refactor: rename and delete some patterns (#2970)

2026-08-18 00:23:32 +00:00 · 2024-05-23 17:57:46 +03:00
parent 399970f8d1
commit 962e8898e0
94 changed files with 972 additions and 3619 deletions
@@ -0,0 +1,96 @@
+---
+title: Command Query Responsibility Segregation
+category: Architectural
+language: en
+tag:
+    - Event-driven
+    - Performance
+    - Scalability
+---
+
+## Also known as
+
+* CQRS
+
+## Intent
+
+Command Query Responsibility Segregation (CQRS) aims to segregate the operations that modify the state of an application (commands) from the operations that read the state (queries). This separation allows for more flexible and optimized designs, especially in complex systems.
+
+## Explanation
+
+Real world example
+
+> Imagine a modern library where the tasks of borrowing and returning books (commands) are handled at the front desk, while the task of searching for books and reading them (queries) happens in the reading area. The front desk optimizes for transaction efficiency and record-keeping, ensuring books are properly checked in and out. Meanwhile, the reading area is optimized for comfort and accessibility, making it easy for readers to find and engage with the books. This separation improves the library's overall efficiency and user experience, much like the CQRS pattern enhances a software system's performance and maintainability.
+
+In plain words
+
+> The CQRS design pattern separates the actions of modifying data (commands) from the actions of retrieving data (queries) to enhance performance, scalability, and maintainability in software systems.
+
+**Programmatic Example**
+
+One way to implement the Command Query Responsibility Segregation (CQRS) pattern is to separate the read and write operations into different services.
+
+1. Command Service: The `CommandServiceImpl` class is used for write operations. It provides methods to create authors and books, and to add books to authors. Here's a snippet of how it's used:
+
+```java
+var commands=new CommandServiceImpl();
+        commands.authorCreated(AppConstants.E_EVANS,"Eric Evans","evans@email.com");
+        commands.bookAddedToAuthor("Domain-Driven Design",60.08,AppConstants.E_EVANS);
+```
+
+2. Query Service: The `QueryServiceImpl` class is used for read operations. It provides methods to get author and book details. Here's a snippet of how it's used:
+
+```java
+var queries=new QueryServiceImpl();
+        var evans=queries.getAuthorByUsername(AppConstants.E_EVANS);
+        var dddBook=queries.getBook("Domain-Driven Design");
+```
+
+This separation of concerns allows for flexibility in how the application handles data access and manipulation, and is a key aspect of the CQRS pattern.
+
+## Class diagram
+
+![alt text](./etc/cqrs.png "CQRS")
+
+## Applicability
+
+* Systems requiring distinct models for read and write operations for scalability and maintainability.
+* Complex domain models where the task of updating objects differs significantly from the task of reading object data.
+* Scenarios where performance optimization for read operations is crucial, and the system can benefit from different data models or databases for reads and writes.
+
+## Known Uses
+
+* Distributed Systems and Microservices Architecture, where different services manage read and write responsibilities.
+* Event-Sourced Systems, where changes to the application state are stored as a sequence of events.
+* High-Performance Web Applications, segregating read and write databases to optimize load handling.
+
+## Consequences
+
+Benefits:
+
+* Scalability: By separating read and write models, each can be scaled independently according to their specific demands.
+* Optimization: Allows for the optimization of read models for query efficiency and write models for transactional integrity.
+* Maintainability: Reduces complexity by separating the concerns, leading to cleaner, more maintainable code.
+* Flexibility: Offers the flexibility to choose different technologies for the read and write sides according to their requirements.
+
+Trade-Offs:
+
+* Complexity: Introduces complexity due to synchronization between read and write models, especially in consistency maintenance.
+* Overhead: Might be an overkill for simple systems where the benefits do not outweigh the additional complexity.
+* Learning Curve: Requires a deeper understanding and careful design to implement effectively, increasing the initial learning curve.
+
+## Related Patterns
+
+* [Event Sourcing](https://java-design-patterns.com/patterns/event-sourcing/): Often used in conjunction with CQRS, where changes to the application state are stored as a sequence of events.
+* Domain-Driven Design (DDD): CQRS fits well within the DDD context, providing clear boundaries and separation of concerns.
+* [Repository](https://java-design-patterns.com/patterns/repository/): Can be used to abstract the data layer, providing a more seamless integration between the command and query sides.
