xref: /dports/devel/hadoop2/hadoop-2.7.2-src/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/BlockReaderFactory.java

/**
 * 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;

import static org.apache.hadoop.hdfs.protocol.proto.DataTransferProtos.ShortCircuitFdResponse.USE_RECEIPT_VERIFICATION;

import java.io.BufferedOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.InetSocketAddress;

import org.apache.commons.lang.mutable.MutableBoolean;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.StorageType;
import org.apache.hadoop.hdfs.net.DomainPeer;
import org.apache.hadoop.hdfs.net.Peer;
import org.apache.hadoop.hdfs.protocol.DatanodeInfo;
import org.apache.hadoop.hdfs.protocol.ExtendedBlock;
import org.apache.hadoop.hdfs.protocol.datatransfer.InvalidEncryptionKeyException;
import org.apache.hadoop.hdfs.protocol.datatransfer.Sender;
import org.apache.hadoop.hdfs.protocol.proto.DataTransferProtos.BlockOpResponseProto;
import org.apache.hadoop.hdfs.protocolPB.PBHelper;
import org.apache.hadoop.hdfs.security.token.block.BlockTokenIdentifier;
import org.apache.hadoop.hdfs.security.token.block.InvalidBlockTokenException;
import org.apache.hadoop.hdfs.server.datanode.CachingStrategy;
import org.apache.hadoop.hdfs.shortcircuit.DomainSocketFactory;
import org.apache.hadoop.hdfs.shortcircuit.ShortCircuitCache;
import org.apache.hadoop.hdfs.shortcircuit.ShortCircuitCache.ShortCircuitReplicaCreator;
import org.apache.hadoop.hdfs.shortcircuit.ShortCircuitReplica;
import org.apache.hadoop.hdfs.shortcircuit.ShortCircuitReplicaInfo;
import org.apache.hadoop.hdfs.shortcircuit.ShortCircuitShm.Slot;
import org.apache.hadoop.hdfs.shortcircuit.ShortCircuitShm.SlotId;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.ipc.RemoteException;
import org.apache.hadoop.net.unix.DomainSocket;
import org.apache.hadoop.security.AccessControlException;
import org.apache.hadoop.security.UserGroupInformation;
import org.apache.hadoop.security.token.SecretManager.InvalidToken;
import org.apache.hadoop.security.token.Token;
import org.apache.hadoop.util.PerformanceAdvisory;
import org.apache.hadoop.util.Time;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;


/**
 * Utility class to create BlockReader implementations.
 */
@InterfaceAudience.Private
public class BlockReaderFactory implements ShortCircuitReplicaCreator {
  static final Log LOG = LogFactory.getLog(BlockReaderFactory.class);

  public static class FailureInjector {
    public void injectRequestFileDescriptorsFailure() throws IOException {
      // do nothing
    }
    public boolean getSupportsReceiptVerification() {
      return true;
    }
  }

  @VisibleForTesting
  static ShortCircuitReplicaCreator
      createShortCircuitReplicaInfoCallback = null;

  private final DFSClient.Conf conf;

  /**
   * Injects failures into specific operations during unit tests.
   */
  private final FailureInjector failureInjector;

  /**
   * The file name, for logging and debugging purposes.
   */
  private String fileName;

  /**
   * The block ID and block pool ID to use.
   */
  private ExtendedBlock block;

  /**
   * The block token to use for security purposes.
   */
  private Token<BlockTokenIdentifier> token;

  /**
   * The offset within the block to start reading at.
   */
  private long startOffset;

  /**
   * If false, we won't try to verify the block checksum.
   */
  private boolean verifyChecksum;

  /**
   * The name of this client.
   */
  private String clientName;

  /**
   * The DataNode we're talking to.
   */
  private DatanodeInfo datanode;

  /**
   * StorageType of replica on DataNode.
   */
  private StorageType storageType;

  /**
   * If false, we won't try short-circuit local reads.
   */
  private boolean allowShortCircuitLocalReads;

  /**
   * The ClientContext to use for things like the PeerCache.
   */
  private ClientContext clientContext;

  /**
   * Number of bytes to read.  -1 indicates no limit.
   */
  private long length = -1;

