撸一撸Spring Cloud Ribbon的原理

说起负载均衡一般都会想到服务端的负载均衡，常用产品包括LBS硬件或云服务、Nginx等，都是耳熟能详的产品。

而Spring Cloud提供了让服务调用端具备负载均衡能力的Ribbon，通过和Eureka的紧密结合，不用在服务集群内再架设负载均衡服务，很大程度简化了服务集群内的架构。

具体也不想多写虚的介绍，反正哪里都能看得到相关的介绍。

直接开撸代码，通过代码来看Ribbon是如何实现的。

配置

详解：

1.RibbonAutoConfiguration配置生成RibbonLoadBalancerClient实例。

代码位置：

spring-cloud-netflix-core-1.3.5.RELEASE.jar
org.springframework.cloud.netflix.ribbon
RibbonAutoConfiguration.class

@Configuration
@ConditionalOnClass({ IClient.class, RestTemplate.class, AsyncRestTemplate.class, Ribbon.class})
@RibbonClients
@AutoConfigureAfter(name = "org.springframework.cloud.netflix.eureka.EurekaClientAutoConfiguration")
@AutoConfigureBefore({LoadBalancerAutoConfiguration.class, AsyncLoadBalancerAutoConfiguration.class})
@EnableConfigurationProperties(RibbonEagerLoadProperties.class)
public class RibbonAutoConfiguration {

    // 略

    @Bean
    @ConditionalOnMissingBean(LoadBalancerClient.class)
    public LoadBalancerClient loadBalancerClient() {
        return new RibbonLoadBalancerClient(springClientFactory());
    }

        // 略
}

先看配置条件项，RibbonAutoConfiguration配置必须在LoadBalancerAutoConfiguration配置前执行，因为在LoadBalancerAutoConfiguration配置中会使用RibbonLoadBalancerClient实例。

RibbonLoadBalancerClient继承自LoadBalancerClient接口，是负载均衡客户端，也是负载均衡策略的调用方。

2.LoadBalancerInterceptorConfig配置生成：
1).负载均衡拦截器LoadBalancerInterceptor实例
包含：
  LoadBalancerClient实现类的RibbonLoadBalancerClient实例
  负载均衡的请求创建工厂LoadBalancerRequestFactory:实例
2).RestTemplate自定义的RestTemplateCustomizer实例

代码位置：

spring-cloud-commons-1.2.4.RELEASE.jar
org.springframework.cloud.client.loadbalancer
LoadBalancerAutoConfiguration.class

@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {

    //  略

    @Bean
    @ConditionalOnMissingBean
    public LoadBalancerRequestFactory loadBalancerRequestFactory(
            LoadBalancerClient loadBalancerClient) {
        return new LoadBalancerRequestFactory(loadBalancerClient, transformers);
    }

    @Configuration
    @ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
    static class LoadBalancerInterceptorConfig {
        @Bean
        public LoadBalancerInterceptor ribbonInterceptor(
                LoadBalancerClient loadBalancerClient,
                LoadBalancerRequestFactory requestFactory) {
            return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
        }

        @Bean
        @ConditionalOnMissingBean
        public RestTemplateCustomizer restTemplateCustomizer(
                final LoadBalancerInterceptor loadBalancerInterceptor) {
            return new RestTemplateCustomizer() {
                @Override
                public void customize(RestTemplate restTemplate) {
                    List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                            restTemplate.getInterceptors());
                    list.add(loadBalancerInterceptor);
                    restTemplate.setInterceptors(list);
                }
            };
        }
    }

    //  略

}

先看配置条件项：

要求在项目环境中必须要有RestTemplate类。

要求必须要有LoadBalancerClient接口的实现类的实例，也就是上一步生成的RibbonLoadBalancerClient。

3.通过上面一步创建的RestTemplateCustomizer配置所有RestTemplate实例，就是将负载均衡拦截器设置给RestTemplate实例。

@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {

    // 略

    @Bean
    public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
            final List<RestTemplateCustomizer> customizers) {
        return new SmartInitializingSingleton() {
            @Override
            public void afterSingletonsInstantiated() {
                for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
                    for (RestTemplateCustomizer customizer : customizers) {
                        customizer.customize(restTemplate);
                    }
                }
            }
        };
    }