+
+## Credits
+
+* [Patterns, Principles, and Practices of Domain-Driven Design](https://amzn.to/3vNV4Hm)
+* [Implementing Domain-Driven Design](https://amzn.to/3TJN2HH)
+* [Microsoft .NET: Architecting Applications for the Enterprise](https://amzn.to/4aktRes)
+* [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)
+* [Command and Query Responsibility Segregation (CQRS) pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/cqrs)
@@ -0,0 +1,136 @@
+@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 {
+    - LOGGER : Logger {static}
+    + 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)
+    # canEqual(other : Object) : boolean
+    + equals(o : Object) : boolean
+    + getEmail() : String
+    + getName() : String
+    + getUsername() : String
+    + hashCode() : int
+    + toString() : String
+  }
+  class Book {
+    - price : double
+    - title : String
+    + Book()
+    + Book(title : String, price : double)
+    # canEqual(other : Object) : boolean
+    + equals(o : Object) : boolean
+    + getPrice() : double
+    + getTitle() : String
+    + hashCode() : int
+    + toString() : String
+  }
+}
+package com.iluwatar.cqrs.commandes {
+  interface CommandService {
+    + 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}
+  }
+  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
+  }
+}
+package com.iluwatar.cqrs.queries {
+  interface QueryService {
+    + 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.constants {
+  class AppConstants {
+    + E_EVANS : String {static}
+    + J_BLOCH : String {static}
+    + M_FOWLER : String {static}
+    + USER_NAME : String {static}
+    + AppConstants()
+  }
+}
+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 ..|> CommandService 
+QueryServiceImpl ..|> QueryService 
+@enduml
@@ -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>
@@ -0,0 +1,134 @@
+@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 {
+    - LOGGER : Logger {static}
+    + 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.constants {
+  class AppConstants {
+    + E_EVANS : String {static}
+    + J_BLOCH : String {static}
+    + M_FOWLER : String {static}
+    + USER_NAME : String {static}
+    + AppConstants()
+  }
+}
+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
@@ -0,0 +1,78 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 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 xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>command-query-responsibility-segregation</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.h2database</groupId>
+            <artifactId>h2</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.hibernate</groupId>
+            <artifactId>hibernate-core</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.glassfish.jaxb</groupId>
+            <artifactId>jaxb-runtime</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>javax.xml.bind</groupId>
+            <artifactId>jaxb-api</artifactId>
+        </dependency>
+    </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-assembly-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <configuration>
+                            <archive>
+                                <manifest>
+                                    <mainClass>com.iluwatar.cqrs.app.App</mainClass>
+                                </manifest>
+                            </archive>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>
@@ -0,0 +1,89 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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 com.iluwatar.cqrs.commandes.CommandServiceImpl;
+import com.iluwatar.cqrs.constants.AppConstants;
+import com.iluwatar.cqrs.queries.QueryServiceImpl;
+import com.iluwatar.cqrs.util.HibernateUtil;
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * 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.
+ *
+ * <p>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).
+ */
+@Slf4j
+public class App {
+
+  /**
+   * Program entry point.
