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-2.6.6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×tamp=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。
