通信架构

首先，看下hdfs的交互图：

可以看到通信方面是有几个角色的：客户端（client)、NameNode、SecondaryNamenode、DataNode;其中SecondaryNamenode只与NameNode交互，其余的三种之间可以相互交互，所以便有了client—>NameNode，client—>DataNode，DataNode—>NameNode,DataNode—>DataNode，外加SecondaryNamenode<—>NameNode这几种交互方式，我们统一通过源码方式来先确定个大概轮廓。

交互协议

其实在Hadoop中存在两种通信协议，一种是基于Hadoop RPC的，其实底层是用到了ProtoBuf，这个主要用于进程间的通信，是的进程间通信形同本地调用一样的；另一种时流式通信，毕竟Hadoop是用来存储文件的数据，这就需要流式方式来读写，因为文件不能中断具备连续性底层是基于Tcp来实现的。这篇文章主要还是在于介绍各类角色间的通信，属于进程间，所以主要还是RPC接口，当然具体实现后面开篇讲清楚，目前只是给个大概轮廓。

Hadoop RPC接口主要是定义在org.apache.hadoop.hdfs.server.protocol和org.apache.hadoop.hdfs.protocol两个包中。其中主要包括如下几个接口：

• ClientProtocol：ClientProtocol定义了客户端和NameNode之间的交互，这个接口方法是非常多的，客户端对文件系统的所有操作都需要通过这个接口，同时客户端读写文件等操作也需要先通过这个接口与NameNode协商之后，在进行数据的读出和写入。
• ClientDatanodeProtocol：客户端与DataNode之间的交互，该接口定义的方法主要用于客户端获取数据节点信息时调用，而真正进行数据读写交互的则是后续要讲到的流式接口。
• DatanodeProtocol：DataNode通过这个接口与NameNode通信，同时NameNode会通过该接口中方法的返回值向DataNode下发指令。注意，这是NameNode与DataNode通信的唯一方式，DataNode会通过这个接口向NameNode注册、汇报数据块的全量以及增量的存储情况。同时，NameNode也会通过这个接口中的方法返回值，指令DataNode执行诸如数据块复制、删除以及恢复等操作。
• InterDatanodeProtocol：DataNode之间的通信，主要用于数据块的恢复，以及同步DataNode上存储的数据块副本的信息。
• NamenodeProtocol：SecondaryNamenode与NameNode之间的通信，因为引入了HA机制，checkpoint操作不再由SecondaryNamenode执行，所以这个接口不太需要详细介绍。
• 其他接口：主要包括安全相关的RefreshAuthorizationPolicyProtocol、RefreshUserMappingsProtocol，HA相关的接口HAServiceProtocol等。

接下来一一介绍。

1、ClientProtocol

ClientProtocol定义了所有由客户端发起，由NameNode响应的操作。这个接口非常大，大概80几个方法：
查看代码

/**
 * 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.protocol;

import java.io.IOException;
import java.util.EnumSet;
import java.util.List;
import java.util.Map;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.crypto.CryptoProtocolVersion;
import org.apache.hadoop.fs.BatchedRemoteIterator.BatchedEntries;
import org.apache.hadoop.fs.PathIsNotEmptyDirectoryException;
import org.apache.hadoop.ha.HAServiceProtocol;
import org.apache.hadoop.hdfs.AddBlockFlag;
import org.apache.hadoop.fs.CacheFlag;
import org.apache.hadoop.fs.ContentSummary;
import org.apache.hadoop.fs.CreateFlag;
import org.apache.hadoop.fs.FsServerDefaults;
import org.apache.hadoop.fs.Options;
import org.apache.hadoop.fs.QuotaUsage;
import org.apache.hadoop.fs.StorageType;
import org.apache.hadoop.fs.XAttr;
import org.apache.hadoop.fs.XAttrSetFlag;
import org.apache.hadoop.fs.permission.AclEntry;
import org.apache.hadoop.fs.permission.AclStatus;
import org.apache.hadoop.fs.permission.FsAction;
import org.apache.hadoop.fs.permission.FsPermission;
import org.apache.hadoop.hdfs.inotify.EventBatchList;
import org.apache.hadoop.hdfs.protocol.HdfsConstants.ReencryptAction;
import org.apache.hadoop.hdfs.protocol.HdfsConstants.RollingUpgradeAction;
import org.apache.hadoop.hdfs.protocol.OpenFilesIterator.OpenFilesType;
import org.apache.hadoop.hdfs.security.token.block.DataEncryptionKey;
import org.apache.hadoop.hdfs.security.token.delegation.DelegationTokenIdentifier;
import org.apache.hadoop.hdfs.security.token.delegation.DelegationTokenSelector;
import org.apache.hadoop.hdfs.server.namenode.ha.ReadOnly;
import org.apache.hadoop.hdfs.server.protocol.DatanodeStorageReport;
import org.apache.hadoop.io.EnumSetWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.retry.AtMostOnce;
import org.apache.hadoop.io.retry.Idempotent;
import org.apache.hadoop.security.AccessControlException;
import org.apache.hadoop.security.KerberosInfo;
import org.apache.hadoop.security.token.Token;
import org.apache.hadoop.security.token.TokenInfo;

import static org.apache.hadoop.hdfs.client.HdfsClientConfigKeys.DFS_NAMENODE_KERBEROS_PRINCIPAL_KEY;

/**********************************************************************
 * ClientProtocol is used by user code via the DistributedFileSystem class to
 * communicate with the NameNode.  User code can manipulate the directory
 * namespace, as well as open/close file streams, etc.
 *
 **********************************************************************/
@InterfaceAudience.Private
@InterfaceStability.Evolving
@KerberosInfo(
    serverPrincipal = DFS_NAMENODE_KERBEROS_PRINCIPAL_KEY)
@TokenInfo(DelegationTokenSelector.class)
public interface ClientProtocol {

