/*
    *
    * 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.hbase.zookeeper;
  
  import static org.apache.hadoop.hbase.HConstants.DEFAULT_META_REPLICA_NUM;
  import static org.apache.hadoop.hbase.HConstants.META_REPLICAS_NUM;
  import static org.apache.hadoop.hbase.HRegionInfo.DEFAULT_REPLICA_ID;
  import static org.apache.hadoop.hbase.zookeeper.ZKUtil.joinZNode;
  import com.google.common.collect.ImmutableMap;
  import java.io.Closeable;
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.ExecutorService;
  import java.util.concurrent.Executors;
  import java.util.concurrent.TimeUnit;
  import java.util.regex.Matcher;
  import java.util.regex.Pattern;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.Abortable;
  import org.apache.hadoop.hbase.AuthUtil;
  import org.apache.hadoop.hbase.HConstants;
  import org.apache.hadoop.hbase.ZooKeeperConnectionException;
  import org.apache.hadoop.hbase.security.Superusers;
  import org.apache.hadoop.hbase.util.EnvironmentEdgeManager;
  import org.apache.hadoop.hbase.util.Threads;
  import org.apache.hadoop.security.UserGroupInformation;
  import org.apache.zookeeper.AsyncCallback;
  import org.apache.zookeeper.KeeperException;
  import org.apache.zookeeper.WatchedEvent;
  import org.apache.zookeeper.Watcher;
  import org.apache.zookeeper.ZooDefs;
  import org.apache.zookeeper.ZooDefs.Ids;
  import org.apache.zookeeper.ZooDefs.Perms;
  import org.apache.zookeeper.data.ACL;
  import org.apache.zookeeper.data.Id;
  import org.apache.zookeeper.data.Stat;
  
  /**
   * Acts as the single ZooKeeper Watcher.  One instance of this is instantiated
   * for each Master, RegionServer, and client process.
   *
   * <p>This is the only class that implements {@link Watcher}.  Other internal
   * classes which need to be notified of ZooKeeper events must register with
   * the local instance of this watcher via {@link #registerListener}.
   *
   * <p>This class also holds and manages the connection to ZooKeeper.  Code to
   * deal with connection related events and exceptions are handled here.
   */
  @InterfaceAudience.Private
  public class ZooKeeperWatcher implements Watcher, Abortable, Closeable {
    private static final Log LOG = LogFactory.getLog(ZooKeeperWatcher.class);
  
    public static final String META_ZNODE_PREFIX_CONF_KEY = "zookeeper.znode.metaserver";
    public static final String META_ZNODE_PREFIX = "meta-region-server";
  
    // Identifier for this watcher (for logging only).  It is made of the prefix
    // passed on construction and the zookeeper sessionid.
    private String prefix;
    private String identifier;
  
    // zookeeper quorum
    private String quorum;
  
    // zookeeper connection
    private final RecoverableZooKeeper recoverableZooKeeper;
  
    // abortable in case of zk failure
    protected Abortable abortable;
    // Used if abortable is null
    private boolean aborted = false;
  
    // listeners to be notified
    private final List<ZooKeeperListener> listeners =
      new CopyOnWriteArrayList<ZooKeeperListener>();
 
   /**
    * znodes containing the locations of the servers hosting the meta replicas
    */
   private final ImmutableMap<Integer, String> metaReplicaZNodes;
 
   // Single threaded executor pool that processes event notifications from Zookeeper. Events are
   // processed in the order in which they arrive (pool backed by an unbounded fifo queue). We do
   // this to decouple the event processing from Zookeeper's ClientCnxn's EventThread context.
   // EventThread internally runs a single while loop to serially process all the events. When events
   // are processed by the listeners in the same thread, that blocks the EventThread from processing
   // subsequent events. Processing events in a separate thread frees up the event thread to continue
   // and further prevents deadlocks if the process method itself makes other zookeeper calls.
   // It is ok to do it in a single thread because the Zookeeper ClientCnxn already serializes the
   // requests using a single while loop and hence there is no performance degradation.
   private final ExecutorService zkEventProcessor =
       Executors.newSingleThreadExecutor(Threads.getNamedThreadFactory("zk-event-processor"));
 
