Ribbon 是netflix 公司开源的基于客户端的负载均衡组件，是Spring Cloud大家庭中非常重要的一个模块；Ribbon应该也是整个大家庭中相对而言比较复杂的模块，直接影响到服务调度的质量和性能。
全面掌握Ribbon可以帮助我们了解在分布式微服务集群工作模式下，服务调度应该考虑到的每个环节。
本文将详细地剖析Ribbon的设计原理，帮助大家对Spring Cloud 有一个更好的认知。 愿意了解源码的朋友直接求求交流分享技术 一七九一七四三三八零
一. Spring集成下的Ribbon工作结构
先贴一张总览图，说明一下Spring如何集成Ribbon的，如下所示：
2.jpg
Spring Cloud集成模式下的Ribbon有以下几个特征：
Ribbon 服务配置方式
每一个服务配置都有一个Spring ApplicationContext上下文，用于加载各自服务的实例。
和Feign的集成模式
在使用Feign作为客户端时，最终请求会转发成 http://<服务名称>/的格式，通过LoadBalancerFeignClient，
提取出服务标识<服务名称>，然后根据服务名称在上下文中查找对应服务的负载均衡器FeignLoadBalancer，负载均衡器负责根据既有的服务实例的统计信息，挑选出最合适的服务实例
二、Spring Cloud模式下和Feign的集成实现方式
和Feign结合的场景下，Feign的调用会被包装成调用请求LoadBalancerCommand，然后底层通过Rxjava基于事件的编码风格，发送请求；Spring Cloud框架通过 Feigin 请求的URL，提取出服务名称，然后在上下文中找到对应服务的的负载均衡器实现FeignLoadBalancer,然后通过负载均衡器中挑选一个合适的Server实例,然后将调用请求转发到该Server实例上，完成调用，在此过程中，记录对应Server实例的调用统计信息。
/**
* Create an {@link Observable} that once subscribed execute network call asynchronously with a server chosen by load balancer.
* If there are any errors that are indicated as retriable by the {@link RetryHandler}, they will be consumed internally by the
* function and will not be observed by the {@link Observer} subscribed to the returned {@link Observable}. If number of retries has
* exceeds the maximal allowed, a final error will be emitted by the returned {@link Observable}. Otherwise, the first successful
* result during execution and retries will be emitted.
*/
public Observable<T> submit(final ServerOperation<T> operation) {
final ExecutionInfoContext context = new ExecutionInfoContext();
if (listenerInvoker != null) {
try {
listenerInvoker.onExecutionStart();
} catch (AbortExecutionException e) {
return Observable.error(e);
}
}
// 同一Server最大尝试次数
final int maxRetrysSame = retryHandler.getMaxRetriesOnSameServer();
//下一Server最大尝试次数
final int maxRetrysNext = retryHandler.getMaxRetriesOnNextServer();
// Use the load balancer
// 使用负载均衡器，挑选出合适的Server，然后执行Server请求，将请求的数据和行为整合到ServerStats中
Observable<T> o =
(server == null ? selectServer() : Observable.just(server))
.concatMap(new Func1<Server, Observable<T>>() {
@Override
// Called for each server being selected
public Observable<T> call(Server server) {
// 获取Server的统计值
context.setServer(server);
final ServerStats stats = loadBalancerContext.getServerStats(server);
// Called for each attempt and retry 服务调用
Observable<T> o = Observable
.just(server)
.concatMap(new Func1<Server, Observable<T>>() {
@Override
public Observable<T> call(final Server server) {
context.incAttemptCount();//重试计数
loadBalancerContext.noteOpenConnection(stats);//链接统计
if (listenerInvoker != null) {
try {
listenerInvoker.onStartWithServer(context.toExecutionInfo());
} catch (AbortExecutionException e) {
return Observable.error(e);
}
}
//执行监控器，记录执行时间
final Stopwatch tracer = loadBalancerContext.getExecuteTracer().start();
//找到合适的server后，开始执行请求
//底层调用有结果后，做消息处理
return operation.call(server).doOnEach(new Observer<T>() {
private T entity;
@Override
public void onCompleted() {
recordStats(tracer, stats, entity, null);
// 记录统计信息
}
@Override
public void onError(Throwable e) {
recordStats(tracer, stats, null, e);//记录异常信息
logger.debug("Got error {} when executed on server {}", e, server);
if (listenerInvoker != null) {
listenerInvoker.onExceptionWithServer(e, context.toExecutionInfo());
}
}
@Override
public void onNext(T entity) {
this.entity = entity;//返回结果值
if (listenerInvoker != null) {
listenerInvoker.onExecutionSuccess(entity, context.toExecutionInfo());
}
}
private void recordStats(Stopwatch tracer, ServerStats stats, Object entity, Throwable exception) {
tracer.stop();//结束计时
//标记请求结束，更新统计信息
loadBalancerContext.noteRequestCompletion(stats, entity, exception, tracer.getDuration(TimeUnit.MILLISECONDS), retryHandler);
}
});
}
});
//如果失败，根据重试策略触发重试逻辑
// 使用observable 做重试逻辑，根据predicate 做逻辑判断，这里做
if (maxRetrysSame > 0)
o = o.retry(retryPolicy(maxRetrysSame, true));
return o;
}
});
// next请求处理，基于重试器操作
if (maxRetrysNext > 0 && server == null)
o = o.retry(retryPolicy(maxRetrysNext, false));
return o.onErrorResumeNext(new Func1<Throwable, Observable<T>>() {
@Override
public Observable<T> call(Throwable e) {
if (context.getAttemptCount() > 0) {
if (maxRetrysNext > 0 && context.getServerAttemptCount() == (maxRetrysNext + 1)) {
e = new ClientException(ClientException.ErrorType.NUMBEROF_RETRIES_NEXTSERVER_EXCEEDED,
"Number of retries on next server exceeded max " + maxRetrysNext
+ " retries, while making a call for: " + context.getServer(), e);
}
else if (maxRetrysSame > 0 && context.getAttemptCount() == (maxRetrysSame + 1)) {
e = new ClientException(ClientException.ErrorType.NUMBEROF_RETRIES_EXEEDED,
"Number of retries exceeded max " + maxRetrysSame
+ " retries, while making a call for: " + context.getServer(), e);
}
}
