feat(lb): deterministic subsetting algorithm

docs/modules/ROOT/pages/spring-cloud-commons/loadbalancer.adoc

@@ -379,6 +379,34 @@ For `WebClient`, you need to implement and define `LoadBalancerClientRequestTran
 If multiple transformers are defined, they are applied in the order in which Beans are defined.
 Alternatively, you can use `LoadBalancerRequestTransformer.DEFAULT_ORDER` or `LoadBalancerClientRequestTransformer.DEFAULT_ORDER` to specify the order.
 
+[[loadbalancer-subset]]
+== Spring Cloud LoadBalancer Subset
+
+`SubsetServiceInstanceListSupplier` implements a https://sre.google/sre-book/load-balancing-datacenter/[deterministic subsetting algorithm] to select a limited number of instances in the `ServiceInstanceListSupplier` delegates hierarchy.
+
+You can configure it either by setting the value of `spring.cloud.loadbalancer.configurations` to `subset` or by providing your own `ServiceInstanceListSupplier` bean -- for example:
+
+[[subset-custom-loadbalancer-configuration-example]]
+[source,java,indent=0]
+----
+public class CustomLoadBalancerConfiguration {
+
+	@Bean
+	public ServiceInstanceListSupplier discoveryClientServiceInstanceListSupplier(
+			ConfigurableApplicationContext context) {
+		return ServiceInstanceListSupplier.builder()
+					.withDiscoveryClient()
+					.withSubset()
+					.withCaching()
+					.build(context);
+	    }
+	}
+----
+
+TIP: By default, each service instance is assigned a unique `instanceId`, and different `instanceId` values often select different subsets. Normally, you need not pay attention to it. However, if you need to have multiple instances select the same subset, you can set it with `spring.cloud.loadbalancer.subset.instance-id` (which supports placeholders).
+
+TIP: By default, the size of the subset is set to 100. You can also set it with `spring.cloud.loadbalancer.subset.size`.
+
 [[spring-cloud-loadbalancer-starter]]
 == Spring Cloud LoadBalancer Starter
 

spring-cloud-commons/src/main/java/org/springframework/cloud/client/loadbalancer/LoadBalancerProperties.java

@@ -29,6 +29,8 @@
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.cloud.client.ServiceInstance;
 import org.springframework.cloud.client.loadbalancer.reactive.ReactiveLoadBalancer;
+import org.springframework.cloud.commons.util.IdUtils;
+import org.springframework.core.env.PropertyResolver;
 import org.springframework.http.HttpMethod;
 import org.springframework.util.LinkedCaseInsensitiveMap;
 
@@ -40,6 +42,7 @@
  *
  * @author Olga Maciaszek-Sharma
  * @author Gandhimathi Velusamy
+ * @author Zhuozhi Ji
  * @since 2.2.1
  */
 public class LoadBalancerProperties {
@@ -85,6 +88,12 @@ public class LoadBalancerProperties {
 	 */
 	private boolean callGetWithRequestOnDelegates = true;
 
+	/**
+	 * Properties for
+	 * {@link org.springframework.cloud.loadbalancer.core.SubsetServiceInstanceListSupplier}.
+	 */
+	private Subset subset = new Subset();
+
 	public HealthCheck getHealthCheck() {
 		return healthCheck;
 	}
@@ -142,6 +151,14 @@ public boolean isCallGetWithRequestOnDelegates() {
 		return callGetWithRequestOnDelegates;
 	}
 
+	public Subset getSubset() {
+		return subset;
+	}
+
+	public void setSubset(Subset subset) {
+		this.subset = subset;
+	}
+
 	public void setCallGetWithRequestOnDelegates(boolean callGetWithRequestOnDelegates) {
 		this.callGetWithRequestOnDelegates = callGetWithRequestOnDelegates;
 	}
@@ -490,4 +507,35 @@ public void setEnabled(boolean enabled) {
 
 	}
 
+	public static class Subset {
+
+		/**
+		 * Instance id of deterministic subsetting. If not set,
+		 * {@link IdUtils#getDefaultInstanceId(PropertyResolver)} will be used.
+		 */
+		private String instanceId = "";
+
+		/**
+		 * Max subset size of deterministic subsetting.
+		 */
+		private int size = 100;
+
+		public String getInstanceId() {
+			return instanceId;
+		}
+
+		public void setInstanceId(String instanceId) {
+			this.instanceId = instanceId;
+		}
+
+		public int getSize() {
+			return size;
+		}
+
+		public void setSize(int size) {
+			this.size = size;
+		}
+
+	}
+
 }

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

@@ -143,6 +143,15 @@ public ServiceInstanceListSupplier weightedServiceInstanceListSupplier(Configura
 					.build(context);
 		}
 