  /**
   * Caching strategy to use when reading the block.
   */
  private CachingStrategy cachingStrategy;

  /**
   * Socket address to use to connect to peer.
   */
  private InetSocketAddress inetSocketAddress;

  /**
   * Remote peer factory to use to create a peer, if needed.
   */
  private RemotePeerFactory remotePeerFactory;

  /**
   * UserGroupInformation  to use for legacy block reader local objects, if needed.
   */
  private UserGroupInformation userGroupInformation;

  /**
   * Configuration to use for legacy block reader local objects, if needed.
   */
  private Configuration configuration;

  /**
   * Information about the domain socket path we should use to connect to the
   * local peer-- or null if we haven't examined the local domain socket.
   */
  private DomainSocketFactory.PathInfo pathInfo;

  /**
   * The remaining number of times that we'll try to pull a socket out of the
   * cache.
   */
  private int remainingCacheTries;

  public BlockReaderFactory(DFSClient.Conf conf) {
    this.conf = conf;
    this.failureInjector = conf.brfFailureInjector;
    this.remainingCacheTries = conf.nCachedConnRetry;
  }

  public BlockReaderFactory setFileName(String fileName) {
    this.fileName = fileName;
    return this;
  }

  public BlockReaderFactory setBlock(ExtendedBlock block) {
    this.block = block;
    return this;
  }

  public BlockReaderFactory setBlockToken(Token<BlockTokenIdentifier> token) {
    this.token = token;
    return this;
  }

  public BlockReaderFactory setStartOffset(long startOffset) {
    this.startOffset = startOffset;
    return this;
  }

  public BlockReaderFactory setVerifyChecksum(boolean verifyChecksum) {
    this.verifyChecksum = verifyChecksum;
    return this;
  }

  public BlockReaderFactory setClientName(String clientName) {
    this.clientName = clientName;
    return this;
  }

  public BlockReaderFactory setDatanodeInfo(DatanodeInfo datanode) {
    this.datanode = datanode;
    return this;
  }

  public BlockReaderFactory setStorageType(StorageType storageType) {
    this.storageType = storageType;
    return this;
  }

  public BlockReaderFactory setAllowShortCircuitLocalReads(
      boolean allowShortCircuitLocalReads) {
    this.allowShortCircuitLocalReads = allowShortCircuitLocalReads;
    return this;
  }

  public BlockReaderFactory setClientCacheContext(
      ClientContext clientContext) {
    this.clientContext = clientContext;
    return this;
  }

  public BlockReaderFactory setLength(long length) {
    this.length = length;
    return this;
  }

  public BlockReaderFactory setCachingStrategy(
      CachingStrategy cachingStrategy) {
    this.cachingStrategy = cachingStrategy;
    return this;
  }

  public BlockReaderFactory setInetSocketAddress (
      InetSocketAddress inetSocketAddress) {
    this.inetSocketAddress = inetSocketAddress;
    return this;
  }

  public BlockReaderFactory setUserGroupInformation(
      UserGroupInformation userGroupInformation) {
    this.userGroupInformation = userGroupInformation;
    return this;
  }

  public BlockReaderFactory setRemotePeerFactory(
      RemotePeerFactory remotePeerFactory) {
    this.remotePeerFactory = remotePeerFactory;
    return this;
  }

  public BlockReaderFactory setConfiguration(
      Configuration configuration) {
    this.configuration = configuration;
    return this;
  }