    // 略

    @Configuration
    @ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
    static class LoadBalancerInterceptorConfig {
        @Bean
        public LoadBalancerInterceptor ribbonInterceptor(
                LoadBalancerClient loadBalancerClient,
                LoadBalancerRequestFactory requestFactory) {
            return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
        }

        @Bean
        @ConditionalOnMissingBean
        public RestTemplateCustomizer restTemplateCustomizer(
                final LoadBalancerInterceptor loadBalancerInterceptor) {
            return new RestTemplateCustomizer() {
                @Override
                public void customize(RestTemplate restTemplate) {
                    List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                            restTemplate.getInterceptors());
                    list.add(loadBalancerInterceptor);
                    restTemplate.setInterceptors(list);
                }
            };
        }
    }

    // 略
}

restTemplate.setInterceptors(list)这个地方就是注入负载均衡拦截器的地方LoadBalancerInterceptor。

从这个地方实际上也可以猜出来，RestTemplate可以通过注入的拦截器来构建相应的请求实现负载均衡。

也能看出来可以自定义拦截器实现其他目的。

4.RibbonClientConfiguration配置生成ZoneAwareLoadBalancer实例

代码位置：

spring-cloud-netflix-core-1.3.5.RELEASE.jar
org.springframework.cloud.netflix.ribbon
RibbonClientConfiguration.class

@SuppressWarnings("deprecation")
@Configuration
@EnableConfigurationProperties
//Order is important here, last should be the default, first should be optional
// see https://github.com/spring-cloud/spring-cloud-netflix/issues/2086#issuecomment-316281653
@Import({OkHttpRibbonConfiguration.class, RestClientRibbonConfiguration.class, HttpClientRibbonConfiguration.class})
public class RibbonClientConfiguration {

    // 略

    @Bean
    @ConditionalOnMissingBean
    public ILoadBalancer ribbonLoadBalancer(IClientConfig config,
            ServerList<Server> serverList, ServerListFilter<Server> serverListFilter,
            IRule rule, IPing ping, ServerListUpdater serverListUpdater) {
        if (this.propertiesFactory.isSet(ILoadBalancer.class, name)) {
            return this.propertiesFactory.get(ILoadBalancer.class, config, name);
        }
        return new ZoneAwareLoadBalancer<>(config, rule, ping, serverList,
                serverListFilter, serverListUpdater);
    }

    // 略
}

ZoneAwareLoadBalancer继承自ILoadBalancer接口，该接口有一个方法：

    /**
     * Choose a server from load balancer.
     *
     * @param key An object that the load balancer may use to determine which server to return. null if
     *         the load balancer does not use this parameter.
     * @return server chosen
     */
    public Server chooseServer(Object key);

ZoneAwareLoadBalancer就是一个具体的负载均衡实现类，也是默认的负载均衡类，通过对chooseServer方法的实现选取某个服务实例。

拦截&请求

1.使用RestTemplate进行Get、Post等各种请求，都是通过doExecute方法实现

代码位置：
spring-web-4.3.12.RELEASE.jar
org.springframework.web.client
RestTemplate.class

public class RestTemplate extends InterceptingHttpAccessor implements RestOperations {

    // 略

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

        Assert.notNull(url, "'url' must not be null");
        Assert.notNull(method, "'method' must not be null");
        ClientHttpResponse response = null;
        try {
            ClientHttpRequest request = createRequest(url, method);
            if (requestCallback != null) {
                requestCallback.doWithRequest(request);
            }
            response = request.execute();
            handleResponse(url, method, response);
            if (responseExtractor != null) {
                return responseExtractor.extractData(response);
            }
            else {
                return 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();
            }
        }
    }

    // 略

}

支持的各种http请求方法最终都是调用doExecute方法，该方法内调用创建方法创建请求实例，并执行请求得到响应对象。

2.生成请求实例创建工厂

上一步代码中，调用createRequest方法创建请求实例，这个方法是定义在父类中。

先整理出主要的继承关系：

createRequest方法实际是定义在HttpAccessor抽象类中。

public abstract class HttpAccessor {

    private ClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();

    public void setRequestFactory(ClientHttpRequestFactory requestFactory) {
        Assert.notNull(requestFactory, "ClientHttpRequestFactory must not be null");
        this.requestFactory = requestFactory;
    }

    public ClientHttpRequestFactory getRequestFactory() {
        return this.requestFactory;
    }

    protected ClientHttpRequest createRequest(URI url, HttpMethod method) throws IOException {
        ClientHttpRequest request = getRequestFactory().createRequest(url, method);
        if (logger.isDebugEnabled()) {
            logger.debug("Created " + method.name() + " request for \"" + url + "\"");
        }
        return request;
    }

