content: /*
   * The Alluxio Open Foundation licenses this work under the Apache License, version 2.0
   * (the "License"). You may not use this work except in compliance with the License, which is
   * available at www.apache.org/licenses/LICENSE-2.0
   *
   * This software is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
   * either express or implied, as more fully set forth in the License.
   *
   * See the NOTICE file distributed with this work for information regarding copyright ownership.
   */
  
  package alluxio.master.file.meta;
  
  import alluxio.AlluxioURI;
  import alluxio.conf.Configuration;
  import alluxio.conf.PropertyKey;
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import java.time.Clock;
  import java.util.List;
  
  /**
   * Cache for recording information about paths that are not present in UFS.
   */
  public interface UfsAbsentPathCache {
    long ALWAYS = -1;
    long NEVER = Long.MAX_VALUE;
  
    /**
     * Processes the given path for the cache. This will asynchronously and sequentially walk down
     * the path to find components which do and do not exist in the UFS, and updates the cache
     * accordingly.
     *
     * @param path the path to process for the cache
     * @param prefixInodes the existing inodes for the path prefix
     */
    void processAsync(AlluxioURI path, List<Inode> prefixInodes);
  
    /**
     * Add a single path to the absent cache synchronously.
     * @param path the path to process for the cache
     */
    void addSinglePath(AlluxioURI path);
  
    /**
     * Processes the given path that already exists. This will sequentially walk down the path and
     * update the cache accordingly.
     *
     * @param path the path to process for the cache
     */
    void processExisting(AlluxioURI path);
  
    /**
     * Returns true if the given path was found to be absent since absentSince, according to this
     * cache.
     * A path is absent if one of its ancestors is absent.
     *
     * @param path the path to check
     * @param absentSince the time when the cache entry would be considered valid
     * @return true if the path is absent according to the cache
     */
    boolean isAbsentSince(AlluxioURI path, long absentSince);
  
    /**
     * Factory class for {@link UfsAbsentPathCache}.
     */
    final class Factory {
      private static final Logger LOG = LoggerFactory.getLogger(UfsAbsentPathCache.Factory.class);
  
      private Factory() {} // prevent instantiation
  
      /**
       * @param mountTable the mount table
       * @param clock the clock to use to compute the sync times
       * @return {@link UfsAbsentPathCache}
       */
      public static UfsAbsentPathCache create(MountTable mountTable, Clock clock) {
        int numThreads = Configuration.getInt(PropertyKey.MASTER_UFS_PATH_CACHE_THREADS);
        if (numThreads <= 0) {
          LOG.info("UfsAbsentPathCache is disabled. {}: {}",
              PropertyKey.MASTER_UFS_PATH_CACHE_THREADS, numThreads);
          return new NoopUfsAbsentPathCache();
        }
        return new AsyncUfsAbsentPathCache(mountTable, numThreads, clock);
      }
    }
  }
  