  /**
   * Build a BlockReader with the given options.
   *
   * This function will do the best it can to create a block reader that meets
   * all of our requirements.  We prefer short-circuit block readers
   * (BlockReaderLocal and BlockReaderLocalLegacy) over remote ones, since the
   * former avoid the overhead of socket communication.  If short-circuit is
   * unavailable, our next fallback is data transfer over UNIX domain sockets,
   * if dfs.client.domain.socket.data.traffic has been enabled.  If that doesn't
   * work, we will try to create a remote block reader that operates over TCP
   * sockets.
   *
   * There are a few caches that are important here.
   *
   * The ShortCircuitCache stores file descriptor objects which have been passed
   * from the DataNode.
   *
   * The DomainSocketFactory stores information about UNIX domain socket paths
   * that we not been able to use in the past, so that we don't waste time
   * retrying them over and over.  (Like all the caches, it does have a timeout,
   * though.)
   *
   * The PeerCache stores peers that we have used in the past.  If we can reuse
   * one of these peers, we avoid the overhead of re-opening a socket.  However,
   * if the socket has been timed out on the remote end, our attempt to reuse
   * the socket may end with an IOException.  For that reason, we limit our
   * attempts at socket reuse to dfs.client.cached.conn.retry times.  After
   * that, we create new sockets.  This avoids the problem where a thread tries
   * to talk to a peer that it hasn't talked to in a while, and has to clean out
   * every entry in a socket cache full of stale entries.
   *
   * @return The new BlockReader.  We will not return null.
   *
   * @throws InvalidToken
   *             If the block token was invalid.
   *         InvalidEncryptionKeyException
   *             If the encryption key was invalid.
   *         Other IOException
   *             If there was another problem.
   */
  public BlockReader build() throws IOException {
    BlockReader reader = null;

    Preconditions.checkNotNull(configuration);
    if (conf.shortCircuitLocalReads && allowShortCircuitLocalReads) {
      if (clientContext.getUseLegacyBlockReaderLocal()) {
        reader = getLegacyBlockReaderLocal();
        if (reader != null) {
          if (LOG.isTraceEnabled()) {
            LOG.trace(this + ": returning new legacy block reader local.");
          }
          return reader;
        }
      } else {
        reader = getBlockReaderLocal();
        if (reader != null) {
          if (LOG.isTraceEnabled()) {
            LOG.trace(this + ": returning new block reader local.");
          }
          return reader;
        }
      }
    }
    if (conf.domainSocketDataTraffic) {
      reader = getRemoteBlockReaderFromDomain();
      if (reader != null) {
        if (LOG.isTraceEnabled()) {
          LOG.trace(this + ": returning new remote block reader using " +
              "UNIX domain socket on " + pathInfo.getPath());
        }
        return reader;
      }
    }
    Preconditions.checkState(!DFSInputStream.tcpReadsDisabledForTesting,
        "TCP reads were disabled for testing, but we failed to " +
        "do a non-TCP read.");
    return getRemoteBlockReaderFromTcp();
  }

  /**
   * Get {@link BlockReaderLocalLegacy} for short circuited local reads.
   * This block reader implements the path-based style of local reads
   * first introduced in HDFS-2246.
   */
  private BlockReader getLegacyBlockReaderLocal() throws IOException {
    if (LOG.isTraceEnabled()) {
      LOG.trace(this + ": trying to construct BlockReaderLocalLegacy");
    }
    if (!DFSClient.isLocalAddress(inetSocketAddress)) {
      if (LOG.isTraceEnabled()) {
        LOG.trace(this + ": can't construct BlockReaderLocalLegacy because " +
            "the address " + inetSocketAddress + " is not local");
      }
      return null;
    }
    if (clientContext.getDisableLegacyBlockReaderLocal()) {
      PerformanceAdvisory.LOG.debug(this + ": can't construct " +
          "BlockReaderLocalLegacy because " +
          "disableLegacyBlockReaderLocal is set.");
      return null;
    }
    IOException ioe = null;
    try {
      return BlockReaderLocalLegacy.newBlockReader(conf,
          userGroupInformation, configuration, fileName, block, token,
          datanode, startOffset, length, storageType);
    } catch (RemoteException remoteException) {
      ioe = remoteException.unwrapRemoteException(
                InvalidToken.class, AccessControlException.class);
    } catch (IOException e) {
      ioe = e;
    }
    if ((!(ioe instanceof AccessControlException)) &&
        isSecurityException(ioe)) {
      // Handle security exceptions.
      // We do not handle AccessControlException here, since
      // BlockReaderLocalLegacy#newBlockReader uses that exception to indicate
      // that the user is not in dfs.block.local-path-access.user, a condition
      // which requires us to disable legacy SCR.
      throw ioe;
    }
    LOG.warn(this + ": error creating legacy BlockReaderLocal.  " +
        "Disabling legacy local reads.", ioe);
    clientContext.setDisableLegacyBlockReaderLocal();
    return null;
  }

