/**
 * 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.hdfs.server.namenode.ha;

import static org.junit.Assert.assertEquals;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ThreadLocalRandom;

import com.google.common.base.Supplier;
import com.google.common.collect.Lists;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataOutputStream;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.fs.StorageType;
import org.apache.hadoop.hdfs.AppendTestUtil;
import org.apache.hadoop.hdfs.DFSConfigKeys;
import org.apache.hadoop.hdfs.DFSTestUtil;
import org.apache.hadoop.hdfs.MiniDFSCluster;
import org.apache.hadoop.hdfs.MiniDFSNNTopology;
import org.apache.hadoop.hdfs.protocol.ExtendedBlock;
import org.apache.hadoop.hdfs.protocolPB.DatanodeProtocolClientSideTranslatorPB;
import org.apache.hadoop.hdfs.server.blockmanagement.BlockManager;
import org.apache.hadoop.hdfs.server.blockmanagement.BlockManagerTestUtil;
import org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy;
import org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault;
import org.apache.hadoop.hdfs.server.blockmanagement.DatanodeStorageInfo;
import org.apache.hadoop.hdfs.server.datanode.DataNode;
import org.apache.hadoop.hdfs.server.datanode.DataNodeTestUtils;
import org.apache.hadoop.hdfs.server.datanode.InternalDataNodeTestUtils;
import org.apache.hadoop.hdfs.server.namenode.NameNode;
import org.apache.hadoop.hdfs.server.namenode.NameNodeAdapter;
import org.apache.hadoop.hdfs.server.protocol.BlockReportContext;
import org.apache.hadoop.hdfs.server.protocol.DatanodeRegistration;
import org.apache.hadoop.hdfs.server.protocol.StorageBlockReport;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.test.GenericTestUtils;
import org.apache.hadoop.test.GenericTestUtils.DelayAnswer;
import org.apache.log4j.Level;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import org.mockito.Mockito;
import org.mockito.invocation.InvocationOnMock;


public class TestDNFencing {
  
  protected static final Log LOG = LogFactory.getLog(TestDNFencing.class);
  private static final String TEST_FILE = "/testStandbyIsHot";
  private static final Path TEST_FILE_PATH = new Path(TEST_FILE);
  private static final int SMALL_BLOCK = 1024;
  
  private Configuration conf;
  private MiniDFSCluster cluster;
  private NameNode nn1, nn2;
  private FileSystem fs;

  static {
    DFSTestUtil.setNameNodeLogLevel(Level.ALL);
  }
  
  @Before
  public void setupCluster() throws Exception {
    conf = new Configuration();
    conf.setInt(DFSConfigKeys.DFS_BLOCK_SIZE_KEY, SMALL_BLOCK);
    // Bump up redundancy interval so that we only run low redundancy
    // checks explicitly.
    conf.setInt(DFSConfigKeys.DFS_NAMENODE_REDUNDANCY_INTERVAL_SECONDS_KEY,
        600);
    // Increase max streams so that we re-replicate quickly.
    conf.setInt(DFSConfigKeys.DFS_NAMENODE_REPLICATION_MAX_STREAMS_KEY, 1000);
    // See RandomDeleterPolicy javadoc.
    conf.setClass(DFSConfigKeys.DFS_BLOCK_REPLICATOR_CLASSNAME_KEY,
        RandomDeleterPolicy.class, BlockPlacementPolicy.class); 
    conf.setInt(DFSConfigKeys.DFS_HA_TAILEDITS_PERIOD_KEY, 1);
    cluster = new MiniDFSCluster.Builder(conf)
      .nnTopology(MiniDFSNNTopology.simpleHATopology())
      .numDataNodes(3)
      .build();
    nn1 = cluster.getNameNode(0);
    nn2 = cluster.getNameNode(1);
    
    cluster.waitActive();
    cluster.transitionToActive(0);
    // Trigger block reports so that the first NN trusts all
    // of the DNs, and will issue deletions
    cluster.triggerBlockReports();
    fs = HATestUtil.configureFailoverFs(cluster, conf);
  }
  
