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Merge pull request iluwatar#595 from isabiq/master
CQRS pattern
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‎cqrs/README.md

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---
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layout: pattern
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title: CQRS
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folder: cqrs
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permalink: /patterns/cqrs/
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pumlid: 7SPR4a0m3030gt00pR_RH6I8QQFouFgC_TfHb6gkd5Q7FQBx363ub4rYpoMTZKuDrYXqDX37HIuuyCPfPPTDfuuHREhGqBy0NUR0GNzAMYizMtq1
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categories: Architectural
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tags:
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- Java
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- Difficulty-Intermediate
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---
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## Intent
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CQRS Command Query Responsibility Segregation - Separate the query side from the command side.
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![alt text](./etc/cqrs.png "CQRS")
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## Applicability
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Use the CQRS pattern when
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* you want to scale the queries and commands independently.
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* you want to use different data models for queries and commands. Useful when dealing with complex domains.
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* you want to use architectures like event sourcing or task based UI.
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## Credits
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* [Greg Young - CQRS, Task Based UIs, Event Sourcing agh!](http://codebetter.com/gregyoung/2010/02/16/cqrs-task-based-uis-event-sourcing-agh/)
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* [Martin Fowler - CQRS](https://martinfowler.com/bliki/CQRS.html)
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* [Oliver Wolf - CQRS for Great Good](https://www.youtube.com/watch?v=Ge53swja9Dw)

‎cqrs/etc/cqrs.png

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‎cqrs/etc/cqrs.ucls

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.2.0" icons="true" always-add-relationships="false" generalizations="true" realizations="true"
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associations="true" dependencies="false" nesting-relationships="true" router="FAN">
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<interface id="1" language="java" name="com.iluwatar.cqrs.commandes.ICommandService" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/commandes/ICommandService.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="291" y="-49"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</interface>
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<class id="2" language="java" name="com.iluwatar.cqrs.commandes.CommandServiceImpl" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/commandes/CommandServiceImpl.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="263" width="256" x="170" y="87"/>
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<display autosize="false" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<interface id="3" language="java" name="com.iluwatar.cqrs.queries.IQueryService" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/queries/IQueryService.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="182" width="248" x="176" y="428"/>
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<display autosize="false" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</interface>
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<class id="4" language="java" name="com.iluwatar.cqrs.queries.QueryServiceImpl" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/queries/QueryServiceImpl.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="258" width="253" x="169" y="665"/>
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<display autosize="false" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="5" language="java" name="com.iluwatar.cqrs.domain.model.Book" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/domain/model/Book.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="326" width="158" x="778" y="-93"/>
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<display autosize="false" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="6" language="java" name="com.iluwatar.cqrs.dto.Book" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/dto/Book.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="219" width="150" x="541" y="607"/>
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<display autosize="false" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="7" language="java" name="com.iluwatar.cqrs.domain.model.Author" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/domain/model/Author.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="608" y="70"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="8" language="java" name="com.iluwatar.cqrs.dto.Author" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/dto/Author.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="834" y="719"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="9" language="java" name="com.iluwatar.cqrs.util.HibernateUtil" project="cqrs"
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file="/cqrs/src/main/java/com/iluwatar/cqrs/util/HibernateUtil.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="662" y="412"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<dependency id="10">
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<end type="SOURCE" refId="4"/>
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<end type="TARGET" refId="9"/>
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</dependency>
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<dependency id="11">
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<end type="SOURCE" refId="2"/>
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<end type="TARGET" refId="9"/>
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</dependency>
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<realization id="12">
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<end type="SOURCE" refId="4"/>
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<end type="TARGET" refId="3"/>
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</realization>
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<association id="13">
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<end type="SOURCE" refId="5" navigable="false">
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<attribute id="14" name="author"/>
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<multiplicity id="15" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="7" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<realization id="16">
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<end type="SOURCE" refId="2"/>
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<end type="TARGET" refId="1"/>
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</realization>
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<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</classifier-display>
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<association-display labels="true" multiplicity="true"/>
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</class-diagram>