}

在createRequest方法中调用getRequestFactory方法获得请求实例创建工厂，实际上getRequestFactory并不是当前HttpAccessor类中定义的，而是在子类InterceptingHttpAccessor中定义的。

public abstract class InterceptingHttpAccessor extends HttpAccessor {

    private List<ClientHttpRequestInterceptor> interceptors = new ArrayList<ClientHttpRequestInterceptor>();

    public void setInterceptors(List<ClientHttpRequestInterceptor> interceptors) {
        this.interceptors = interceptors;
    }

    public List<ClientHttpRequestInterceptor> getInterceptors() {
        return interceptors;
    }

    @Override
    public ClientHttpRequestFactory getRequestFactory() {
        ClientHttpRequestFactory delegate = super.getRequestFactory();
        if (!CollectionUtils.isEmpty(getInterceptors())) {
            return new InterceptingClientHttpRequestFactory(delegate, getInterceptors());
        }
        else {
            return delegate;
        }
    }

}

在这里做了个小动作，首先还是通过HttpAccessor类创建并获得SimpleClientHttpRequestFactory工厂，这个工厂主要就是在没有拦截器的时候创建基本请求实例。

其次，在有拦截器注入的情况下，创建InterceptingClientHttpRequestFactory工厂，该工厂就是创建带拦截器的请求实例，因为注入了负载均衡拦截器，所以这里就从InterceptingClientHttpRequestFactory工厂创建。

3.通过工厂创建请求实例

创建实例就看工厂的createRequest方法。

public class InterceptingClientHttpRequestFactory extends AbstractClientHttpRequestFactoryWrapper {

    private final List<ClientHttpRequestInterceptor> interceptors;

    public InterceptingClientHttpRequestFactory(ClientHttpRequestFactory requestFactory,
            List<ClientHttpRequestInterceptor> interceptors) {

        super(requestFactory);
        this.interceptors = (interceptors != null ? interceptors : Collections.<ClientHttpRequestInterceptor>emptyList());
    }

    @Override
    protected ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod, ClientHttpRequestFactory requestFactory) {
        return new InterceptingClientHttpRequest(requestFactory, this.interceptors, uri, httpMethod);
    }

}

就是new了个InterceptingClientHttpRequest实例，并且把拦截器、基本请求实例创建工厂注进去。

4.请求实例调用配置阶段注入的负载均衡拦截器的拦截方法intercept

可从第1步看出，创建完请求实例后，通过执行请求实例的execute方法执行请求。

ClientHttpRequest request = createRequest(url, method);
if (requestCallback != null) {
    requestCallback.doWithRequest(request);
}
response = request.execute();

实际请求实例是InterceptingClientHttpRequest，execute实际是在它的父类中。

类定义位置：

spring-web-4.3.12.RELEASE.jar
org.springframework.http.client
InterceptingClientHttpRequest.class

看一下它们的继承关系。

在execute方法中实际调用了子类实现的executeInternal方法。

public abstract class AbstractClientHttpRequest implements ClientHttpRequest {

    private final HttpHeaders headers = new HttpHeaders();

    private boolean executed = false;

    @Override
    public final HttpHeaders getHeaders() {
        return (this.executed ? HttpHeaders.readOnlyHttpHeaders(this.headers) : this.headers);
    }

    @Override
    public final OutputStream getBody() throws IOException {
        assertNotExecuted();
        return getBodyInternal(this.headers);
    }

    @Override
    public final ClientHttpResponse execute() throws IOException {
        assertNotExecuted();
        ClientHttpResponse result = executeInternal(this.headers);
        this.executed = true;
        return result;
    }

    protected void assertNotExecuted() {
        Assert.state(!this.executed, "ClientHttpRequest already executed");
    }

    protected abstract OutputStream getBodyInternal(HttpHeaders headers) throws IOException;

    protected abstract ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException;

}

其实就是InterceptingClientHttpRequest类的executeInternal方法，其中，又调用了一个执行器InterceptingRequestExecution的execute，通关判断如果有拦截器注入进来过，就调用拦截器的intercept方法。

