Support same service instance preference

docs/src/main/asciidoc/spring-cloud-commons.adoc

@@ -995,6 +995,28 @@ public class CustomLoadBalancerConfiguration {
 
 WARNING: `HealthCheckServiceInstanceListSupplier` has its own caching mechanism based on Reactor Flux `replay()`. Therefore, if it's being used, you may want to skip wrapping that supplier with `CachingServiceInstanceListSupplier`.
 
+=== Same instance preference for LoadBalancer
+
+You can set up the LoadBalancer in such a way that it prefers the instance that was previously selected, if that instance is available.
+
+For that, you need to use `SameInstancePreferenceServiceInstanceListSupplier`. You can configure it either by setting the value of `spring.cloud.loadbalancer.configurations` to `same-instance-preference` or by providing your own `ServiceInstanceListSupplier` bean -- for example:
+
+[source,java,indent=0]
+----
+public class CustomLoadBalancerConfiguration {
+
+	@Bean
+	public ServiceInstanceListSupplier discoveryClientServiceInstanceListSupplier(
+			ConfigurableApplicationContext context) {
+		return ServiceInstanceListSupplier.builder()
+					.withDiscoveryClient()
+					.withSameInstancePreference()
+					.build(context);
+	    }
+	}
+----
+
+TIP: This is also a replacement for Zookeeper `StickyRule`.
 
 [[spring-cloud-loadbalancer-starter]]
 === Spring Cloud LoadBalancer Starter

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

@@ -100,6 +100,17 @@ public ServiceInstanceListSupplier healthCheckDiscoveryClientServiceInstanceList
 					.withHealthChecks().build(context);
 		}
 
+		@Bean
+		@ConditionalOnBean(ReactiveDiscoveryClient.class)
+		@ConditionalOnMissingBean
+		@ConditionalOnProperty(value = "spring.cloud.loadbalancer.configurations",
+				havingValue = "same-instance-preference")
+		public ServiceInstanceListSupplier sameInstancePreferenceServiceInstanceListSupplier(
+				ConfigurableApplicationContext context) {
+			return ServiceInstanceListSupplier.builder().withDiscoveryClient()
+					.withSameInstancePreference().build(context);
+		}
+
 		@Bean
 		@ConditionalOnBean(ReactiveDiscoveryClient.class)
 		@ConditionalOnMissingBean
@@ -157,6 +168,17 @@ public ServiceInstanceListSupplier healthCheckDiscoveryClientServiceInstanceList
 					.withHealthChecks().build(context);
 		}
 
+		@Bean
+		@ConditionalOnBean(DiscoveryClient.class)
+		@ConditionalOnMissingBean
+		@ConditionalOnProperty(value = "spring.cloud.loadbalancer.configurations",
+				havingValue = "same-instance-preference")
+		public ServiceInstanceListSupplier sameInstancePreferenceServiceInstanceListSupplier(
+				ConfigurableApplicationContext context) {
+			return ServiceInstanceListSupplier.builder().withBlockingDiscoveryClient()
+					.withSameInstancePreference().build(context);
+		}
+
 		@Bean
 		@ConditionalOnBean(DiscoveryClient.class)
 		@ConditionalOnMissingBean

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

@@ -115,13 +115,28 @@ public Mono<Response<ServiceInstance>> choose(Request request) {
 		if (serviceInstanceListSupplierProvider != null) {
 			ServiceInstanceListSupplier supplier = serviceInstanceListSupplierProvider
 					.getIfAvailable(NoopServiceInstanceListSupplier::new);
-			return supplier.get().next().map(this::getInstanceResponse);
+			return supplier.get().next()
+					.map(serviceInstances -> processInstanceResponse(supplier,
+							serviceInstances));
 		}
 		ServiceInstanceSupplier supplier = this.serviceInstanceSupplier
 				.getIfAvailable(NoopServiceInstanceSupplier::new);
 		return supplier.get().collectList().map(this::getInstanceResponse);
 	}
 
