hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/net/DNSToSwitchMappingWithDependency.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.hadoop.net;

import java.util.List;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * An interface that must be implemented to allow pluggable
 * DNS-name/IP-address to RackID resolvers.
 *
 */
@InterfaceAudience.Private
@InterfaceStability.Evolving

public interface DNSToSwitchMappingWithDependency extends DNSToSwitchMapping {
  /**
   * Get a list of dependent DNS-names for a given DNS-name/IP-address.
   * Dependent DNS-names fall into the same fault domain which must be
   * taken into account when placing replicas. This is intended to be used for
   * cross node group dependencies when node groups are not sufficient to 
   * distinguish data nodes by fault domains. In practice, this is needed when
   * a compute server runs VMs which use shared storage (as opposite to 
   * directly attached storage). In this case data nodes fall in two different
   * fault domains. One fault domain is defined by a compute server and 
   * the other is defined by storage. With node groups we can group data nodes
   * either by server fault domain or by storage fault domain. However one of
   * the fault domains cannot be handled and there we need to define cross node
   * group dependencies. These dependencies are applied in block placement 
   * polices which ensure that no two replicas will be on two dependent nodes. 
   * @param name - host name or IP address of a data node. Input host name 
   * parameter must take a value of dfs.datanode.hostname config value if this
   * config property is set. Otherwise FQDN of the data node is used.
   * @return list of dependent host names. If dfs.datanode.hostname config
   * property is set, then its value must be returned.
   * Otherwise, FQDN is returned. 
   */
  public List<String> getDependency(String name);
}