  @After
  public void shutdownCluster() throws Exception {
    if (cluster != null) {
      banner("Shutting down cluster. NN1 metadata:");
      doMetasave(nn1);
      banner("Shutting down cluster. NN2 metadata:");
      doMetasave(nn2);
      cluster.shutdown();
      cluster = null;
    }
  }
  

  @Test
  public void testDnFencing() throws Exception {
    // Create a file with replication level 3.
    DFSTestUtil.createFile(fs, TEST_FILE_PATH, 30*SMALL_BLOCK, (short)3, 1L);
    ExtendedBlock block = DFSTestUtil.getFirstBlock(fs, TEST_FILE_PATH);
    
    // Drop its replication count to 1, so it becomes over-replicated.
    // Then compute the invalidation of the extra blocks and trigger
    // heartbeats so the invalidations are flushed to the DNs.
    nn1.getRpcServer().setReplication(TEST_FILE, (short) 1);
    BlockManagerTestUtil.computeInvalidationWork(
        nn1.getNamesystem().getBlockManager());
    cluster.triggerHeartbeats();
    
    // Transition nn2 to active even though nn1 still thinks it's active.
    banner("Failing to NN2 but let NN1 continue to think it's active");
    NameNodeAdapter.abortEditLogs(nn1);
    NameNodeAdapter.enterSafeMode(nn1, false);
    cluster.transitionToActive(1);
    
    // Check that the standby picked up the replication change.
    assertEquals(1,
        nn2.getRpcServer().getFileInfo(TEST_FILE).getReplication());

    // Dump some info for debugging purposes.
    banner("NN2 Metadata immediately after failover");
    doMetasave(nn2);
    
    banner("Triggering heartbeats and block reports so that fencing is completed");
    cluster.triggerHeartbeats();
    cluster.triggerBlockReports();
    
    banner("Metadata after nodes have all block-reported");
    doMetasave(nn2);

    // Force a rescan of postponedMisreplicatedBlocks.
    BlockManager nn2BM = nn2.getNamesystem().getBlockManager();
    BlockManagerTestUtil.checkHeartbeat(nn2BM);
    BlockManagerTestUtil.rescanPostponedMisreplicatedBlocks(nn2BM);

    // The blocks should no longer be postponed.
    assertEquals(0, nn2.getNamesystem().getPostponedMisreplicatedBlocks());
    
    // Wait for NN2 to enact its deletions (redundancy monitor has to run, etc)
    BlockManagerTestUtil.computeInvalidationWork(
        nn2.getNamesystem().getBlockManager());
    cluster.triggerHeartbeats();
    HATestUtil.waitForDNDeletions(cluster);
    cluster.triggerDeletionReports();
    assertEquals(0, nn2.getNamesystem().getUnderReplicatedBlocks());
    assertEquals(0, nn2.getNamesystem().getPendingReplicationBlocks());
    
    banner("Making sure the file is still readable");
    FileSystem fs2 = cluster.getFileSystem(1);
    DFSTestUtil.readFile(fs2, TEST_FILE_PATH);

    banner("Waiting for the actual block files to get deleted from DNs.");
    waitForTrueReplication(cluster, block, 1);
  }
  
  /**
   * Test case which restarts the standby node in such a way that,
   * when it exits safemode, it will want to invalidate a bunch
   * of over-replicated block replicas. Ensures that if we failover
   * at this point it won't lose data.
   */
  @Test
  public void testNNClearsCommandsOnFailoverAfterStartup()
      throws Exception {
    // Make lots of blocks to increase chances of triggering a bug.
    DFSTestUtil.createFile(fs, TEST_FILE_PATH, 30*SMALL_BLOCK, (short)3, 1L);

    banner("Shutting down NN2");
    cluster.shutdownNameNode(1);

    banner("Setting replication to 1, rolling edit log.");
    nn1.getRpcServer().setReplication(TEST_FILE, (short) 1);
    nn1.getRpcServer().rollEditLog();
    
    // Start NN2 again. When it starts up, it will see all of the
    // blocks as over-replicated, since it has the metadata for
    // replication=1, but the DNs haven't yet processed the deletions.
    banner("Starting NN2 again.");
    cluster.restartNameNode(1);
    nn2 = cluster.getNameNode(1);
    
    banner("triggering BRs");
    cluster.triggerBlockReports();