  /**
   * Until version 69, this class ClientProtocol served as both
   * the client interface to the NN AND the RPC protocol used to
   * communicate with the NN.
   *
   * This class is used by both the DFSClient and the
   * NN server side to insulate from the protocol serialization.
   *
   * If you are adding/changing this interface then you need to
   * change both this class and ALSO related protocol buffer
   * wire protocol definition in ClientNamenodeProtocol.proto.
   *
   * For more details on protocol buffer wire protocol, please see
   * .../org/apache/hadoop/hdfs/protocolPB/overview.html
   *
   * The log of historical changes can be retrieved from the svn).
   * 69: Eliminate overloaded method names.
   *
   * 69L is the last version id when this class was used for protocols
   *  serialization. DO not update this version any further.
   */
  long versionID = 69L;

  ///////////////////////////////////////
  // File contents
  ///////////////////////////////////////
  /**
   * Get locations of the blocks of the specified file
   * within the specified range.
   * DataNode locations for each block are sorted by
   * the proximity to the client.
   * <p>
   * Return {@link LocatedBlocks} which contains
   * file length, blocks and their locations.
   * DataNode locations for each block are sorted by
   * the distance to the client's address.
   * <p>
   * The client will then have to contact
   * one of the indicated DataNodes to obtain the actual data.
   *
   * @param src file name
   * @param offset range start offset
   * @param length range length
   *
   * @return file length and array of blocks with their locations
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException If file <code>src</code> does not
   *           exist
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  @ReadOnly(atimeAffected = true, isCoordinated = true)
  LocatedBlocks getBlockLocations(String src, long offset, long length)
      throws IOException;

  /**
   * Get server default values for a number of configuration params.
   * @return a set of server default configuration values
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  FsServerDefaults getServerDefaults() throws IOException;

  /**
   * Create a new file entry in the namespace.
   * <p>
   * This will create an empty file specified by the source path.
   * The path should reflect a full path originated at the root.
   * The name-node does not have a notion of "current" directory for a client.
   * <p>
   * Once created, the file is visible and available for read to other clients.
   * Although, other clients cannot {@link #delete(String, boolean)}, re-create
   * or {@link #rename(String, String)} it until the file is completed
   * or explicitly as a result of lease expiration.
   * <p>
   * Blocks have a maximum size.  Clients that intend to create
   * multi-block files must also use
   * {@link #addBlock}
   *
   * @param src path of the file being created.
   * @param masked masked permission.
   * @param clientName name of the current client.
   * @param flag indicates whether the file should be overwritten if it already
   *             exists or create if it does not exist or append, or whether the
   *             file should be a replicate file, no matter what its ancestor's
   *             replication or erasure coding policy is.
   * @param createParent create missing parent directory if true
   * @param replication block replication factor.
   * @param blockSize maximum block size.
   * @param supportedVersions CryptoProtocolVersions supported by the client
   * @param ecPolicyName the name of erasure coding policy. A null value means
   *                     this file will inherit its parent directory's policy,
   *                     either traditional replication or erasure coding
   *                     policy. ecPolicyName and SHOULD_REPLICATE CreateFlag
   *                     are mutually exclusive. It's invalid to set both
   *                     SHOULD_REPLICATE flag and a non-null ecPolicyName.
   *@param storagePolicy the name of the storage policy.
   *
   * @return the status of the created file, it could be null if the server
   *           doesn't support returning the file status
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws AlreadyBeingCreatedException if the path does not exist.
   * @throws DSQuotaExceededException If file creation violates disk space
   *           quota restriction
   * @throws org.apache.hadoop.fs.FileAlreadyExistsException If file
   *           <code>src</code> already exists
   * @throws java.io.FileNotFoundException If parent of <code>src</code> does
   *           not exist and <code>createParent</code> is false
   * @throws org.apache.hadoop.fs.ParentNotDirectoryException If parent of
   *           <code>src</code> is not a directory.
   * @throws NSQuotaExceededException If file creation violates name space
   *           quota restriction
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException create not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   *
   * RuntimeExceptions:
   * @throws org.apache.hadoop.fs.InvalidPathException Path <code>src</code> is
   *           invalid
   * <p>
   * <em>Note that create with {@link CreateFlag#OVERWRITE} is idempotent.</em>
   */
  @AtMostOnce
  HdfsFileStatus create(String src, FsPermission masked,
      String clientName, EnumSetWritable<CreateFlag> flag,
      boolean createParent, short replication, long blockSize,
      CryptoProtocolVersion[] supportedVersions, String ecPolicyName,
      String storagePolicy)
      throws IOException;

  /**
   * Append to the end of the file.
   * @param src path of the file being created.
   * @param clientName name of the current client.
   * @param flag indicates whether the data is appended to a new block.
   * @return wrapper with information about the last partial block and file
   *    status if any
   * @throws org.apache.hadoop.security.AccessControlException if permission to
   * append file is denied by the system. As usually on the client side the
   * exception will be wrapped into
   * {@link org.apache.hadoop.ipc.RemoteException}.
   * Allows appending to an existing file if the server is
   * configured with the parameter dfs.support.append set to true, otherwise
   * throws an IOException.
   *
   * @throws org.apache.hadoop.security.AccessControlException If permission to
   *           append to file is denied
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws DSQuotaExceededException If append violates disk space quota
   *           restriction
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException append not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred.
   *
   * RuntimeExceptions:
   * @throws UnsupportedOperationException if append is not supported
   */
  @AtMostOnce
  LastBlockWithStatus append(String src, String clientName,
      EnumSetWritable<CreateFlag> flag) throws IOException;

  /**
   * Set replication for an existing file.
   * <p>
   * The NameNode sets replication to the new value and returns.
   * The actual block replication is not expected to be performed during
   * this method call. The blocks will be populated or removed in the
   * background as the result of the routine block maintenance procedures.
   *
   * @param src file name
   * @param replication new replication
   *
   * @return true if successful;
   *         false if file does not exist or is a directory
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws DSQuotaExceededException If replication violates disk space
   *           quota restriction
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>src</code>
   *           contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  boolean setReplication(String src, short replication)
      throws IOException;

  /**
   * Get all the available block storage policies.
   * @return All the in-use block storage policies currently.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  BlockStoragePolicy[] getStoragePolicies() throws IOException;

