
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
+  private long id;
+  private String username;
+  private String name;
+  private String email;
+
+  /**
+   * 
+   * @param username
+   *          username of the author
+   * @param name
+   *          name of the author
+   * @param email
+   *          email of the author
+   */
+  public Author(String username, String name, String email) {
+    super();
+    this.username = username;
+    this.name = name;
+    this.email = email;
+  }
+
+  protected Author() {
+    super();
+  }
+
+  public long getId() {
+    return id;
+  }
+
+  public void setId(long id) {
+    this.id = id;
+  }
+
+  public String getUsername() {
+    return username;
+  }
+
+  public void setUsername(String username) {
+    this.username = username;
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  public void setName(String name) {
+    this.name = name;
+  }
+
+  public String getEmail() {
+    return email;
+  }
+
+  public void setEmail(String email) {
+    this.email = email;
+  }
+
+  @Override
+  public String toString() {
+    return "Author [name=" + name + ", email=" + email + "]";
+  }
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/domain/model/Book.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/domain/model/Book.java
@ -0,0 +1,80 @@
+package com.iluwatar.cqrs.domain.model;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+
+/**
+ * This is a Book entity. It is used by Hibernate for persistence. Many books can be written by one {@link Author}
+ *
+ */
+@Entity
+public class Book {
+  @Id
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
+  private long id;
+  private String title;
+  private double price;
+  @ManyToOne
+  private Author author;
+
+  /**
+   * 
+   * @param title
+   *          title of the book
+   * @param price
+   *          price of the book
+   * @param author
+   *          author of the book
+   */
+  public Book(String title, double price, Author author) {
+    super();
+    this.title = title;
+    this.price = price;
+    this.author = author;
+  }
+
+  protected Book() {
+    super();
+  }
+
+  public long getId() {
+    return id;
+  }
+
+  public void setId(long id) {
+    this.id = id;
+  }
+
+  public String getTitle() {
+    return title;
+  }
+
+  public void setTitle(String title) {
+    this.title = title;
+  }
+
+  public double getPrice() {
+    return price;
+  }
+
+  public void setPrice(double price) {
+    this.price = price;
+  }
+
+  public Author getAuthor() {
+    return author;
+  }
+
+  public void setAuthor(Author author) {
+    this.author = author;
+  }
+
+  @Override
+  public String toString() {
+    return "Book [title=" + title + ", price=" + price + ", author=" + author + "]";
+  }
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/dto/Author.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/dto/Author.java
@ -0,0 +1,71 @@
+package com.iluwatar.cqrs.dto;
+
+import java.util.Objects;
+
+/**
+ * 
+ * This is a DTO (Data Transfer Object) author, contains only useful information to be returned
+ *
+ */
+public class Author {
+
+  private String name;
+  private String email;
+  private String username;
+
+  /**
+   * 
+   * @param name
+   *          name of the author
+   * @param email
+   *          email of the author
+   * @param username
+   *          username of the author
+   */
+  public Author(String name, String email, String username) {
+    super();
+    this.name = name;
+    this.email = email;
+    this.username = username;
+  }
+
+  public Author() {
+    super();
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  public String getEmail() {
+    return email;
+  }
+
+  public String getUsername() {
+    return username;
+  }
+
+  @Override
+  public String toString() {
+    return "AuthorDTO [name=" + name + ", email=" + email + ", username=" + username + "]";
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(username, name, email);
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (this == obj) {