+		@Bean
+		@ConditionalOnBean(ReactiveDiscoveryClient.class)
+		@ConditionalOnMissingBean
+		@Conditional(SubsetConfigurationCondition.class)
+		public ServiceInstanceListSupplier subsetServiceInstanceListSupplier(ConfigurableApplicationContext context) {
+			return ServiceInstanceListSupplier.builder().withDiscoveryClient().withSubset().withCaching()
+					.build(context);
+		}
+
 	}
 
 	@Configuration(proxyBeanMethods = false)
@@ -208,6 +217,15 @@ public ServiceInstanceListSupplier weightedServiceInstanceListSupplier(Configura
 					.build(context);
 		}
 
+		@Bean
+		@ConditionalOnBean(DiscoveryClient.class)
+		@ConditionalOnMissingBean
+		@Conditional(SubsetConfigurationCondition.class)
+		public ServiceInstanceListSupplier subsetServiceInstanceListSupplier(ConfigurableApplicationContext context) {
+			return ServiceInstanceListSupplier.builder().withBlockingDiscoveryClient().withSubset().withCaching()
+					.build(context);
+		}
+
 	}
 
 	@Configuration(proxyBeanMethods = false)
@@ -353,4 +371,14 @@ public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata)
 
 	}
 
+	static class SubsetConfigurationCondition implements Condition {
+
+		@Override
+		public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
+			return LoadBalancerEnvironmentPropertyUtils.equalToForClientOrDefault(context.getEnvironment(),
+					"configurations", "subset");
+		}
+
+	}
+
 }

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

@@ -35,6 +35,7 @@
 import org.springframework.cloud.loadbalancer.config.LoadBalancerZoneConfig;
 import org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory;
 import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.core.env.PropertyResolver;
 import org.springframework.http.HttpStatus;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -301,6 +302,16 @@ public ServiceInstanceListSupplierBuilder withHints() {
 		return this;
 	}
 
+	public ServiceInstanceListSupplierBuilder withSubset() {
+		DelegateCreator creator = (context, delegate) -> {
+			PropertyResolver resolver = context.getBean(PropertyResolver.class);
+			LoadBalancerClientFactory factory = context.getBean(LoadBalancerClientFactory.class);
+			return new SubsetServiceInstanceListSupplier(delegate, resolver, factory);
+		};
+		creators.add(creator);
+		return this;
+	}
+
 	/**
 	 * Support {@link ServiceInstanceListSupplierBuilder} can be added to the expansion
 	 * implementation of {@link ServiceInstanceListSupplier} by this method.

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

@@ -0,0 +1,93 @@
+/*
+ * Copyright 2012-2023 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.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Random;
+
+import reactor.core.publisher.Flux;
+
+import org.springframework.cloud.client.ServiceInstance;
+import org.springframework.cloud.client.loadbalancer.LoadBalancerProperties;
+import org.springframework.cloud.client.loadbalancer.reactive.ReactiveLoadBalancer;
+import org.springframework.cloud.commons.util.IdUtils;
+import org.springframework.core.env.PropertyResolver;
+import org.springframework.util.StringUtils;
+
+/**
+ * A {@link ServiceInstanceListSupplier} implementation that uses
+ * <a href="https://sre.google/sre-book/load-balancing-datacenter/">deterministic
+ * subsetting algorithm</a> to limit the number of instances provided by delegate.
+ *
+ * @author Zhuozhi Ji
+ * @since 4.1.0
+ */
+public class SubsetServiceInstanceListSupplier extends DelegatingServiceInstanceListSupplier {
+
+	private final String instanceId;
+
+	private final int size;
+
+	public SubsetServiceInstanceListSupplier(ServiceInstanceListSupplier delegate, PropertyResolver resolver,
+			ReactiveLoadBalancer.Factory<ServiceInstance> factory) {
+		super(delegate);
+		LoadBalancerProperties properties = factory.getProperties(getServiceId());
+		this.instanceId = resolveInstanceId(properties, resolver);
+		this.size = properties.getSubset().getSize();
+	}
+
+	@Override
+	public Flux<List<ServiceInstance>> get() {
+		return delegate.get().map(instances -> {
+			if (instances.size() <= size) {
+				return instances;
+			}
+
+			instances = new ArrayList<>(instances);
+
+			int instanceId = this.instanceId.hashCode() & Integer.MAX_VALUE;
+			int count = instances.size() / size;
+			int round = instanceId / count;
+
+			Random random = new Random(round);
+			Collections.shuffle(instances, random);
+
+			int bucket = instanceId % count;
+			int start = bucket * size;
+			return instances.subList(start, start + size);
+		});
+	}
+
+	private static String resolveInstanceId(LoadBalancerProperties properties, PropertyResolver resolver) {
+		String instanceId = properties.getSubset().getInstanceId();
+		if (StringUtils.hasText(instanceId)) {
+			return resolver.resolvePlaceholders(properties.getSubset().getInstanceId());
+		}
+		return IdUtils.getDefaultInstanceId(resolver);
+	}
+
+	public String getInstanceId() {
+		return instanceId;
+	}
+
+	public int getSize() {
+		return size;
+	}
+
+}