  /**
   * Set the storage policy for a file/directory.
   * @param src Path of an existing file/directory.
   * @param policyName The name of the storage policy
   * @throws SnapshotAccessControlException If access is denied
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>src</code>
   *           contains a symlink
   * @throws java.io.FileNotFoundException If file/dir <code>src</code> is not
   *           found
   * @throws QuotaExceededException If changes violate the quota restriction
   */
  @Idempotent
  void setStoragePolicy(String src, String policyName)
      throws IOException;

  /**
   * Unset the storage policy set for a given file or directory.
   * @param src Path of an existing file/directory.
   * @throws SnapshotAccessControlException If access is denied
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>src</code>
   *           contains a symlink
   * @throws java.io.FileNotFoundException If file/dir <code>src</code> is not
   *           found
   * @throws QuotaExceededException If changes violate the quota restriction
   */
  @Idempotent
  void unsetStoragePolicy(String src) throws IOException;

  /**
   * Get the storage policy for a file/directory.
   * @param path
   *          Path of an existing file/directory.
   * @throws AccessControlException
   *           If access is denied
   * @throws org.apache.hadoop.fs.UnresolvedLinkException
   *           if <code>src</code> contains a symlink
   * @throws java.io.FileNotFoundException
   *           If file/dir <code>src</code> is not found
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  BlockStoragePolicy getStoragePolicy(String path) throws IOException;

  /**
   * Set permissions for an existing file/directory.
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  void setPermission(String src, FsPermission permission)
      throws IOException;

  /**
   * Set Owner of a path (i.e. a file or a directory).
   * The parameters username and groupname cannot both be null.
   * @param src file path
   * @param username If it is null, the original username remains unchanged.
   * @param groupname If it is null, the original groupname remains unchanged.
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  void setOwner(String src, String username, String groupname)
      throws IOException;

  /**
   * The client can give up on a block by calling abandonBlock().
   * The client can then either obtain a new block, or complete or abandon the
   * file.
   * Any partial writes to the block will be discarded.
   *
   * @param b         Block to abandon
   * @param fileId    The id of the file where the block resides.  Older clients
   *                    will pass GRANDFATHER_INODE_ID here.
   * @param src       The path of the file where the block resides.
   * @param holder    Lease holder.
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException file <code>src</code> is not found
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  void abandonBlock(ExtendedBlock b, long fileId,
      String src, String holder)
      throws IOException;

  /**
   * A client that wants to write an additional block to the
   * indicated filename (which must currently be open for writing)
   * should call addBlock().
   *
   * addBlock() allocates a new block and datanodes the block data
   * should be replicated to.
   *
   * addBlock() also commits the previous block by reporting
   * to the name-node the actual generation stamp and the length
   * of the block that the client has transmitted to data-nodes.
   *
   * @param src the file being created
   * @param clientName the name of the client that adds the block
   * @param previous  previous block
   * @param excludeNodes a list of nodes that should not be
   * allocated for the current block
   * @param fileId the id uniquely identifying a file
   * @param favoredNodes the list of nodes where the client wants the blocks.
   *          Nodes are identified by either host name or address.
   * @param addBlockFlags flags to advise the behavior of allocating and placing
   *                      a new block.
   *
   * @return LocatedBlock allocated block information.
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws org.apache.hadoop.hdfs.server.namenode.NotReplicatedYetException
   *           previous blocks of the file are not replicated yet.
   *           Blocks cannot be added until replication completes.
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException create not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  LocatedBlock addBlock(String src, String clientName,
      ExtendedBlock previous, DatanodeInfo[] excludeNodes, long fileId,
      String[] favoredNodes, EnumSet<AddBlockFlag> addBlockFlags)
      throws IOException;

  /**
   * Get a datanode for an existing pipeline.
   *
   * @param src the file being written
   * @param fileId the ID of the file being written
   * @param blk the block being written
   * @param existings the existing nodes in the pipeline
   * @param excludes the excluded nodes
   * @param numAdditionalNodes number of additional datanodes
   * @param clientName the name of the client
   *
   * @return the located block.
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException create not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  LocatedBlock getAdditionalDatanode(final String src,
      final long fileId, final ExtendedBlock blk,
      final DatanodeInfo[] existings,
      final String[] existingStorageIDs,
      final DatanodeInfo[] excludes,
      final int numAdditionalNodes, final String clientName
      ) throws IOException;

  /**
   * The client is done writing data to the given filename, and would
   * like to complete it.
   *
   * The function returns whether the file has been closed successfully.
   * If the function returns false, the caller should try again.
   *
   * close() also commits the last block of file by reporting
   * to the name-node the actual generation stamp and the length
   * of the block that the client has transmitted to data-nodes.
   *
   * A call to complete() will not return true until all the file's
   * blocks have been replicated the minimum number of times.  Thus,
   * DataNode failures may cause a client to call complete() several
   * times before succeeding.
   *
   * @param src the file being created
   * @param clientName the name of the client that adds the block
   * @param last the last block info
   * @param fileId the id uniquely identifying a file
   *
   * @return true if all file blocks are minimally replicated or false otherwise
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException create not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  boolean complete(String src, String clientName,
                          ExtendedBlock last, long fileId)
      throws IOException;

  /**
   * The client wants to report corrupted blocks (blocks with specified
   * locations on datanodes).
   * @param blocks Array of located blocks to report
   */
  @Idempotent
  void reportBadBlocks(LocatedBlock[] blocks) throws IOException;

  ///////////////////////////////////////
  // Namespace management
  ///////////////////////////////////////
  /**
   * Rename an item in the file system namespace.
   * @param src existing file or directory name.
   * @param dst new name.
   * @return true if successful, or false if the old name does not exist
   * or if the new name already belongs to the namespace.
   *
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException an I/O error occurred
   */
  @AtMostOnce
  boolean rename(String src, String dst)
      throws IOException;

  /**
   * Moves blocks from srcs to trg and delete srcs.
   *
   * @param trg existing file
   * @param srcs - list of existing files (same block size, same replication)
   * @throws IOException if some arguments are invalid
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>trg</code> or
   *           <code>srcs</code> contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   */
  @AtMostOnce
  void concat(String trg, String[] srcs)
      throws IOException;

