CQRS框架：AxonFramework 之 Hello World

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：

CQRS架构简介 

浅谈命令查询职责分离(CQRS)模式

DDD CQRS架构和传统架构的优缺点比较

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：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:axon="http://www.axonframework.org/schema/core"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
                           http://www.axonframework.org/schema/core http://www.axonframework.org/schema/axon-core-2.0.xsd">

    <axon:command-bus id="commandBus"/>
    <axon:event-bus id="eventBus"/>

    <axon:event-sourcing-repository id="toDoRepository"
                                    aggregate-type="demo.axon.model.ToDoItem"/>

    <axon:aggregate-command-handler id="toDoItemHandler"
                                    aggregate-type="demo.axon.model.ToDoItem"
                                    repository="toDoRepository"
                                    command-bus="commandBus"/>

    <axon:filesystem-event-store id="eventStore" base-dir="events"/>

    <axon:annotation-config />

    <bean class="demo.axon.handler.ToDoEventHandler"/>

    <bean class="org.axonframework.commandhandling.gateway.CommandGatewayFactoryBean">
        <property name="commandBus" ref="commandBus"/>
    </bean>

</beans>

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 <demo.axon.handler.ToDoEventHandler>  INFO [main]: We've got something to do: Need to do this (3126f293-67fd-4bb7-b152-069acba775b6)
12:01:36,114 <org.axonframework.commandhandling.callbacks.LoggingCallback>  INFO [main]: Command executed successfully: demo.axon.command.CreateToDoItemCommand
12:01:36,205 <demo.axon.handler.ToDoEventHandler>  INFO [main]: We've completed a task: 3126f293-67fd-4bb7-b152-069acba775b6
12:01:36,206 <org.axonframework.commandhandling.callbacks.LoggingCallback>  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。

最后 github上还有一个比较复杂的示例项目：https://github.com/AxonFramework/Axon-trader，想深入了解的可以研究下