微服务 Choreography-based saga example

分析https://github.com/eventuate-tram/eventuate-tram-sagas-examples-customers-and-orders中的例子.

例子中使用Eventuate Tram framework,spring boot ,jpa。实现的逻辑是上一篇文章讲到的电子商务网站的例子。

体系图

这个例子中有两个service。一个order service ,一个customer service。

关于order service,

1. 首先为order建立一个domain对象，每一个order会存储一个OrderState的字段,OrderState有PENDING, APPROVED, REJECTED三种状态。Order的初始状态为PENDING。

   package io.eventuate.examples.tram.ordersandcustomers.orders.domain;
   
   
   import io.eventuate.examples.tram.ordersandcustomers.commondomain.OrderCreatedEvent;
   import io.eventuate.examples.tram.ordersandcustomers.commondomain.OrderDetails;
   import io.eventuate.examples.tram.ordersandcustomers.commondomain.OrderState;
   import io.eventuate.tram.events.ResultWithEvents;
   
   import javax.persistence.*;
   
   import static java.util.Collections.singletonList;
   
   @Entity
   @Table(name="orders")
   @Access(AccessType.FIELD)
   public class Order {
   
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     private Long id;
   
     private OrderState state;
   
     @Embedded
     private OrderDetails orderDetails;
   
     public Order() {
     }
   
     public Order(OrderDetails orderDetails) {
       this.orderDetails = orderDetails;
       this.state = OrderState.PENDING;
     }
   
     public static ResultWithEvents createOrder(OrderDetails orderDetails) {
       Order order = new Order(orderDetails);
       OrderCreatedEvent orderCreatedEvent = new OrderCreatedEvent(orderDetails);
       return new ResultWithEvents<>(order, singletonList(orderCreatedEvent));
     }
   
     public Long getId() {
       return id;
     }
   
     public void noteCreditReserved() {
       this.state = OrderState.APPROVED;
     }
   
     public void noteCreditReservationFailed() {
       this.state = OrderState.REJECTED;
     }
   
     public OrderState getState() {
       return state;
     }
   
     public OrderDetails getOrderDetails() {
       return orderDetails;
     }
   }
   

    

2. 当OrderService创建一个order时，会先创建一个ResultWithEvents对象，然后将order存储到数据库中，然后使用domainEventPublisher发布关于这个order的event。                                                                                                                            可以看到createOrder方法上有@Transactional注解，说明

   orderRepository.save(order);
   和
   domainEventPublisher.publish(Order.class, order.getId(), orderWithEvents.events);                                这两行代码要在一个事务中，保证原子性，order一旦保存，orderCreatedEvent事件也必须也要进行持久化

   @Transactional
     public Order createOrder(OrderDetails orderDetails) {
       ResultWithEvents orderWithEvents = Order.createOrder(orderDetails);
       Order order = orderWithEvents.result;
       orderRepository.save(order);
       domainEventPublisher.publish(Order.class, order.getId(), orderWithEvents.events);
       return order;
     }

   public static ResultWithEvents createOrder(OrderDetails orderDetails) {
       Order order = new Order(orderDetails);
       OrderCreatedEvent orderCreatedEvent = new OrderCreatedEvent(orderDetails);
       return new ResultWithEvents<>(order, singletonList(orderCreatedEvent));
     }

    

3. 当order service发布了orderCreateEvent事件后，customer service会消费orderCreateEvent事件，customer service会先冻结 orderCreatedEvent事件中的订单金额，当冻结成功后，customer service会发布一个customerCreditReservedEvent,很明显，这个事件是用来通知order service已成功为用户冻结金额。

   package io.eventuate.examples.tram.ordersandcustomers.customers.service;
   
   import io.eventuate.examples.tram.ordersandcustomers.commondomain.CustomerCreditReservationFailedEvent;
   import io.eventuate.examples.tram.ordersandcustomers.commondomain.CustomerCreditReservedEvent;
   import io.eventuate.examples.tram.ordersandcustomers.commondomain.OrderCreatedEvent;
   import io.eventuate.examples.tram.ordersandcustomers.customers.domain.Customer;
   import io.eventuate.examples.tram.ordersandcustomers.customers.domain.CustomerCreditLimitExceededException;
   import io.eventuate.examples.tram.ordersandcustomers.customers.domain.CustomerRepository;
   import io.eventuate.tram.events.publisher.DomainEventPublisher;
   import io.eventuate.tram.events.subscriber.DomainEventEnvelope;
   import io.eventuate.tram.events.subscriber.DomainEventHandlers;
   import io.eventuate.tram.events.subscriber.DomainEventHandlersBuilder;
   import org.springframework.beans.factory.annotation.Autowired;
   
   import java.util.Collections;
   
   
   public class OrderEventConsumer {
   
     @Autowired
     private CustomerRepository customerRepository;
   