    // We expect that both NN1 and NN2 will have some number of
    // deletions queued up for the DNs.
    banner("computing invalidation on nn1");
    BlockManagerTestUtil.computeInvalidationWork(
        nn1.getNamesystem().getBlockManager());

    banner("computing invalidation on nn2");
    BlockManagerTestUtil.computeInvalidationWork(
        nn2.getNamesystem().getBlockManager());
    
    // Dump some info for debugging purposes.
    banner("Metadata immediately before failover");
    doMetasave(nn2);


    // Transition nn2 to active even though nn1 still thinks it's active
    banner("Failing to NN2 but let NN1 continue to think it's active");
    NameNodeAdapter.abortEditLogs(nn1);
    NameNodeAdapter.enterSafeMode(nn1, false);

    cluster.transitionToActive(1);

    // Check that the standby picked up the replication change.
    assertEquals(1,
        nn2.getRpcServer().getFileInfo(TEST_FILE).getReplication());

    // Dump some info for debugging purposes.
    banner("Metadata immediately after failover");
    doMetasave(nn2);
    
    banner("Triggering heartbeats and block reports so that fencing is completed");
    cluster.triggerHeartbeats();
    cluster.triggerBlockReports();
    
    banner("Metadata after nodes have all block-reported");
    doMetasave(nn2);

    // Force a rescan of postponedMisreplicatedBlocks.
    BlockManager nn2BM = nn2.getNamesystem().getBlockManager();
    BlockManagerTestUtil.checkHeartbeat(nn2BM);
    BlockManagerTestUtil.rescanPostponedMisreplicatedBlocks(nn2BM);

    // The block should no longer be postponed.
    assertEquals(0, nn2.getNamesystem().getPostponedMisreplicatedBlocks());
    
    // Wait for NN2 to enact its deletions (redundancy monitor has to run, etc)
    BlockManagerTestUtil.computeInvalidationWork(
        nn2.getNamesystem().getBlockManager());

    HATestUtil.waitForNNToIssueDeletions(nn2);
    cluster.triggerHeartbeats();
    HATestUtil.waitForDNDeletions(cluster);
    cluster.triggerDeletionReports();
    assertEquals(0, nn2.getNamesystem().getUnderReplicatedBlocks());
    assertEquals(0, nn2.getNamesystem().getPendingReplicationBlocks());
    
    banner("Making sure the file is still readable");
    FileSystem fs2 = cluster.getFileSystem(1);
    DFSTestUtil.readFile(fs2, TEST_FILE_PATH);
  }
  
  /**
   * Test case that reduces replication of a file with a lot of blocks
   * and then fails over right after those blocks enter the DN invalidation
   * queues on the active. Ensures that fencing is correct and no replicas
   * are lost.
   */
  @Test
  public void testNNClearsCommandsOnFailoverWithReplChanges()
      throws Exception {
    // Make lots of blocks to increase chances of triggering a bug.
    DFSTestUtil.createFile(fs, TEST_FILE_PATH, 30*SMALL_BLOCK, (short)1, 1L);

    banner("rolling NN1's edit log, forcing catch-up");
    HATestUtil.waitForStandbyToCatchUp(nn1, nn2);
    
    // Get some new replicas reported so that NN2 now considers
    // them over-replicated and schedules some more deletions
    nn1.getRpcServer().setReplication(TEST_FILE, (short) 2);
    while (BlockManagerTestUtil.getComputedDatanodeWork(
        nn1.getNamesystem().getBlockManager()) > 0) {
      LOG.info("Getting more replication work computed");
    }
    BlockManager bm1 = nn1.getNamesystem().getBlockManager();
    while (bm1.getPendingReconstructionBlocksCount() > 0) {
      BlockManagerTestUtil.updateState(bm1);
      cluster.triggerHeartbeats();
      Thread.sleep(1000);
    }
    
    banner("triggering BRs");
    cluster.triggerBlockReports();
    
    nn1.getRpcServer().setReplication(TEST_FILE, (short) 1);

    
    banner("computing invalidation on nn1");

    BlockManagerTestUtil.computeInvalidationWork(
        nn1.getNamesystem().getBlockManager());
    doMetasave(nn1);

    banner("computing invalidation on nn2");
    BlockManagerTestUtil.computeInvalidationWork(
        nn2.getNamesystem().getBlockManager());
    doMetasave(nn2);