   // Used by ZKUtil:waitForZKConnectionIfAuthenticating to wait for SASL
   // negotiation to complete
   public CountDownLatch saslLatch = new CountDownLatch(1);
 
   // node names
 
   // base znode for this cluster
   public String baseZNode;
   //znodes containing the locations of the servers hosting the meta replicas
   private Map<Integer,String> metaReplicaZnodes = new HashMap<Integer, String>();
   // znode containing ephemeral nodes of the regionservers
   public String rsZNode;
   // znode containing ephemeral nodes of the draining regionservers
   public String drainingZNode;
   // znode of currently active master
   private String masterAddressZNode;
   // znode of this master in backup master directory, if not the active master
   public String backupMasterAddressesZNode;
   // znode containing the current cluster state
   public String clusterStateZNode;
   // znode used for region transitioning and assignment
   public String assignmentZNode;
   // znode used for table disabling/enabling
   public String tableZNode;
   // znode containing the unique cluster ID
   public String clusterIdZNode;
   // znode used for log splitting work assignment
   public String splitLogZNode;
   // znode containing the state of the load balancer
   public String balancerZNode;
   // znode containing the state of region normalizer
   private String regionNormalizerZNode;
   // znode containing the state of all switches, currently there are split and merge child node.
   private String switchZNode;
   // znode containing the lock for the tables
   public String tableLockZNode;
   // znode containing the state of the snapshot auto-cleanup
   String snapshotCleanupZNode;
   // znode containing the state of recovering regions
   public String recoveringRegionsZNode;
   // znode containing namespace descriptors
   public static String namespaceZNode = "namespace";
   // znode of indicating master maintenance mode
   public static String masterMaintZNode = "masterMaintenance";
 
   /**
    * The prefix of meta znode. Does not include baseZNode.
    * Its a 'prefix' because meta replica id integer can be tagged on the end (if
    * no number present, it is 'default' replica).
    */
   private final String metaZNodePrefix;
 
   // Certain ZooKeeper nodes need to be world-readable
   public static final ArrayList<ACL> CREATOR_ALL_AND_WORLD_READABLE =
     new ArrayList<ACL>() { {
       add(new ACL(ZooDefs.Perms.READ,ZooDefs.Ids.ANYONE_ID_UNSAFE));
       add(new ACL(ZooDefs.Perms.ALL,ZooDefs.Ids.AUTH_IDS));
     }};
 
   private static final String DEFAULT_SNAPSHOT_CLEANUP_ZNODE = "snapshot-cleanup";
 
   private final Configuration conf;
 
   private final long zkSyncTimeout;
 
   /* A pattern that matches a Kerberos name, borrowed from Hadoop's KerberosName */
   private static final Pattern NAME_PATTERN = Pattern.compile("([^/@]*)(/([^/@]*))?@([^/@]*)");
 
   /**
    * Instantiate a ZooKeeper connection and watcher.
    * @param identifier string that is passed to RecoverableZookeeper to be used as identifier for
    *          this instance. Use null for default.
    * @throws IOException
    * @throws ZooKeeperConnectionException
    */
   public ZooKeeperWatcher(Configuration conf, String identifier, Abortable abortable)
       throws ZooKeeperConnectionException, IOException {
     this(conf, identifier, abortable, false);
   }
 
