/*
    * 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.master;
  
  import static org.apache.hadoop.hbase.zookeeper.ZKUtil.joinZNode;
  import com.google.common.util.concurrent.ThreadFactoryBuilder;
  import java.util.ArrayList;
  import java.util.List;
  import java.util.concurrent.ConcurrentNavigableMap;
  import java.util.concurrent.ThreadFactory;
  import org.apache.hadoop.hbase.HRegionLocation;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.exceptions.DeserializationException;
  import org.apache.hadoop.hbase.protobuf.ProtobufUtil;
  import org.apache.hadoop.hbase.types.CopyOnWriteArrayMap;
  import org.apache.hadoop.hbase.util.RetryCounter;
  import org.apache.hadoop.hbase.util.RetryCounterFactory;
  import org.apache.hadoop.hbase.zookeeper.ZKUtil;
  import org.apache.hadoop.hbase.zookeeper.ZooKeeperListener;
  import org.apache.hadoop.hbase.zookeeper.ZooKeeperWatcher;
  import org.apache.zookeeper.KeeperException;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  /**
   * A cache of meta region location metadata. Registers a listener on ZK to track changes to the
   * meta table znodes. Clients are expected to retry if the meta information is stale. This class
   * is thread-safe (a single instance of this class can be shared by multiple threads without race
   * conditions).
   */
  @InterfaceAudience.Private
  public class MetaRegionLocationCache extends ZooKeeperListener {
  
    private static final Logger LOG = LoggerFactory.getLogger(MetaRegionLocationCache.class);
  
    /**
     * Maximum number of times we retry when ZK operation times out.
     */
    private static final int MAX_ZK_META_FETCH_RETRIES = 10;
    /**
     * Sleep interval ms between ZK operation retries.
     */
    private static final int SLEEP_INTERVAL_MS_BETWEEN_RETRIES = 1000;
    private static final int SLEEP_INTERVAL_MS_MAX = 10000;
    private final RetryCounterFactory retryCounterFactory =
        new RetryCounterFactory(MAX_ZK_META_FETCH_RETRIES, SLEEP_INTERVAL_MS_BETWEEN_RETRIES);
  
    /**
     * Cached meta region locations indexed by replica ID.
     * CopyOnWriteArrayMap ensures synchronization during updates and a consistent snapshot during
     * client requests. Even though CopyOnWriteArrayMap copies the data structure for every write,
     * that should be OK since the size of the list is often small and mutations are not too often
     * and we do not need to block client requests while mutations are in progress.
     */
    private final CopyOnWriteArrayMap<Integer, HRegionLocation> cachedMetaLocations;
  
    private enum ZNodeOpType {
      INIT,
      CREATED,
      CHANGED,
      DELETED
    }
  
    public MetaRegionLocationCache(ZooKeeperWatcher zkWatcher) {
      super(zkWatcher);
      cachedMetaLocations = new CopyOnWriteArrayMap<>();
      watcher.registerListener(this);
      // Populate the initial snapshot of data from meta znodes.
      // This is needed because stand-by masters can potentially start after the initial znode
      // creation. It blocks forever until the initial meta locations are loaded from ZK and watchers
      // are established. Subsequent updates are handled by the registered listener. Also, this runs
      // in a separate thread in the background to not block master init.
      ThreadFactory threadFactory = new ThreadFactoryBuilder().setDaemon(true).build();
      final RetryCounterFactory retryFactory = new RetryCounterFactory(
          Integer.MAX_VALUE, SLEEP_INTERVAL_MS_BETWEEN_RETRIES, SLEEP_INTERVAL_MS_MAX);
      threadFactory.newThread(
          new Runnable() {
            @Override
            public void run() {
              MetaRegionLocationCache.this.loadMetaLocationsFromZk(
                  retryFactory.create(), ZNodeOpType.INIT);
            }
          }).start();
    }
 
   /**
    * Populates the current snapshot of meta locations from ZK. If no meta znodes exist, it registers
    * a watcher on base znode to check for any CREATE/DELETE events on the children.
    * @param retryCounter controls the number of retries and sleep between retries.
    */
   private void loadMetaLocationsFromZk(RetryCounter retryCounter, ZNodeOpType opType) {
     List<String> znodes = null;
     while (retryCounter.shouldRetry()) {
       try {
         znodes = watcher.getMetaReplicaNodesAndWatchChildren();
         break;
       } catch (KeeperException ke) {
         LOG.debug("Error populating initial meta locations", ke);
         if (!retryCounter.shouldRetry()) {
           // Retries exhausted and watchers not set. This is not a desirable state since the cache
           // could remain stale forever. Propagate the exception.
           watcher.abort("Error populating meta locations", ke);
           return;
         }
         try {
           retryCounter.sleepUntilNextRetry();
         } catch (InterruptedException ie) {
           LOG.error("Interrupted while loading meta locations from ZK", ie);
           Thread.currentThread().interrupt();
           return;
         }
       }
     }
     if (znodes == null || znodes.isEmpty()) {
       // No meta znodes exist at this point but we registered a watcher on the base znode to listen
       // for updates. They will be handled via nodeChildrenChanged().
       return;
     }
     if (znodes.size() == cachedMetaLocations.size()) {
       // No new meta znodes got added.
       return;
     }
     for (String znode: znodes) {
       String path = joinZNode(watcher.baseZNode, znode);
       updateMetaLocation(path, opType);
     }
   }
 
