feat(lb): deterministic subsetting algorithm (#1289)

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