   /**
    * Instantiate a ZooKeeper connection and watcher.
    * @param conf
    * @param identifier string that is passed to RecoverableZookeeper to be used as identifier for
    *          this instance. Use null for default.
    * @param abortable Can be null if there is on error there is no host to abort: e.g. client
    *          context.
    * @param canCreateBaseZNode
    * @throws IOException
    * @throws ZooKeeperConnectionException
    */
   public ZooKeeperWatcher(Configuration conf, String identifier,
       Abortable abortable, boolean canCreateBaseZNode)
   throws IOException, ZooKeeperConnectionException {
     this.conf = conf;
     this.quorum = ZKConfig.getZKQuorumServersString(conf);
     this.prefix = identifier;
     // Identifier will get the sessionid appended later below down when we
     // handle the syncconnect event.
     this.identifier = identifier + "0x0";
     this.abortable = abortable;
     setNodeNames(conf);
     PendingWatcher pendingWatcher = new PendingWatcher();
     this.recoverableZooKeeper = ZKUtil.connect(conf, quorum, pendingWatcher, identifier);
     pendingWatcher.prepare(this);
     ImmutableMap.Builder<Integer, String> builder = ImmutableMap.builder();
     metaZNodePrefix = conf.get(META_ZNODE_PREFIX_CONF_KEY, META_ZNODE_PREFIX);
     String defaultMetaReplicaZNode = joinZNode(baseZNode, metaZNodePrefix);
     builder.put(DEFAULT_REPLICA_ID, defaultMetaReplicaZNode);
     int numMetaReplicas = conf.getInt(META_REPLICAS_NUM, DEFAULT_META_REPLICA_NUM);
     for (int i = 1; i < numMetaReplicas; i++) {
       builder.put(i, defaultMetaReplicaZNode + "-" + i);
     }
     metaReplicaZNodes = builder.build();
     if (canCreateBaseZNode) {
       try {
         createBaseZNodes();
       } catch (ZooKeeperConnectionException zce) {
         try {
           this.recoverableZooKeeper.close();
         } catch (InterruptedException ie) {
           LOG.debug("Encountered InterruptedException when closing " + this.recoverableZooKeeper);
           Thread.currentThread().interrupt();
         }
         throw zce;
       }
     }
     this.zkSyncTimeout = conf.getLong(HConstants.ZK_SYNC_BLOCKING_TIMEOUT_MS,
         HConstants.ZK_SYNC_BLOCKING_TIMEOUT_DEFAULT_MS);
   }
 
   /**
    * @return true if the znode is a meta region replica
    */
   public boolean isAnyMetaReplicaZNode(String node) {
     return this.metaReplicaZNodes.containsValue(node);
   }
 
   private void createBaseZNodes() throws ZooKeeperConnectionException {
     try {
       // Create all the necessary "directories" of znodes
       ZKUtil.createWithParents(this, baseZNode);
       if (conf.getBoolean("hbase.assignment.usezk", true)) {
         ZKUtil.createAndFailSilent(this, assignmentZNode);
       }
       ZKUtil.createAndFailSilent(this, rsZNode);
       ZKUtil.createAndFailSilent(this, drainingZNode);
       ZKUtil.createAndFailSilent(this, tableZNode);
       ZKUtil.createAndFailSilent(this, splitLogZNode);
       ZKUtil.createAndFailSilent(this, backupMasterAddressesZNode);
       ZKUtil.createAndFailSilent(this, tableLockZNode);
       ZKUtil.createAndFailSilent(this, recoveringRegionsZNode);
       ZKUtil.createAndFailSilent(this, masterMaintZNode);
     } catch (KeeperException e) {
       throw new ZooKeeperConnectionException(
           prefix("Unexpected KeeperException creating base node"), e);
     }
   }
 
   /** Returns whether the znode is supposed to be readable by the client
    * and DOES NOT contain sensitive information (world readable).*/
   public boolean isClientReadable(String node) {
     // Developer notice: These znodes are world readable. DO NOT add more znodes here UNLESS
     // all clients need to access this data to work. Using zk for sharing data to clients (other
     // than service lookup case is not a recommended design pattern.
     return
         node.equals(baseZNode) ||
         isAnyMetaReplicaZnode(node) ||
         node.equals(getMasterAddressZNode()) ||
         node.equals(clusterIdZNode)||
         node.equals(rsZNode) ||
         // /hbase/table and /hbase/table/foo is allowed, /hbase/table-lock is not
         node.equals(tableZNode) ||
         node.startsWith(tableZNode + "/");
   }
 
   /**
    * On master start, we check the znode ACLs under the root directory and set the ACLs properly
    * if needed. If the cluster goes from an unsecure setup to a secure setup, this step is needed
    * so that the existing znodes created with open permissions are now changed with restrictive
    * perms.
    */
   public void checkAndSetZNodeAcls() {
     if (!ZKUtil.isSecureZooKeeper(getConfiguration())) {
       LOG.info("not a secure deployment, proceeding");
       return;
     }
 