  private BlockReader getBlockReaderLocal() throws InvalidToken {
    if (LOG.isTraceEnabled()) {
      LOG.trace(this + ": trying to construct a BlockReaderLocal " +
          "for short-circuit reads.");
    }
    if (pathInfo == null) {
      pathInfo = clientContext.getDomainSocketFactory().
                      getPathInfo(inetSocketAddress, conf);
    }
    if (!pathInfo.getPathState().getUsableForShortCircuit()) {
      PerformanceAdvisory.LOG.debug(this + ": " + pathInfo + " is not " +
          "usable for short circuit; giving up on BlockReaderLocal.");
      return null;
    }
    ShortCircuitCache cache = clientContext.getShortCircuitCache();
    ExtendedBlockId key = new ExtendedBlockId(block.getBlockId(), block.getBlockPoolId());
    ShortCircuitReplicaInfo info = cache.fetchOrCreate(key, this);
    InvalidToken exc = info.getInvalidTokenException();
    if (exc != null) {
      if (LOG.isTraceEnabled()) {
        LOG.trace(this + ": got InvalidToken exception while trying to " +
            "construct BlockReaderLocal via " + pathInfo.getPath());
      }
      throw exc;
    }
    if (info.getReplica() == null) {
      if (LOG.isTraceEnabled()) {
        PerformanceAdvisory.LOG.debug(this + ": failed to get " +
            "ShortCircuitReplica. Cannot construct " +
            "BlockReaderLocal via " + pathInfo.getPath());
      }
      return null;
    }
    return new BlockReaderLocal.Builder(conf).
        setFilename(fileName).
        setBlock(block).
        setStartOffset(startOffset).
        setShortCircuitReplica(info.getReplica()).
        setVerifyChecksum(verifyChecksum).
        setCachingStrategy(cachingStrategy).
        setStorageType(storageType).
        build();
  }

  /**
   * Fetch a pair of short-circuit block descriptors from a local DataNode.
   *
   * @return    Null if we could not communicate with the datanode,
   *            a new ShortCircuitReplicaInfo object otherwise.
   *            ShortCircuitReplicaInfo objects may contain either an InvalidToken
   *            exception, or a ShortCircuitReplica object ready to use.
   */
  @Override
  public ShortCircuitReplicaInfo createShortCircuitReplicaInfo() {
    if (createShortCircuitReplicaInfoCallback != null) {
      ShortCircuitReplicaInfo info =
        createShortCircuitReplicaInfoCallback.createShortCircuitReplicaInfo();
      if (info != null) return info;
    }
    if (LOG.isTraceEnabled()) {
      LOG.trace(this + ": trying to create ShortCircuitReplicaInfo.");
    }
    BlockReaderPeer curPeer;
    while (true) {
      curPeer = nextDomainPeer();
      if (curPeer == null) break;
      if (curPeer.fromCache) remainingCacheTries--;
      DomainPeer peer = (DomainPeer)curPeer.peer;
      Slot slot = null;
      ShortCircuitCache cache = clientContext.getShortCircuitCache();
      try {
        MutableBoolean usedPeer = new MutableBoolean(false);
        slot = cache.allocShmSlot(datanode, peer, usedPeer,
            new ExtendedBlockId(block.getBlockId(), block.getBlockPoolId()),
            clientName);
        if (usedPeer.booleanValue()) {
          if (LOG.isTraceEnabled()) {
            LOG.trace(this + ": allocShmSlot used up our previous socket " +
              peer.getDomainSocket() + ".  Allocating a new one...");
          }
          curPeer = nextDomainPeer();
          if (curPeer == null) break;
          peer = (DomainPeer)curPeer.peer;
        }
        ShortCircuitReplicaInfo info = requestFileDescriptors(peer, slot);
        clientContext.getPeerCache().put(datanode, peer);
        return info;
      } catch (IOException e) {
        if (slot != null) {
          cache.freeSlot(slot);
        }
        if (curPeer.fromCache) {
          // Handle an I/O error we got when using a cached socket.
          // These are considered less serious, because the socket may be stale.
          if (LOG.isDebugEnabled()) {
            LOG.debug(this + ": closing stale domain peer " + peer, e);
          }
          IOUtils.cleanup(LOG, peer);
        } else {
          // Handle an I/O error we got when using a newly created socket.
          // We temporarily disable the domain socket path for a few minutes in
          // this case, to prevent wasting more time on it.
          LOG.warn(this + ": I/O error requesting file descriptors.  " +
              "Disabling domain socket " + peer.getDomainSocket(), e);
          IOUtils.cleanup(LOG, peer);
          clientContext.getDomainSocketFactory()
              .disableDomainSocketPath(pathInfo.getPath());
          return null;
        }
      }
    }
    return null;
  }

