「DUBBO系列」服务降级源码分析

Posted 2021-04-05 JAVA前线

IT徐胖子原创本文未授权请勿转载

1 文章概述

假设服务提供者提供A服务，但是A服务并不稳定，那么如果服务消费者无法正常消费A服务就需要做降级处理，不再消费A服务而是返回一个mock值，这就是所谓服务降级。我们通过分析源码讲解服务降级策略，首先看一个消费者代码实例。

public class Consumer { public static void main(String[] args) throws Exception { ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[] { "classpath*:META-INF/spring/dubbo-consumer.xml" }); context.start(); HelloService helloService = (HelloService) context.getBean("helloService"); String result = helloService.sayHello("IT徐胖子"); System.out.println("==========客户端收到结果==========" + result); }}

<beans> <dubbo:application name="xpz-consumer" /> <dubbo:registry address="zookeeper://127.0.0.1:2181" /> <dubbo:reference id="helloService" interface="com.itxpz.dubbo.demo.provider.HelloService" /></beans>

MockClusterInvoker正是服务降级核心

// 通过动态代理机制生成代理对象Proxy// Proxy持有InvokerInvocationHandler// InvokerInvocationHandler持有invoker = MockClusterInvoker(FailoverClusterInvoker)HelloService helloService = (HelloService) context.getBean("helloService")

2 服务降级策略

2.1 不配置降级策略

<dubbo:reference id="helloService" interface="com.itxpz.dubbo.demo.provider.HelloService" />

2.2 强制降级策略

<dubbo:reference id="helloService" mock="force:return 1" interface="com.itxpz.dubbo.demo.provider.HelloService" />

2.3 异常降级策略

<dubbo:reference id="helloService" mock="throw com.itxpz.BizException" interface="com.itxpz.dubbo.demo.provider.HelloService" />

2.4 自定义降级策略

package com.itxpz.dubbo.demo.consumer;import com.itxpz.dubbo.demo.provider.HelloService;public class HelloServiceMock implements HelloService { @Override public String sayHello(String name) throws Exception { return "mock"; }}

<dubbo:reference id="helloService" mock="com.itxpz.dubbo.demo.consumer.HelloServiceMock" interface="com.itxpz.dubbo.demo.provider.HelloService" />

3 源码分析

public class MockClusterInvoker<T> implements Invoker<T> {
 @Override public Result invoke(Invocation invocation) throws RpcException { Result result = null;
 // 检查是否有mock属性 String value = directory.getUrl().getMethodParameter(invocation.getMethodName(), Constants.MOCK_KEY, Boolean.FALSE.toString()).trim();
 // 没有mock属性直接执行消费逻辑 if (value.length() == 0 || value.equalsIgnoreCase("false")) {
 // 服务消费默认执行FailoverClusterInvoker result = this.invoker.invoke(invocation); }
 // 不执行消费逻辑直接返回 else if (value.startsWith("force")) { if (logger.isWarnEnabled()) { logger.warn("force-mock: " + invocation.getMethodName() + " force-mock enabled , url : " + directory.getUrl()); } // 直接执行mock逻辑 result = doMockInvoke(invocation, null); } else { try { // 服务消费默认执行FailoverClusterInvoker result = this.invoker.invoke(invocation); } catch (RpcException e) { if (e.isBiz()) { throw e; } if (logger.isWarnEnabled()) { logger.warn("fail-mock: " + invocation.getMethodName() + " fail-mock enabled , url : " + directory.getUrl(), e); } // 服务消费失败执行mock逻辑 result = doMockInvoke(invocation, e); } } return result; }}

public class MockInvoker<T> implements Invoker<T> {
 @Override public Result invoke(Invocation invocation) throws RpcException { String mock = getUrl().getParameter(invocation.getMethodName() + "." + Constants.MOCK_KEY); if (invocation instanceof RpcInvocation) { ((RpcInvocation) invocation).setInvoker(this); } if (StringUtils.isBlank(mock)) { mock = getUrl().getParameter(Constants.MOCK_KEY); }
 if (StringUtils.isBlank(mock)) { throw new RpcException(new IllegalAccessException("mock can not be null. url :" + url)); } mock = normalizeMock(URL.decode(mock));
 // <mock="force:return 1">直接包装返回结果 if (mock.startsWith(Constants.RETURN_PREFIX)) { mock = mock.substring(Constants.RETURN_PREFIX.length()).trim(); try { Type[] returnTypes = RpcUtils.getReturnTypes(invocation); Object value = parseMockValue(mock, returnTypes); return new RpcResult(value); } catch (Exception ew) { throw new RpcException("mock return invoke error. method :" + invocation.getMethodName() + ", mock:" + mock + ", url: " + url, ew); } }
 // <mock="throw">抛出异常 else if (mock.startsWith(Constants.THROW_PREFIX)) { mock = mock.substring(Constants.THROW_PREFIX.length()).trim(); if (StringUtils.isBlank(mock)) { throw new RpcException("mocked exception for service degradation."); } else { // 获取自定义异常 Throwable t = getThrowable(mock); throw new RpcException(RpcException.BIZ_EXCEPTION, t); } }
 // <mock="com.itxpz.HelloServiceMock">自定义mock策略 else { try { Invoker<T> invoker = getInvoker(mock); return invoker.invoke(invocation); } catch (Throwable t) { throw new RpcException("Failed to create mock implementation class " + mock, t); } } }}

4 文章总结

本文我们分析了服务消费者降级策略源码，服务降级是分布式系统的重要策略，可以有效防止系统雪崩从而保护系统。

长按二维码关注更多精彩文章