  /**
   * Rename src to dst.
   * <ul>
   * <li>Fails if src is a file and dst is a directory.
   * <li>Fails if src is a directory and dst is a file.
   * <li>Fails if the parent of dst does not exist or is a file.
   * </ul>
   * <p>
   * Without OVERWRITE option, rename fails if the dst already exists.
   * With OVERWRITE option, rename overwrites the dst, if it is a file
   * or an empty directory. Rename fails if dst is a non-empty directory.
   * <p>
   * This implementation of rename is atomic.
   * <p>
   * @param src existing file or directory name.
   * @param dst new name.
   * @param options Rename options
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws DSQuotaExceededException If rename violates disk space
   *           quota restriction
   * @throws org.apache.hadoop.fs.FileAlreadyExistsException If <code>dst</code>
   *           already exists and <code>options</code> has
   *           {@link org.apache.hadoop.fs.Options.Rename#OVERWRITE} option
   *           false.
   * @throws java.io.FileNotFoundException If <code>src</code> does not exist
   * @throws NSQuotaExceededException If rename violates namespace
   *           quota restriction
   * @throws org.apache.hadoop.fs.ParentNotDirectoryException If parent of
   *           <code>dst</code> is not a directory
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException rename not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code> or
   *           <code>dst</code> contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   */
  @AtMostOnce
  void rename2(String src, String dst, Options.Rename... options)
      throws IOException;

  /**
   * Truncate file src to new size.
   * <ul>
   * <li>Fails if src is a directory.
   * <li>Fails if src does not exist.
   * <li>Fails if src is not closed.
   * <li>Fails if new size is greater than current size.
   * </ul>
   * <p>
   * This implementation of truncate is purely a namespace operation if truncate
   * occurs at a block boundary. Requires DataNode block recovery otherwise.
   * <p>
   * @param src  existing file
   * @param newLength  the target size
   *
   * @return true if client does not need to wait for block recovery,
   * false if client needs to wait for block recovery.
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException truncate
   *           not allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  boolean truncate(String src, long newLength, String clientName)
      throws IOException;

  /**
   * Delete the given file or directory from the file system.
   * <p>
   * same as delete but provides a way to avoid accidentally
   * deleting non empty directories programmatically.
   * @param src existing name
   * @param recursive if true deletes a non empty directory recursively,
   * else throws an exception.
   * @return true only if the existing file or directory was actually removed
   * from the file system.
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws java.io.FileNotFoundException If file <code>src</code> is not found
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException create not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws PathIsNotEmptyDirectoryException if path is a non-empty directory
   *           and <code>recursive</code> is set to false
   * @throws IOException If an I/O error occurred
   */
  @AtMostOnce
  boolean delete(String src, boolean recursive)
      throws IOException;

  /**
   * Create a directory (or hierarchy of directories) with the given
   * name and permission.
   *
   * @param src The path of the directory being created
   * @param masked The masked permission of the directory being created
   * @param createParent create missing parent directory if true
   *
   * @return True if the operation success.
   *
   * @throws org.apache.hadoop.security.AccessControlException If access is
   *           denied
   * @throws org.apache.hadoop.fs.FileAlreadyExistsException If <code>src</code>
   *           already exists
   * @throws java.io.FileNotFoundException If parent of <code>src</code> does
   *           not exist and <code>createParent</code> is false
   * @throws NSQuotaExceededException If file creation violates quota
   *           restriction
   * @throws org.apache.hadoop.fs.ParentNotDirectoryException If parent of
   *           <code>src</code> is not a directory
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException create not
   *           allowed in safemode
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred.
   *
   * RunTimeExceptions:
   * @throws org.apache.hadoop.fs.InvalidPathException If <code>src</code> is
   *           invalid
   */
  @Idempotent
  boolean mkdirs(String src, FsPermission masked, boolean createParent)
      throws IOException;

  /**
   * Get a partial listing of the indicated directory.
   *
   * @param src the directory name
   * @param startAfter the name to start listing after encoded in java UTF8
   * @param needLocation if the FileStatus should contain block locations
   *
   * @return a partial listing starting after startAfter
   *
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>src</code> is not found
   * @throws org.apache.hadoop.fs.UnresolvedLinkException If <code>src</code>
   *           contains a symlink
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  DirectoryListing getListing(String src, byte[] startAfter,
      boolean needLocation) throws IOException;

  /**
   * Get a partial listing of the input directories
   *
   * @param srcs the input directories
   * @param startAfter the name to start listing after encoded in Java UTF8
   * @param needLocation if the FileStatus should contain block locations
   *
   * @return a partial listing starting after startAfter. null if the input is
   *   empty
   * @throws IOException if an I/O error occurred
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  BatchedDirectoryListing getBatchedListing(
      String[] srcs,
      byte[] startAfter,
      boolean needLocation) throws IOException;

  /**
   * Get the list of snapshottable directories that are owned
   * by the current user. Return all the snapshottable directories if the
   * current user is a super user.
   * @return The list of all the current snapshottable directories.
   * @throws IOException If an I/O error occurred.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  SnapshottableDirectoryStatus[] getSnapshottableDirListing()
      throws IOException;

  ///////////////////////////////////////
  // System issues and management
  ///////////////////////////////////////

  /**
   * Client programs can cause stateful changes in the NameNode
   * that affect other clients.  A client may obtain a file and
   * neither abandon nor complete it.  A client might hold a series
   * of locks that prevent other clients from proceeding.
   * Clearly, it would be bad if a client held a bunch of locks
   * that it never gave up.  This can happen easily if the client
   * dies unexpectedly.
   * <p>
   * So, the NameNode will revoke the locks and live file-creates
   * for clients that it thinks have died.  A client tells the
   * NameNode that it is still alive by periodically calling
   * renewLease().  If a certain amount of time passes since
   * the last call to renewLease(), the NameNode assumes the
   * client has died.
   *
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  void renewLease(String clientName) throws IOException;

  /**
   * Start lease recovery.
   * Lightweight NameNode operation to trigger lease recovery
   *
   * @param src path of the file to start lease recovery
   * @param clientName name of the current client
   * @return true if the file is already closed
   * @throws IOException
   */
  @Idempotent
  boolean recoverLease(String src, String clientName) throws IOException;

