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 java.util.HashMap;
  import java.util.Map;
  import java.util.concurrent.Semaphore;
  import java.util.concurrent.TimeUnit;
  
  import org.apache.hadoop.conf.Configuration;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  /**
   * Base fairness policy that implements @RouterRpcFairnessPolicyController.
   * Internally a map of nameservice to Semaphore is used to control permits.
   */
  public class AbstractRouterRpcFairnessPolicyController
      implements RouterRpcFairnessPolicyController {
  
    private static final Logger LOG =
        LoggerFactory.getLogger(AbstractRouterRpcFairnessPolicyController.class);
  
    /** Hash table to hold semaphore for each configured name service. */
    private Map<String, Semaphore> permits;
  
    public void init(Configuration conf) {
      this.permits = new HashMap<>();
    }
  
    @Override
    public boolean acquirePermit(String nsId) {
      try {
        LOG.debug("Taking lock for nameservice {}", nsId);
        return this.permits.get(nsId).tryAcquire(1, TimeUnit.SECONDS);
      } catch (InterruptedException e) {
        LOG.debug("Cannot get a permit for nameservice {}", nsId);
      }
      return false;
    }
  
    @Override
    public void releasePermit(String nsId) {
      this.permits.get(nsId).release();
    }
  
    @Override
    public void shutdown() {
      // drain all semaphores
      for (Semaphore sema: this.permits.values()) {
        sema.drainPermits();
      }
    }
  
    protected void insertNameServiceWithPermits(String nsId, int maxPermits) {
      this.permits.put(nsId, new Semaphore(maxPermits));
    }
  
    protected int getAvailablePermits(String nsId) {
      return this.permits.get(nsId).availablePermits();
    }
  }
  