+   *
+   * @param args command line args
+   */
+  public static void main(String[] args) {
+    var commands = new CommandServiceImpl();
+
+    // Create Authors and Books using CommandService
+    commands.authorCreated(AppConstants.E_EVANS, "Eric Evans", "evans@email.com");
+    commands.authorCreated(AppConstants.J_BLOCH, "Joshua Bloch", "jBloch@email.com");
+    commands.authorCreated(AppConstants.M_FOWLER, "Martin Fowler", "mFowler@email.com");
+
+    commands.bookAddedToAuthor("Domain-Driven Design", 60.08, AppConstants.E_EVANS);
+    commands.bookAddedToAuthor("Effective Java", 40.54, AppConstants.J_BLOCH);
+    commands.bookAddedToAuthor("Java Puzzlers", 39.99, AppConstants.J_BLOCH);
+    commands.bookAddedToAuthor("Java Concurrency in Practice", 29.40, AppConstants.J_BLOCH);
+    commands.bookAddedToAuthor("Patterns of Enterprise"
+        + " Application Architecture", 54.01, AppConstants.M_FOWLER);
+    commands.bookAddedToAuthor("Domain Specific Languages", 48.89, AppConstants.M_FOWLER);
+    commands.authorNameUpdated(AppConstants.E_EVANS, "Eric J. Evans");
+
+    var queries = new QueryServiceImpl();
+
+    // Query the database using QueryService
+    var nullAuthor = queries.getAuthorByUsername("username");
+    var evans = queries.getAuthorByUsername(AppConstants.E_EVANS);
+    var blochBooksCount = queries.getAuthorBooksCount(AppConstants.J_BLOCH);
+    var authorsCount = queries.getAuthorsCount();
+    var dddBook = queries.getBook("Domain-Driven Design");
+    var blochBooks = queries.getAuthorBooks(AppConstants.J_BLOCH);
+
+    LOGGER.info("Author username : {}", nullAuthor);
+    LOGGER.info("Author evans : {}", evans);
+    LOGGER.info("jBloch number of books : {}", blochBooksCount);
+    LOGGER.info("Number of authors : {}", authorsCount);
+    LOGGER.info("DDD book : {}", dddBook);
+    LOGGER.info("jBloch books : {}", blochBooks);
+
+    HibernateUtil.getSessionFactory().close();
+  }
+
+}
@@ -0,0 +1,46 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.commandes;
+
+/**
+ * This interface represents the commands of the CQRS pattern.
+ */
+public interface CommandService {
+
+  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);
+
+}
@@ -0,0 +1,144 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.commandes;
+
+import com.iluwatar.cqrs.domain.model.Author;
+import com.iluwatar.cqrs.domain.model.Book;
+import com.iluwatar.cqrs.util.HibernateUtil;
+import org.hibernate.SessionFactory;
+
+/**
+ * This class is an implementation of {@link CommandService} interface. It uses Hibernate as an api
+ * for persistence.
+ */
+public class CommandServiceImpl implements CommandService {
+
+  private final SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
+
+  private Author getAuthorByUsername(String username) {
+    Author author;
+    try (var session = sessionFactory.openSession()) {
+      var 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;
+    try (var session = sessionFactory.openSession()) {
+      var 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) {
+    var author = new Author(username, name, email);
+    try (var session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.save(author);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void bookAddedToAuthor(String title, double price, String username) {
+    var author = getAuthorByUsername(username);
+    var book = new Book(title, price, author);
+    try (var session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.save(book);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void authorNameUpdated(String username, String name) {
+    var author = getAuthorByUsername(username);
+    author.setName(name);
+    try (var session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(author);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void authorUsernameUpdated(String oldUsername, String newUsername) {
+    var author = getAuthorByUsername(oldUsername);
+    author.setUsername(newUsername);
+    try (var session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(author);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void authorEmailUpdated(String username, String email) {
+    var author = getAuthorByUsername(username);
+    author.setEmail(email);
+    try (var session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(author);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void bookTitleUpdated(String oldTitle, String newTitle) {
+    var book = getBookByTitle(oldTitle);
+    book.setTitle(newTitle);
+    try (var session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(book);
+      session.getTransaction().commit();
+    }
+  }
+
+  @Override
+  public void bookPriceUpdated(String title, double price) {
+    var book = getBookByTitle(title);
+    book.setPrice(price);
+    try (var session = sessionFactory.openSession()) {
+      session.beginTransaction();
+      session.update(book);
+      session.getTransaction().commit();
+    }
+  }
+
+}
@@ -0,0 +1,37 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.constants;
+
+/**
+ * Class to define the constants.
+ */
+public class AppConstants {
+
+  public static final String E_EVANS = "eEvans";
+  public static final String J_BLOCH = "jBloch";
+  public static final String M_FOWLER = "mFowler";
+  public static final String USER_NAME = "username";
+
+}
@@ -0,0 +1,66 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.domain.model;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import lombok.Getter;
+import lombok.Setter;
+import lombok.ToString;
+
+/**
+ * This is an Author entity. It is used by Hibernate for persistence.
+ */
+@ToString
+@Getter
+@Setter
+@Entity
+public class Author {
+  @Id
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
+  private long id;
+  private String username;
+  private String name;
+  private String email;
+
+  /**
+   * Constructor.