    // Dump some info for debugging purposes.
    banner("Metadata immediately before failover");
    doMetasave(nn2);


    // Transition nn2 to active even though nn1 still thinks it's active
    banner("Failing to NN2 but let NN1 continue to think it's active");
    NameNodeAdapter.abortEditLogs(nn1);
    NameNodeAdapter.enterSafeMode(nn1, false);

    
    BlockManagerTestUtil.computeInvalidationWork(
        nn2.getNamesystem().getBlockManager());
    cluster.transitionToActive(1);

    // Check that the standby picked up the replication change.
    assertEquals(1,
        nn2.getRpcServer().getFileInfo(TEST_FILE).getReplication());

    // Dump some info for debugging purposes.
    banner("Metadata immediately after failover");
    doMetasave(nn2);
    
    banner("Triggering heartbeats and block reports so that fencing is completed");
    cluster.triggerHeartbeats();
    cluster.triggerBlockReports();
    
    banner("Metadata after nodes have all block-reported");
    doMetasave(nn2);
    
    // Force a rescan of postponedMisreplicatedBlocks.
    BlockManager nn2BM = nn2.getNamesystem().getBlockManager();
    BlockManagerTestUtil.checkHeartbeat(nn2BM);
    BlockManagerTestUtil.rescanPostponedMisreplicatedBlocks(nn2BM);

    // The block should no longer be postponed.
    assertEquals(0, nn2.getNamesystem().getPostponedMisreplicatedBlocks());
    
    // Wait for NN2 to enact its deletions (redundancy monitor has to run, etc)
    BlockManagerTestUtil.computeInvalidationWork(
        nn2.getNamesystem().getBlockManager());

    HATestUtil.waitForNNToIssueDeletions(nn2);
    cluster.triggerHeartbeats();
    HATestUtil.waitForDNDeletions(cluster);
    cluster.triggerDeletionReports();
    assertEquals(0, nn2.getNamesystem().getUnderReplicatedBlocks());
    assertEquals(0, nn2.getNamesystem().getPendingReplicationBlocks());
    
    banner("Making sure the file is still readable");
    FileSystem fs2 = cluster.getFileSystem(1);
    DFSTestUtil.readFile(fs2, TEST_FILE_PATH);
  }
  
  /**
   * Regression test for HDFS-2742. The issue in this bug was:
   * - DN does a block report while file is open. This BR contains
   *   the block in RBW state.
   * - Standby queues the RBW state in PendingDatanodeMessages
   * - Standby processes edit logs during failover. Before fixing
   *   this bug, it was mistakenly applying the RBW reported state
   *   after the block had been completed, causing the block to get
   *   marked corrupt. Instead, we should now be applying the RBW
   *   message on OP_ADD, and then the FINALIZED message on OP_CLOSE.
   */
  @Test
  public void testBlockReportsWhileFileBeingWritten() throws Exception {
    FSDataOutputStream out = fs.create(TEST_FILE_PATH);
    try {
      AppendTestUtil.write(out, 0, 10);
      out.hflush();
      
      // Block report will include the RBW replica, but will be
      // queued on the StandbyNode.
      cluster.triggerBlockReports();
      
    } finally {
      IOUtils.closeStream(out);
    }

    cluster.transitionToStandby(0);
    cluster.transitionToActive(1);
    
    // Verify that no replicas are marked corrupt, and that the
    // file is readable from the failed-over standby.
    BlockManagerTestUtil.updateState(nn1.getNamesystem().getBlockManager());
    BlockManagerTestUtil.updateState(nn2.getNamesystem().getBlockManager());
    assertEquals(0, nn1.getNamesystem().getCorruptReplicaBlocks());
    assertEquals(0, nn2.getNamesystem().getCorruptReplicaBlocks());
    
    DFSTestUtil.readFile(fs, TEST_FILE_PATH);
  }
  
  /**
   * Test that, when a block is re-opened for append, the related
   * datanode messages are correctly queued by the SBN because
   * they have future states and genstamps.
   */
  @Test
  public void testQueueingWithAppend() throws Exception {
    int numQueued = 0;
    int numDN = cluster.getDataNodes().size();
    
