开篇

• Dubbo路由规则在发起一次RPC调用前起到过滤目标服务器地址的作用，过滤后的地址列表，将作为消费端最终发起RPC调用的备选地址。
• 目前支持的路由包括：
  条件路由。支持以服务或Consumer应用为粒度配置路由规则。

标签路由。以Provider应用为粒度配置路由规则。

• 这篇文章的分析是基于Dubbo-2.6.x版本的，不同的版本实现方法会有些不一样，官方的链接路由规则。

Dubbo标签路由

• 标签路由通过将某一个或多个服务的提供者划分到同一个分组，约束流量只在指定分组中流转，从而实现流量隔离的目的，可以作为蓝绿发布、灰度发布等场景的能力基础。
• 标签主要是指对Provider端应用实例的分组，目前有两种方式可以完成实例分组，分别是动态规则打标和静态规则打标，其中动态规则相较于静态规则优先级更高，而当两种规则同时存在且出现冲突时，将以动态规则为准。
• 请求标签的作用域为每一次 invocation，使用 attachment 来传递请求标签，注意保存在 attachment 中的值将会在一次完整的远程调用中持续传递，得益于这样的特性，我们只需要在起始调用时，通过一行代码的设置，达到标签的持续传递。
• 降级约定

consumer携带request.tag=tag1 时优先选择 标记了tag=tag1 的 provider。若集群中不存在与请求标记对应的服务，默认将降级请求 tag为空的provider；如果要改变这种默认行为，即找不到匹配tag1的provider返回异常，需设置request.tag.force=true。

comsumer侧request.tag未设置时，只会匹配tag为空的provider。即使集群中存在可用的服务，若tag不匹配也就无法调用，这与约定1不同，携带标签的请求可以降级访问到无标签的服务，但不携带标签/携带其他种类标签的请求永远无法访问到其他标签的服务。

Dubbo标签路由选择过程

public class TagRouter extends AbstractRouter {

    @Override
    public <T> List<Invoker<T>> route(List<Invoker<T>> invokers, URL url, Invocation invocation) throws RpcException {
        // filter
        List<Invoker<T>> result = new ArrayList<Invoker<T>>();
        // Dynamic param
        String tag = RpcContext.getContext().getAttachment(Constants.TAG_KEY);
        // 处理consumer携带tag的情况
        if (!StringUtils.isEmpty(tag)) {
            // 优先选择携带标签的provider
            for (Invoker<T> invoker : invokers) {
                if (tag.equals(invoker.getUrl().getParameter(Constants.TAG_KEY))) {
                    result.add(invoker);
                }
            }
        }
        // 如果未指定标签tag或者携带了标签但是未找到匹配的provider的情况
        if (result.isEmpty()) {
            // 未强制指定FORCE_USE_TAG的逻辑
            String forceTag = RpcContext.getContext().getAttachment(Constants.FORCE_USE_TAG);
            if (StringUtils.isEmpty(forceTag) || "false".equals(forceTag)) {
                for (Invoker<T> invoker : invokers) {
                    // 获取没有携带标签的provider对象
                    if (StringUtils.isEmpty(invoker.getUrl().getParameter(Constants.TAG_KEY))) {
                        result.add(invoker);
                    }
                }
            }
        }
        return result;
    }
}

• 携带标签的请求优先访问带标签的provider，再不存在携带标签的情况下降级访问到无标签的provider。
• 针对不携带标签的请求只能访问无标签的provider。

Dubbo标签路由的举例

Provider

public class DemoServiceImpl implements DemoService {

    @Override
    public String sayHello(String name) {
        System.out.println("[" + new SimpleDateFormat("HH:mm:ss").format(new Date()) + "] Hello " + name + ", request from consumer: " + RpcContext.getContext().getRemoteAddress());
        return "Hello " + name + ", response from provider: " + RpcContext.getContext().getLocalAddress();
    }

}


public class Provider {

    public static void main(String[] args) throws Exception {
        //Prevent to get IPV6 address,this way only work in debug mode
        //But you can pass use -Djava.net.preferIPv4Stack=true,then it work well whether in debug mode or not
        System.setProperty("java.net.preferIPv4Stack", "true");
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-provider.xml"});
        context.start();

