Ensure that instances don't restart at the same time upon config change

Fixes: #350

spring-cloud-kubernetes-config/src/main/java/org/springframework/cloud/kubernetes/config/reload/ConfigReloadAutoConfiguration.java

@@ -16,6 +16,8 @@
 
 package org.springframework.cloud.kubernetes.config.reload;
 
+import java.util.concurrent.ThreadLocalRandom;
+
 import io.fabric8.kubernetes.client.KubernetesClient;
 
 import org.springframework.beans.factory.annotation.Autowired;
@@ -114,18 +116,34 @@ public class ConfigReloadAutoConfiguration {
 			switch (properties.getStrategy()) {
 			case RESTART_CONTEXT:
 				return new ConfigurationUpdateStrategy(properties.getStrategy().name(),
-						restarter::restart);
+						() -> {
+							wait(properties);
+							restarter.restart();
+						});
 			case REFRESH:
 				return new ConfigurationUpdateStrategy(properties.getStrategy().name(),
 						refresher::refresh);
 			case SHUTDOWN:
 				return new ConfigurationUpdateStrategy(properties.getStrategy().name(),
-						ctx::close);
+						() -> {
+							wait(properties);
+							ctx.close();
+						});
 			}
 			throw new IllegalStateException("Unsupported configuration update strategy: "
 					+ properties.getStrategy());
 		}
 
+		private static void wait(ConfigReloadProperties properties) {
+			final long waitMillis = ThreadLocalRandom.current()
+					.nextLong(properties.getMaxWaitForRestart().toMillis());
+			try {
+				Thread.sleep(waitMillis);
+			}
+			catch (InterruptedException ignored) {
+			}
+		}
+
 	}
 
 }

spring-cloud-kubernetes-config/src/main/java/org/springframework/cloud/kubernetes/config/reload/ConfigReloadProperties.java

@@ -58,6 +58,16 @@ public class ConfigReloadProperties {
 	 */
 	private Duration period = Duration.ofMillis(15000L);
 
+	/**
+	 * If Restart or Shutdown strategies are used, Spring Cloud Kubernetes waits a random
+	 * amount of time before restarting. This is done in order to avoid having all
+	 * instances of the same application restart at the same time. This property
+	 * configures the maximum of amount of wait time from the moment the signal is
+	 * received that a restart is needed until the moment the restart is actually
+	 * triggered
+	 */
+	private Duration maxWaitForRestart = Duration.ofSeconds(2);
+
 	public ConfigReloadProperties() {
 	}
 
@@ -109,6 +119,14 @@ public class ConfigReloadProperties {
 		this.period = period;
 	}
 
+	public Duration getMaxWaitForRestart() {
+		return maxWaitForRestart;
+	}
+
+	public void setMaxWaitForRestart(Duration maxWaitForRestart) {
+		this.maxWaitForRestart = maxWaitForRestart;
+	}
+
 	/**
 	 * Reload strategies.
 	 */