     // Check the base znodes permission first. Only do the recursion if base znode's perms are not
     // correct.
     try {
       List<ACL> actualAcls = recoverableZooKeeper.getAcl(baseZNode, new Stat());
 
       if (!isBaseZnodeAclSetup(actualAcls)) {
         LOG.info("setting znode ACLs");
         setZnodeAclsRecursive(baseZNode);
       }
     } catch(KeeperException.NoNodeException nne) {
       return;
     } catch(InterruptedException ie) {
       interruptedExceptionNoThrow(ie, false);
     } catch (IOException|KeeperException e) {
       LOG.warn("Received exception while checking and setting zookeeper ACLs", e);
     }
   }
 
   /**
    * Set the znode perms recursively. This will do post-order recursion, so that baseZnode ACLs
    * will be set last in case the master fails in between.
    * @param znode
    */
   private void setZnodeAclsRecursive(String znode) throws KeeperException, InterruptedException {
     List<String> children = recoverableZooKeeper.getChildren(znode, false);
 
     for (String child : children) {
       setZnodeAclsRecursive(joinZNode(znode, child));
     }
     List<ACL> acls = ZKUtil.createACL(this, znode, true);
     LOG.info("Setting ACLs for znode:" + znode + " , acl:" + acls);
     recoverableZooKeeper.setAcl(znode, acls, -1);
   }
 
   /**
    * Checks whether the ACLs returned from the base znode (/hbase) is set for secure setup.
    * @param acls acls from zookeeper
    * @return whether ACLs are set for the base znode
    * @throws IOException
    */
   private boolean isBaseZnodeAclSetup(List<ACL> acls) throws IOException {
     if (LOG.isDebugEnabled()) {
       LOG.debug("Checking znode ACLs");
     }
     String[] superUsers = conf.getStrings(Superusers.SUPERUSER_CONF_KEY);
     // Check whether ACL set for all superusers
     if (superUsers != null && !checkACLForSuperUsers(superUsers, acls)) {
       return false;
     }
 
     // this assumes that current authenticated user is the same as zookeeper client user
     // configured via JAAS
     String hbaseUser = UserGroupInformation.getCurrentUser().getShortUserName();
 
     if (acls.isEmpty()) {
       if (LOG.isDebugEnabled()) {
         LOG.debug("ACL is empty");
       }
       return false;
     }
 
     for (ACL acl : acls) {
       int perms = acl.getPerms();
       Id id = acl.getId();
       // We should only set at most 3 possible ACLs for 3 Ids. One for everyone, one for superuser
       // and one for the hbase user
       if (Ids.ANYONE_ID_UNSAFE.equals(id)) {
         if (perms != Perms.READ) {
           if (LOG.isDebugEnabled()) {
             LOG.debug(String.format("permissions for '%s' are not correct: have 0x%x, want 0x%x",
               id, perms, Perms.READ));
           }
           return false;
         }
       } else if (superUsers != null && isSuperUserId(superUsers, id)) {
         if (perms != Perms.ALL) {
           if (LOG.isDebugEnabled()) {
             LOG.debug(String.format("permissions for '%s' are not correct: have 0x%x, want 0x%x",
               id, perms, Perms.ALL));
           }
           return false;
         }
       } else if ("sasl".equals(id.getScheme())) {
         String name = id.getId();
         // If ZooKeeper recorded the Kerberos full name in the ACL, use only the shortname
         Matcher match = NAME_PATTERN.matcher(name);
         if (match.matches()) {
           name = match.group(1);
         }
         if (name.equals(hbaseUser)) {
           if (perms != Perms.ALL) {
             if (LOG.isDebugEnabled()) {
               LOG.debug(String.format("permissions for '%s' are not correct: have 0x%x, want 0x%x",
                 id, perms, Perms.ALL));
             }
             return false;
           }
         } else {
           if (LOG.isDebugEnabled()) {
             LOG.debug("Unexpected shortname in SASL ACL: " + id);
           }
           return false;
         }
       } else {
         if (LOG.isDebugEnabled()) {
           LOG.debug("unexpected ACL id '" + id + "'");
         }
         return false;
       }
     }
     return true;
   }
   