+      return true;
+    }
+    if (!(obj instanceof Author)) {
+      return false;
+    }
+    Author other = (Author) obj;
+    return username.equals(other.getUsername()) && email.equals(other.getEmail()) && name.equals(other.getName());
+
+  }
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/dto/Book.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/dto/Book.java
@ -0,0 +1,62 @@
+package com.iluwatar.cqrs.dto;
+
+import java.util.Objects;
+
+/**
+ * 
+ * This is a DTO (Data Transfer Object) book, contains only useful information to be returned
+ *
+ */
+public class Book {
+
+  private String title;
+  private double price;
+
+  /**
+   * 
+   * @param title
+   *          title of the book
+   * @param price
+   *          price of the book
+   */
+  public Book(String title, double price) {
+    super();
+    this.title = title;
+    this.price = price;
+  }
+
+  public Book() {
+    super();
+  }
+
+  public String getTitle() {
+    return title;
+  }
+
+  public double getPrice() {
+    return price;
+  }
+
+  @Override
+  public String toString() {
+    return "BookDTO [title=" + title + ", price=" + price + "]";
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(title, price);
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (this == obj) {
+      return true;
+    }
+    if (!(obj instanceof Book)) {
+      return false;
+    }
+    Book book = (Book) obj;
+    return title.equals(book.getTitle()) && price == book.getPrice();
+  }
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/queries/IQueryService.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/queries/IQueryService.java
@ -0,0 +1,26 @@
+package com.iluwatar.cqrs.queries;
+
+import java.math.BigInteger;
+import java.util.List;
+
+import com.iluwatar.cqrs.dto.Author;
+import com.iluwatar.cqrs.dto.Book;
+
+/**
+ * 
+ * This interface represents the query methods of the CQRS pattern
+ *
+ */
+public interface IQueryService {
+
+  Author getAuthorByUsername(String username);
+
+  Book getBook(String title);
+
+  List<Book> getAuthorBooks(String username);
+
+  BigInteger getAuthorBooksCount(String username);
+
+  BigInteger getAuthorsCount();
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/queries/QueryServiceImpl.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/queries/QueryServiceImpl.java
@ -0,0 +1,83 @@
+package com.iluwatar.cqrs.queries;
+
+import java.math.BigInteger;
+import java.util.List;
+
+import org.hibernate.SQLQuery;
+import org.hibernate.Session;
+import org.hibernate.SessionFactory;
+import org.hibernate.transform.Transformers;
+
+import com.iluwatar.cqrs.dto.Author;
+import com.iluwatar.cqrs.dto.Book;
+import com.iluwatar.cqrs.util.HibernateUtil;
+
+/**
+ * This class is an implementation of {@link IQueryService}. It uses Hibernate native queries to return DTOs from the
+ * database.
+ *
+ */
+public class QueryServiceImpl implements IQueryService {
+
+  private SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
+
+  @Override
+  public Author getAuthorByUsername(String username) {
+    Author authorDTo = null;
+    try (Session session = sessionFactory.openSession()) {
+      SQLQuery sqlQuery = session
+          .createSQLQuery("SELECT a.username as \"username\", a.name as \"name\", a.email as \"email\""
+              + "FROM Author a where a.username=:username");
+      sqlQuery.setParameter("username", username);
+      authorDTo = (Author) sqlQuery.setResultTransformer(Transformers.aliasToBean(Author.class)).uniqueResult();
+    }
+    return authorDTo;
+  }
+
+  @Override
+  public Book getBook(String title) {
+    Book bookDTo = null;
+    try (Session session = sessionFactory.openSession()) {
+      SQLQuery sqlQuery = session
+          .createSQLQuery("SELECT b.title as \"title\", b.price as \"price\"" + " FROM Book b where b.title=:title");
+      sqlQuery.setParameter("title", title);
+      bookDTo = (Book) sqlQuery.setResultTransformer(Transformers.aliasToBean(Book.class)).uniqueResult();
+    }
+    return bookDTo;
+  }
+
+  @Override
