实战：基于CQRS微服务通信
Axon Framework是一个适用于Java的、基于事件驱动的轻量级CQRS框架，既支持直接持久化Aggregate状态，也支持采用EventSourcing。
Axon Framework的应用架构如下图所示。
本节，我们将基于Axon Framework来实现一个CQRS应用“axon-cqrs”。该应用展示了：“开通银行账户，取钱”这样一个逻辑业务。
配置
配置依赖如下。
分布式系统开发实战：微服务架构，实战：基于CQRS微服务通信
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<spring.version>5.2.3.RELEASE</spring.version>
<axon.version>3.4.3</axon.version>
<log4j.version>2.13.0</log4j.version>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework</groupId><artifactId>spring-context</artifactId>
<version>${
spring.version}</version>
</dependency>
<dependency>
<groupId>org.axonframework</groupId>
<artifactId>axon-core</artifactId>
<version>${
axon.version}</version>
</dependency>
<dependency>
<groupId>org.axonframework</groupId>
<artifactId>axon-spring</artifactId>
<version>${
axon.version}</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>${
log4j.version}</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-jcl</artifactId>
<version>${
log4j.version}</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-slf4j-impl</artifactId>
<version>${
log4j.version}</version>
</dependency>
</dependencies>
其中，采用了Axon Framework以及日志框架。
Aggregate
BankAccount是DDD中的Aggregate，代表了银行账户。
package com.waylau.axon.cqrs.command.aggregates;
import static org.axonframework.commandhandling.model.AggregateLifecycle.apply;
import static org.slf4j.LoggerFactory.getLogger;
import java.math.BigDecimal;
import org.axonframework.commandhandling.CommandHandler;
import org.axonframework.commandhandling.model.AggregateIdentifier;
import org.axonframework.eventhandling.EventHandler;
import org.slf4j.Logger;
import com.waylau.axon.cqrs.command.commands.CreateAccountCommand;
import com.waylau.axon.cqrs.command.commands.WithdrawMoneyCommand;
import com.waylau.axon.cqrs.common.domain.AccountId;
import com.waylau.axon.cqrs.common.events.CreateAccountEvent;
import com.waylau.axon.cqrs.common.events.WithdrawMoneyEvent;
public class BankAccount {
private static final Logger LOGGER = getLogger(BankAccount.class);@AggregateIdentifier
private AccountId accountId;
private String accountName;
private BigDecimal balance;
public BankAccount() {
}
@CommandHandler
public BankAccount(CreateAccountCommand command){
LOGGER.debug("Construct a new BankAccount");
apply(new CreateAccountEvent(command.getAccountId(),
command.getAccountName(),
command.getAmount()));
}
@CommandHandler
public void handle(WithdrawMoneyCommand command){
apply(new WithdrawMoneyEvent(command.getAccountId(),
command.getAmount()));
}
@EventHandler
public void on(CreateAccountEvent event){
this.accountId = event.getAccountId();
this.accountName = event.getAccountName();
this.balance = new BigDecimal(event.getAmount());
LOGGER.info("Account {} is created with balance {}",
accountId,
this.balance);
}
@EventHandler
public void on(WithdrawMoneyEvent event){
BigDecimal result = this.balance.subtract(
new BigDecimal(event.getAmount()));
if(result.compareTo(BigDecimal.ZERO)<0)
LOGGER.error("Cannot withdraw more money than the balance!");
else {
this.balance = result;
LOGGER.info("Withdraw {} from account {}, balance result: {}",
event.getAmount(), accountId, balance);
}
}
}
其中，注解@CommandHandler和@EventHandler代表了对Command和Event的处理。@AggregateIdentifier代表的AccountId，是一个Aggregate的全局唯一的标识符。
AccountId声明如下。
package com.waylau.axon.cqrs.common.domain;
import org.axonframework.common.Assert;
import org.axonframework.common.IdentifierFactory;
import java.io.Serializable;
public class AccountId implements Serializable {
private static final long serialVersionUID = 7119961474083133148L;
private final String identifier;
private final int hashCode;
public AccountId() {
this.identifier =
IdentifierFactory.getInstance().generateIdentifier();
this.hashCode = identifier.hashCode();
}
public AccountId(String identifier) {
Assert.notNull(identifier, ()->"Identifier may not be null");
this.identifier = identifier;
this.hashCode = identifier.hashCode();
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
AccountId accountId = (AccountId) o;
return identifier.equals(accountId.identifier);
}
@Override
public int hashCode() {
return hashCode;
}
@Override
public String toString() {
return identifier;
}
}
其中：
·实现equal和hashCode方法，因为它会被拿来与其他标识对比。
·实现toString方法，其结果也应该是全局唯一的。
·实现Serializable接口以表明可序列化。
Command
该应用共有两个Command：创建账户和取钱。
CreateAccountCommand代表了创建账户。
package com.waylau.axon.cqrs.command.commands;
import com.waylau.axon.cqrs.common.domain.AccountId;
public class CreateAccountCommand {
private AccountId accountId;
private String accountName;
private long amount;public CreateAccountCommand(AccountId accountId,
String accountName,
long amount) {
this.accountId = accountId;
this.accountName = accountName;
this.amount = amount;
}
public AccountId getAccountId() {
return accountId;
}
public String getAccountName() {
return accountName;
}
public long getAmount() {