   /*
    * Validate whether ACL set for all superusers.
    */
   private boolean checkACLForSuperUsers(String[] superUsers, List<ACL> acls) {
     for (String user : superUsers) {
       boolean hasAccess = false;
       // TODO: Validate super group members also when ZK supports setting node ACL for groups.
       if (!AuthUtil.isGroupPrincipal(user)) {
         for (ACL acl : acls) {
           if (user.equals(acl.getId().getId())) {
             if (acl.getPerms() == Perms.ALL) {
               hasAccess = true;
             } else {
               if (LOG.isDebugEnabled()) {
                 LOG.debug(String.format(
                   "superuser '%s' does not have correct permissions: have 0x%x, want 0x%x",
                   acl.getId().getId(), acl.getPerms(), Perms.ALL));
               }
             }
             break;
           }
         }
         if (!hasAccess) {
           return false;
         }
       }
     }
     return true;
   }
   
   /*
    * Validate whether ACL ID is superuser.
    */
   public static boolean isSuperUserId(String[] superUsers, Id id) {
     for (String user : superUsers) {
       // TODO: Validate super group members also when ZK supports setting node ACL for groups.
       if (!AuthUtil.isGroupPrincipal(user) && new Id("sasl", user).equals(id)) {
         return true;
       }
     }
     return false;
   }
 
   @Override
   public String toString() {
     return this.identifier + ", quorum=" + quorum + ", baseZNode=" + baseZNode;
   }
 
   /**
    * Adds this instance's identifier as a prefix to the passed <code>str</code>
    * @param str String to amend.
    * @return A new string with this instance's identifier as prefix: e.g.
    * if passed 'hello world', the returned string could be
    */
   public String prefix(final String str) {
     return this.toString() + " " + str;
   }
 
   /**
    * Set the local variable node names using the specified configuration.
    */
   private void setNodeNames(Configuration conf) {
     baseZNode = conf.get(HConstants.ZOOKEEPER_ZNODE_PARENT,
         HConstants.DEFAULT_ZOOKEEPER_ZNODE_PARENT);
     metaReplicaZnodes.put(0, joinZNode(baseZNode,
            conf.get("zookeeper.znode.metaserver", "meta-region-server")));
     int numMetaReplicas = conf.getInt(META_REPLICAS_NUM,
             DEFAULT_META_REPLICA_NUM);
     for (int i = 1; i < numMetaReplicas; i++) {
       String str = joinZNode(baseZNode,
         conf.get("zookeeper.znode.metaserver", "meta-region-server") + "-" + i);
       metaReplicaZnodes.put(i, str);
     }
     rsZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.rs", "rs"));
     drainingZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.draining.rs", "draining"));
     masterAddressZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.master", "master"));
     backupMasterAddressesZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.backup.masters", "backup-masters"));
     clusterStateZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.state", "running"));
     assignmentZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.unassigned", "region-in-transition"));
     tableZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.tableEnableDisable", "table"));
     clusterIdZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.clusterId", "hbaseid"));
     splitLogZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.splitlog", HConstants.SPLIT_LOGDIR_NAME));
     balancerZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.balancer", "balancer"));
     regionNormalizerZNode = joinZNode(baseZNode,
       conf.get("zookeeper.znode.regionNormalizer", "normalizer"));
     switchZNode = joinZNode(baseZNode, conf.get("zookeeper.znode.switch", "switch"));
     tableLockZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.tableLock", "table-lock"));
     snapshotCleanupZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.snapshot.cleanup", DEFAULT_SNAPSHOT_CLEANUP_ZNODE));
     recoveringRegionsZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.recovering.regions", "recovering-regions"));
     namespaceZNode = joinZNode(baseZNode,
         conf.get("zookeeper.znode.namespace", "namespace"));
     masterMaintZNode = joinZNode(baseZNode,
       conf.get("zookeeper.znode.masterMaintenance", "master-maintenance"));
   }
 
   /**
    * Is the znode of any meta replica
    * @param node
    * @return true or false
    */
   public boolean isAnyMetaReplicaZnode(String node) {
     if (metaReplicaZnodes.values().contains(node)) {
       return true;
     }
     return false;
   }
 