    // case 1: create file and call hflush after write
    FSDataOutputStream out = fs.create(TEST_FILE_PATH);
    try {
      AppendTestUtil.write(out, 0, 10);
      out.hflush();

      // Opening the file will report RBW replicas, but will be
      // queued on the StandbyNode.
      // However, the delivery of RBW messages is delayed by HDFS-7217 fix.
      // Apply cluster.triggerBlockReports() to trigger the reporting sooner.
      //
      cluster.triggerBlockReports();
      numQueued += numDN; // RBW messages

      // The cluster.triggerBlockReports() call above does a full 
      // block report that incurs 3 extra RBW messages
      numQueued += numDN; // RBW messages      
    } finally {
      IOUtils.closeStream(out);
      numQueued += numDN; // blockReceived messages
    }

    cluster.triggerBlockReports();
    numQueued += numDN;
    assertEquals(numQueued, cluster.getNameNode(1).getNamesystem().
        getPendingDataNodeMessageCount());

    // case 2: append to file and call hflush after write
    try {
      out = fs.append(TEST_FILE_PATH);
      AppendTestUtil.write(out, 10, 10);
      out.hflush();
      cluster.triggerBlockReports();
      numQueued += numDN * 2; // RBW messages, see comments in case 1
    } finally {
      IOUtils.closeStream(out);
      cluster.triggerHeartbeats();
      numQueued += numDN; // blockReceived
    }
    assertEquals(numQueued, cluster.getNameNode(1).getNamesystem().
        getPendingDataNodeMessageCount());

    // case 3: similar to case 2, except no hflush is called.
    try {
      out = fs.append(TEST_FILE_PATH);
      AppendTestUtil.write(out, 20, 10);
    } finally {
      // The write operation in the try block is buffered, thus no RBW message
      // is reported yet until the closeStream call here. When closeStream is
      // called, before HDFS-7217 fix, there would be three RBW messages
      // (blockReceiving), plus three FINALIZED messages (blockReceived)
      // delivered to NN. However, because of HDFS-7217 fix, the reporting of
      // RBW  messages is postponed. In this case, they are even overwritten 
      // by the blockReceived messages of the same block when they are waiting
      // to be delivered. All this happens within the closeStream() call.
      // What's delivered to NN is the three blockReceived messages. See 
      //    BPServiceActor#addPendingReplicationBlockInfo 
      //
      IOUtils.closeStream(out);
      numQueued += numDN; // blockReceived
    }

    cluster.triggerBlockReports();
    numQueued += numDN;

    LOG.info("Expect " + numQueued + " and got: " + cluster.getNameNode(1).getNamesystem().
        getPendingDataNodeMessageCount());      

    assertEquals(numQueued, cluster.getNameNode(1).getNamesystem().
        getPendingDataNodeMessageCount());

    cluster.transitionToStandby(0);
    cluster.transitionToActive(1);
    
    // Verify that no replicas are marked corrupt, and that the
    // file is readable from the failed-over standby.
    BlockManagerTestUtil.updateState(nn1.getNamesystem().getBlockManager());
    BlockManagerTestUtil.updateState(nn2.getNamesystem().getBlockManager());
    assertEquals(0, nn1.getNamesystem().getCorruptReplicaBlocks());
    assertEquals(0, nn2.getNamesystem().getCorruptReplicaBlocks());
    
    AppendTestUtil.check(fs, TEST_FILE_PATH, 30);
  }
  
  /**
   * Another regression test for HDFS-2742. This tests the following sequence:
   * - DN does a block report while file is open. This BR contains
   *   the block in RBW state.
   * - The block report is delayed in reaching the standby.
   * - The file is closed.
   * - The standby processes the OP_ADD and OP_CLOSE operations before
   *   the RBW block report arrives.
   * - The standby should not mark the block as corrupt.
   */
  @Test
  public void testRBWReportArrivesAfterEdits() throws Exception {
    final CountDownLatch brFinished = new CountDownLatch(1);
    DelayAnswer delayer = new GenericTestUtils.DelayAnswer(LOG) {
      @Override
      protected Object passThrough(InvocationOnMock invocation)
          throws Throwable {
        try {
          return super.passThrough(invocation);
        } finally {
          // inform the test that our block report went through.
          brFinished.countDown();
        }
      }
    };

