DATAES-488 - Add reactive Elasticsearch client support.

Initial implementation of a ReactiveElasticsearchClient using WebClient to connect to cluster nodes.

ReactiveElasticsearchClient client = ElasticsearchClients.createClient()
  .connectedTo("http://localhost:9200/", "http://localhost:9201/")
  .reactive();
A HostProvider selects active nodes and routes requests.

client.index(request ->

  request.index("spring-data")
    .type("elasticsearch")
    .id(randomUUID().toString())
    .source(singletonMap("feature", "reactive-client"))
    .setRefreshPolicy(IMMEDIATE);
);
This implementation provides the first building block for reactive Template and Repository support to be added subsequently.

Along the lines we upgraded to Elasticsearch 6.5.

Original Pull Request: spring-projects#226

Author: christophstrobl

.gitignore

@@ -1,3 +1,7 @@
+.DS_Store
+*.graphml
+.springBeans
+
 atlassian-ide-plugin.xml
 
 ## Ignore svn files

pom.xml

@@ -19,7 +19,7 @@
     <properties>
         <commonscollections>3.2.1</commonscollections>
         <commonslang>2.6</commonslang>
-        <elasticsearch>6.3.0</elasticsearch>
+        <elasticsearch>6.5.0</elasticsearch>
         <log4j>2.9.1</log4j>
         <springdata.commons>2.2.0.BUILD-SNAPSHOT</springdata.commons>
         <java-module-name>spring.data.elasticsearch</java-module-name>
@@ -51,6 +51,24 @@
             <version>${springdata.commons}</version>
         </dependency>
 
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-webflux</artifactId>
+            <version>5.1.0.RELEASE</version>
+        </dependency>
+
+        <dependency>
+            <groupId>io.projectreactor.netty</groupId>
+            <artifactId>reactor-netty</artifactId>
+            <version>0.8.0.RELEASE</version>
+        </dependency>
+
+        <dependency>
+            <groupId>io.projectreactor</groupId>
+            <artifactId>reactor-test</artifactId>
+            <version>3.2.0.RELEASE</version>
+        </dependency>
+
         <!-- APACHE -->
         <dependency>
             <groupId>commons-lang</groupId>

src/main/java/org/springframework/data/elasticsearch/client/ElasticsearchClients.java

@@ -0,0 +1,197 @@
+/*
+ * Copyright 2018 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
+ *
+ *      http://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.data.elasticsearch.client;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
+import org.apache.http.Header;
+import org.apache.http.HttpHost;
+import org.apache.http.message.BasicHeader;
+import org.elasticsearch.client.RestClient;
+import org.elasticsearch.client.RestClientBuilder;
+import org.elasticsearch.client.RestHighLevelClient;
+import org.springframework.data.elasticsearch.client.reactive.DefaultReactiveElasticsearchClient;
+import org.springframework.data.elasticsearch.client.reactive.ReactiveElasticsearchClient;
+import org.springframework.http.HttpHeaders;
+import org.springframework.util.Assert;
+
+/**
+ * Utility class for common access to Elasticsearch clients. {@link ElasticsearchClients} consolidates set up routines
+ * for the various drivers into a single place.
+ *
+ * @author Christoph Strobl
+ * @since 4.0
+ */
+public final class ElasticsearchClients {
+
+	private ElasticsearchClients() {}
+
+	/**
+	 * Start here to create a new client tailored to your needs.
+	 *
+	 * @return new instance of {@link ClientBuilderWithRequiredHost}.
+	 */
+	public static ClientBuilderWithRequiredHost createClient() {
+		return new ElasticsearchClientBuilderImpl();
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public interface ElasticsearchClientBuilder {
+
+		/**
+		 * Apply the configuration to create a {@link ReactiveElasticsearchClient}.
+		 *
+		 * @return new instance of {@link ReactiveElasticsearchClient}.
+		 */
+		ReactiveElasticsearchClient reactive();
+
+		/**
+		 * Apply the configuration to create a {@link RestHighLevelClient}.
+		 *
+		 * @return new instance of {@link RestHighLevelClient}.
+		 */
+		RestHighLevelClient rest();
+
+		/**
+		 * Apply the configuration to create a {@link RestClient}.
+		 *
+		 * @return new instance of {@link RestClient}.
+		 */
+		default RestClient lowLevelRest() {
+			return rest().getLowLevelClient();
+		}
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public interface ClientBuilderWithRequiredHost {
+
+		/**
+		 * @param host the {@literal host} and {@literal port} formatted as String {@literal host:port}. You may leave out
+		 *          {@literal http / https} and use {@link MaybeSecureClientBuilder#viaSsl() viaSsl}.
+		 * @return the {@link MaybeSecureClientBuilder}.
+		 */
+		default MaybeSecureClientBuilder connectedTo(String host) {
+			return connectedTo(new String[] { host });
+		}
+
+		/**
+		 * @param hosts the list of {@literal host} and {@literal port} combinations formatted as String
+		 *          {@literal host:port}. You may leave out {@literal http / https} and use
+		 *          {@link MaybeSecureClientBuilder#viaSsl() viaSsl}.
+		 * @return the {@link MaybeSecureClientBuilder}.
+		 */
+		MaybeSecureClientBuilder connectedTo(String... hosts);
+
+		/**
+		 * Obviously for testing.
+		 *
+		 * @return the {@link MaybeSecureClientBuilder}.
+		 */
+		default MaybeSecureClientBuilder connectedToLocalhost() {
+			return connectedTo("localhost:9200");
+		}
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public interface MaybeSecureClientBuilder extends ClientBuilderWithOptionalDefaultHeaders {
+
+		/**
+		 * Connect via {@literal https} <br />
+		 * <strong>NOTE</strong> You need to leave out the protocol in
+		 * {@link ClientBuilderWithRequiredHost#connectedTo(String)}.
+		 * 
+		 * @return the {@link ClientBuilderWithOptionalDefaultHeaders}.
+		 */
+		ClientBuilderWithOptionalDefaultHeaders viaSsl();
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public interface ClientBuilderWithOptionalDefaultHeaders extends ElasticsearchClientBuilder {
+
+		/**
+		 * @param defaultHeaders
+		 * @return the {@link ElasticsearchClientBuilder}
+		 */
+		ElasticsearchClientBuilder withDefaultHeaders(HttpHeaders defaultHeaders);
+	}
+
+	private static class ElasticsearchClientBuilderImpl
+			implements ElasticsearchClientBuilder, ClientBuilderWithRequiredHost, MaybeSecureClientBuilder {
+
+		private List<String> hosts = new ArrayList<>();
+		private HttpHeaders headers = HttpHeaders.EMPTY;
+		private String protocoll = "http";
+
+		@Override
+		public ReactiveElasticsearchClient reactive() {
+			return DefaultReactiveElasticsearchClient.create(headers, formattedHosts().toArray(new String[0]));
+		}
+
+		@Override
+		public RestHighLevelClient rest() {
+
+			HttpHost[] httpHosts = formattedHosts().stream().map(HttpHost::create).toArray(HttpHost[]::new);
+			RestClientBuilder builder = RestClient.builder(httpHosts);
+
+			if (!headers.isEmpty()) {
+
+				Header[] httpHeaders = headers.toSingleValueMap().entrySet().stream()
+						.map(it -> new BasicHeader(it.getKey(), it.getValue())).toArray(Header[]::new);
+				builder = builder.setDefaultHeaders(httpHeaders);
+			}
+
+			return new RestHighLevelClient(builder);
+		}
+
+		@Override
+		public MaybeSecureClientBuilder connectedTo(String... hosts) {
+
+			Assert.notEmpty(hosts, "At least one host is required.");
+			this.hosts.addAll(Arrays.asList(hosts));
+			return this;
+		}
+
+		@Override
+		public ClientBuilderWithOptionalDefaultHeaders withDefaultHeaders(HttpHeaders defaultHeaders) {
+
+			Assert.notNull(defaultHeaders, "DefaultHeaders must not be null!");
+			this.headers = defaultHeaders;
+			return this;
+		}
+
+		List<String> formattedHosts() {
+			return hosts.stream().map(it -> it.startsWith("http") ? it : protocoll + "://" + it).collect(Collectors.toList());
+		}
+
+		@Override
+		public ClientBuilderWithOptionalDefaultHeaders viaSsl() {
+			this.protocoll = "https";
+			return this;
+		}
+	}
+}

