Add function-based health check. Add restTemplate-based health-check beans to default config

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

@@ -968,6 +968,9 @@ public class CustomLoadBalancerConfiguration {
 	}
 ----
 
+TIP: For the non-reactive stack, create this supplier with the `withBlockingHealthChecks()`.
+You can also pass your own `WebClient` or `RestTemplate` instance to be used for the checks.
+
 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

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

@@ -43,6 +43,8 @@
 import org.springframework.core.annotation.Order;
 import org.springframework.core.env.Environment;
 import org.springframework.retry.support.RetryTemplate;
+import org.springframework.web.client.RestTemplate;
+import org.springframework.web.reactive.function.client.WebClient;
 
 /**
  * @author Spencer Gibb
@@ -90,7 +92,7 @@ public ServiceInstanceListSupplier zonePreferenceDiscoveryClientServiceInstanceL
 		}
 
 		@Bean
-		@ConditionalOnBean(ReactiveDiscoveryClient.class)
+		@ConditionalOnBean({ ReactiveDiscoveryClient.class, WebClient.Builder.class })
 		@ConditionalOnMissingBean
 		@ConditionalOnProperty(value = "spring.cloud.loadbalancer.configurations", havingValue = "health-check")
 		public ServiceInstanceListSupplier healthCheckDiscoveryClientServiceInstanceListSupplier(
@@ -148,12 +150,12 @@ public ServiceInstanceListSupplier zonePreferenceDiscoveryClientServiceInstanceL
 		}
 
 		@Bean
-		@ConditionalOnBean(DiscoveryClient.class)
+		@ConditionalOnBean({ DiscoveryClient.class, RestTemplate.class })
 		@ConditionalOnMissingBean
 		@ConditionalOnProperty(value = "spring.cloud.loadbalancer.configurations", havingValue = "health-check")
 		public ServiceInstanceListSupplier healthCheckDiscoveryClientServiceInstanceListSupplier(
 				ConfigurableApplicationContext context) {
-			return ServiceInstanceListSupplier.builder().withBlockingDiscoveryClient().withHealthChecks()
+			return ServiceInstanceListSupplier.builder().withBlockingDiscoveryClient().withBlockingHealthChecks()
 					.build(context);
 		}
 

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

@@ -19,6 +19,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.function.BiFunction;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -31,14 +32,11 @@
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.cloud.client.ServiceInstance;
 import org.springframework.cloud.client.loadbalancer.LoadBalancerProperties;
-import org.springframework.http.HttpStatus;
-import org.springframework.web.reactive.function.client.WebClient;
-import org.springframework.web.util.UriComponentsBuilder;
 
 /**
  * A {@link ServiceInstanceListSupplier} implementation that verifies whether the
- * instances are alive and only returns the healthy one, unless there are none. Uses
- * {@link WebClient} to ping the <code>health</code> endpoint of the instances.
+ * instances are alive and only returns the healthy one, unless there are none. Uses a
+ * user-provided function to ping the <code>health</code> endpoint of the instances.
  *
  * @author Olga Maciaszek-Sharma
  * @author Roman Matiushchenko
@@ -51,19 +49,20 @@ public class HealthCheckServiceInstanceListSupplier extends DelegatingServiceIns
 
 	private final LoadBalancerProperties.HealthCheck healthCheck;
 
-	private final WebClient webClient;
-
 	private final String defaultHealthCheckPath;
 
 	private final Flux<List<ServiceInstance>> aliveInstancesReplay;
 
 	private Disposable healthCheckDisposable;
 
+	private final BiFunction<ServiceInstance, String, Mono<Boolean>> aliveFunction;
+
 	public HealthCheckServiceInstanceListSupplier(ServiceInstanceListSupplier delegate,
-			LoadBalancerProperties.HealthCheck healthCheck, WebClient webClient) {
+			LoadBalancerProperties.HealthCheck healthCheck,
+			BiFunction<ServiceInstance, String, Mono<Boolean>> aliveFunction) {
 		super(delegate);
 		defaultHealthCheckPath = healthCheck.getPath().getOrDefault("default", "/actuator/health");
-		this.webClient = webClient;
+		this.aliveFunction = aliveFunction;
 		this.healthCheck = healthCheck;
 		Repeat<Object> aliveInstancesReplayRepeat = Repeat
 				.onlyIf(repeatContext -> this.healthCheck.getRefetchInstances())
@@ -129,10 +128,7 @@ public Flux<List<ServiceInstance>> get() {
 	protected Mono<Boolean> isAlive(ServiceInstance serviceInstance) {
 		String healthCheckPropertyValue = healthCheck.getPath().get(serviceInstance.getServiceId());
 		String healthCheckPath = healthCheckPropertyValue != null ? healthCheckPropertyValue : defaultHealthCheckPath;
-		return webClient.get()
-				.uri(UriComponentsBuilder.fromUri(serviceInstance.getUri()).path(healthCheckPath).build().toUri())
-				.exchange().flatMap(clientResponse -> clientResponse.releaseBody()
-						.thenReturn(HttpStatus.OK.value() == clientResponse.rawStatusCode()));
+		return aliveFunction.apply(serviceInstance, healthCheckPath);
 	}
 
