Gh 576 use reactive load balancer (#584)

* Add `ReactorLoadBalancerClient` interface and its default implementation.

* Add initial `ReactorLoadBalancerExchangeFilterFunction` implementation.
Add `ReactorLoadBalancerClientAutoConfiguration`.
Refactor `ReactorLoadBalancerClient` interface and default implementation.

* Implement configuration changes to make default `ReactorLoadBalancer` and
`ReactorLoadBalancerClient` work out of the box with `@LoadBalanced
WebClient.Builder`.

* Add more tests for ReactorLoadBalancerExchangeFilterFunction and
DefaultReactorLoadBalancer.

* Fix configuration. Add tests. Add documentation.

* Add information on caching to the documentation.

* Add fixes after code review.

* Small refactoring after code review.

* Switch from handle(response, sink) to map(response).

* Remove redundant cast.

* Add link to caching in Springboot reference to the docs.

* Add more information on working with spring-cloud-loadbalancer vs. spring-cloud-starter-netflix-ribbon to the docs.

* Fix after code review.

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/annotation/LoadBalancerClientConfiguration.java

@@ -17,25 +17,34 @@
 package org.springframework.cloud.loadbalancer.annotation;
 
 import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.cache.CacheManager;
+import org.springframework.cloud.client.ConditionalOnDiscoveryEnabled;
+import org.springframework.cloud.client.ServiceInstance;
 import org.springframework.cloud.client.discovery.DiscoveryClient;
 import org.springframework.cloud.loadbalancer.core.CachingServiceInstanceSupplier;
 import org.springframework.cloud.loadbalancer.core.DiscoveryClientServiceInstanceSupplier;
+import org.springframework.cloud.loadbalancer.core.ReactorLoadBalancer;
+import org.springframework.cloud.loadbalancer.core.RoundRobinLoadBalancer;
 import org.springframework.cloud.loadbalancer.core.ServiceInstanceSupplier;
+import org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.env.Environment;
 
 /**
  * @author Spencer Gibb
+ * @author Olga Maciaszek-Sharma
  */
 @Configuration
 @EnableConfigurationProperties
+@ConditionalOnDiscoveryEnabled
 public class LoadBalancerClientConfiguration {
 
 	@Bean
+	@ConditionalOnBean(DiscoveryClient.class)
 	@ConditionalOnMissingBean
 	public ServiceInstanceSupplier discoveryClientServiceInstanceSupplier(
 			DiscoveryClient discoveryClient, Environment env,
@@ -50,4 +59,14 @@ public class LoadBalancerClientConfiguration {
 		return delegate;
 	}
 
+	@Bean
+	@ConditionalOnMissingBean
+	public ReactorLoadBalancer<ServiceInstance> reactorServiceInstanceLoadBalancer(
+			Environment environment,
+			LoadBalancerClientFactory loadBalancerClientFactory) {
+		String name = environment.getProperty(LoadBalancerClientFactory.PROPERTY_NAME);
+		return new RoundRobinLoadBalancer(name, loadBalancerClientFactory
+				.getLazyProvider(name, ServiceInstanceSupplier.class));
+	}
+
 }

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/config/LoadBalancerAutoConfiguration.java

@@ -20,6 +20,9 @@ import java.util.Collections;
 import java.util.List;
 
 import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.autoconfigure.AutoConfigureBefore;
+import org.springframework.cloud.client.loadbalancer.reactive.ReactiveLoadBalancerAutoConfiguration;
+import org.springframework.cloud.client.loadbalancer.reactive.ReactorLoadBalancerClientAutoConfiguration;
 import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClientSpecification;
 import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClients;
 import org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory;
@@ -28,9 +31,12 @@ import org.springframework.context.annotation.Configuration;
 
 /**
  * @author Spencer Gibb
+ * @author Olga Maciaszek-Sharma
  */
 @Configuration
 @LoadBalancerClients
+@AutoConfigureBefore({ ReactorLoadBalancerClientAutoConfiguration.class,
+		ReactiveLoadBalancerAutoConfiguration.class })
 // @EnableCaching //TODO: how to enforce, or check conditions?
 // @AutoConfigureBefore(CacheAutoConfiguration.class)
 public class LoadBalancerAutoConfiguration {

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/core/ReactorServiceInstanceLoadBalancer.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.loadbalancer.core;
+
+import org.springframework.cloud.client.ServiceInstance;
+
+/**
+ * A marker interface for {@link ReactorLoadBalancer} that allows selecting
+ * {@link ServiceInstance} objects.
+ *
+ * @author Olga Maciaszek-Sharma
+ * @since 2.2.0
+ */
+public interface ReactorServiceInstanceLoadBalancer
+		extends ReactorLoadBalancer<ServiceInstance> {
+
+}

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/core/RoundRobinLoadBalancer.java

@@ -33,7 +33,7 @@ import org.springframework.cloud.client.loadbalancer.reactive.Response;
 /**
  * @author Spencer Gibb
  */
-public class RoundRobinLoadBalancer implements ReactorLoadBalancer<ServiceInstance> {
+public class RoundRobinLoadBalancer implements ReactorServiceInstanceLoadBalancer {
 
 	private static final Log log = LogFactory.getLog(RoundRobinLoadBalancer.class);
 

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/support/LoadBalancerClientFactory.java

@@ -16,9 +16,12 @@
 
 package org.springframework.cloud.loadbalancer.support;
 
+import org.springframework.cloud.client.ServiceInstance;
+import org.springframework.cloud.client.loadbalancer.reactive.ReactiveLoadBalancer;
 import org.springframework.cloud.context.named.NamedContextFactory;
 import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClientConfiguration;
 import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClientSpecification;
+import org.springframework.cloud.loadbalancer.core.ReactorServiceInstanceLoadBalancer;
 import org.springframework.core.env.Environment;
 
 /**
@@ -28,9 +31,11 @@ import org.springframework.core.env.Environment;
  *
  * @author Spencer Gibb
  * @author Dave Syer
+ * @author Olga Maciaszek-Sharma
  */
 public class LoadBalancerClientFactory
-		extends NamedContextFactory<LoadBalancerClientSpecification> {
+		extends NamedContextFactory<LoadBalancerClientSpecification>
+		implements ReactiveLoadBalancer.Factory<ServiceInstance> {
 
 	/**
 	 * Property source name for load balancer.
@@ -50,4 +55,9 @@ public class LoadBalancerClientFactory
 		return environment.getProperty(PROPERTY_NAME);
 	}
 
+	@Override
+	public ReactiveLoadBalancer<ServiceInstance> getInstance(String serviceId) {
+		return getInstance(serviceId, ReactorServiceInstanceLoadBalancer.class);
+	}
+
 }