+   *
+   * @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) {
+    this.username = username;
+    this.name = name;
+    this.email = email;
+  }
+
+  protected Author() {
+  }
+
+}
@@ -0,0 +1,69 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.domain.model;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+import lombok.Getter;
+import lombok.Setter;
+import lombok.ToString;
+
+/**
+ * This is a Book entity. It is used by Hibernate for persistence. Many books can be written by one
+ * {@link Author}
+ */
+@ToString
+@Setter
+@Getter
+@Entity
+public class Book {
+  @Id
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
+  private long id;
+  private String title;
+  private double price;
+  @ManyToOne
+  private Author author;
+
+  /**
+   * Constructor.
+   *
+   * @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) {
+    this.title = title;
+    this.price = price;
+    this.author = author;
+  }
+
+  protected Book() {
+  }
+
+}
@@ -0,0 +1,47 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.dto;
+
+import lombok.AllArgsConstructor;
+import lombok.EqualsAndHashCode;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.ToString;
+
+/**
+ * This is a DTO (Data Transfer Object) author, contains only useful information to be returned.
+ */
+@ToString
+@EqualsAndHashCode
+@Getter
+@NoArgsConstructor
+@AllArgsConstructor
+public class Author {
+
+  private String name;
+  private String email;
+  private String username;
+
+}
@@ -0,0 +1,46 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.dto;
+
+import lombok.AllArgsConstructor;
+import lombok.EqualsAndHashCode;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.ToString;
+
+/**
+ * This is a DTO (Data Transfer Object) book, contains only useful information to be returned.
+ */
+@ToString
+@EqualsAndHashCode
+@Getter
+@AllArgsConstructor
+@NoArgsConstructor
+public class Book {
+
+  private String title;
+  private double price;
+
+}
@@ -0,0 +1,47 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.queries;
+
+import com.iluwatar.cqrs.dto.Author;
+import com.iluwatar.cqrs.dto.Book;
+import java.math.BigInteger;
+import java.util.List;
+
+/**
+ * This interface represents the query methods of the CQRS pattern.
+ */
+public interface QueryService {
+
+  Author getAuthorByUsername(String username);
+
+  Book getBook(String title);
+
+  List<Book> getAuthorBooks(String username);
+
+  BigInteger getAuthorBooksCount(String username);
+
+  BigInteger getAuthorsCount();
+
+}
@@ -0,0 +1,107 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.queries;
+
+import com.iluwatar.cqrs.constants.AppConstants;
+import com.iluwatar.cqrs.dto.Author;
+import com.iluwatar.cqrs.dto.Book;
+import com.iluwatar.cqrs.util.HibernateUtil;
+import java.math.BigInteger;
+import java.util.List;
+import org.hibernate.SessionFactory;
+import org.hibernate.query.Query;
+
+/**
+ * This class is an implementation of {@link QueryService}. It uses Hibernate native queries to
+ * return DTOs from the database.
+ */
+public class QueryServiceImpl implements QueryService {
+
+  private final SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
+
+  @Override
+  public Author getAuthorByUsername(String username) {
+    Author authorDto;
+    try (var session = sessionFactory.openSession()) {
+      Query<Author> sqlQuery = session.createQuery(
+              "select new com.iluwatar.cqrs.dto.Author(a.name, a.email, a.username)"
+                      + " from com.iluwatar.cqrs.domain.model.Author a where a.username=:username");
+      sqlQuery.setParameter(AppConstants.USER_NAME, username);
+      authorDto = sqlQuery.uniqueResult();
+    }
+    return authorDto;
+  }
+
+  @Override
+  public Book getBook(String title) {
+    Book bookDto;
+    try (var session = sessionFactory.openSession()) {
+      Query<Book> sqlQuery = session.createQuery(
+              "select new com.iluwatar.cqrs.dto.Book(b.title, b.price)"
+                      + " from com.iluwatar.cqrs.domain.model.Book b where b.title=:title");
+      sqlQuery.setParameter("title", title);
+      bookDto = sqlQuery.uniqueResult();
+    }
+    return bookDto;
+  }
+
+  @Override
+  public List<Book> getAuthorBooks(String username) {
+    List<Book> bookDtos;
+    try (var session = sessionFactory.openSession()) {
+      Query<Book> sqlQuery = session.createQuery(
+              "select new com.iluwatar.cqrs.dto.Book(b.title, b.price)"
+                      + " from com.iluwatar.cqrs.domain.model.Author a, com.iluwatar.cqrs.domain.model.Book b "