 	@Override

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

@@ -16,13 +16,15 @@
 
 package org.springframework.cloud.loadbalancer.core;
 
+import java.net.URI;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.function.BiFunction;
 import java.util.function.Function;
 
 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.discovery.DiscoveryClient;
@@ -31,8 +33,11 @@
 import org.springframework.cloud.loadbalancer.cache.LoadBalancerCacheManager;
 import org.springframework.cloud.loadbalancer.config.LoadBalancerZoneConfig;
 import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.http.HttpStatus;
 import org.springframework.util.Assert;
+import org.springframework.web.client.RestTemplate;
 import org.springframework.web.reactive.function.client.WebClient;
+import org.springframework.web.util.UriComponentsBuilder;
 
 /**
  * A Builder for creating a {@link ServiceInstanceListSupplier} hierarchy to be used in
@@ -110,7 +115,22 @@ public ServiceInstanceListSupplierBuilder withHealthChecks() {
 		DelegateCreator creator = (context, delegate) -> {
 			LoadBalancerProperties properties = context.getBean(LoadBalancerProperties.class);
 			WebClient.Builder webClient = context.getBean(WebClient.Builder.class);
-			return new HealthCheckServiceInstanceListSupplier(delegate, properties.getHealthCheck(), webClient.build());
+			return healthCheckServiceInstanceListSupplier(webClient.build(), delegate, properties);
+		};
+		this.creators.add(creator);
+		return this;
+	}
+
+	/**
+	 * Adds a {@link HealthCheckServiceInstanceListSupplier} that uses user-provided
+	 * {@link WebClient} instance to the {@link ServiceInstanceListSupplier} hierarchy.
+	 * @param webClient a user-provided {@link WebClient} instance
+	 * @return the {@link ServiceInstanceListSupplierBuilder} object
+	 */
+	public ServiceInstanceListSupplierBuilder withHealthChecks(WebClient webClient) {
+		DelegateCreator creator = (context, delegate) -> {
+			LoadBalancerProperties properties = context.getBean(LoadBalancerProperties.class);
+			return healthCheckServiceInstanceListSupplier(webClient, delegate, properties);
 		};
 		this.creators.add(creator);
 		return this;
@@ -130,14 +150,29 @@ public ServiceInstanceListSupplierBuilder withSameInstancePreference() {
 
 	/**
 	 * Adds a {@link HealthCheckServiceInstanceListSupplier} that uses user-provided
-	 * {@link WebClient} instance to the {@link ServiceInstanceListSupplier} hierarchy.
-	 * @param webClient a user-provided {@link WebClient} instance
+	 * {@link RestTemplate} instance to the {@link ServiceInstanceListSupplier} hierarchy.
 	 * @return the {@link ServiceInstanceListSupplierBuilder} object
 	 */
-	public ServiceInstanceListSupplierBuilder withHealthChecks(WebClient webClient) {
+	public ServiceInstanceListSupplierBuilder withBlockingHealthChecks() {
+		DelegateCreator creator = (context, delegate) -> {
+			RestTemplate restTemplate = context.getBean(RestTemplate.class);
+			LoadBalancerProperties properties = context.getBean(LoadBalancerProperties.class);
+			return blockingHealthCheckServiceInstanceListSupplier(restTemplate, delegate, properties);
+		};
+		this.creators.add(creator);
+		return this;
+	}
+
+	/**
+	 * Adds a {@link HealthCheckServiceInstanceListSupplier} that uses user-provided
+	 * {@link RestTemplate} instance to the {@link ServiceInstanceListSupplier} hierarchy.
+	 * @param restTemplate a user-provided {@link RestTemplate} instance
+	 * @return the {@link ServiceInstanceListSupplierBuilder} object
+	 */
+	public ServiceInstanceListSupplierBuilder withBlockingHealthChecks(RestTemplate restTemplate) {
 		DelegateCreator creator = (context, delegate) -> {
 			LoadBalancerProperties properties = context.getBean(LoadBalancerProperties.class);
-			return new HealthCheckServiceInstanceListSupplier(delegate, properties.getHealthCheck(), webClient);
+			return blockingHealthCheckServiceInstanceListSupplier(restTemplate, delegate, properties);
 		};
 		this.creators.add(creator);
 		return this;
@@ -225,6 +260,32 @@ public ServiceInstanceListSupplier build(ConfigurableApplicationContext context)
 		return supplier;
 	}
 
