Ribbon负载均衡的原理

Ribbon

依赖

    <parent>
        <groupId>org.springframework.bootgroupId>
        <artifactId>spring-boot-starter-parentartifactId>
        <version>2.3.12.RELEASEversion>
        <relativePath/> 
    parent>
    <properties>
        <java.version>1.8java.version>
        <spring-cloud.version>Hoxton.SR12spring-cloud.version>
    properties>

    
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloudgroupId>
            <artifactId>spring-cloud-starter-netflix-eureka-clientartifactId>
        dependency>
    dependencies>

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloudgroupId>
                <artifactId>spring-cloud-dependenciesartifactId>
                <version>${spring-cloud.version}version>
                <type>pomtype>
                <scope>importscope>
            dependency>
        dependencies>
    dependencyManagement>
project>

SpringCloud2020.0.1版本之后，去除了ribbbon依赖，使用Load balancer进行负载均衡

负载均衡配置

spring:
  application:
    name: ribbon   #项目名字

# eureka client配置
eureka:
  client:
    registryFetchIntervalSeconds: 5
    serviceUrl:
      defaultZone: http://localhost:8098/eureka/  #eureka服务端提供的注册地址 参考服务端配置的这个路径
  instance:
    hostname: ribbon #此实例注册到eureka服务端的唯一的实例ID
    prefer-ip-address: true #是否显示IP地址
    leaseRenewalIntervalInSeconds: 10 #eureka客户需要多长时间发送心跳给eureka服务器，表明它仍然活着,默认为30 秒 (与下面配置的单位都是秒)
    leaseExpirationDurationInSeconds: 30 #Eureka服务器在接收到实例的最后一次发出的心跳后，需要等待多久才可以将此实例删除，默认为90秒
    health-check-url-path: /actuator/health

ribbon:
  MaxAutoRetries: 2 #最大重试次数，当Eureka中可以找到服务，但是服务连不上时将会重试
  MaxAutoRetriesNextServer: 3 #切换实例的重试次数
  OkToRetryOnAllOperations: false  #对所有操作请求都进行重试，如果是get则可以，如果是post，put等操作没有实现幂等的情况下是很危险的,所以设置为false
  ConnectTimeout: 5000  #请求连接的超时时间
  ReadTimeout: 6000 #请求处理的超时时间
  
# 调用USER-MGT微服务时使用随机策略
USER-MGT:
  ribbon:
    NFLoadBalancerRuleClassName: com.netflix.loadbalancer.RandomRule

负载均衡原理

以调用下面的接口为例讲解ribbon怎么进行负载均衡

@RequestMapping(value = "/strategy", method = RequestMethod.GET, produces = "application/json")
public String testRibbonStrategy() {
    ResponseEntity<String> forEntity =
            restTemplate.getForEntity("http://USER-MGT/sequence/number/port", String.class);
    return forEntity.getBody();
}

首先，restTemplate.getForEntity方法

@Override
public <T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Object... uriVariables)
      throws RestClientException {
   RequestCallback requestCallback = acceptHeaderRequestCallback(responseType);
   ResponseExtractor<ResponseEntity<T>> responseExtractor = responseEntityExtractor(responseType);
    // execute方法中进行HTTP调用
   return nonNull(execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables));
}

execute方法会调用到doExecute方法，他会创建一个客户端HTTP请求，然后进行调用

protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback,
      @Nullable ResponseExtractor<T> responseExtractor) throws RestClientException {

   Assert.notNull(url, "URI is required");
   Assert.notNull(method, "HttpMethod is required");
   ClientHttpResponse response = null;
   try {
       // 创建客户端HTTP请求
      ClientHttpRequest request = createRequest(url, method);
      if (requestCallback != null) {
         requestCallback.doWithRequest(request);
      }
       // 执行客户端HTTP调用
      response = request.execute();
      handleResponse(url, method, response);
      return (responseExtractor != null ? responseExtractor.extractData(response) : null);
   }
   catch (IOException ex) {
      String resource = url.toString();
      String query = url.getRawQuery();
      resource = (query != null ? resource.substring(0, resource.indexOf('?')) : resource);
      throw new ResourceAccessException("I/O error on " + method.name() +
            " request for \"" + resource + "\": " + ex.getMessage(), ex);
   }
   finally {
      if (response != null) {
         response.close();
      }
   }
}

request.execute()方法会调用到抽象类AbstractClientHttpRequest#execute

public final ClientHttpResponse execute() throws IOException {
   assertNotExecuted();
    // 调用到AbstractBufferingClientHttpRequest
   ClientHttpResponse result = executeInternal(this.headers);
   this.executed = true;
   return result;
}

executeInternal方法会调用到AbstractBufferingClientHttpRequest#executeInternal

protected ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException {
   byte[] bytes = this.bufferedOutput.toByteArray();
   if (headers.getContentLength() < 0) {
      headers.setContentLength(bytes.length);
   }
    // 调用InterceptingClientHttpRequest#executeInternal
   ClientHttpResponse result = executeInternal(headers, bytes);
   this.bufferedOutput = new ByteArrayOutputStream(0);
   return result;
}