这里的拦截器实际上就是在配置阶段注入进RestTemplate实例的负载均衡拦截器LoadBalancerInterceptor实例，可参考上面配置阶段的第2步。

class InterceptingClientHttpRequest extends AbstractBufferingClientHttpRequest {

    // 略

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

    private class InterceptingRequestExecution implements ClientHttpRequestExecution {

        private final Iterator<ClientHttpRequestInterceptor> iterator;

        public InterceptingRequestExecution() {
            this.iterator = interceptors.iterator();
        }

        @Override
        public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {
            if (this.iterator.hasNext()) {
                ClientHttpRequestInterceptor nextInterceptor = this.iterator.next();
                return nextInterceptor.intercept(request, body, this);
            }
            else {
                ClientHttpRequest delegate = requestFactory.createRequest(request.getURI(), request.getMethod());
                for (Map.Entry<String, List<String>> entry : request.getHeaders().entrySet()) {
                    List<String> values = entry.getValue();
                    for (String value : values) {
                        delegate.getHeaders().add(entry.getKey(), value);
                    }
                }
                if (body.length > 0) {
                    StreamUtils.copy(body, delegate.getBody());
                }
                return delegate.execute();
            }
        }
    }

}

5.负载均衡拦截器调用负载均衡客户端

在负载均衡拦截器LoadBalancerInterceptor类的intercept方法中，又调用了负载均衡客户端LoadBalancerClient实现类的execute方法。

public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {

    private LoadBalancerClient loadBalancer;
    private LoadBalancerRequestFactory requestFactory;

    public LoadBalancerInterceptor(LoadBalancerClient loadBalancer, LoadBalancerRequestFactory requestFactory) {
        this.loadBalancer = loadBalancer;
        this.requestFactory = requestFactory;
    }

    public LoadBalancerInterceptor(LoadBalancerClient loadBalancer) {
        // for backwards compatibility
        this(loadBalancer, new LoadBalancerRequestFactory(loadBalancer));
    }

    @Override
    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);
        return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
    }
}

在配置阶段的第1步，可以看到实现类是RibbonLoadBalancerClient。

6.负载均衡客户端调用负载均衡策略选取目标服务实例并发起请求

在RibbonLoadBalancerClient的第一个execute方法以及getServer方法中可以看到，实际上是通过ILoadBalancer的负载均衡器实现类作的chooseServer方法选取一个服务，交给接下来的请求对象发起一个请求。

这里的负载均衡实现类默认是ZoneAwareLoadBalancer区域感知负载均衡器实例，其内部通过均衡策略选择一个服务。

ZoneAwareLoadBalancer的创建可以参考配置阶段的第4步。

public class RibbonLoadBalancerClient implements LoadBalancerClient {
    @Override
    public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
        ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
        Server server = getServer(loadBalancer);
        if (server == null) {
            throw new IllegalStateException("No instances available for " + serviceId);
        }
        RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server,
                serviceId), serverIntrospector(serviceId).getMetadata(server));

        return execute(serviceId, ribbonServer, request);
    }

    @Override
    public <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException {
        Server server = null;
        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 {
            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;
    }

    // 略 

    protected Server getServer(ILoadBalancer loadBalancer) {
        if (loadBalancer == null) {
            return null;
        }
        return loadBalancer.chooseServer("default"); // TODO: better handling of key
    }

    protected ILoadBalancer getLoadBalancer(String serviceId) {
        return this.clientFactory.getLoadBalancer(serviceId);
    }

    public static class RibbonServer implements ServiceInstance {
        private final String serviceId;
        private final Server server;
        private final boolean secure;
        private Map<String, String> metadata;

        public RibbonServer(String serviceId, Server server) {
            this(serviceId, server, false, Collections.<String, String> emptyMap());
        }

        public RibbonServer(String serviceId, Server server, boolean secure,
                Map<String, String> metadata) {
            this.serviceId = serviceId;
            this.server = server;
            this.secure = secure;
            this.metadata = metadata;
        }

        // 略
    }

}

代码撸完，总结下。

普通使用RestTemplate请求其他服务时，内部使用的就是常规的http请求实例发送请求。

为RestTemplate增加了@LoanBalanced 注解后，实际上通过配置，为RestTemplate注入负载均衡拦截器，让负载均衡器选择根据其对应的策略选择合适的服务后，再发送请求。

End