    FSDataOutputStream out = fs.create(TEST_FILE_PATH);
    try {
      AppendTestUtil.write(out, 0, 10);
      out.hflush();

      DataNode dn = cluster.getDataNodes().get(0);
      DatanodeProtocolClientSideTranslatorPB spy =
        InternalDataNodeTestUtils.spyOnBposToNN(dn, nn2);

      Mockito.doAnswer(delayer)
        .when(spy).blockReport(
          Mockito.<DatanodeRegistration>anyObject(),
          Mockito.anyString(),
          Mockito.<StorageBlockReport[]>anyObject(),
          Mockito.<BlockReportContext>anyObject());
      dn.scheduleAllBlockReport(0);
      delayer.waitForCall();
      
    } finally {
      IOUtils.closeStream(out);
    }

    cluster.transitionToStandby(0);
    cluster.transitionToActive(1);
    
    delayer.proceed();
    brFinished.await();
    
    // Verify that no replicas are marked corrupt, and that the
    // file is readable from the failed-over standby.
    BlockManagerTestUtil.updateState(nn1.getNamesystem().getBlockManager());
    BlockManagerTestUtil.updateState(nn2.getNamesystem().getBlockManager());
    assertEquals(0, nn1.getNamesystem().getCorruptReplicaBlocks());
    assertEquals(0, nn2.getNamesystem().getCorruptReplicaBlocks());
    
    DFSTestUtil.readFile(fs, TEST_FILE_PATH);
  }

  /**
   * Print a big banner in the test log to make debug easier.
   */
  private void banner(String string) {
    LOG.info("\n\n\n\n================================================\n" +
        string + "\n" +
        "==================================================\n\n");
  }

  private void doMetasave(NameNode nn2) {
    nn2.getNamesystem().writeLock();
    try {
      PrintWriter pw = new PrintWriter(System.err);
      nn2.getNamesystem().getBlockManager().metaSave(pw);
      pw.flush();
    } finally {
      nn2.getNamesystem().writeUnlock();
    }
  }

  private void waitForTrueReplication(final MiniDFSCluster cluster,
      final ExtendedBlock block, final int waitFor) throws Exception {
    GenericTestUtils.waitFor(new Supplier<Boolean>() {
      @Override
      public Boolean get() {
        try {
          return getTrueReplication(cluster, block) == waitFor;
        } catch (IOException e) {
          throw new RuntimeException(e);
        }
      }
    }, 500, 10000);
  }

  private int getTrueReplication(MiniDFSCluster cluster, ExtendedBlock block)
      throws IOException {
    int count = 0;
    for (DataNode dn : cluster.getDataNodes()) {
      if (DataNodeTestUtils.getFSDataset(dn).getStoredBlock(
          block.getBlockPoolId(), block.getBlockId()) != null) {
        count++;
      }
    }
    return count;
  }

  /**
   * A BlockPlacementPolicy which, rather than using space available, makes
   * random decisions about which excess replica to delete. This is because,
   * in the test cases, the two NNs will usually (but not quite always)
   * make the same decision of which replica to delete. The fencing issues
   * are exacerbated when the two NNs make different decisions, which can
   * happen in "real life" when they have slightly out-of-sync heartbeat
   * information regarding disk usage.
   */
  public static class RandomDeleterPolicy extends BlockPlacementPolicyDefault {

    public RandomDeleterPolicy() {
      super();
    }

    @Override
    public DatanodeStorageInfo chooseReplicaToDelete(
        Collection<DatanodeStorageInfo> moreThanOne,
        Collection<DatanodeStorageInfo> exactlyOne,
        List<StorageType> excessTypes,
        Map<String, List<DatanodeStorageInfo>> rackMap) {

      Collection<DatanodeStorageInfo> chooseFrom = !moreThanOne.isEmpty() ?
          moreThanOne : exactlyOne;

      List<DatanodeStorageInfo> l = Lists.newArrayList(chooseFrom);
      return l.get(ThreadLocalRandom.current().nextInt(l.size()));
    }
  }

}
