Command Query Responsibility Segregation,CQRS 这个架构好象最近博客园里讨论得比较多，有几篇园友的文章很有深度，推荐阅读：

比较有趣的是，以往一断谈及架构思路、OO这些，往往都是java大佬们的专长，而CQRS这个话题，好象.NET占了上风。园友汤雪华的ENODE开源大作，在github上人气也很旺。

于是，我逆向思路搜索了下java的类似项目，果然有一个AxonFramework，甚至还有一个专门的网站。按文档上的介绍，弄了一个hello world，记录一下：

CRQS是基于事件驱动的，其主要架构并不复杂，见下图：

简单来讲，对数据库的修改操作，UI层只管发送各种命令(Command)，触发事件(Event)，然后由EventHandler去异步处理，最终写入master DB，对于数据库的查询，则查询slave DB(注：这里的master db, slave db只是一个逻辑上的区分，可以是真正的主-从库，也可以都是一个库)。 这样的架构，很容易实现读写分离，也易于大型项目的扩展。

项目结构：

package的名称上大概就能看出用途：

command包定义各种命令，

event包定义各种事件，

handler包定义事件处理逻辑，

model包相当于领域模型

最外层的ToDOItemRunner相当于应用程序入口。

gradle依赖项：

group 'yjmyzz'

version '1.0'

apply plugin: 'java'

apply plugin: 'application'

sourceCompatibility = 1.8

repositories {

mavenLocal()

maven {

url 'http://maven.oschina.net/content/groups/public/'

}

mavenCentral()

}

dependencies {

testCompile group: 'junit', name: 'junit', version: '4.12'

compile "org.axonframework:axon:2.4.3"

compile "org.axonframework:axon-core:2.4.3"

compile "org.axonframework:axon-test:2.4.3"

compile 'org.springframework:spring-core:4.2.3.RELEASE'

compile 'org.springframework:spring-beans:4.2.3.RELEASE'

compile 'org.springframework:spring-context:4.2.3.RELEASE'

compile 'org.springframework:spring-context-support:4.2.3.RELEASE'

compile 'org.springframework:spring-aop:4.2.3.RELEASE'

compile 'org.springframework:spring-test:4.2.3.RELEASE'

compile 'org.apache.logging.log4j:log4j-slf4j-impl:2.5'

compile 'org.apache.logging.log4j:log4j-core:2.5'

compile 'javax.persistence:persistence-api:2.1'

}

mainClassName='demo.axon.ToDoItemRunner'

command命令：

这里我们假设了二个命令：创建命令、完成命令

CreateToDoItemCommand

package demo.axon.command;

import org.axonframework.commandhandling.annotation.TargetAggregateIdentifier;

public class CreateToDoItemCommand {

@TargetAggregateIdentifier

private final String todoId;

private final String description;

public CreateToDoItemCommand(String todoId, String description) {

this.todoId = todoId;

this.description = description;

}

public String getTodoId() {

return todoId;

}

public String getDescription() {

return description;

}

}

MarkCompletedCommand

package demo.axon.command;

import org.axonframework.commandhandling.annotation.TargetAggregateIdentifier;

public class MarkCompletedCommand {

@TargetAggregateIdentifier

private final String todoId;

public MarkCompletedCommand(String todoId) {

this.todoId = todoId;

}

public String getTodoId() {

return todoId;

}

}

Event事件：

ToDoItemCreatedEvent

package demo.axon.event;

public class ToDoItemCreatedEvent {

private final String todoId;

private final String description;

public ToDoItemCreatedEvent(String todoId, String description) {

this.todoId = todoId;

this.description = description;

}

public String getTodoId() {

return todoId;

}

public String getDescription() {

return description;

}

}

ToDoItemCompletedEvent

package demo.axon.event;

public class ToDoItemCompletedEvent {

private final String todoId;

public ToDoItemCompletedEvent(String todoId) {

this.todoId = todoId;

}

public String getTodoId() {

return todoId;

}

}

EventHandler事件处理

package demo.axon.handler;

import demo.axon.event.ToDoItemCompletedEvent;

import demo.axon.event.ToDoItemCreatedEvent;

import org.axonframework.eventhandling.annotation.EventHandler;

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

public class ToDoEventHandler {

Logger logger = LoggerFactory.getLogger(ToDoEventHandler.class);

@EventHandler

public void handle(ToDoItemCreatedEvent event) {

logger.info("We've got something to do: " + event.getDescription() + " (" + event.getTodoId() + ")");

}

@EventHandler

public void handle(ToDoItemCompletedEvent event) {

logger.info("We've completed a task: " + event.getTodoId());