        System.in.read(); // press any key to exit
    }

}

<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:dubbo="http://dubbo.apache.org/schema/dubbo"
       xmlns="http://www.springframework.org/schema/beans"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.3.xsd
       http://dubbo.apache.org/schema/dubbo http://dubbo.apache.org/schema/dubbo/dubbo.xsd">

    <!-- provider's application name, used for tracing dependency relationship -->
    <dubbo:application name="demo-provider"/>

    <!-- use multicast registry center to export service -->
    <dubbo:registry address="multicast://224.5.6.7:1234"/>

    <!-- use dubbo protocol to export service on port 20880 -->
    <dubbo:protocol name="dubbo" port="20880"/>

    <!-- service implementation, as same as regular local bean -->
    <bean id="demoService" class="com.alibaba.dubbo.demo.provider.DemoServiceImpl"/>

    <!-- declare the service interface to be exported -->
    <dubbo:service interface="com.alibaba.dubbo.demo.DemoService" ref="demoService" tag="zzzz"/>

</beans>

consumer

public class Consumer {

    public static void main(String[] args) {
        //Prevent to get IPV6 address,this way only work in debug mode
        //But you can pass use -Djava.net.preferIPv4Stack=true,then it work well whether in debug mode or not
        System.setProperty("java.net.preferIPv4Stack", "true");
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-consumer.xml"});
        context.start();
        DemoService demoService = (DemoService) context.getBean("demoService"); // get remote service proxy
        // 携带tag路由
        RpcContext.getContext().setAttachment("dubbo.tag", "zzzz");

        while (true) {
            try {
                Thread.sleep(1000);
                String hello = demoService.sayHello("world"); // call remote method
                System.out.println(hello); // get result

            } catch (Throwable throwable) {
                throwable.printStackTrace();
            }
        }

    }
}

<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:dubbo="http://dubbo.apache.org/schema/dubbo"
       xmlns="http://www.springframework.org/schema/beans"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.3.xsd
       http://dubbo.apache.org/schema/dubbo http://dubbo.apache.org/schema/dubbo/dubbo.xsd">

    <!-- consumer's application name, used for tracing dependency relationship (not a matching criterion),
    don't set it same as provider -->
    <dubbo:application name="demo-consumer"/>

    <!-- use multicast registry center to discover service -->
    <dubbo:registry address="multicast://224.5.6.7:1234"/>

    <!-- generate proxy for the remote service, then demoService can be used in the same way as the
    local regular interface -->
    <dubbo:reference id="demoService" check="false" interface="com.alibaba.dubbo.demo.DemoService"/>

</beans>

dubbo://192.168.1.5:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true
&application=demo-provider
&bean.name=com.alibaba.dubbo.demo.DemoService&default.dubbo.tag=xxx&dubbo=2.0.2
&dubbo.tag=zzzzzz&generic=false
&interface=com.alibaba.dubbo.demo.DemoService
&methods=sayHello&pid=82561&side=provider&timestamp=1577466134212

Dubbo标签路由的坑

@Activate(group = Constants.CONSUMER, order = -10000)
public class ConsumerContextFilter implements Filter {

    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        RpcContext.getContext()
                .setInvoker(invoker)
                .setInvocation(invocation)
                .setLocalAddress(NetUtils.getLocalHost(), 0)
                .setRemoteAddress(invoker.getUrl().getHost(),
                        invoker.getUrl().getPort());
        if (invocation instanceof RpcInvocation) {
            ((RpcInvocation) invocation).setInvoker(invoker);
        }
        try {
            RpcResult result = (RpcResult) invoker.invoke(invocation);
            RpcContext.getServerContext().setAttachments(result.getAttachments());
            return result;
        } finally {
            // 清空attachments
            RpcContext.getContext().clearAttachments();
        }
    }

}

• dubbo的consumer侧的Filter对象ConsumerContextFilter每次请求后都会清空Attachments，导致再次发起请求就无法找到tag，所以需要在整个生命周期内保存tag。一般通过线程的ThreadLocal进行实现。