alluxio-master-2.x/core/server/master/src/main/java/alluxio/master/file/meta/UfsAbsentPathCache.java
/*
 * 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);
    }
  }
}