  /**
   * Constants to index the array of aggregated stats returned by
   * {@link #getStats()}.
   */
  int GET_STATS_CAPACITY_IDX = 0;
  int GET_STATS_USED_IDX = 1;
  int GET_STATS_REMAINING_IDX = 2;
  /**
   * Use {@link #GET_STATS_LOW_REDUNDANCY_IDX} instead.
   */
  @Deprecated
  int GET_STATS_UNDER_REPLICATED_IDX = 3;
  int GET_STATS_LOW_REDUNDANCY_IDX = 3;
  int GET_STATS_CORRUPT_BLOCKS_IDX = 4;
  int GET_STATS_MISSING_BLOCKS_IDX = 5;
  int GET_STATS_MISSING_REPL_ONE_BLOCKS_IDX = 6;
  int GET_STATS_BYTES_IN_FUTURE_BLOCKS_IDX = 7;
  int GET_STATS_PENDING_DELETION_BLOCKS_IDX = 8;
  int STATS_ARRAY_LENGTH = 9;

  /**
   * Get an array of aggregated statistics combining blocks of both type
   * {@link BlockType#CONTIGUOUS} and {@link BlockType#STRIPED} in the
   * filesystem. Use public constants like {@link #GET_STATS_CAPACITY_IDX} in
   * place of actual numbers to index into the array.
   * <ul>
   * <li> [0] contains the total storage capacity of the system, in bytes.</li>
   * <li> [1] contains the total used space of the system, in bytes.</li>
   * <li> [2] contains the available storage of the system, in bytes.</li>
   * <li> [3] contains number of low redundancy blocks in the system.</li>
   * <li> [4] contains number of corrupt blocks. </li>
   * <li> [5] contains number of blocks without any good replicas left. </li>
   * <li> [6] contains number of blocks which have replication factor
   *          1 and have lost the only replica. </li>
   * <li> [7] contains number of bytes that are at risk for deletion. </li>
   * <li> [8] contains number of pending deletion blocks. </li>
   * </ul>
   */
  @Idempotent
  @ReadOnly
  long[] getStats() throws IOException;

  /**
   * Get statistics pertaining to blocks of type {@link BlockType#CONTIGUOUS}
   * in the filesystem.
   */
  @Idempotent
  @ReadOnly
  ReplicatedBlockStats getReplicatedBlockStats() throws IOException;

  /**
   * Get statistics pertaining to blocks of type {@link BlockType#STRIPED}
   * in the filesystem.
   */
  @Idempotent
  @ReadOnly
  ECBlockGroupStats getECBlockGroupStats() throws IOException;

  /**
   * Get a report on the system's current datanodes.
   * One DatanodeInfo object is returned for each DataNode.
   * Return live datanodes if type is LIVE; dead datanodes if type is DEAD;
   * otherwise all datanodes if type is ALL.
   */
  @Idempotent
  @ReadOnly
  DatanodeInfo[] getDatanodeReport(HdfsConstants.DatanodeReportType type)
      throws IOException;

  /**
   * Get a report on the current datanode storages.
   */
  @Idempotent
  @ReadOnly
  DatanodeStorageReport[] getDatanodeStorageReport(
      HdfsConstants.DatanodeReportType type) throws IOException;

  /**
   * Get the block size for the given file.
   * @param filename The name of the file
   * @return The number of bytes in each block
   * @throws IOException
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if the path contains
   *           a symlink.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  long getPreferredBlockSize(String filename)
      throws IOException;

  /**
   * Enter, leave or get safe mode.
   * <p>
   * Safe mode is a name node state when it
   * <ol><li>does not accept changes to name space (read-only), and</li>
   * <li>does not replicate or delete blocks.</li></ol>
   *
   * <p>
   * Safe mode is entered automatically at name node startup.
   * Safe mode can also be entered manually using
   * {@link #setSafeMode(HdfsConstants.SafeModeAction,boolean)
   * setSafeMode(SafeModeAction.SAFEMODE_ENTER,false)}.
   * <p>
   * At startup the name node accepts data node reports collecting
   * information about block locations.
   * In order to leave safe mode it needs to collect a configurable
   * percentage called threshold of blocks, which satisfy the minimal
   * replication condition.
   * The minimal replication condition is that each block must have at least
   * {@code dfs.namenode.replication.min} replicas.
   * When the threshold is reached the name node extends safe mode
   * for a configurable amount of time
   * to let the remaining data nodes to check in before it
   * will start replicating missing blocks.
   * Then the name node leaves safe mode.
   * <p>
   * If safe mode is turned on manually using
   * {@link #setSafeMode(HdfsConstants.SafeModeAction,boolean)
   * setSafeMode(SafeModeAction.SAFEMODE_ENTER,false)}
   * then the name node stays in safe mode until it is manually turned off
   * using {@link #setSafeMode(HdfsConstants.SafeModeAction,boolean)
   * setSafeMode(SafeModeAction.SAFEMODE_LEAVE,false)}.
   * Current state of the name node can be verified using
   * {@link #setSafeMode(HdfsConstants.SafeModeAction,boolean)
   * setSafeMode(SafeModeAction.SAFEMODE_GET,false)}
   *
   * <p><b>Configuration parameters:</b></p>
   * {@code dfs.safemode.threshold.pct} is the threshold parameter.<br>
   * {@code dfs.safemode.extension} is the safe mode extension parameter.<br>
   * {@code dfs.namenode.replication.min} is the minimal replication parameter.
   *
   * <p><b>Special cases:</b></p>
   * The name node does not enter safe mode at startup if the threshold is
   * set to 0 or if the name space is empty.<br>
   * If the threshold is set to 1 then all blocks need to have at least
   * minimal replication.<br>
   * If the threshold value is greater than 1 then the name node will not be
   * able to turn off safe mode automatically.<br>
   * Safe mode can always be turned off manually.
   *
   * @param action  <ul> <li>0 leave safe mode;</li>
   *                <li>1 enter safe mode;</li>
   *                <li>2 get safe mode state.</li></ul>
   * @param isChecked If true then action will be done only in ActiveNN.
   *
   * @return <ul><li>0 if the safe mode is OFF or</li>
   *         <li>1 if the safe mode is ON.</li></ul>
   *
   * @throws IOException
   */
  @Idempotent
  boolean setSafeMode(HdfsConstants.SafeModeAction action, boolean isChecked)
      throws IOException;