  /**
   * Request file descriptors from a DomainPeer.
   *
   * @param peer   The peer to use for communication.
   * @param slot   If non-null, the shared memory slot to associate with the
   *               new ShortCircuitReplica.
   *
   * @return  A ShortCircuitReplica object if we could communicate with the
   *          datanode; null, otherwise.
   * @throws  IOException If we encountered an I/O exception while communicating
   *          with the datanode.
   */
  private ShortCircuitReplicaInfo requestFileDescriptors(DomainPeer peer,
          Slot slot) throws IOException {
    ShortCircuitCache cache = clientContext.getShortCircuitCache();
    final DataOutputStream out =
        new DataOutputStream(new BufferedOutputStream(peer.getOutputStream()));
    SlotId slotId = slot == null ? null : slot.getSlotId();
    new Sender(out).requestShortCircuitFds(block, token, slotId, 1,
        failureInjector.getSupportsReceiptVerification());
    DataInputStream in = new DataInputStream(peer.getInputStream());
    BlockOpResponseProto resp = BlockOpResponseProto.parseFrom(
        PBHelper.vintPrefixed(in));
    DomainSocket sock = peer.getDomainSocket();
    failureInjector.injectRequestFileDescriptorsFailure();
    switch (resp.getStatus()) {
    case SUCCESS:
      byte buf[] = new byte[1];
      FileInputStream fis[] = new FileInputStream[2];
      sock.recvFileInputStreams(fis, buf, 0, buf.length);
      ShortCircuitReplica replica = null;
      try {
        ExtendedBlockId key =
            new ExtendedBlockId(block.getBlockId(), block.getBlockPoolId());
        if (buf[0] == USE_RECEIPT_VERIFICATION.getNumber()) {
          LOG.trace("Sending receipt verification byte for slot " + slot);
          sock.getOutputStream().write(0);
        }
        replica = new ShortCircuitReplica(key, fis[0], fis[1], cache,
            Time.monotonicNow(), slot);
        return new ShortCircuitReplicaInfo(replica);
      } catch (IOException e) {
        // This indicates an error reading from disk, or a format error.  Since
        // it's not a socket communication problem, we return null rather than
        // throwing an exception.
        LOG.warn(this + ": error creating ShortCircuitReplica.", e);
        return null;
      } finally {
        if (replica == null) {
          IOUtils.cleanup(DFSClient.LOG, fis[0], fis[1]);
        }
      }
    case ERROR_UNSUPPORTED:
      if (!resp.hasShortCircuitAccessVersion()) {
        LOG.warn("short-circuit read access is disabled for " +
            "DataNode " + datanode + ".  reason: " + resp.getMessage());
        clientContext.getDomainSocketFactory()
            .disableShortCircuitForPath(pathInfo.getPath());
      } else {
        LOG.warn("short-circuit read access for the file " +
            fileName + " is disabled for DataNode " + datanode +
            ".  reason: " + resp.getMessage());
      }
      return null;
    case ERROR_ACCESS_TOKEN:
      String msg = "access control error while " +
          "attempting to set up short-circuit access to " +
          fileName + resp.getMessage();
      if (LOG.isDebugEnabled()) {
        LOG.debug(this + ":" + msg);
      }
      return new ShortCircuitReplicaInfo(new InvalidToken(msg));
    default:
      LOG.warn(this + ": unknown response code " + resp.getStatus() +
          " while attempting to set up short-circuit access. " +
          resp.getMessage());
      clientContext.getDomainSocketFactory()
          .disableShortCircuitForPath(pathInfo.getPath());
      return null;
    }
  }

