/**
 * 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.yarn.server.federation.policies.router;

import java.util. ArrayList;
import java.util. Collections;
import java.util. List;
import java.util. Map;

import org.apache.hadoop.yarn.api.records. ApplicationSubmissionContext;
import org.apache.hadoop.yarn.exceptions. YarnException;
import org.apache.hadoop.yarn.server.federation.policies. FederationPolicyInitializationContext;
import org.apache.hadoop.yarn.server.federation.policies. FederationPolicyInitializationContextValidator;
import org.apache.hadoop.yarn.server.federation.policies. FederationPolicyUtils;
import org.apache.hadoop.yarn.server.federation.policies.exceptions. FederationPolicyInitializationException;
import org.apache.hadoop.yarn.server.federation.store.records. SubClusterId;
import org.apache.hadoop.yarn.server.federation.store.records. SubClusterInfo;

/**
 * This {@link FederationRouterPolicy} pick a subcluster based on the hash of
 * the job's queue name.  Useful to provide a default behavior when too many
 * queues exist in a system.  This also ensures that all jobs belonging to a
 * queue are mapped to the same sub-cluster (likely help with locality). 
 */
public class HashBasedRouterPolicy extends AbstractRouterPolicy {

  @Override
  public void reinitialize(
      FederationPolicyInitializationContext federationPolicyContext)
      throws FederationPolicyInitializationException {
    FederationPolicyInitializationContextValidator
        .validate(federationPolicyContext, this.getClass().getCanonicalName());

    // note: this overrides BaseRouterPolicy and ignores the weights
    setPolicyContext(federationPolicyContext);
  }

  /**
   * Simply picks from alphabetically-sorted active subclusters based on the
   * hash of quey name.  Jobs of the same queue will all be routed to the same
   * sub-cluster, as far as the number of active sub-cluster and their names
   * remain the same. 
   *
   * @param appSubmissionContext the {@link ApplicationSubmissionContext} that
   *          has to be routed to an appropriate subCluster for execution. 
   *
   * @param blackListSubClusters the list of subClusters as identified by
   *          {@link SubClusterId} to blackList from the selection of the home
   *          subCluster. 
   *
   * @return a hash-based chosen {@link SubClusterId} that will be the "home"
   *         for this application. 
   *
   * @throws YarnException if there are no active subclusters. 
   */
  @Override
  public SubClusterId getHomeSubcluster(
      ApplicationSubmissionContext appSubmissionContext,
      List<SubClusterId> blackListSubClusters) throws YarnException {

    // throws if no active subclusters available
    Map<SubClusterId, SubClusterInfo> activeSubclusters =
        getActiveSubclusters();

    FederationPolicyUtils.validateSubClusterAvailability(
        new ArrayList<SubClusterId>(activeSubclusters.keySet()),
        blackListSubClusters);

    if (blackListSubClusters ! = null) {

      // Remove from the active SubClusters from StateStore the blacklisted ones
      for (SubClusterId scId : blackListSubClusters) {
        activeSubclusters.remove(scId);
      }
    }

    validate(appSubmissionContext);

    int chosenPosition = Math.abs(
        appSubmissionContext.getQueue().hashCode() % activeSubclusters.size());

    List<SubClusterId> list = new ArrayList<>(activeSubclusters.keySet());
    Collections.sort(list);
    return list.get(chosenPosition);
  }

}