+  public List<Book> getAuthorBooks(String username) {
+    List<Book> bookDTos = null;
+    try (Session session = sessionFactory.openSession()) {
+      SQLQuery sqlQuery = session.createSQLQuery("SELECT b.title as \"title\", b.price as \"price\""
+          + " FROM Author a , Book b where b.author_id = a.id and a.username=:username");
+      sqlQuery.setParameter("username", username);
+      bookDTos = sqlQuery.setResultTransformer(Transformers.aliasToBean(Book.class)).list();
+    }
+    return bookDTos;
+  }
+
+  @Override
+  public BigInteger getAuthorBooksCount(String username) {
+    BigInteger bookcount = null;
+    try (Session session = sessionFactory.openSession()) {
+      SQLQuery sqlQuery = session.createSQLQuery(
+          "SELECT count(b.title)" + " FROM  Book b, Author a where b.author_id = a.id and a.username=:username");
+      sqlQuery.setParameter("username", username);
+      bookcount = (BigInteger) sqlQuery.uniqueResult();
+    }
+    return bookcount;
+  }
+
+  @Override
+  public BigInteger getAuthorsCount() {
+    BigInteger authorcount = null;
+    try (Session session = sessionFactory.openSession()) {
+      SQLQuery sqlQuery = session.createSQLQuery("SELECT count(id) from Author");
+      authorcount = (BigInteger) sqlQuery.uniqueResult();
+    }
+    return authorcount;
+  }
+
+}
--- a/cqrs/src/main/java/com/iluwatar/cqrs/util/HibernateUtil.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/util/HibernateUtil.java
@ -0,0 +1,36 @@
+package com.iluwatar.cqrs.util;
+
+import org.hibernate.SessionFactory;
+import org.hibernate.boot.MetadataSources;
+import org.hibernate.boot.registry.StandardServiceRegistry;
+import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This class simply returns one instance of {@link SessionFactory} initialized when the application is started
+ *
+ */
+public class HibernateUtil {
+
+  private static final SessionFactory SESSIONFACTORY = buildSessionFactory();
+  private static final Logger LOGGER = LoggerFactory.getLogger(HibernateUtil.class);
+
+  private static SessionFactory buildSessionFactory() {
+
+    // configures settings from hibernate.cfg.xml
+    final StandardServiceRegistry registry = new StandardServiceRegistryBuilder().configure().build();
+    try {
+      return new MetadataSources(registry).buildMetadata().buildSessionFactory();
+    } catch (Exception ex) {
+      StandardServiceRegistryBuilder.destroy(registry);
+      LOGGER.error("Initial SessionFactory creation failed." + ex);
+      throw new ExceptionInInitializerError(ex);
+    }
+  }
+
+  public static SessionFactory getSessionFactory() {
+    return SESSIONFACTORY;
+  }
+
+}
--- a/cqrs/src/main/resources/hibernate.cfg.xml
+++ b/cqrs/src/main/resources/hibernate.cfg.xml
@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE hibernate-configuration SYSTEM
+"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
+
+<hibernate-configuration>
+    <session-factory>
+        <property name="dialect">org.hibernate.dialect.H2Dialect</property>
+        <property name="connection.driver_class">org.h2.Driver</property>
+        <property name="connection.url">jdbc:h2:mem:test</property>
+        <property name="connection.username">sa</property>
+        <property name="hbm2ddl.auto">create</property>
+        <mapping class="com.iluwatar.cqrs.domain.model.Author" />
+        <mapping class="com.iluwatar.cqrs.domain.model.Book" />
+    </session-factory>
+</hibernate-configuration>
--- a/cqrs/src/main/resources/logback.xml
+++ b/cqrs/src/main/resources/logback.xml
@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <root level="info">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>
--- a/cqrs/src/test/java/com/iluwatar/cqrs/IntegrationTest.java
+++ b/cqrs/src/test/java/com/iluwatar/cqrs/IntegrationTest.java
@ -0,0 +1,98 @@
+package com.iluwatar.cqrs;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.math.BigInteger;
+import java.util.List;
+
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import com.iluwatar.cqrs.commandes.CommandServiceImpl;