   /**
    * Gets the HRegionLocation for a given meta replica ID. Renews the watch on the znode for
    * future updates.
    * @param replicaId ReplicaID of the region.
    * @return HRegionLocation for the meta replica.
    * @throws KeeperException if there is any issue fetching/parsing the serialized data.
    */
   private HRegionLocation getMetaRegionLocation(int replicaId)
       throws KeeperException {
     RegionState metaRegionState;
     try {
       byte[] data = ZKUtil.getDataAndWatch(watcher,
           watcher.getZNodeForReplica(replicaId));
       metaRegionState = ProtobufUtil.parseMetaRegionStateFrom(data, replicaId);
     } catch (DeserializationException e) {
       throw ZKUtil.convert(e);
     }
     return new HRegionLocation(metaRegionState.getRegion(), metaRegionState.getServerName());
   }
 
   private void updateMetaLocation(String path, ZNodeOpType opType) {
     if (!isValidMetaZNode(path)) {
       return;
     }
     LOG.debug("Updating meta znode for path {}: {}", path, opType.name());
     int replicaId = watcher.getMetaReplicaIdFromPath(path);
     RetryCounter retryCounter = retryCounterFactory.create();
     HRegionLocation location = null;
     while (retryCounter.shouldRetry()) {
       try {
         if (opType == ZNodeOpType.DELETED) {
           if (!ZKUtil.watchAndCheckExists(watcher, path)) {
             // The path does not exist, we've set the watcher and we can break for now.
             break;
           }
           // If it is a transient error and the node appears right away, we fetch the
           // latest meta state.
         }
         location = getMetaRegionLocation(replicaId);
         break;
       } catch (KeeperException e) {
         LOG.debug("Error getting meta location for path {}", path, e);
         if (!retryCounter.shouldRetry()) {
           LOG.warn("Error getting meta location for path {}. Retries exhausted.", path, e);
           break;
         }
         try {
           retryCounter.sleepUntilNextRetry();
         } catch (InterruptedException ie) {
           Thread.currentThread().interrupt();
           return;
         }
       }
     }
     if (location == null) {
       cachedMetaLocations.remove(replicaId);
       return;
     }
     cachedMetaLocations.put(replicaId, location);
   }
 
   /**
    * @return Optional list of HRegionLocations for meta replica(s), null if the cache is empty.
    *
    */
   public List<HRegionLocation> getMetaRegionLocations() {
     ConcurrentNavigableMap<Integer, HRegionLocation> snapshot =
         cachedMetaLocations.tailMap(cachedMetaLocations.firstKey());
     List<HRegionLocation> result = new ArrayList<>();
     if (snapshot.isEmpty()) {
       // This could be possible if the master has not successfully initialized yet or meta region
       // is stuck in some weird state.
       return result;
     }
     // Explicitly iterate instead of new ArrayList<>(snapshot.values()) because the underlying
     // ArrayValueCollection does not implement toArray().
     for (HRegionLocation location: snapshot.values()) {
       result.add(location);
     }
     return result;
   }
 
   /**
    * Helper to check if the given 'path' corresponds to a meta znode. This listener is only
    * interested in changes to meta znodes.
    */
   private boolean isValidMetaZNode(String path) {
     return watcher.isAnyMetaReplicaZNode(path);
   }
 
   @Override
   public void nodeCreated(String path) {
     updateMetaLocation(path, ZNodeOpType.CREATED);
   }
 
   @Override
   public void nodeDeleted(String path) {
     updateMetaLocation(path, ZNodeOpType.DELETED);
   }
 
   @Override
   public void nodeDataChanged(String path) {
     updateMetaLocation(path, ZNodeOpType.CHANGED);
   }
 
   @Override
   public void nodeChildrenChanged(String path) {
     if (!path.equals(watcher.baseZNode)) {
       return;
     }
     loadMetaLocationsFromZk(retryCounterFactory.create(), ZNodeOpType.CHANGED);
   }
 }