  /**
   * Save namespace image.
   * <p>
   * Saves current namespace into storage directories and reset edits log.
   * Requires superuser privilege and safe mode.
   *
   * @param timeWindow NameNode does a checkpoint if the latest checkpoint was
   *                   done beyond the given time period (in seconds).
   * @param txGap NameNode does a checkpoint if the gap between the latest
   *              checkpoint and the latest transaction id is greater this gap.
   * @return whether an extra checkpoint has been done
   *
   * @throws IOException if image creation failed.
   */
  @AtMostOnce
  boolean saveNamespace(long timeWindow, long txGap) throws IOException;

  /**
   * Roll the edit log.
   * Requires superuser privileges.
   *
   * @throws org.apache.hadoop.security.AccessControlException if the superuser
   *           privilege is violated
   * @throws IOException if log roll fails
   * @return the txid of the new segment
   */
  @Idempotent
  long rollEdits() throws IOException;

  /**
   * Enable/Disable restore failed storage.
   * <p>
   * sets flag to enable restore of failed storage replicas
   *
   * @throws org.apache.hadoop.security.AccessControlException if the superuser
   *           privilege is violated.
   */
  @Idempotent
  boolean restoreFailedStorage(String arg) throws IOException;

  /**
   * Tells the namenode to reread the hosts and exclude files.
   * @throws IOException
   */
  @Idempotent
  void refreshNodes() throws IOException;

  /**
   * Finalize previous upgrade.
   * Remove file system state saved during the upgrade.
   * The upgrade will become irreversible.
   *
   * @throws IOException
   */
  @Idempotent
  void finalizeUpgrade() throws IOException;

  /**
   * Get status of upgrade - finalized or not.
   * @return true if upgrade is finalized or if no upgrade is in progress and
   * false otherwise.
   * @throws IOException
   */
  @Idempotent
  boolean upgradeStatus() throws IOException;

  /**
   * Rolling upgrade operations.
   * @param action either query, prepare or finalize.
   * @return rolling upgrade information. On query, if no upgrade is in
   * progress, returns null.
   */
  @Idempotent
  RollingUpgradeInfo rollingUpgrade(RollingUpgradeAction action)
      throws IOException;

  /**
   * @return CorruptFileBlocks, containing a list of corrupt files (with
   *         duplicates if there is more than one corrupt block in a file)
   *         and a cookie
   * @throws IOException
   *
   * Each call returns a subset of the corrupt files in the system. To obtain
   * all corrupt files, call this method repeatedly and each time pass in the
   * cookie returned from the previous call.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  CorruptFileBlocks listCorruptFileBlocks(String path, String cookie)
      throws IOException;

  /**
   * Dumps namenode data structures into specified file. If the file
   * already exists, then append.
   *
   * @throws IOException
   */
  @Idempotent
  void metaSave(String filename) throws IOException;

  /**
   * Tell all datanodes to use a new, non-persistent bandwidth value for
   * dfs.datanode.balance.bandwidthPerSec.
   *
   * @param bandwidth Blanacer bandwidth in bytes per second for this datanode.
   * @throws IOException
   */
  @Idempotent
  void setBalancerBandwidth(long bandwidth) throws IOException;

  /**
   * Get the file info for a specific file or directory.
   * @param src The string representation of the path to the file
   *
   * @return object containing information regarding the file
   *         or null if file not found
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>src</code> is not found
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if the path contains
   *           a symlink.
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  HdfsFileStatus getFileInfo(String src) throws IOException;

  /**
   * Get the close status of a file.
   * @param src The string representation of the path to the file
   *
   * @return return true if file is closed
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>src</code> is not found
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if the path contains
   *           a symlink.
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  boolean isFileClosed(String src) throws IOException;

  /**
   * Get the file info for a specific file or directory. If the path
   * refers to a symlink then the FileStatus of the symlink is returned.
   * @param src The string representation of the path to the file
   *
   * @return object containing information regarding the file
   *         or null if file not found
   *
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>src</code>
   *           contains a symlink
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  HdfsFileStatus getFileLinkInfo(String src) throws IOException;

  /**
   * Get the file info for a specific file or directory with
   * {@link LocatedBlocks}.
   * @param src The string representation of the path to the file
   * @param needBlockToken Generate block tokens for {@link LocatedBlocks}
   * @return object containing information regarding the file
   *         or null if file not found
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>src</code> is not found
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  HdfsLocatedFileStatus getLocatedFileInfo(String src, boolean needBlockToken)
      throws IOException;

  /**
   * Get {@link ContentSummary} rooted at the specified directory.
   * @param path The string representation of the path
   *
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>path</code> is not found
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>path</code>
   *           contains a symlink.
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  ContentSummary getContentSummary(String path) throws IOException;

  /**
   * Set the quota for a directory.
   * @param path  The string representation of the path to the directory
   * @param namespaceQuota Limit on the number of names in the tree rooted
   *                       at the directory
   * @param storagespaceQuota Limit on storage space occupied all the files
   *                       under this directory.
   * @param type StorageType that the space quota is intended to be set on.
   *             It may be null when called by traditional space/namespace
   *             quota. When type is is not null, the storagespaceQuota
   *             parameter is for type specified and namespaceQuota must be
   *             {@link HdfsConstants#QUOTA_DONT_SET}.
   *
   * <br><br>
   *
   * The quota can have three types of values : (1) 0 or more will set
   * the quota to that value, (2) {@link HdfsConstants#QUOTA_DONT_SET}  implies
   * the quota will not be changed, and (3) {@link HdfsConstants#QUOTA_RESET}
   * implies the quota will be reset. Any other value is a runtime error.
   *
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>path</code> is not found
   * @throws QuotaExceededException if the directory size
   *           is greater than the given quota
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if the
   *           <code>path</code> contains a symlink.
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  void setQuota(String path, long namespaceQuota, long storagespaceQuota,
      StorageType type) throws IOException;