return amount;
}
}
WithdrawMoneyCommand代表了取钱。
package com.waylau.axon.cqrs.command.commands;
import org.axonframework.commandhandling.TargetAggregateIdentifier;
import com.waylau.axon.cqrs.common.domain.AccountId;
public class WithdrawMoneyCommand {
@TargetAggregateIdentifier
private AccountId accountId;
private long amount;
public WithdrawMoneyCommand(AccountId accountId, long amount) {
this.accountId = accountId;
this.amount = amount;
}
public AccountId getAccountId() {
return accountId;
}
public long getAmount() {
return amount;
}
}
Event
Event是系统中发生任何改变时产生的事件类，典型的Event就是对Aggregate状态的修改。与Command相对应，会有两个事件。
CreateAccountEvent代表了创建账户的Event。
package com.waylau.axon.cqrs.common.events;import com.waylau.axon.cqrs.common.domain.AccountId;
public class CreateAccountEvent {
private AccountId accountId;
private String accountName;
private long amount;
public CreateAccountEvent(AccountId accountId,
String accountName, long amount) {
this.accountId = accountId;
this.accountName = accountName;
this.amount = amount;
}
public AccountId getAccountId() {
return accountId;
}
public String getAccountName() {
return accountName;
}
public long getAmount() {
return amount;
}
}
WithdrawMoneyEvent代表了取钱的Event。
package com.waylau.axon.cqrs.common.events;
import com.waylau.axon.cqrs.common.domain.AccountId;
public class WithdrawMoneyEvent {
private AccountId accountId;
private long amount;
public WithdrawMoneyEvent(AccountId accountId, long amount) {
this.accountId = accountId;
this.amount = amount;
}
public AccountId getAccountId() {
return accountId;
}
public long getAmount() {
return amount;
}
}
测试
为了方便测试，我们创建一个Application类。
package com.waylau.axon.cqrs;
import static org.slf4j.LoggerFactory.getLogger;
import org.axonframework.config.Configuration;
import org.axonframework.config.DefaultConfigurer;import org.axonframework.eventsourcing.eventstore.inmemory.
InMemoryEventStorageEngine;
import org.slf4j.Logger;
import com.waylau.axon.cqrs.command.aggregates.BankAccount;
import com.waylau.axon.cqrs.command.commands.CreateAccountCommand;
import com.waylau.axon.cqrs.command.commands.WithdrawMoneyCommand;
import com.waylau.axon.cqrs.common.domain.AccountId;
public class Application {
private static final Logger LOGGER = getLogger(Application.class);
public static void main(String args[]) throws InterruptedException{
LOGGER.info("Application is start.");
Configuration config = DefaultConfigurer.defaultConfiguration()
.configureAggregate(BankAccount.class)
.configureEmbeddedEventStore(c -> new InMemoryEventStorageEngine())
.buildConfiguration();
config.start();
AccountId id = new AccountId();
config.commandGateway().send(new CreateAccountCommand(id, "MyAccount",1000));
config.commandGateway().send(new WithdrawMoneyCommand(id, 500));
config.commandGateway().send(new WithdrawMoneyCommand(id, 500));
config.commandGateway().send(new WithdrawMoneyCommand(id, 500));
// 线程先睡5s，等事件处理完
Thread.sleep(5000L);
LOGGER.info("Application is shutdown.");
}
}
在该类中，我们创建了一个账户，并执行了3次取钱动作。由于账户总额为1000，所以，当执行第3次取钱动作时，预期是会因为余额不足而失败。
运行该Application类，能看到以下输出。
22:11:52.432 [main] INFO com.waylau.axon.cqrs.Application - Application is start.
22:11:52.676 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Account cc0653ff-afbd-49f5-b868-38aff296611c is created with balance 1000
22:11:52.719 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Account cc0653ff-afbd-49f5-b868-38aff296611c is created with balance 1000
22:11:52.725 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Withdraw 500 from account cc0653ff-afbd-49f5-b868-38aff296611c, balance result: 500
22:11:52.725 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Account cc0653ff-afbd-49f5-b868-38aff296611c is created with balance 1000
22:11:52.725 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Withdraw 500 from account cc0653ff-afbd-49f5-b868-38aff296611c, balance result: 500
22:11:52.727 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Withdraw 500 from account cc0653ff-afbd-49f5-b868-38aff296611c, balance result: 0
22:11:52.727 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Account cc0653ff-afbd-49f5-b868-38aff296611c is created with balance 1000
22:11:52.728 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Withdraw 500 from account cc0653ff-afbd-49f5-b868-38aff296611c, balance result: 500
22:11:52.728 [main] INFO com.waylau.axon.cqrs.command.aggregates.BankAccount –
Withdraw 500 from account cc0653ff-afbd-49f5-b868-38aff296611c, balance result: 022:11:52.728 [main] ERROR com.waylau.axon.cqrs.command.aggregates.BankAccount - Cannot
withdraw more money than the balance!
22:11:57.729 [main] INFO com.waylau.axon.cqrs.Application - Application is shutdown.
本节示例，可以在axon-cqrs项目下找到。
本章小结
本章介绍了微服务架构的概念及构建微服务常用的技术。同时，也介绍了在微服务中常用的3种通信方式：HTTP、消息、事件驱动。在微服务中，我们可以使用CQRS来降低构建微服务通信的复杂度。
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