+import com.iluwatar.cqrs.commandes.ICommandService;
+import com.iluwatar.cqrs.dto.Author;
+import com.iluwatar.cqrs.dto.Book;
+import com.iluwatar.cqrs.queries.IQueryService;
+import com.iluwatar.cqrs.queries.QueryServiceImpl;
+
+/**
+ * Integration test of IQueryService and ICommandService with h2 data
+ *
+ */
+public class IntegrationTest {
+
+  private static IQueryService queryService;
+  private static ICommandService commandService;
+
+  @BeforeClass
+  public static void initialize() {
+    commandService = new CommandServiceImpl();
+    queryService = new QueryServiceImpl();
+  }
+
+  @BeforeClass
+  public static void populateDatabase() {
+    // create first author1
+    commandService.authorCreated("username1", "name1", "email1");
+
+    // create author1 and update all its data
+    commandService.authorCreated("username2", "name2", "email2");
+    commandService.authorEmailUpdated("username2", "new_email2");
+    commandService.authorNameUpdated("username2", "new_name2");
+    commandService.authorUsernameUpdated("username2", "new_username2");
+
+    // add book1 to author1
+    commandService.bookAddedToAuthor("title1", 10, "username1");
+
+    // add book2 to author1 and update all its data
+    commandService.bookAddedToAuthor("title2", 20, "username1");
+    commandService.bookPriceUpdated("title2", 30);
+    commandService.bookTitleUpdated("title2", "new_title2");
+
+  }
+
+  @Test
+  public void testGetAuthorByUsername() {
+    Author author = queryService.getAuthorByUsername("username1");
+    assertEquals("username1", author.getUsername());
+    assertEquals("name1", author.getName());
+    assertEquals("email1", author.getEmail());
+  }
+
+  @Test
+  public void testGetUpdatedAuthorByUsername() {
+    Author author = queryService.getAuthorByUsername("new_username2");
+    Author expectedAuthor = new Author("new_name2", "new_email2", "new_username2");
+    assertEquals(expectedAuthor, author);
+
+  }
+
+  @Test
+  public void testGetBook() {
+    Book book = queryService.getBook("title1");
+    assertEquals("title1", book.getTitle());
+    assertEquals(10, book.getPrice(), 0);
+  }
+
+  @Test
+  public void testGetAuthorBooks() {
+    List<Book> books = queryService.getAuthorBooks("username1");
+    assertTrue(books.size() == 2);
+    assertTrue(books.contains(new Book("title1", 10)));
+    assertTrue(books.contains(new Book("new_title2", 30)));
+  }
+
+  @Test
+  public void testGetAuthorBooksCount() {
+    BigInteger bookCount = queryService.getAuthorBooksCount("username1");
+    assertEquals(new BigInteger("2"), bookCount);
+  }
+
+  @Test
+  public void testGetAuthorsCount() {
+    BigInteger authorCount = queryService.getAuthorsCount();
+    assertEquals(new BigInteger("2"), authorCount);
+  }
+
+}
--- a/cqrs/src/test/resources/hibernate.cfg.xml
+++ b/cqrs/src/test/resources/hibernate.cfg.xml
@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE hibernate-configuration SYSTEM
+"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
+
+<hibernate-configuration>
+    <session-factory>
+        <property name="dialect">org.hibernate.dialect.H2Dialect</property>
+        <property name="connection.driver_class">org.h2.Driver</property>
+        <property name="connection.url">jdbc:h2:mem:test</property>
+        <property name="connection.username">sa</property>
+        <property name="hbm2ddl.auto">create</property>
+        <mapping class="com.iluwatar.cqrs.domain.model.Author" />
+        <mapping class="com.iluwatar.cqrs.domain.model.Book" />
+    </session-factory>
+</hibernate-configuration>
--- a/cqrs/src/test/resources/logback.xml
+++ b/cqrs/src/test/resources/logback.xml
@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <root level="info">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>
--- a/pom.xml
+++ b/pom.xml
@ -142,6 +142,7 @@
        <module>balking</module>
        <module>extension-objects</module>
        <module>marker</module>
+        <module>cqrs</module>

    </modules>