+	private Response<ServiceInstance> processInstanceResponse(
+			ServiceInstanceListSupplier supplier,
+			List<ServiceInstance> serviceInstances) {
+		Response<ServiceInstance> serviceInstanceResponse = getInstanceResponse(
+				serviceInstances);
+		if (supplier instanceof SelectedInstanceCallback
+				&& serviceInstanceResponse.hasServer()) {
+			((SelectedInstanceCallback) supplier)
+					.selectedServiceInstance(serviceInstanceResponse.getServer());
+		}
+		return serviceInstanceResponse;
+	}
+
 	private Response<ServiceInstance> getInstanceResponse(
 			List<ServiceInstance> instances) {
 		if (instances.isEmpty()) {

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

@@ -0,0 +1,86 @@
+/*
+ * Copyright 2012-2020 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 java.util.Collections;
+import java.util.List;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import reactor.core.publisher.Flux;
+
+import org.springframework.cloud.client.ServiceInstance;
+
+/**
+ * An implementation of {@link ServiceInstanceListSupplier} that selects the previously
+ * chosen instance if it's available.
+ *
+ * @author Olga Maciaszek-Sharma
+ * @since 3.0.0
+ */
+public class SameInstancePreferenceServiceInstanceListSupplier extends
+		DelegatingServiceInstanceListSupplier implements SelectedInstanceCallback {
+
+	private static final Log LOG = LogFactory
+			.getLog(SameInstancePreferenceServiceInstanceListSupplier.class);
+
+	private ServiceInstance previouslyReturnedInstance;
+
+	public SameInstancePreferenceServiceInstanceListSupplier(
+			ServiceInstanceListSupplier delegate) {
+		super(delegate);
+	}
+
+	@Override
+	public String getServiceId() {
+		return delegate.getServiceId();
+	}
+
+	@Override
+	public Flux<List<ServiceInstance>> get() {
+		return delegate.get().map(this::filteredBySameInstancePreference);
+	}
+
+	private List<ServiceInstance> filteredBySameInstancePreference(
+			List<ServiceInstance> serviceInstances) {
+		if (previouslyReturnedInstance != null
+				&& serviceInstances.contains(previouslyReturnedInstance)) {
+			if (LOG.isDebugEnabled()) {
+				LOG.debug(String.format(
+						"Returning previously selected service instance: %s",
+						previouslyReturnedInstance));
+			}
+			return Collections.singletonList(previouslyReturnedInstance);
+		}
+		if (LOG.isDebugEnabled()) {
+			LOG.debug(String.format(
+					"Previously selected service instance %s was not available. Returning all the instances returned by delegate.",
+					previouslyReturnedInstance));
+		}
+		previouslyReturnedInstance = null;
+		return serviceInstances;
+	}
+
+	@Override
+	public void selectedServiceInstance(ServiceInstance serviceInstance) {
+		if (previouslyReturnedInstance == null
+				|| !previouslyReturnedInstance.equals(serviceInstance)) {
+			previouslyReturnedInstance = serviceInstance;
+		}
+	}
+
+}

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

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2012-2020 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 callback interface that allows to pass the selected service instance data from the
+ * LoadBalancer.
+ *
+ * @author Olga Maciaszek-Sharma
+ */
+public interface SelectedInstanceCallback {
+
+	/**
+	 * Passes the selected {@link ServiceInstance} as an argument.
+	 * @param serviceInstance that has been selected
+	 */
+	void selectedServiceInstance(ServiceInstance serviceInstance);
+
+}

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

@@ -125,6 +125,19 @@ public ServiceInstanceListSupplierBuilder withHealthChecks() {
 		return this;
 	}
 
+	/**
+	 * Adds a {@link SameInstancePreferenceServiceInstanceListSupplier} to the
+	 * {@link ServiceInstanceListSupplier} hierarchy.
+	 * @return the {@link ServiceInstanceListSupplierBuilder} object
+	 */
+	public ServiceInstanceListSupplierBuilder withSameInstancePreference() {
+		DelegateCreator creator = (context,
+				delegate) -> new SameInstancePreferenceServiceInstanceListSupplier(
+						delegate);
+		this.creators.add(creator);
+		return this;
+	}
+
 	/**
 	 * Adds a {@link HealthCheckServiceInstanceListSupplier} that uses user-provided
 	 * {@link WebClient} instance to the {@link ServiceInstanceListSupplier} hierarchy.

spring-cloud-loadbalancer/src/test/java/org/springframework/cloud/loadbalancer/core/LoadBalancerTests.java

