code: /**
   * 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.hdfs.server.federation.fairness;
  
  import org.apache.hadoop.classification.InterfaceAudience;
  import org.apache.hadoop.classification.InterfaceStability;
  
  /**
   * Interface to define handlers assignment for specific name services.
   * This is needed to allow handlers provide a certain QoS for all name services
   * configured. Implementations can choose to define any algorithm which
   * would help maintain QoS. This is different when compared to FairCallQueue
   * semantics as fairness has a separate context in router based federation.
   * An implementation for example, could allocate a dedicated set of handlers
   * per name service and allow handlers to continue making downstream name
   * node calls if permissions are available, another implementation could use
   * preemption semantics and dynamically increase or decrease handlers
   * assigned per name service.
   */
  
  @InterfaceAudience.Private
  @InterfaceStability.Evolving
  public interface RouterRpcFairnessPolicyController {
    /**
     * Request permission for a specific name service to continue the call and
     * connect to downstream name node. Controllers based on policies defined
     * and allocations done at start-up through assignHandlersToNameservices,
     * may provide a permission or reject the request by throwing exception.
     *
     * @param nsId NS id for which a permission to continue is requested.
     * @return true or false based on whether permit is given.
     */
    boolean acquirePermit(String nsId);
  
    /**
     * Handler threads are expected to invoke this method that signals
     * controller to release the resources allocated to the thread for the
     * particular name service. This would mean permissions getting available
     * for other handlers to request for this specific name service.
     *
     * @param nsId Name service id for which permission release request is made.
     */
    void releasePermit(String nsId);
  
    /**
     * Shutdown steps to stop accepting new permission requests and clean-up.
     */
    void shutdown();
  }
  