‎cqrs/etc/cqrs.urm.puml

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@startuml
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package com.iluwatar.cqrs.util {
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class HibernateUtil {
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- LOGGER : Logger {static}
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- SESSIONFACTORY : SessionFactory {static}
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+ HibernateUtil()
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- buildSessionFactory() : SessionFactory {static}
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+ getSessionFactory() : SessionFactory {static}
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}
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}
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package com.iluwatar.cqrs.app {
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class App {
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+ App()
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+ main(args : String[]) {static}
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}
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}
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package com.iluwatar.cqrs.dto {
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class Author {
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- email : String
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- name : String
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- username : String
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+ Author()
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+ Author(name : String, email : String, username : String)
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+ equals(obj : Object) : boolean
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+ getEmail() : String
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+ getName() : String
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+ getUsername() : String
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+ hashCode() : int
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+ toString() : String
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}
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class Book {
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- price : double
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- title : String
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+ Book()
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+ Book(title : String, price : double)
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+ equals(obj : Object) : boolean
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+ getPrice() : double
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+ getTitle() : String
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+ hashCode() : int
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+ toString() : String
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}
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}
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package com.iluwatar.cqrs.commandes {
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class CommandServiceImpl {
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- sessionFactory : SessionFactory
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+ CommandServiceImpl()
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+ authorCreated(username : String, name : String, email : String)
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+ authorEmailUpdated(username : String, email : String)
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+ authorNameUpdated(username : String, name : String)
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+ authorUsernameUpdated(oldUsername : String, newUsername : String)
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+ bookAddedToAuthor(title : String, price : double, username : String)
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+ bookPriceUpdated(title : String, price : double)
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+ bookTitleUpdated(oldTitle : String, newTitle : String)
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- getAuthorByUsername(username : String) : Author
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- getBookByTitle(title : String) : Book
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}
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interface ICommandService {
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+ authorCreated(String, String, String) {abstract}
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+ authorEmailUpdated(String, String) {abstract}
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+ authorNameUpdated(String, String) {abstract}
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+ authorUsernameUpdated(String, String) {abstract}
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+ bookAddedToAuthor(String, double, String) {abstract}
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+ bookPriceUpdated(String, double) {abstract}
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+ bookTitleUpdated(String, String) {abstract}
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}
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}
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package com.iluwatar.cqrs.queries {
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interface IQueryService {
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+ getAuthorBooks(String) : List<Book> {abstract}
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+ getAuthorBooksCount(String) : BigInteger {abstract}
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+ getAuthorByUsername(String) : Author {abstract}
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+ getAuthorsCount() : BigInteger {abstract}
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+ getBook(String) : Book {abstract}
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}
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class QueryServiceImpl {
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- sessionFactory : SessionFactory
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+ QueryServiceImpl()
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+ getAuthorBooks(username : String) : List<Book>
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+ getAuthorBooksCount(username : String) : BigInteger
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+ getAuthorByUsername(username : String) : Author
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+ getAuthorsCount() : BigInteger
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+ getBook(title : String) : Book
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}
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}
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package com.iluwatar.cqrs.domain.model {
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class Author {
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- email : String
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- id : long
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- name : String
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- username : String
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# Author()
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+ Author(username : String, name : String, email : String)
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+ getEmail() : String
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+ getId() : long
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+ getName() : String
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+ getUsername() : String
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+ setEmail(email : String)
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+ setId(id : long)
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+ setName(name : String)
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+ setUsername(username : String)
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+ toString() : String
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}
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class Book {
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- author : Author
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- id : long
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- price : double
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- title : String
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# Book()
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+ Book(title : String, price : double, author : Author)
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+ getAuthor() : Author
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+ getId() : long
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+ getPrice() : double
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+ getTitle() : String
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+ setAuthor(author : Author)
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+ setId(id : long)
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+ setPrice(price : double)
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+ setTitle(title : String)
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+ toString() : String
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}
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}
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Book --> "-author" Author
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CommandServiceImpl ..|> ICommandService
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QueryServiceImpl ..|> IQueryService
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@enduml

‎cqrs/pom.xml

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<?xml version="1.0"?>
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<!-- The MIT License Copyright (c) 2014-2016 Ilkka Seppälä Permission is
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hereby granted, free of charge, to any person obtaining a copy of this software
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and associated documentation files (the "Software"), to deal in the Software
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without restriction, including without limitation the rights to use, copy,
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modify, merge, publish, distribute, sublicense, and/or sell copies of the
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Software, and to permit persons to whom the Software is furnished to do so,
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subject to the following conditions: The above copyright notice and this
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permission notice shall be included in all copies or substantial portions
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of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
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KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
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NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
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DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
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ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
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DEALINGS IN THE SOFTWARE. -->
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<project
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"
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xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.17.0-SNAPSHOT</version>
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</parent>
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<artifactId>cqrs</artifactId>
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<dependencies>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>com.h2database</groupId>
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<artifactId>h2</artifactId>
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</dependency>
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<dependency>
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<groupId>org.hibernate</groupId>
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<artifactId>hibernate-core</artifactId>
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</dependency>
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</dependencies>
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</project>

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