  /**
   * Get a RemoteBlockReader that communicates over a UNIX domain socket.
   *
   * @return The new BlockReader, or null if we failed to create the block
   * reader.
   *
   * @throws InvalidToken    If the block token was invalid.
   * Potentially other security-related execptions.
   */
  private BlockReader getRemoteBlockReaderFromDomain() throws IOException {
    if (pathInfo == null) {
      pathInfo = clientContext.getDomainSocketFactory().
                      getPathInfo(inetSocketAddress, conf);
    }
    if (!pathInfo.getPathState().getUsableForDataTransfer()) {
      PerformanceAdvisory.LOG.debug(this + ": not trying to create a " +
          "remote block reader because the UNIX domain socket at " +
          pathInfo + " is not usable.");
      return null;
    }
    if (LOG.isTraceEnabled()) {
      LOG.trace(this + ": trying to create a remote block reader from the " +
          "UNIX domain socket at " + pathInfo.getPath());
    }

    while (true) {
      BlockReaderPeer curPeer = nextDomainPeer();
      if (curPeer == null) break;
      if (curPeer.fromCache) remainingCacheTries--;
      DomainPeer peer = (DomainPeer)curPeer.peer;
      BlockReader blockReader = null;
      try {
        blockReader = getRemoteBlockReader(peer);
        return blockReader;
      } catch (IOException ioe) {
        IOUtils.cleanup(LOG, peer);
        if (isSecurityException(ioe)) {
          if (LOG.isTraceEnabled()) {
            LOG.trace(this + ": got security exception while constructing " +
                "a remote block reader from the unix domain socket at " +
                pathInfo.getPath(), ioe);
          }
          throw ioe;
        }
        if (curPeer.fromCache) {
          // Handle an I/O error we got when using a cached peer.  These are
          // considered less serious, because the underlying socket may be stale.
          if (LOG.isDebugEnabled()) {
            LOG.debug("Closed potentially stale domain peer " + peer, ioe);
          }
        } else {
          // Handle an I/O error we got when using a newly created domain peer.
          // We temporarily disable the domain socket path for a few minutes in
          // this case, to prevent wasting more time on it.
          LOG.warn("I/O error constructing remote block reader.  Disabling " +
              "domain socket " + peer.getDomainSocket(), ioe);
          clientContext.getDomainSocketFactory()
              .disableDomainSocketPath(pathInfo.getPath());
          return null;
        }
      } finally {
        if (blockReader == null) {
          IOUtils.cleanup(LOG, peer);
        }
      }
    }
    return null;
  }

  /**
   * Get a RemoteBlockReader that communicates over a TCP socket.
   *
   * @return The new BlockReader.  We will not return null, but instead throw
   *         an exception if this fails.
   *
   * @throws InvalidToken
   *             If the block token was invalid.
   *         InvalidEncryptionKeyException
   *             If the encryption key was invalid.
   *         Other IOException
   *             If there was another problem.
   */
  private BlockReader getRemoteBlockReaderFromTcp() throws IOException {
    if (LOG.isTraceEnabled()) {
      LOG.trace(this + ": trying to create a remote block reader from a " +
          "TCP socket");
    }
    BlockReader blockReader = null;
    while (true) {
      BlockReaderPeer curPeer = null;
      Peer peer = null;
      try {
        curPeer = nextTcpPeer();
        if (curPeer.fromCache) remainingCacheTries--;
        peer = curPeer.peer;
        blockReader = getRemoteBlockReader(peer);
        return blockReader;
      } catch (IOException ioe) {
        if (isSecurityException(ioe)) {
          if (LOG.isTraceEnabled()) {
            LOG.trace(this + ": got security exception while constructing " +
                "a remote block reader from " + peer, ioe);
          }
          throw ioe;
        }
        if ((curPeer != null) && curPeer.fromCache) {
          // Handle an I/O error we got when using a cached peer.  These are
          // considered less serious, because the underlying socket may be
          // stale.
          if (LOG.isDebugEnabled()) {
            LOG.debug("Closed potentially stale remote peer " + peer, ioe);
          }
        } else {
          // Handle an I/O error we got when using a newly created peer.
          LOG.warn("I/O error constructing remote block reader.", ioe);
          throw ioe;
        }
      } finally {
        if (blockReader == null) {
          IOUtils.cleanup(LOG, peer);
        }
      }
    }
  }

  public static class BlockReaderPeer {
    final Peer peer;
    final boolean fromCache;