该方法继续调用InterceptingClientHttpRequest#executeInternal方法

protected final ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput) throws IOException {
   InterceptingRequestExecution requestExecution = new InterceptingRequestExecution();
   return requestExecution.execute(this, bufferedOutput);
}

requestExecution.execute(this, bufferedOutput)会执行以下方法

public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {
   if (this.iterator.hasNext()) { // 先处理拦截器
       // 获取LoadBalancerInterceptor负载均衡拦截器
      ClientHttpRequestInterceptor nextInterceptor = this.iterator.next();
       // 进行负载均衡拦截
      return nextInterceptor.intercept(request, body, this);
   }else { // 拦截器处理之后，会再次进入到这里
      HttpMethod method = request.getMethod();
      Assert.state(method != null, "No standard HTTP method");
       // 产生HTTP请求执行的代理对象
      ClientHttpRequest delegate = requestFactory.createRequest(request.getURI(), method);
       // 添加请求头
      request.getHeaders().forEach((key, value) -> delegate.getHeaders().addAll(key, value));
      if (body.length > 0) { // 添加body
         if (delegate instanceof StreamingHttpOutputMessage) {
            StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) delegate;
            streamingOutputMessage.setBody(outputStream -> StreamUtils.copy(body, outputStream));
         } else {
            StreamUtils.copy(body, delegate.getBody());
         }
      }
       // 代理对象执行HTTP请求
      return delegate.execute();
   }
}

LoadBalancerInterceptor拦截器是负载均衡的关键实现，他会进行HTTP请求前的拦截，根据负载均衡策略选择合适的服务器实例，将http://USER-MGT/sequence/number/port中的USER-MGT微服务实例名替换为对应的IP和端口号，然后返回，交给HTTP层进行实际的调用

public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
      final ClientHttpRequestExecution execution) throws IOException {
   final URI originalUri = request.getURI();
   String serviceName = originalUri.getHost();
   Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
    // 执行负载均衡拦截
   return this.loadBalancer.execute(serviceName, 
                                    this.requestFactory.createRequest(request, body, execution));
}

public <T> T execute(String serviceId, LoadBalancerRequest<T> request, Object hint)
      throws IOException {
    // 获取拦截器
   ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
    // 获取server，调用BaseLoadBalancer#chooseServer
   Server server = getServer(loadBalancer, hint);
   if (server == null) {
      throw new IllegalStateException("No instances available for " + serviceId);
   }
    // ribbon server
   RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server, serviceId),
         serverIntrospector(serviceId).getMetadata(server));
    // 执行
   return execute(serviceId, ribbonServer, request);
}

getServer(loadBalancer, hint)这里会调用BaseLoadBalancer#chooseServer，他会使用设置的负载均衡策略选取服务器实例

public Server chooseServer(Object key) {
    if (counter == null) {
        counter = createCounter();
    }
    counter.increment();
    if (rule == null) {
        return null;
    } else {
        try {
            // 根据负载均衡策略选择服务实例
            return rule.choose(key);
        } catch (Exception e) {
            logger.warn("LoadBalancer [{}]:  Error choosing server for key {}", name, key, e);
            return null;
        }
    }
}

rule.choose(key)会调用负载均衡策略的选择实例方法，这里配置的是随机策略RandomRule，即调用RandomRule#choose

public Server choose(ILoadBalancer lb, Object key) {
    if (lb == null) {
        return null;
    }
    Server server = null;

    while (server == null) {
        if (Thread.interrupted()) {
            return null;
        }
        // 可达服务实例列表
        List<Server> upList = lb.getReachableServers();
        // 所有服务实例列表
        List<Server> allList = lb.getAllServers();

        int serverCount = allList.size();
        if (serverCount == 0) {
            /*
             * No servers. End regardless of pass, because subsequent passes
             * only get more restrictive.
             */
            return null;
        }
        // 根据服务实例数产生随机值
        int index = chooseRandomInt(serverCount);
        // 选择服务实例
        server = upList.get(index);

        if (server == null) {
            /*
             * The only time this should happen is if the server list were
             * somehow trimmed. This is a transient condition. Retry after
             * yielding.
             */
            Thread.yield();
            continue;
        }
        // 服务可用，则返回
        if (server.isAlive()) {
            return (server);
        }
        // Shouldn't actually happen.. but must be transient or a bug.
        server = null;
        Thread.yield();
    }
    return server;
}

LoadBalancerInterceptor拦截之后执行execute(serviceId, ribbonServer, request)会调用如下方法，

public <T> T execute(String serviceId, ServiceInstance serviceInstance,
      LoadBalancerRequest<T> request) throws IOException {
   Server server = null;
    // 获取server服务器信息
   if (serviceInstance instanceof RibbonServer) {
      server = ((RibbonServer) serviceInstance).getServer();
   }
   if (server == null) {
      throw new IllegalStateException("No instances available for " + serviceId);
   }

   RibbonLoadBalancerContext context = this.clientFactory.getLoadBalancerContext(serviceId);
   RibbonStatsRecorder statsRecorder = new RibbonStatsRecorder(context, server);

   try {
       // 进行HTTP实际调用，这里会回到InterceptingClientHttpRequest#execute拦截器之后的处理逻辑，进行真正的HTTP调用
      T returnVal = request.apply(serviceInstance);
      statsRecorder.recordStats(returnVal);
      return returnVal;
   }
   // catch IOException and rethrow so RestTemplate behaves correctly
   catch (IOException ex) {
      statsRecorder.recordStats(ex);
      throw ex;
   }
   catch (Exception ex) {
      statsRecorder.recordStats(ex);
      ReflectionUtils.rethrowRuntimeException(ex);
   }
   return null;
}