solr/solrj/src/java/org/apache/solr/client/solrj/impl/LBHttp2SolrClient.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.solr.client.solrj.impl;

import java.util.Arrays;

import org.apache.solr.client.solrj.SolrClient;

/**
 * LBHttp2SolrClient or "LoadBalanced LBHttp2SolrClient" is a load balancing wrapper around
 * {@link Http2SolrClient}. This is useful when you
 * have multiple Solr servers and the requests need to be Load Balanced among them.
 *
 * Do <b>NOT</b> use this class for indexing in leader/follower scenarios since documents must be sent to the
 * correct leader; no inter-node routing is done.
 *
 * In SolrCloud (leader/replica) scenarios, it is usually better to use
 * {@link CloudSolrClient}, but this class may be used
 * for updates because the server will forward them to the appropriate leader.
 *
 * <p>
 * It offers automatic failover when a server goes down and it detects when the server comes back up.
 * <p>
 * Load balancing is done using a simple round-robin on the list of servers.
 * <p>
 * If a request to a server fails by an IOException due to a connection timeout or read timeout then the host is taken
 * off the list of live servers and moved to a 'dead server list' and the request is resent to the next live server.
 * This process is continued till it tries all the live servers. If at least one server is alive, the request succeeds,
 * and if not it fails.
 * <blockquote><pre>
 * SolrClient lbHttp2SolrClient = new LBHttp2SolrClient(http2SolrClient, "http://host1:8080/solr/", "http://host2:8080/solr", "http://host2:8080/solr");
 * </pre></blockquote>
 * This detects if a dead server comes alive automatically. The check is done in fixed intervals in a dedicated thread.
 * This interval can be set using {@link #setAliveCheckInterval} , the default is set to one minute.
 * <p>
 * <b>When to use this?</b><br> This can be used as a software load balancer when you do not wish to setup an external
 * load balancer. Alternatives to this code are to use
 * a dedicated hardware load balancer or using Apache httpd with mod_proxy_balancer as a load balancer. See <a
 * href="http://en.wikipedia.org/wiki/Load_balancing_(computing)">Load balancing on Wikipedia</a>
 *
 * @lucene.experimental
 * @since solr 8.0
 */
public class LBHttp2SolrClient extends LBSolrClient {
  private Http2SolrClient httpClient;

  public LBHttp2SolrClient(Http2SolrClient httpClient, String... baseSolrUrls) {
    super(Arrays.asList(baseSolrUrls));
    this.httpClient = httpClient;
  }
  @Override
  protected SolrClient getClient(String baseUrl) {
    return httpClient;
  }
}