     @Autowired
     private DomainEventPublisher domainEventPublisher;
     //订阅OrderCreatedEvent事件
     public DomainEventHandlers domainEventHandlers() {
       return DomainEventHandlersBuilder
               .forAggregateType("io.eventuate.examples.tram.ordersandcustomers.orders.domain.Order")
               .onEvent(OrderCreatedEvent.class, this::orderCreatedEventHandler)
               .build();
     }
   
     private void orderCreatedEventHandler(DomainEventEnvelope domainEventEnvelope) {
   
       Long orderId = Long.parseLong(domainEventEnvelope.getAggregateId());
   
       OrderCreatedEvent orderCreatedEvent = domainEventEnvelope.getEvent();
   
       Customer customer = customerRepository
               .findOne(orderCreatedEvent.getOrderDetails().getCustomerId());
   
       try {
       //冻结用户的钱
         customer.reserveCredit(orderId, orderCreatedEvent.getOrderDetails().getOrderTotal());
   
         CustomerCreditReservedEvent customerCreditReservedEvent =
                 new CustomerCreditReservedEvent(orderId, orderCreatedEvent.getOrderDetails());
          //发布用户金额冻结成功的事件
         domainEventPublisher.publish(Customer.class,
                 customer.getId(),
                 Collections.singletonList(customerCreditReservedEvent));
   
       } catch (CustomerCreditLimitExceededException e) {
           //如果冻结失败，发布冻结失败事件
         CustomerCreditReservationFailedEvent customerCreditReservationFailedEvent =
                 new CustomerCreditReservationFailedEvent(orderId, orderCreatedEvent.getOrderDetails());
   
         domainEventPublisher.publish(Customer.class,
                 customer.getId(),
                 Collections.singletonList(customerCreditReservationFailedEvent));
       }
     }
   }
   

    

 

4.customer service发布完事件之后，会被order service消费掉

package io.eventuate.examples.tram.ordersandcustomers.orders.service;

import io.eventuate.examples.tram.ordersandcustomers.commondomain.CustomerCreditReservationFailedEvent;
import io.eventuate.examples.tram.ordersandcustomers.commondomain.CustomerCreditReservedEvent;
import io.eventuate.tram.events.subscriber.DomainEventEnvelope;
import io.eventuate.tram.events.subscriber.DomainEventHandlers;
import io.eventuate.tram.events.subscriber.DomainEventHandlersBuilder;
import org.springframework.beans.factory.annotation.Autowired;


public class CustomerEventConsumer {

  @Autowired
  private OrderService orderService;
//订阅CustomerCreditReservedEvent和CustomerCreditReservationFailedEvent事件
  public DomainEventHandlers domainEventHandlers() {
    return DomainEventHandlersBuilder
            .forAggregateType("io.eventuate.examples.tram.ordersandcustomers.customers.domain.Customer")
            .onEvent(CustomerCreditReservedEvent.class, this::customerCreditReservedEventHandler)
            .onEvent(CustomerCreditReservationFailedEvent.class, this::customerCreditReservationFailedEventHandler)
            .build();
  }

  private void customerCreditReservedEventHandler(DomainEventEnvelope domainEventEnvelope) {
//用户金钱冻结成功，就修改订单状态为APPROVED
    orderService.approveOrder(domainEventEnvelope.getEvent().getOrderId());
  }

  private void customerCreditReservationFailedEventHandler(DomainEventEnvelope domainEventEnvelope) {
//用户如果金钱不足，冻结失败，修改订单状态为REJECTED
    orderService.rejectOrder(domainEventEnvelope.getEvent().getOrderId());
  }
}

所以整体看来要实现Choreography-based saga模式还是挺复杂的，在借助与 Eventuate Tram framework框架之下，要为order 和customer创建域对象，新建order和orderCreatedEvent要保证原子性， Eventuate Tram framework会读取transaction log，然后发布事件到kafka中，order service和customer service订阅相应的事件，然后进行处理。order service要编写补偿事务代码。

 

总结一下，编写Choreography-based saga需要注意的几点：

1. 编写带有状态字段的域对象
2. 保证修改域对象的字段和发布相应的事件在同一个事务里
3. 上游的service要编写补偿事务代码
4. service订阅相应的事件进行处理。