项目场景：

Spring Cloud Feign 学习过程中遇到Feign访问的时候报错Load balancer does not have available server for client

问题描述：

完整报错日志如下

path [] threw exception [Request processing failed; nested exception is java.lang.RuntimeException: com.netflix.client.ClientException: Load balancer does not have available server for client: MS-CUSTOMER] with root causecom.netflix.client.ClientException: Load balancer does not have available server for client: MS-CUSTOMERat com.netflix.loadbalancer.LoadBalancerContext.getServerFromLoadBalancer(LoadBalancerContext.java:483) ~[ribbon-loadbalancer-2.2.2.jar:2.2.2]at com.netflix.loadbalancer.reactive.LoadBalancerCommand$1.call(LoadBalancerCommand.java:184) ~[ribbon-loadbalancer-2.2.2.jar:2.2.2]at com.netflix.loadbalancer.reactive.LoadBalancerCommand$1.call(LoadBalancerCommand.java:180) ~[ribbon-loadbalancer-2.2.2.jar:2.2.2]at rx.Observable.unsafeSubscribe(Observable.java:10211) ~[rxjava-1.1.10.jar:1.1.10]at rx.internal.operators.OnSubscribeConcatMap.call(OnSubscribeConcatMap.java:94) ~[rxjava-1.1.10.jar:1.1.10]at rx.internal.operators.OnSubscribeConcatMap.call(OnSubscribeConcatMap.java:42) ~[rxjava-1.1.10.jar:1.1.10]at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:48) ~[rxjava-1.1.10.jar:1.1.10]at rx.internal.operators.OnSubscribeLift.call(OnSubscribeLift.java:30) ~[rxjava-1.1.10.jar:1.1.10]

原因分析：

从报错信息上看Load balancer does not have available server for client: MS-CUSTOMER，表示负载均衡器没有找到有效的服务信息,此异常是 LoadBalancerContext从LoadBalancer对象中获取服务的时候，没有找到所调用的服务信息

Server svc = lb.chooseServer(loadBalancerKey);
if (svc == null){throw new ClientException(ClientException.ErrorType.GENERAL,"Load balancer does not have available server for client: "+ clientName);
}

我们查询lb.chooseServer 实现的功能其实是从LoadBalancer对象中的到所有的服务，然后根据规则从服务中获取一个匹配的服务（相关规则信息可以查看文章 Spring Cloud Ribbon负载均衡策略详解_学然后知不足！-CSDN博客）

List<Server> serverList = getLoadBalancer().getAllServers();

再看一下getAllServer接口，这个接口返回的是可用的和非可用的服务

    /*** @return All known servers, both reachable and unreachable.*/public List<Server> getAllServers();

查看BaseLoadBalancer 如何实现的此方法发现，服务取自List<Server>集合

    protected volatile List<Server> allServerList = Collections.synchronizedList(new ArrayList<Server>());@Overridepublic List<Server> getAllServers() {return Collections.unmodifiableList(allServerList);}/*** Add a server to the 'allServer' list; does not verify uniqueness, so you* could give a server a greater share by adding it more than once.*/public void addServer(Server newServer) {if (newServer != null) {try {ArrayList<Server> newList = new ArrayList<Server>();newList.addAll(allServerList);newList.add(newServer);setServersList(newList);} catch (Exception e) {logger.error("LoadBalancer [{}]: Error adding newServer {}", name, newServer.getHost(), e);}}}/*** Add a list of servers to the 'allServer' list; does not verify* uniqueness, so you could give a server a greater share by adding it more* than once*/@Overridepublic void addServers(List<Server> newServers) {if (newServers != null && newServers.size() > 0) {try {ArrayList<Server> newList = new ArrayList<Server>();newList.addAll(allServerList);newList.addAll(newServers);setServersList(newList);} catch (Exception e) {logger.error("LoadBalancer [{}]: Exception while adding Servers", name, e);}}}

解决方案：

从上面的源码分析可知，请求处理时候会从会先从一个缓存集合List<Server> 中得到可用和不可用的服务，然后根据规则Rule 过滤获可访问的服务信息并返回Server，那么获取Server为空引起报错的可能场景有如下几个

1. Feign 启动后还没有从注册中心得到List<Server> 的时候lb#chooseServer获取结果为空

第一种可能：客户端没有开启从Eureka 中获取服务列表 ,所以需要检查配置是否错误配置成了 false

eureka.client.register-with-eureka=true
eureka.client.fetch-registry=true

第二种可能：客户端已经开启了从注册中心获取服务列表，获取列表是通过任务获取，此时任务还没执行List<Server>为空，只需要等一会应用从注册中心检索服务后，问题会自行解决。

第三种可能：检索后发现没有得到服务，此时很有可能是你访问的服务应用没有正确注册到注册中心引起，正确注册后Eureka注册中心可以正确看到发布的服务信息，发布失败一般也是上面的配置出错或其他配置问题

2. 根据Rule规则过滤从List<Server>中没有找到可用的Server

第一种可能：因为getAllServers()返回的是可用和非可用， Rule规则会过滤到可用，所以如果你的服务已经Down那么会找不到服务，可以通过健康检查查看服务状态是否正常

第二种可能：检查Fegin接口配置的服务名称和要访问的服务名称是否相同，注意字母顺序

3. 其他未知原因，只能通过断点调试了

上一篇：Spring Cloud Feign 示例笔记

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Spring Cloud Feign启动Load balancer does not have available server for client分析

项目场景：

问题描述：

原因分析：

解决方案：

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