   /**
    * Is it the default meta replica's znode
    * @param node
    * @return true or false
    */
   public boolean isDefaultMetaReplicaZnode(String node) {
     if (getZNodeForReplica(DEFAULT_REPLICA_ID).equals(node)) {
       return true;
     }
     return false;
   }
 
   /**
    * Get the znodes corresponding to the meta replicas from ZK
    * @return list of znodes
    * @throws KeeperException
    */
   public List<String> getMetaReplicaNodes() throws KeeperException {
     List<String> childrenOfBaseNode = ZKUtil.listChildrenNoWatch(this, baseZNode);
     List<String> metaReplicaNodes = new ArrayList<String>(2);
     if (childrenOfBaseNode != null) {
       String pattern = conf.get("zookeeper.znode.metaserver","meta-region-server");
       for (String child : childrenOfBaseNode) {
         if (child.startsWith(pattern)) metaReplicaNodes.add(child);
       }
     }
     return metaReplicaNodes;
   }
 
   /**
    * Get the znode string corresponding to a replicaId
    * @param replicaId
    * @return znode
    */
   public String getZNodeForReplica(int replicaId) {
     String str = metaReplicaZnodes.get(replicaId);
     // return a newly created path but don't update the cache of paths
     // This is mostly needed for tests that attempt to create meta replicas
     // from outside the master
     if (str == null) {
       str = joinZNode(baseZNode,
           conf.get("zookeeper.znode.metaserver", "meta-region-server") + "-" + replicaId);
     }
     return str;
   }
 
   /**
    * Parse the meta replicaId from the passed znode
    * @param znode
    * @return replicaId
    */
   public int getMetaReplicaIdFromZnode(String znode) {
     String pattern = conf.get("zookeeper.znode.metaserver","meta-region-server");
     if (znode.equals(pattern)) {
       return DEFAULT_REPLICA_ID;
     }
     // the non-default replicas are of the pattern meta-region-server-<replicaId>
     String nonDefaultPattern = pattern + "-";
     return Integer.parseInt(znode.substring(nonDefaultPattern.length()));
   }
 
   /**
    * Register the specified listener to receive ZooKeeper events.
    * @param listener
    */
   public void registerListener(ZooKeeperListener listener) {
     listeners.add(listener);
   }
 
   /**
    * Register the specified listener to receive ZooKeeper events and add it as
    * the first in the list of current listeners.
    * @param listener
    */
   public void registerListenerFirst(ZooKeeperListener listener) {
     listeners.add(0, listener);
   }
 
   public void unregisterListener(ZooKeeperListener listener) {
     listeners.remove(listener);
   }
 
   /**
    * Clean all existing listeners
    */
   public void unregisterAllListeners() {
     listeners.clear();
   }
 
   /**
    * Get a copy of current registered listeners
    */
   public List<ZooKeeperListener> getListeners() {
     return new ArrayList<ZooKeeperListener>(listeners);
   }
 
   /**
    * @return The number of currently registered listeners
    */
   public int getNumberOfListeners() {
     return listeners.size();
   }
 
   /**
    * Get the connection to ZooKeeper.
    * @return connection reference to zookeeper
    */
   public RecoverableZooKeeper getRecoverableZooKeeper() {
     return recoverableZooKeeper;
   }
 
   public void reconnectAfterExpiration() throws IOException, KeeperException, InterruptedException {
     recoverableZooKeeper.reconnectAfterExpiration();
   }
 
   /**
    * Get the quorum address of this instance.
    * @return quorum string of this zookeeper connection instance
    */
   public String getQuorum() {
     return quorum;
   }
 
   /**
    * @return the base znode of this zookeeper connection instance.
    */
   public String getBaseZNode() {
     return baseZNode;
   }
 
   private void processEvent(WatchedEvent event) {
     switch(event.getType()) {
       // If event type is NONE, this is a connection status change
       case None: {
         connectionEvent(event);
         break;
       }
       // Otherwise pass along to the listeners
       case NodeCreated: {
         for(ZooKeeperListener listener : listeners) {
           listener.nodeCreated(event.getPath());
         }
         break;
       }
       case NodeDeleted: {
         for(ZooKeeperListener listener : listeners) {
           listener.nodeDeleted(event.getPath());
         }
         break;
       }
       case NodeDataChanged: {
         for(ZooKeeperListener listener : listeners) {
           listener.nodeDataChanged(event.getPath());
         }
         break;
       }
       case NodeChildrenChanged: {
         for(ZooKeeperListener listener : listeners) {
           listener.nodeChildrenChanged(event.getPath());
         }
         break;
       }
       default: {
         LOG.error(String.format("Invalid event of type %s received for path %s. Ignoring",
             event.getType(), event.getPath()));
         break;
       }
     }
   }
 