+                      + "where b.author.id = a.id and a.username=:username");
+      sqlQuery.setParameter(AppConstants.USER_NAME, username);
+      bookDtos = sqlQuery.list();
+    }
+    return bookDtos;
+  }
+
+  @Override
+  public BigInteger getAuthorBooksCount(String username) {
+    BigInteger bookcount;
+    try (var session = sessionFactory.openSession()) {
+      var sqlQuery = session.createNativeQuery(
+              "SELECT count(b.title)" + " FROM  Book b, Author a"
+                      + " where b.author_id = a.id and a.username=:username");
+      sqlQuery.setParameter(AppConstants.USER_NAME, username);
+      bookcount = (BigInteger) sqlQuery.uniqueResult();
+    }
+    return bookcount;
+  }
+
+  @Override
+  public BigInteger getAuthorsCount() {
+    BigInteger authorcount;
+    try (var session = sessionFactory.openSession()) {
+      var sqlQuery = session.createNativeQuery("SELECT count(id) from Author");
+      authorcount = (BigInteger) sqlQuery.uniqueResult();
+    }
+    return authorcount;
+  }
+
+}
@@ -0,0 +1,58 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.util;
+
+import lombok.extern.slf4j.Slf4j;
+import org.hibernate.SessionFactory;
+import org.hibernate.boot.MetadataSources;
+import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
+
+/**
+ * This class simply returns one instance of {@link SessionFactory} initialized when the application
+ * is started.
+ */
+@Slf4j
+public class HibernateUtil {
+
+  private static final SessionFactory SESSIONFACTORY = buildSessionFactory();
+
+  private static SessionFactory buildSessionFactory() {
+
+    // configures settings from hibernate.cfg.xml
+    final var 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;
+  }
+
+}
@@ -0,0 +1,37 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2021 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.
+
+-->
+<!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>
@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2021 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.
+
+-->
+<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>
@@ -0,0 +1,113 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.iluwatar.cqrs.commandes.CommandServiceImpl;
+import com.iluwatar.cqrs.dto.Author;
+import com.iluwatar.cqrs.dto.Book;
+import com.iluwatar.cqrs.queries.QueryService;
+import com.iluwatar.cqrs.queries.QueryServiceImpl;
+import java.math.BigInteger;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Integration test of IQueryService and ICommandService with h2 data
+ */
+class IntegrationTest {
+
+  private static QueryService queryService;
+
+  @BeforeAll
+  static void initializeAndPopulateDatabase() {
+    var commandService = new CommandServiceImpl();
+    queryService = new QueryServiceImpl();
+
+    // 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
+  void testGetAuthorByUsername() {
+    var author = queryService.getAuthorByUsername("username1");
+    assertEquals("username1", author.getUsername());
+    assertEquals("name1", author.getName());
+    assertEquals("email1", author.getEmail());
+  }
+
+  @Test
+  void testGetUpdatedAuthorByUsername() {
+    var author = queryService.getAuthorByUsername("new_username2");
+    var expectedAuthor = new Author("new_name2", "new_email2", "new_username2");
+    assertEquals(expectedAuthor, author);
+
+  }
+
+  @Test
+  void testGetBook() {
+    var book = queryService.getBook("title1");
+    assertEquals("title1", book.getTitle());
+    assertEquals(10, book.getPrice(), 0.01);
+  }
+
+  @Test
+  void testGetAuthorBooks() {
+    var books = queryService.getAuthorBooks("username1");
+    assertEquals(2, books.size());
+    assertTrue(books.contains(new Book("title1", 10)));
+    assertTrue(books.contains(new Book("new_title2", 30)));
+  }
+
+  @Test
+  void testGetAuthorBooksCount() {
+    var bookCount = queryService.getAuthorBooksCount("username1");
+    assertEquals(new BigInteger("2"), bookCount);
+  }
+
+  @Test
+  void testGetAuthorsCount() {
+    var authorCount = queryService.getAuthorsCount();
+    assertEquals(new BigInteger("2"), authorCount);
+  }
+
+}
@@ -0,0 +1,37 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2021 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.
+
+-->
+<!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>
@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2021 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.
+
+-->
+<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>