+	private ServiceInstanceListSupplier healthCheckServiceInstanceListSupplier(WebClient webClient,
+			ServiceInstanceListSupplier delegate, LoadBalancerProperties properties) {
+		return new HealthCheckServiceInstanceListSupplier(delegate, properties.getHealthCheck(),
+				(serviceInstance, healthCheckPath) -> webClient.get()
+						.uri(UriComponentsBuilder.fromUri(serviceInstance.getUri()).path(healthCheckPath).build()
+								.toUri())
+						.exchange().flatMap(clientResponse -> clientResponse.releaseBody()
+								.thenReturn(HttpStatus.OK.value() == clientResponse.rawStatusCode())));
+	}
+
+	private ServiceInstanceListSupplier blockingHealthCheckServiceInstanceListSupplier(RestTemplate restTemplate,
+			ServiceInstanceListSupplier delegate, LoadBalancerProperties properties) {
+		return new HealthCheckServiceInstanceListSupplier(delegate, properties.getHealthCheck(),
+				(serviceInstance, healthCheckPath) -> Mono.defer(() -> {
+					URI uri = UriComponentsBuilder.fromUri(serviceInstance.getUri()).path(healthCheckPath).build()
+							.toUri();
+					try {
+						return Mono
+								.just(HttpStatus.OK.equals(restTemplate.getForEntity(uri, Void.class).getStatusCode()));
+					}
+					catch (Exception ignored) {
+						return Mono.just(false);
+					}
+				}));
+	}
+
 	/**
 	 * Allows creating a {@link ServiceInstanceListSupplier} instance based on provided
 	 * {@link ConfigurableApplicationContext}.

spring-cloud-loadbalancer/src/test/java/org/springframework/cloud/loadbalancer/annotation/LoadBalancerClientConfigurationTests.java

@@ -37,6 +37,7 @@
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.retry.support.RetryTemplate;
+import org.springframework.web.client.RestTemplate;
 import org.springframework.web.reactive.function.client.WebClient;
 
 import static org.assertj.core.api.BDDAssertions.then;
@@ -170,6 +171,17 @@ void shouldNotWrapWithRetryAwareSupplierWhenRetryTemplateOnClasspath() {
 
 	}
 
+	@Test
+	void shouldInstantiateBlockingHealthCheckServiceInstanceListSupplier() {
+		blockingDiscoveryClientRunner.withUserConfiguration(RestTemplateTestConfig.class)
+				.withPropertyValues("spring.cloud.loadbalancer.configurations=health-check").run(context -> {
+					ServiceInstanceListSupplier supplier = context.getBean(ServiceInstanceListSupplier.class);
+					then(supplier).isInstanceOf(HealthCheckServiceInstanceListSupplier.class);
+					then(((DelegatingServiceInstanceListSupplier) supplier).getDelegate())
+							.isInstanceOf(DiscoveryClientServiceInstanceListSupplier.class);
+				});
+	}
+
 	@Configuration
 	protected static class TestConfig {
 
@@ -181,4 +193,14 @@ WebClient.Builder webClientBuilder() {
 
 	}
 
+	@Configuration
+	protected static class RestTemplateTestConfig {
+
+		@Bean
+		RestTemplate restTemplate() {
+			return new RestTemplate();
+		}
+
+	}
+
 }

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

@@ -41,6 +41,7 @@
 
 import static java.time.Duration.ofMillis;
 import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively;
+import static org.springframework.cloud.loadbalancer.core.ServiceInstanceListSuppliersTestUtils.healthCheckFunction;
 
 /**
  * Tests for {@link CachingServiceInstanceListSupplier}.
@@ -140,7 +141,7 @@ private static class TestHealthCheckServiceInstanceListSupplier extends HealthCh
 
 			TestHealthCheckServiceInstanceListSupplier(ServiceInstanceListSupplier delegate,
 					LoadBalancerProperties.HealthCheck healthCheck, WebClient webClient) {
-				super(delegate, healthCheck, webClient);
+				super(delegate, healthCheck, healthCheckFunction(webClient));
 			}
 
 			@Override