}

}

上面的代码只是演示，将事件信息输出而已，真实应用中，这里可以完成对db的更新操作。

领域模型model

package demo.axon.model;

import demo.axon.command.CreateToDoItemCommand;

import demo.axon.command.MarkCompletedCommand;

import demo.axon.event.ToDoItemCompletedEvent;

import demo.axon.event.ToDoItemCreatedEvent;

import org.axonframework.commandhandling.annotation.CommandHandler;

import org.axonframework.eventhandling.annotation.EventHandler;

import org.axonframework.eventsourcing.annotation.AbstractAnnotatedAggregateRoot;

import org.axonframework.eventsourcing.annotation.AggregateIdentifier;

public class ToDoItem extends AbstractAnnotatedAggregateRoot {

@AggregateIdentifier

private String id;

public ToDoItem() {

}

@CommandHandler

public ToDoItem(CreateToDoItemCommand command) {

apply(new ToDoItemCreatedEvent(command.getTodoId(), command.getDescription()));

}

@EventHandler

public void on(ToDoItemCreatedEvent event) {

this.id = event.getTodoId();

}

@CommandHandler

public void markCompleted(MarkCompletedCommand command) {

apply(new ToDoItemCompletedEvent(id));

}

}

然后让Spring将这些东西串在一起，配置文件如下：

1 <?xml version="1.0" encoding="UTF-8"?>

2

3 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

4 xmlns:axon="http://www.axonframework.org/schema/core"

5 xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd6 http://www.axonframework.org/schema/core http://www.axonframework.org/schema/axon-core-2.0.xsd">

7

8

9

10

11

12 aggregate-type="demo.axon.model.ToDoItem"/>

13

14

15 aggregate-type="demo.axon.model.ToDoItem"

16 repository="toDoRepository"

17 command-bus="commandBus"/>

18

19

20

21

22

23

24

25

26

27

28

29

View Code

最后，提供一个舞台，让整个应用run起来：

package demo.axon;

import demo.axon.command.CreateToDoItemCommand;

import demo.axon.command.MarkCompletedCommand;

import org.axonframework.commandhandling.gateway.CommandGateway;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import java.util.UUID;

public class ToDoItemRunner {

private CommandGateway commandGateway;

public ToDoItemRunner(CommandGateway commandGateway) {

this.commandGateway = commandGateway;

}

public static void main(String[] args) {

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("sampleContext.xml");

ToDoItemRunner runner = new ToDoItemRunner(applicationContext.getBean(CommandGateway.class));

runner.run();

}

private void run() {

final String itemId = UUID.randomUUID().toString();

commandGateway.send(new CreateToDoItemCommand(itemId, "Need to do this"));

commandGateway.send(new MarkCompletedCommand(itemId));

}

}

输出结果：

12:01:36,113 INFO [main]: We've got something to do: Need to do this (3126f293-67fd-4bb7-b152-069acba775b6)

12:01:36,114 INFO [main]: Command executed successfully: demo.axon.command.CreateToDoItemCommand

12:01:36,205 INFO [main]: We've completed a task: 3126f293-67fd-4bb7-b152-069acba775b6

12:01:36,206 INFO [main]: Command executed successfully: demo.axon.command.MarkCompletedCommand

axon框架测试也很容易：

package test.demo.axon;

import demo.axon.command.CreateToDoItemCommand;

import demo.axon.command.MarkCompletedCommand;

import demo.axon.event.ToDoItemCompletedEvent;

import demo.axon.event.ToDoItemCreatedEvent;

import demo.axon.model.ToDoItem;

import org.axonframework.test.FixtureConfiguration;

import org.axonframework.test.Fixtures;

import org.junit.Before;

import org.junit.Test;

public class ToDoItemTest {

private FixtureConfiguration fixture;

@Before

public void setUp() throws Exception {

fixture = Fixtures.newGivenWhenThenFixture(ToDoItem.class);

}

@Test

public void testCreateToDoItem() throws Exception {

fixture.given()

.when(new CreateToDoItemCommand("todo1", "need to implement the aggregate"))

.expectEvents(new ToDoItemCreatedEvent("todo1", "need to implement the aggregate"));

}

@Test

public void testMarkToDoItemAsCompleted() throws Exception {

fixture.given(new ToDoItemCreatedEvent("todo1", "need to implement the aggregate"))

.when(new MarkCompletedCommand("todo1"))

.expectEvents(new ToDoItemCompletedEvent("todo1"));

}

}

given/when/expectEvents的意思是，给(given)一个事件，然后当(when)某个命令被调用时，期待(expectEvents)某个事件被触发。