@@ -17,10 +17,12 @@
 package org.springframework.cloud.loadbalancer.core;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 
 import org.junit.Test;
 import org.junit.runner.RunWith;
+import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
@@ -45,13 +47,17 @@
 import org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory;
 import org.springframework.cloud.loadbalancer.support.ServiceInstanceListSuppliers;
 import org.springframework.cloud.loadbalancer.support.ServiceInstanceSuppliers;
+import org.springframework.cloud.loadbalancer.support.SimpleObjectProvider;
 import org.springframework.context.annotation.Bean;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.env.Environment;
 import org.springframework.test.context.junit4.SpringRunner;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.BDDAssertions.then;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 /**
  * @author Spencer Gibb
@@ -165,6 +171,22 @@ public void canPassHintViaRequest() {
 		assertThat(serviceInstance.getServiceId()).isEqualTo("test2");
 	}
 
+	@Test
+	public void selectedInstanceCallback() {
+		String serviceId = "test1";
+		ServiceInstance serviceInstance = instance(serviceId, "1host", false);
+		SameInstancePreferenceServiceInstanceListSupplier supplier = mock(
+				SameInstancePreferenceServiceInstanceListSupplier.class);
+		when(supplier.get())
+				.thenReturn(Flux.just(Collections.singletonList(serviceInstance)));
+		RoundRobinLoadBalancer loadBalancer = new RoundRobinLoadBalancer(
+				new SimpleObjectProvider<>(supplier), serviceId);
+
+		loadBalancer.choose().block();
+
+		verify(supplier).selectedServiceInstance(serviceInstance);
+	}
+
 	private static class TestHintLoadBalancer extends RoundRobinLoadBalancer {
 
 		TestHintLoadBalancer(

spring-cloud-loadbalancer/src/test/java/org/springframework/cloud/loadbalancer/core/SameInstancePreferenceServiceInstanceListSupplierTests.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright 2012-2020 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 java.util.Arrays;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Flux;
+
+import org.springframework.cloud.client.DefaultServiceInstance;
+import org.springframework.cloud.client.ServiceInstance;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+/**
+ * Tests for {@link SameInstancePreferenceServiceInstanceListSupplier}.
+ *
+ * @author Olga Maciaszek-Sharma
+ */
+class SameInstancePreferenceServiceInstanceListSupplierTests {
+
+	private final DiscoveryClientServiceInstanceListSupplier delegate = mock(
+			DiscoveryClientServiceInstanceListSupplier.class);
+
+	private final SameInstancePreferenceServiceInstanceListSupplier supplier = new SameInstancePreferenceServiceInstanceListSupplier(
+			delegate);
+
+	private final ServiceInstance first = serviceInstance("test-1");
+
+	private final ServiceInstance second = serviceInstance("test-2");
+
+	private final ServiceInstance third = serviceInstance("test-3");
+
+	@Test
+	void shouldReturnPreviouslySelectedInstanceIfAvailable() {
+		when(delegate.get()).thenReturn(Flux.just(Arrays.asList(first, second, third)));
+		supplier.selectedServiceInstance(first);
+
+		List<ServiceInstance> instances = supplier.get().blockFirst();
+
+		assertThat(instances).hasSize(1);
+		assertThat(instances.get(0)).isEqualTo(first);
+	}
+
+	@Test
+	void shouldReturnAllInstancesFromDelegateIfNoPreviouslySelectedInstance() {
+		when(delegate.get()).thenReturn(Flux.just(Arrays.asList(first, second, third)));
+
+		List<ServiceInstance> instances = supplier.get().blockFirst();
+
+		assertThat(instances).hasSize(3);
+	}
+
+	@Test
+	void shouldReturnAllInstancesFromDelegateIfPreviouslySelectedInstanceIfAvailable() {
+		when(delegate.get()).thenReturn(Flux.just(Arrays.asList(second, third)));
+		supplier.selectedServiceInstance(first);
+
+		List<ServiceInstance> instances = supplier.get().blockFirst();
+
+		assertThat(instances).hasSize(2);
+	}
+
+	private DefaultServiceInstance serviceInstance(String instanceId) {
+		return new DefaultServiceInstance(instanceId, "test", "http://test.test", 9080,
+				false);
+	}
+
+}