  /**
   * Write all metadata for this file into persistent storage.
   * The file must be currently open for writing.
   * @param src The string representation of the path
   * @param inodeId The inode ID, or GRANDFATHER_INODE_ID if the client is
   *                too old to support fsync with inode IDs.
   * @param client The string representation of the client
   * @param lastBlockLength The length of the last block (under construction)
   *                        to be reported to NameNode
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>src</code> is not found
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>src</code>
   *           contains a symlink.
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  void fsync(String src, long inodeId, String client, long lastBlockLength)
      throws IOException;

  /**
   * Sets the modification and access time of the file to the specified time.
   * @param src The string representation of the path
   * @param mtime The number of milliseconds since Jan 1, 1970.
   *              Setting negative mtime means that modification time should not
   *              be set by this call.
   * @param atime The number of milliseconds since Jan 1, 1970.
   *              Setting negative atime means that access time should not be
   *              set by this call.
   *
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>src</code> is not found
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>src</code>
   *           contains a symlink.
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  void setTimes(String src, long mtime, long atime) throws IOException;

  /**
   * Create symlink to a file or directory.
   * @param target The path of the destination that the
   *               link points to.
   * @param link The path of the link being created.
   * @param dirPerm permissions to use when creating parent directories
   * @param createParent - if true then missing parent dirs are created
   *                       if false then parent must exist
   *
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws org.apache.hadoop.fs.FileAlreadyExistsException If file
   *           <code>link</code> already exists
   * @throws java.io.FileNotFoundException If parent of <code>link</code> does
   *           not exist and <code>createParent</code> is false
   * @throws org.apache.hadoop.fs.ParentNotDirectoryException If parent of
   *           <code>link</code> is not a directory.
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>link</code>
   *           contains a symlink.
   * @throws SnapshotAccessControlException if path is in RO snapshot
   * @throws IOException If an I/O error occurred
   */
  @AtMostOnce
  void createSymlink(String target, String link, FsPermission dirPerm,
      boolean createParent) throws IOException;

  /**
   * Return the target of the given symlink. If there is an intermediate
   * symlink in the path (ie a symlink leading up to the final path component)
   * then the given path is returned with this symlink resolved.
   *
   * @param path The path with a link that needs resolution.
   * @return The path after resolving the first symbolic link in the path.
   * @throws org.apache.hadoop.security.AccessControlException permission denied
   * @throws java.io.FileNotFoundException If <code>path</code> does not exist
   * @throws IOException If the given path does not refer to a symlink
   *           or an I/O error occurred
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  String getLinkTarget(String path) throws IOException;

  /**
   * Get a new generation stamp together with an access token for
   * a block under construction
   *
   * This method is called only when a client needs to recover a failed
   * pipeline or set up a pipeline for appending to a block.
   *
   * @param block a block
   * @param clientName the name of the client
   * @return a located block with a new generation stamp and an access token
   * @throws IOException if any error occurs
   */
  @Idempotent
  LocatedBlock updateBlockForPipeline(ExtendedBlock block,
      String clientName) throws IOException;

  /**
   * Update a pipeline for a block under construction.
   *
   * @param clientName the name of the client
   * @param oldBlock the old block
   * @param newBlock the new block containing new generation stamp and length
   * @param newNodes datanodes in the pipeline
   * @throws IOException if any error occurs
   */
  @AtMostOnce
  void updatePipeline(String clientName, ExtendedBlock oldBlock,
      ExtendedBlock newBlock, DatanodeID[] newNodes, String[] newStorageIDs)
      throws IOException;

  /**
   * Get a valid Delegation Token.
   *
   * @param renewer the designated renewer for the token
   * @throws IOException
   */
  @Idempotent
  Token<DelegationTokenIdentifier> getDelegationToken(Text renewer)
      throws IOException;

  /**
   * Renew an existing delegation token.
   *
   * @param token delegation token obtained earlier
   * @return the new expiration time
   * @throws IOException
   */
  @Idempotent
  long renewDelegationToken(Token<DelegationTokenIdentifier> token)
      throws IOException;

  /**
   * Cancel an existing delegation token.
   *
   * @param token delegation token
   * @throws IOException
   */
  @Idempotent
  void cancelDelegationToken(Token<DelegationTokenIdentifier> token)
      throws IOException;

  /**
   * @return encryption key so a client can encrypt data sent via the
   *         DataTransferProtocol to/from DataNodes.
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  DataEncryptionKey getDataEncryptionKey() throws IOException;

  /**
   * Create a snapshot.
   * @param snapshotRoot the path that is being snapshotted
   * @param snapshotName name of the snapshot created
   * @return the snapshot path.
   * @throws IOException
   */
  @AtMostOnce
  String createSnapshot(String snapshotRoot, String snapshotName)
      throws IOException;

  /**
   * Delete a specific snapshot of a snapshottable directory.
   * @param snapshotRoot  The snapshottable directory
   * @param snapshotName Name of the snapshot for the snapshottable directory
   * @throws IOException
   */
  @AtMostOnce
  void deleteSnapshot(String snapshotRoot, String snapshotName)
      throws IOException;

  /**
   * Rename a snapshot.
   * @param snapshotRoot the directory path where the snapshot was taken
   * @param snapshotOldName old name of the snapshot
   * @param snapshotNewName new name of the snapshot
   * @throws IOException
   */
  @AtMostOnce
  void renameSnapshot(String snapshotRoot, String snapshotOldName,
      String snapshotNewName) throws IOException;

  /**
   * Allow snapshot on a directory.
   * @param snapshotRoot the directory to be snapped
   * @throws IOException on error
   */
  @Idempotent
  void allowSnapshot(String snapshotRoot)
      throws IOException;

  /**
   * Disallow snapshot on a directory.
   * @param snapshotRoot the directory to disallow snapshot
   * @throws IOException on error
   */
  @Idempotent
  void disallowSnapshot(String snapshotRoot)
      throws IOException;

  /**
   * Get the difference between two snapshots, or between a snapshot and the
   * current tree of a directory.
   *
   * @param snapshotRoot
   *          full path of the directory where snapshots are taken
   * @param fromSnapshot
   *          snapshot name of the from point. Null indicates the current
   *          tree
   * @param toSnapshot
   *          snapshot name of the to point. Null indicates the current
   *          tree.
   * @return The difference report represented as a {@link SnapshotDiffReport}.
   * @throws IOException on error
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  SnapshotDiffReport getSnapshotDiffReport(String snapshotRoot,
      String fromSnapshot, String toSnapshot) throws IOException;

