POC for a session/cookie based sitcky load balancer implementation.

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

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+/*
+ * 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.List;
+import java.util.Objects;
+import java.util.Optional;
+
+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;
+import org.springframework.http.HttpCookie;
+import org.springframework.http.ResponseCookie;
+import org.springframework.web.server.ServerWebExchange;
+
+/**
+ * A session cookie based implementation of {@link ReactorServiceInstanceLoadBalancer}
+ * that ensures requests from the same client are routed to the same server.
+ *
+ * @author Andrew Fitzgerald
+ * @author Spencer Gibb
+ * @author Olga Maciaszek-Sharma
+ */
+public class StickySessionLoadBalancer implements ReactorServiceInstanceLoadBalancer {
+
+	private static final Log log = LogFactory.getLog(StickySessionLoadBalancer.class);
+
+	private final ReactorServiceInstanceLoadBalancer delegate;
+
+	private final String serviceId;
+
+	// TODO make this configurable
+	private final String cookieName = "scg-instance-id";
+
+	private ObjectProvider<ServiceInstanceListSupplier> serviceInstanceListSupplierProvider;
+
+	/**
+	 * @param serviceInstanceListSupplierProvider a provider of
+	 * {@link ServiceInstanceListSupplier} that will be used to get available instances
+	 * @param serviceId id of the service for which to choose an instance
+	 */
+	public StickySessionLoadBalancer(ObjectProvider<ServiceInstanceListSupplier> serviceInstanceListSupplierProvider,
+			String serviceId) {
+		this(serviceInstanceListSupplierProvider, serviceId,
+				new RoundRobinLoadBalancer(serviceInstanceListSupplierProvider, serviceId));
+	}
+
+	/**
+	 * @param serviceInstanceListSupplierProvider a provider of
+	 * {@link ServiceInstanceListSupplier} that will be used to get available instances
+	 * @param serviceId id of the service for which to choose an instance
+	 * @param delegate The delegate load balancer to use if the incoming request does not
+	 * already have a valid instance selected
+	 */
+	public StickySessionLoadBalancer(ObjectProvider<ServiceInstanceListSupplier> serviceInstanceListSupplierProvider,
+			String serviceId, ReactorServiceInstanceLoadBalancer delegate) {
+		this.serviceId = serviceId;
+		this.serviceInstanceListSupplierProvider = serviceInstanceListSupplierProvider;
+		this.delegate = delegate;
+	}
+
+	@SuppressWarnings("rawtypes")
+	@Override
+	public Mono<Response<ServiceInstance>> choose(Request request) {
+		ServiceInstanceListSupplier supplier = serviceInstanceListSupplierProvider
+				.getIfAvailable(NoopServiceInstanceListSupplier::new);
+		return supplier.get().next().flatMap(list -> getInstanceResponse(list, request));
+	}
+
+	private Mono<Response<ServiceInstance>> getInstanceResponse(List<ServiceInstance> instances, Request request) {
+		if (instances.isEmpty()) {
+			log.warn("No servers available for service: " + this.serviceId);
+			return Mono.just(new EmptyResponse());
+		}
+		Object context = request.getContext();
+		if (!(context instanceof ServerWebExchange)) {
+			throw new IllegalArgumentException("The context must be a ServerWebExchange");
+		}
+		ServerWebExchange exchange = (ServerWebExchange) context;
+
+		// Check if the exchange has a cookie that points to a valid ServiceInstance
+		return serviceInstanceFromCookie(exchange, instances)
+				// if it does, then route to that server
+				.map(instance -> Mono.just((Response<ServiceInstance>) new DefaultResponse(instance)))
+				// otherwise we'll let the delegate pick a server
+				.orElseGet(() -> delegate.choose(request))
+				// either way we should set/renew the cookie
+				.doOnNext(response -> setCookie(exchange, response));
+	}
+
+	private Optional<ServiceInstance> serviceInstanceFromCookie(ServerWebExchange exchange,
+			List<ServiceInstance> instances) {
+		HttpCookie cookie = exchange.getRequest().getCookies().getFirst(cookieName);
+		if (cookie == null) {
+			return Optional.empty();
+		}
+		String cookieInstanceId = cookie.getValue();
+		return instances.stream().filter(instance -> Objects.equals(instance.getInstanceId(), cookieInstanceId))
+				.findFirst();
+	}
+
+	private void setCookie(ServerWebExchange exchange, Response<ServiceInstance> response) {
+		if (!response.hasServer()) {
+			return;
+		}
+		exchange.getResponse().addCookie(ResponseCookie.from(cookieName, response.getServer().getInstanceId()).build());
+	}
+
+}