   /**
    * Method called from ZooKeeper for events and connection status.
    * <p>
    * Valid events are passed along to listeners.  Connection status changes
    * are dealt with locally.
    */
   @Override
   public void process(final WatchedEvent event) {
     LOG.debug(prefix("Received ZooKeeper Event, " +
         "type=" + event.getType() + ", " +
         "state=" + event.getState() + ", " +
         "path=" + event.getPath()));
     zkEventProcessor.submit(new Runnable() {
       @Override
       public void run() {
         processEvent(event);
       }
     });
   }
 
   // Connection management
 
   /**
    * Called when there is a connection-related event via the Watcher callback.
    * <p>
    * If Disconnected or Expired, this should shutdown the cluster. But, since
    * we send a KeeperException.SessionExpiredException along with the abort
    * call, it's possible for the Abortable to catch it and try to create a new
    * session with ZooKeeper. This is what the client does in HCM.
    * <p>
    * @param event
    */
   private void connectionEvent(WatchedEvent event) {
     switch(event.getState()) {
       case SyncConnected:
         this.identifier = this.prefix + "-0x" +
           Long.toHexString(this.recoverableZooKeeper.getSessionId());
         // Update our identifier.  Otherwise ignore.
         LOG.debug(this.identifier + " connected");
         break;
 
       // Abort the server if Disconnected or Expired
       case Disconnected:
         LOG.debug(prefix("Received Disconnected from ZooKeeper, ignoring"));
         break;
 
       case Expired:
         String msg = prefix(this.identifier + " received expired from " +
           "ZooKeeper, aborting");
         // TODO: One thought is to add call to ZooKeeperListener so say,
         // ZooKeeperNodeTracker can zero out its data values.
         if (this.abortable != null) {
           this.abortable.abort(msg, new KeeperException.SessionExpiredException());
         }
         break;
 
       case ConnectedReadOnly:
       case SaslAuthenticated:
       case AuthFailed:
         break;
 
       default:
         throw new IllegalStateException("Received event is not valid: " + event.getState());
     }
   }
 
   /**
    * Forces a synchronization of this ZooKeeper client connection within a timeout. Enforcing a
    * timeout lets the callers fail-fast rather than wait forever for the sync to finish.
    * <p>
    * Executing this method before running other methods will ensure that the
    * subsequent operations are up-to-date and consistent as of the time that
    * the sync is complete.
    * <p>
    * This is used for compareAndSwap type operations where we need to read the
    * data of an existing node and delete or transition that node, utilizing the
    * previously read version and data.  We want to ensure that the version read
    * is up-to-date from when we begin the operation.
    */
   public void syncOrTimeout(String path) throws KeeperException {
     final CountDownLatch latch = new CountDownLatch(1);
     long startTime = EnvironmentEdgeManager.currentTime();
     this.recoverableZooKeeper.sync(path, new AsyncCallback.VoidCallback() {
       @Override
       public void processResult(int i, String s, Object o) {
         latch.countDown();
       }
     }, null);
     try {
       if (!latch.await(zkSyncTimeout, TimeUnit.MILLISECONDS)) {
         LOG.warn(String.format("sync() operation to ZK timed out. Configured timeout: %s ms. " +
             "This usually points to a ZK side issue. Check ZK server logs and metrics.",
             zkSyncTimeout));
         throw new KeeperException.OperationTimeoutException();
       }
     } catch (InterruptedException e) {
       LOG.warn("Interrupted waiting for ZK sync() to finish.", e);
       Thread.currentThread().interrupt();
       return;
     }
     if (LOG.isDebugEnabled()) {
       // TODO: Switch to a metric once server side ZK watcher metrics are implemented. This is a
       // useful metric to have since the latency of sync() impacts the callers.
       LOG.debug(String.format("ZK sync() operation took %d ms",
           EnvironmentEdgeManager.currentTime() - startTime));
     }
   }
 