if (listenerInvoker != null) {
listenerInvoker.onExecutionFailed(e, context.toFinalExecutionInfo());
}
return Observable.error(e);
}
});
}
从一组ServerList 列表中挑选合适的Server
/**
* Compute the final URI from a partial URI in the request. The following steps are performed:
* <ul>
* <li> 如果host尚未指定，则从负载均衡器中选定 host/port
* <li> 如果host 尚未指定并且尚未找到负载均衡器，则尝试从 虚拟地址中确定host/port
* <li> 如果指定了HOST,并且URI的授权部分通过虚拟地址设置，并且存在负载均衡器，则通过负载就均衡器中确定host/port（指定的HOST将会被忽略）
* <li> 如果host已指定，但是尚未指定负载均衡器和虚拟地址配置，则使用真实地址作为host
* <li> if host is missing but none of the above applies, throws ClientException
* </ul>
*
* @param original Original URI passed from caller
*/
public Server getServerFromLoadBalancer(@Nullable URI original, @Nullable Object loadBalancerKey) throws ClientException {
String host = null;
int port = -1;
if (original != null) {
host = original.getHost();
}
if (original != null) {
Pair<String, Integer> schemeAndPort = deriveSchemeAndPortFromPartialUri(original);
port = schemeAndPort.second();
}
// Various Supported Cases
// The loadbalancer to use and the instances it has is based on how it was registered
// In each of these cases, the client might come in using Full Url or Partial URL
ILoadBalancer lb = getLoadBalancer();
if (host == null) {
// 提供部分URI，缺少HOST情况下
// well we have to just get the right instances from lb - or we fall back
if (lb != null){
Server svc = lb.chooseServer(loadBalancerKey);// 使用负载均衡器选择Server
if (svc == null){
throw new ClientException(ClientException.ErrorType.GENERAL,
"Load balancer does not have available server for client: "
+ clientName);
}
//通过负载均衡器选择的结果中选择host
host = svc.getHost();
if (host == null){
throw new ClientException(ClientException.ErrorType.GENERAL,
"Invalid Server for :" + svc);
}
logger.debug("{} using LB returned Server: {} for request {}", new Object[]{clientName, svc, original});
return svc;
} else {
// No Full URL - and we dont have a LoadBalancer registered to
// obtain a server
// if we have a vipAddress that came with the registration, we
// can use that else we
// bail out
// 通过虚拟地址配置解析出host配置返回
if (vipAddresses != null && vipAddresses.contains(",")) {
throw new ClientException(
ClientException.ErrorType.GENERAL,
"Method is invoked for client " + clientName + " with partial URI of ("
+ original
+ ") with no load balancer configured."
+ " Also, there are multiple vipAddresses and hence no vip address can be chosen"
+ " to complete this partial uri");
} else if (vipAddresses != null) {
try {
Pair<String,Integer> hostAndPort = deriveHostAndPortFromVipAddress(vipAddresses);
host = hostAndPort.first();
port = hostAndPort.second();
} catch (URISyntaxException e) {
throw new ClientException(
ClientException.ErrorType.GENERAL,
"Method is invoked for client " + clientName + " with partial URI of ("
+ original
+ ") with no load balancer configured. "
+ " Also, the configured/registered vipAddress is unparseable (to determine host and port)");
}
} else {
throw new ClientException(
ClientException.ErrorType.GENERAL,
this.clientName
+ " has no LoadBalancer registered and passed in a partial URL request (with no host:port)."
+ " Also has no vipAddress registered");
}
}
} else {
// Full URL Case URL中指定了全地址，可能是虚拟地址或者是hostAndPort
// This could either be a vipAddress or a hostAndPort or a real DNS
// if vipAddress or hostAndPort, we just have to consult the loadbalancer
// but if it does not return a server, we should just proceed anyways
// and assume its a DNS
// For restClients registered using a vipAddress AND executing a request
// by passing in the full URL (including host and port), we should only
// consult lb IFF the URL passed is registered as vipAddress in Discovery
boolean shouldInterpretAsVip = false;
if (lb != null) {
shouldInterpretAsVip = isVipRecognized(original.getAuthority());
}
if (shouldInterpretAsVip) {
Server svc = lb.chooseServer(loadBalancerKey);
if (svc != null){
host = svc.getHost();
if (host == null){
throw new ClientException(ClientException.ErrorType.GENERAL,
"Invalid Server for :" + svc);
}
logger.debug("using LB returned Server: {} for request: {}", svc, original);
return svc;
} else {
// just fall back as real DNS
logger.debug("{}:{} assumed to be a valid VIP address or exists in the DNS", host, port);
}
} else {
// consult LB to obtain vipAddress backed instance given full URL
//Full URL execute request - where url!=vipAddress
logger.debug("Using full URL passed in by caller (not using load balancer): {}", original);
}
}
// end of creating final URL
if (host == null){
throw new ClientException(ClientException.ErrorType.GENERAL,"Request contains no HOST to talk to");
}
// just verify that at this point we have a full URL
return new Server(host, port);
}
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