Draft POC.

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

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+package org.springframework.cloud.loadbalancer.core;
+
+import org.springframework.cloud.client.ServiceInstance;
+
+/**
+ * @author Olga Maciaszek-Sharma
+ */
+public interface ConnectionTrackingServiceInstance extends ServiceInstance {
+
+	int getConnectionCount();
+
+	void addConnection();
+
+}

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

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+package org.springframework.cloud.loadbalancer.core;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+import reactor.core.publisher.Mono;
+
+import org.springframework.cloud.client.discovery.DiscoveryClient;
+import org.springframework.core.env.Environment;
+
+import static org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory.PROPERTY_NAME;
+
+/**
+ * @author Olga Maciaszek-Sharma
+ */
+public class DiscoveryClientServiceInstanceListSupplier implements ServiceInstanceListSupplier<ConnectionTrackingServiceInstance> {
+
+	private final DiscoveryClient delegate;
+
+	private final String serviceId;
+
+	public DiscoveryClientServiceInstanceListSupplier(DiscoveryClient delegate,
+			Environment environment) {
+		this.delegate = delegate;
+		this.serviceId = environment.getProperty(PROPERTY_NAME);
+	}
+
+	@Override
+	public Mono<List<ConnectionTrackingServiceInstance>> get() {
+		List<ConnectionTrackingServiceInstance> instances = this.delegate
+				.getInstances(this.serviceId)
+				.stream()
+				// switch to a more sensible conversion
+				.map(serviceInstance -> (ConnectionTrackingServiceInstance) serviceInstance)
+				.collect(Collectors.toList());
+		return Mono.just(instances);
+	}
+
+	public String getServiceId() {
+		return this.serviceId;
+	}
+}

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

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+package org.springframework.cloud.loadbalancer.core;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
+
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.cloud.client.ServiceInstance;
+import org.springframework.cloud.client.loadbalancer.reactive.DefaultResponse;
+import org.springframework.cloud.client.loadbalancer.reactive.EmptyResponse;
+import org.springframework.cloud.client.loadbalancer.reactive.Request;
+import org.springframework.cloud.client.loadbalancer.reactive.Response;
+
+/**
+ * @author Olga Maciaszek-Sharma
+ */
+public class PowerOfTwoChoicesPOCLoadBalancer implements ReactorServiceInstanceLoadBalancer {
+
+	private static final Log log = LogFactory.getLog(RoundRobinLoadBalancer.class);
+
+	private final ObjectProvider<ServiceInstanceListSupplier<ConnectionTrackingServiceInstance>> serviceInstanceListSupplier;
+
+	private final String serviceId;
+
+	private List<ConnectionTrackingServiceInstance> instances;
+
+	public PowerOfTwoChoicesPOCLoadBalancer(String serviceId,
+			ObjectProvider<ServiceInstanceListSupplier<ConnectionTrackingServiceInstance>> serviceInstanceListSupplier) {
+		this.serviceId = serviceId;
+		this.serviceInstanceListSupplier = serviceInstanceListSupplier;
+		resetInstances();
+	}
+
+	private void resetInstances() {
+		Schedulers.fromExecutorService(Executors.newSingleThreadScheduledExecutor())
+				.schedulePeriodically(() -> instances = serviceInstanceListSupplier
+						// maybe we don't have to block at all?
+						// TODO:  sensible interval defaults + config
+						.getIfAvailable().get().block(), 0, 10, TimeUnit.MINUTES);
+	}
+
+	// TODO: optimise
+	@Override
+	public Mono<Response<ServiceInstance>> choose(Request request) {
+		if (instances.isEmpty()) {
+			log.warn("No servers available for service: " + this.serviceId);
+			return Mono.just(new EmptyResponse());
+		}
+		if (instances.size() == 1 || instances.get(0).getConnectionCount() < instances
+				.get(1)
+				.getConnectionCount()) {
+			instances.get(0).addConnection();
+			return Mono.just(new DefaultResponse(instances.get(0)));
+		}
+		Collections.shuffle(instances);
+
+		instances.get(1).addConnection();
+		return Mono.just(new DefaultResponse(instances.get(1)));
+	}
+
+}

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

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+package org.springframework.cloud.loadbalancer.core;
+
+import java.util.Random;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import reactor.core.publisher.Mono;
+
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.cloud.client.ServiceInstance;
+import org.springframework.cloud.client.loadbalancer.reactive.DefaultResponse;
+import org.springframework.cloud.client.loadbalancer.reactive.EmptyResponse;
+import org.springframework.cloud.client.loadbalancer.reactive.Request;
+import org.springframework.cloud.client.loadbalancer.reactive.Response;
+
+/**
+ * @author Olga Maciaszek-Sharma
+ */
+public class RoundRobinListLoadBalancer implements ReactorServiceInstanceLoadBalancer {
+
+	private static final Log log = LogFactory.getLog(RoundRobinLoadBalancer.class);
+
+	private final AtomicInteger position;
+
+	private final ObjectProvider<ServiceInstanceListSupplier<ServiceInstance>> serviceInstanceListSupplier;
+
+	private final String serviceId;
+
+	public RoundRobinListLoadBalancer(String serviceId,
+			ObjectProvider<ServiceInstanceListSupplier<ServiceInstance>> serviceInstanceListSupplier) {
+		this(serviceId, serviceInstanceListSupplier, new Random().nextInt(1000));
+	}
+
+	public RoundRobinListLoadBalancer(String serviceId,
+			ObjectProvider<ServiceInstanceListSupplier<ServiceInstance>> serviceInstanceListSupplier,
+			int seedPosition) {
+		this.serviceId = serviceId;
+		this.serviceInstanceListSupplier = serviceInstanceListSupplier;
+		this.position = new AtomicInteger(seedPosition);
+	}
+
+	@Override
+	// see original
+	// https://github.com/Netflix/ocelli/blob/master/ocelli-core/
+	// src/main/java/netflix/ocelli/loadbalancer/RoundRobinLoadBalancer.java
+	public Mono<Response<ServiceInstance>> choose(Request request) {
+		// TODO: move supplier to Request?
+		ServiceInstanceListSupplier<ServiceInstance> supplier = this.serviceInstanceListSupplier.getIfAvailable();
+		return supplier.get()
+				.map(instances -> {
+			if (instances.isEmpty()) {
+				log.warn("No servers available for service: " + this.serviceId);
+				return new EmptyResponse();
+			}
+			// TODO: enforce order?
+			int pos = Math.abs(this.position.incrementAndGet());
+
+			ServiceInstance instance = instances.get(pos % instances.size());
+
+			return new DefaultResponse(instance);
+		});
+	}
+}

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

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+package org.springframework.cloud.loadbalancer.core;
+
+import java.util.List;
+import java.util.function.Supplier;
+
+import reactor.core.publisher.Mono;
+
+import org.springframework.cloud.client.ServiceInstance;
+
+/**
+ * @author Olga Maciaszek-Sharma
+ */
+public interface ServiceInstanceListSupplier<T extends ServiceInstance> extends Supplier<Mono<List<T>>> {
+
+	String getServiceId();
+}