   /**
    * Handles KeeperExceptions in client calls.
    * <p>
    * This may be temporary but for now this gives one place to deal with these.
    * <p>
    * TODO: Currently this method rethrows the exception to let the caller handle
    * <p>
    * @param ke
    * @throws KeeperException
    */
   public void keeperException(KeeperException ke)
   throws KeeperException {
     LOG.error(prefix("Received unexpected KeeperException, re-throwing exception"), ke);
     throw ke;
   }
 
   /**
    * Handles InterruptedExceptions in client calls.
    * @param ie the InterruptedException instance thrown
    * @throws KeeperException the exception to throw, transformed from the InterruptedException
    */
   public void interruptedException(InterruptedException ie) throws KeeperException {
     interruptedExceptionNoThrow(ie, true);
     // Throw a system error exception to let upper level handle it
     throw new KeeperException.SystemErrorException();
   }
 
   /**
    * Log the InterruptedException and interrupt current thread
    * @param ie The IterruptedException to log
    * @param throwLater Whether we will throw the exception latter
    */
   public void interruptedExceptionNoThrow(InterruptedException ie, boolean throwLater) {
     LOG.debug(prefix("Received InterruptedException, will interrupt current thread"
         + (throwLater ? " and rethrow a SystemErrorException" : "")),
       ie);
     // At least preserve interrupt.
     Thread.currentThread().interrupt();
   }
 
   /**
    * Close the connection to ZooKeeper.
    *
    */
   @Override
   public void close() {
     try {
       recoverableZooKeeper.close();
     } catch (InterruptedException e) {
       Thread.currentThread().interrupt();
     } finally {
       zkEventProcessor.shutdownNow();
     }
   }
 
   public Configuration getConfiguration() {
     return conf;
   }
 
   @Override
   public void abort(String why, Throwable e) {
     if (this.abortable != null) this.abortable.abort(why, e);
     else this.aborted = true;
   }
 
   @Override
   public boolean isAborted() {
     return this.abortable == null? this.aborted: this.abortable.isAborted();
   }
 
   /**
    * @return Path to the currently active master.
    */
   public String getMasterAddressZNode() {
     return this.masterAddressZNode;
   }
 
   /**
    * @return ZooKeeper znode for region normalizer state
    */
   public String getRegionNormalizerZNode() {
     return regionNormalizerZNode;
   }
 
   /**
    *  @return ZK node for switch
    * */
   public String getSwitchZNode() {
     return switchZNode;
   }
 
   /**
    * Parses the meta replicaId from the passed path.
    * @param path the name of the full path which includes baseZNode.
    * @return replicaId
    */
   public int getMetaReplicaIdFromPath(String path) {
     // Extract the znode from path. The prefix is of the following format.
     // baseZNode + PATH_SEPARATOR.
     int prefixLen = baseZNode.length() + 1;
     return getMetaReplicaIdFromZnode(path.substring(prefixLen));
   }
 
   /**
    * Same as {@link #getMetaReplicaNodes()} except that this also registers a watcher on base znode
    * for subsequent CREATE/DELETE operations on child nodes.
    */
   public List<String> getMetaReplicaNodesAndWatchChildren() throws KeeperException {
     List<String> childrenOfBaseNode =
         ZKUtil.listChildrenAndWatchForNewChildren(this, baseZNode);
     return filterMetaReplicaNodes(childrenOfBaseNode);
   }
 
   /**
    * @param nodes Input list of znodes
    * @return Filtered list of znodes from nodes that belong to meta replica(s).
    */
   private List<String> filterMetaReplicaNodes(List<String> nodes) {
     if (nodes == null || nodes.isEmpty()) {
       return new ArrayList<>();
     }
     List<String> metaReplicaNodes = new ArrayList<>(2);
     String pattern = conf.get(META_ZNODE_PREFIX_CONF_KEY, META_ZNODE_PREFIX);
     for (String child : nodes) {
       if (child.startsWith(pattern)) {
         metaReplicaNodes.add(child);
       }
     }
     return metaReplicaNodes;
   }
 
 }