    BlockReaderPeer(Peer peer, boolean fromCache) {
      this.peer = peer;
      this.fromCache = fromCache;
    }
  }

  /**
   * Get the next DomainPeer-- either from the cache or by creating it.
   *
   * @return the next DomainPeer, or null if we could not construct one.
   */
  private BlockReaderPeer nextDomainPeer() {
    if (remainingCacheTries > 0) {
      Peer peer = clientContext.getPeerCache().get(datanode, true);
      if (peer != null) {
        if (LOG.isTraceEnabled()) {
          LOG.trace("nextDomainPeer: reusing existing peer " + peer);
        }
        return new BlockReaderPeer(peer, true);
      }
    }
    DomainSocket sock = clientContext.getDomainSocketFactory().
        createSocket(pathInfo, conf.socketTimeout);
    if (sock == null) return null;
    return new BlockReaderPeer(new DomainPeer(sock), false);
  }

  /**
   * Get the next TCP-based peer-- either from the cache or by creating it.
   *
   * @return the next Peer, or null if we could not construct one.
   *
   * @throws IOException  If there was an error while constructing the peer
   *                      (such as an InvalidEncryptionKeyException)
   */
  private BlockReaderPeer nextTcpPeer() throws IOException {
    if (remainingCacheTries > 0) {
      Peer peer = clientContext.getPeerCache().get(datanode, false);
      if (peer != null) {
        if (LOG.isTraceEnabled()) {
          LOG.trace("nextTcpPeer: reusing existing peer " + peer);
        }
        return new BlockReaderPeer(peer, true);
      }
    }
    try {
      Peer peer = remotePeerFactory.newConnectedPeer(inetSocketAddress, token,
        datanode);
      if (LOG.isTraceEnabled()) {
        LOG.trace("nextTcpPeer: created newConnectedPeer " + peer);
      }
      return new BlockReaderPeer(peer, false);
    } catch (IOException e) {
      if (LOG.isTraceEnabled()) {
        LOG.trace("nextTcpPeer: failed to create newConnectedPeer " +
                  "connected to " + datanode);
      }
      throw e;
    }
  }

  /**
   * Determine if an exception is security-related.
   *
   * We need to handle these exceptions differently than other IOExceptions.
   * They don't indicate a communication problem.  Instead, they mean that there
   * is some action the client needs to take, such as refetching block tokens,
   * renewing encryption keys, etc.
   *
   * @param ioe    The exception
   * @return       True only if the exception is security-related.
   */
  private static boolean isSecurityException(IOException ioe) {
    return (ioe instanceof InvalidToken) ||
            (ioe instanceof InvalidEncryptionKeyException) ||
            (ioe instanceof InvalidBlockTokenException) ||
            (ioe instanceof AccessControlException);
  }

  @SuppressWarnings("deprecation")
  private BlockReader getRemoteBlockReader(Peer peer) throws IOException {
    if (conf.useLegacyBlockReader) {
      return RemoteBlockReader.newBlockReader(fileName,
          block, token, startOffset, length, conf.ioBufferSize,
          verifyChecksum, clientName, peer, datanode,
          clientContext.getPeerCache(), cachingStrategy);
    } else {
      return RemoteBlockReader2.newBlockReader(
          fileName, block, token, startOffset, length,
          verifyChecksum, clientName, peer, datanode,
          clientContext.getPeerCache(), cachingStrategy);
    }
  }

  @Override
  public String toString() {
    return "BlockReaderFactory(fileName=" + fileName + ", block=" + block + ")";
  }

  /**
   * File name to print when accessing a block directly (from servlets)
   * @param s Address of the block location
   * @param poolId Block pool ID of the block
   * @param blockId Block ID of the block
   * @return string that has a file name for debug purposes
   */
  public static String getFileName(final InetSocketAddress s,
      final String poolId, final long blockId) {
    return s.toString() + ":" + poolId + ":" + blockId;
  }
}