content: /**
   * 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 com.alibaba.wasp;
  
  import com.alibaba.wasp.zookeeper.MasterAddressTracker;
  import com.alibaba.wasp.zookeeper.ZooKeeperWatcher;
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.Abortable;
  
  import java.io.BufferedReader;
  import java.io.BufferedWriter;
  import java.io.File;
  import java.io.FileReader;
  import java.io.FileWriter;
  import java.io.IOException;
  
  /**
   * <p>
   * Contains a set of methods for the collaboration between the start/stop
   * scripts and the servers. It allows to delete immediately the znode when the
   * master or the entityGroups server crashes. The entityGroup server / master
   * writes a specific file when it starts / becomes main master. When they end
   * properly, they delete the file.
   * </p>
   * <p>
   * In the script, we check for the existence of these files when the program
   * ends. If they still exist we conclude that the server crashed, likely without
   * deleting their znode. To have a faster recovery we delete immediately the
   * znode.
   * </p>
   * <p>
   * The strategy depends on the server type. For a entityGroup server we store
   * the znode path in the file, and use it to delete it. for a master, as the
   * znode path constant whatever the server, we check its content to make sure
   * that the backup server is not now in charge.
   * </p>
   */
  public class ZNodeClearer {
    public static final Log LOG = LogFactory.getLog(ZNodeClearer.class);
  
    private ZNodeClearer() {
    }
  
    /**
     * Logs the errors without failing on exception.
     */
    public static void writeMyEphemeralNodeOnDisk(String fileContent) {
      String fileName = ZNodeClearer.getMyEphemeralNodeFileName();
  
      if (fileName == null) {
        LOG.warn("No filename given to save the znode used, it won't be saved "
            + "(Environment variable WASP_ZNODE_FILE is not set).");
        return;
      }
  
      FileWriter fstream;
      try {
        fstream = new FileWriter(fileName);
      } catch (IOException e) {
        LOG.warn("Can't write znode file " + fileName, e);
        return;
      }
  
      BufferedWriter out = new BufferedWriter(fstream);
  
      try {
        try {
          out.write(fileContent + "\n");
        } finally {
          try {
            out.close();
          } finally {
            fstream.close();
          }
        }
      } catch (IOException e) {
        LOG.warn("Can't write znode file " + fileName, e);
      }
    }
  
    /**
     * read the content of znode file, expects a single line.
     */
    public static String readMyEphemeralNodeOnDisk() throws IOException {
      String fileName = getMyEphemeralNodeFileName();
      if (fileName == null) {
        throw new IOException("No filename");
      }
      FileReader znodeFile = new FileReader(fileName);
      BufferedReader br = new BufferedReader(znodeFile);
      String file_content = br.readLine();
      br.close();
      return file_content;
    }
  
    /**
     * Get the name of the file used to store the znode contents
     */
    public static String getMyEphemeralNodeFileName() {
      return System.getenv().get("WASP_ZNODE_FILE");
    }
  
    /**
     * delete the znode file
     */
    public static void deleteMyEphemeralNodeOnDisk() {
      String fileName = getMyEphemeralNodeFileName();
  
      if (fileName != null) {
        new File(fileName).delete();
      }
    }
  
    /**
     * Delete the master znode if its content (ServerName string) is the same as
     * the one in the znode file. (env: HBASE_ZNODE_FILE).
     * 
     * @return true on successful deletion, false otherwise.
     */
    public static boolean clear(Configuration conf) {
      Configuration tempConf = new Configuration(conf);
      tempConf.setInt("zookeeper.recovery.retry", 0);
  
      ZooKeeperWatcher zkw;
      try {
        zkw = new ZooKeeperWatcher(tempConf, "clean znode for master",
            new Abortable() {
              @Override
              public void abort(String why, Throwable e) {
              }
  
              @Override
              public boolean isAborted() {
                return false;
              }
            });
      } catch (IOException e) {
        LOG.warn("Can't connect to zookeeper to read the master znode", e);
        return false;
      }
  
      String znodeFileContent;
      try {
        znodeFileContent = ZNodeClearer.readMyEphemeralNodeOnDisk();
      } catch (IOException e) {
        LOG.warn("Can't read the content of the znode file", e);
        return false;
      }
  
      return MasterAddressTracker.deleteIfEquals(zkw, znodeFileContent);
    }
  }
  