src/main/java/org/springframework/data/elasticsearch/client/ElasticsearchHost.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2018 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
+ *
+ *      http://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.data.elasticsearch.client;
+
+import java.time.Instant;
+
+/**
+ * Value Object containing information about Elasticsearch cluster nodes.
+ *
+ * @author Christoph Strobl
+ * @since 4.0
+ */
+public class ElasticsearchHost {
+
+	private final String host;
+	private final State state;
+	private final Instant timestamp;
+
+	public ElasticsearchHost(String host, State state) {
+
+		this.host = host;
+		this.state = state;
+		this.timestamp = Instant.now();
+	}
+
+	/**
+	 * @param host must not be {@literal null}.
+	 * @return new instance of {@link ElasticsearchHost}.
+	 */
+	public static ElasticsearchHost online(String host) {
+		return new ElasticsearchHost(host, State.ONLINE);
+	}
+
+	/**
+	 * @param host must not be {@literal null}.
+	 * @return new instance of {@link ElasticsearchHost}.
+	 */
+	public static ElasticsearchHost offline(String host) {
+		return new ElasticsearchHost(host, State.OFFLINE);
+	}
+
+	/**
+	 * @return {@literal true} if the last known {@link State} was {@link State#ONLINE}
+	 */
+	public boolean isOnline() {
+		return State.ONLINE.equals(state);
+	}
+
+	/**
+	 * @return never {@literal null}.
+	 */
+	public String getHost() {
+		return host;
+	}
+
+	/**
+	 * @return the last known {@link State}.
+	 */
+	public State getState() {
+		return state;
+	}
+
+	/**
+	 * @return the {@link Instant} the information was captured.
+	 */
+	public Instant getTimestamp() {
+		return timestamp;
+	}
+
+	@Override
+	public String toString() {
+		return "ElasticsearchHost(" + host + ", " + state.name() + ")";
+	}
+
+	public enum State {
+		ONLINE, OFFLINE, UNKNOWN
+	}
+}

src/main/java/org/springframework/data/elasticsearch/client/NoReachableHostException.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2018 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
+ *
+ *      http://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.data.elasticsearch.client;
+
+import java.util.Set;
+
+/**
+ * {@link RuntimeException} to be emitted / thrown when the cluster is down (aka none of the known nodes is reachable).
+ *
+ * @author Christoph Strobl
+ * @since 4.0
+ */
+public class NoReachableHostException extends RuntimeException {
+
+	public NoReachableHostException(Set<ElasticsearchHost> hosts) {
+		super(createMessage(hosts));
+	}
+
+	public NoReachableHostException(Set<ElasticsearchHost> hosts, Throwable cause) {
+		super(createMessage(hosts), cause);
+	}
+
+	private static String createMessage(Set<ElasticsearchHost> hosts) {
+
+		if (hosts.size() == 1) {
+			return String.format("Host '%s' not reachable. Cluster state is offline.", hosts.iterator().next().getHost());
+		}
+
+		return String.format("No active host found in cluster. (%s) of (%s) nodes offline.", hosts.size(), hosts.size());
+	}
+}