  /**
   * Get the difference between two snapshots of a directory iteratively.
   *
   * @param snapshotRoot
   *          full path of the directory where snapshots are taken
   * @param fromSnapshot
   *          snapshot name of the from point. Null indicates the current
   *          tree
   * @param toSnapshot
   *          snapshot name of the to point. Null indicates the current
   *          tree.
   * @param startPath
   *          path relative to the snapshottable root directory from where the
   *          snapshotdiff computation needs to start across multiple rpc calls
   * @param index
   *           index in the created or deleted list of the directory at which
   *           the snapshotdiff computation stopped during the last rpc call
   *           as the no of entries exceeded the snapshotdiffentry limit. -1
   *           indicates, the snapshotdiff compuatation needs to start right
   *           from the startPath provided.
   * @return The difference report represented as a {@link SnapshotDiffReport}.
   * @throws IOException on error
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  SnapshotDiffReportListing getSnapshotDiffReportListing(String snapshotRoot,
      String fromSnapshot, String toSnapshot, byte[] startPath, int index)
      throws IOException;

  /**
   * Add a CacheDirective to the CacheManager.
   *
   * @param directive A CacheDirectiveInfo to be added
   * @param flags {@link CacheFlag}s to use for this operation.
   * @return A CacheDirectiveInfo associated with the added directive
   * @throws IOException if the directive could not be added
   */
  @AtMostOnce
  long addCacheDirective(CacheDirectiveInfo directive,
      EnumSet<CacheFlag> flags) throws IOException;

  /**
   * Modify a CacheDirective in the CacheManager.
   *
   * @param flags {@link CacheFlag}s to use for this operation.
   * @throws IOException if the directive could not be modified
   */
  @AtMostOnce
  void modifyCacheDirective(CacheDirectiveInfo directive,
      EnumSet<CacheFlag> flags) throws IOException;

  /**
   * Remove a CacheDirectiveInfo from the CacheManager.
   *
   * @param id of a CacheDirectiveInfo
   * @throws IOException if the cache directive could not be removed
   */
  @AtMostOnce
  void removeCacheDirective(long id) throws IOException;

  /**
   * List the set of cached paths of a cache pool. Incrementally fetches results
   * from the server.
   *
   * @param prevId The last listed entry ID, or -1 if this is the first call to
   *               listCacheDirectives.
   * @param filter Parameters to use to filter the list results,
   *               or null to display all directives visible to us.
   * @return A batch of CacheDirectiveEntry objects.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  BatchedEntries<CacheDirectiveEntry> listCacheDirectives(
      long prevId, CacheDirectiveInfo filter) throws IOException;

  /**
   * Add a new cache pool.
   *
   * @param info Description of the new cache pool
   * @throws IOException If the request could not be completed.
   */
  @AtMostOnce
  void addCachePool(CachePoolInfo info) throws IOException;

  /**
   * Modify an existing cache pool.
   *
   * @param req
   *          The request to modify a cache pool.
   * @throws IOException
   *          If the request could not be completed.
   */
  @AtMostOnce
  void modifyCachePool(CachePoolInfo req) throws IOException;

  /**
   * Remove a cache pool.
   *
   * @param pool name of the cache pool to remove.
   * @throws IOException if the cache pool did not exist, or could not be
   *           removed.
   */
  @AtMostOnce
  void removeCachePool(String pool) throws IOException;

  /**
   * List the set of cache pools. Incrementally fetches results from the server.
   *
   * @param prevPool name of the last pool listed, or the empty string if this
   *          is the first invocation of listCachePools
   * @return A batch of CachePoolEntry objects.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  BatchedEntries<CachePoolEntry> listCachePools(String prevPool)
      throws IOException;

  /**
   * Modifies ACL entries of files and directories.  This method can add new ACL
   * entries or modify the permissions on existing ACL entries.  All existing
   * ACL entries that are not specified in this call are retained without
   * changes.  (Modifications are merged into the current ACL.)
   */
  @Idempotent
  void modifyAclEntries(String src, List<AclEntry> aclSpec)
      throws IOException;

  /**
   * Removes ACL entries from files and directories.  Other ACL entries are
   * retained.
   */
  @Idempotent
  void removeAclEntries(String src, List<AclEntry> aclSpec)
      throws IOException;

  /**
   * Removes all default ACL entries from files and directories.
   */
  @Idempotent
  void removeDefaultAcl(String src) throws IOException;

  /**
   * Removes all but the base ACL entries of files and directories.  The entries
   * for user, group, and others are retained for compatibility with permission
   * bits.
   */
  @Idempotent
  void removeAcl(String src) throws IOException;

  /**
   * Fully replaces ACL of files and directories, discarding all existing
   * entries.
   */
  @Idempotent
  void setAcl(String src, List<AclEntry> aclSpec) throws IOException;

  /**
   * Gets the ACLs of files and directories.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  AclStatus getAclStatus(String src) throws IOException;

  /**
   * Create an encryption zone.
   */
  @AtMostOnce
  void createEncryptionZone(String src, String keyName)
    throws IOException;

  /**
   * Get the encryption zone for a path.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  EncryptionZone getEZForPath(String src)
    throws IOException;

  /**
   * Used to implement cursor-based batched listing of {@link EncryptionZone}s.
   *
   * @param prevId ID of the last item in the previous batch. If there is no
   *               previous batch, a negative value can be used.
   * @return Batch of encryption zones.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  BatchedEntries<EncryptionZone> listEncryptionZones(
      long prevId) throws IOException;

  /**
   * Used to implement re-encryption of encryption zones.
   *
   * @param zone the encryption zone to re-encrypt.
   * @param action the action for the re-encryption.
   * @throws IOException
   */
  @AtMostOnce
  void reencryptEncryptionZone(String zone, ReencryptAction action)
      throws IOException;

  /**
   * Used to implement cursor-based batched listing of
   * {@link ZoneReencryptionStatus}s.
   *
   * @param prevId ID of the last item in the previous batch. If there is no
   *               previous batch, a negative value can be used.
   * @return Batch of encryption zones.
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  BatchedEntries<ZoneReencryptionStatus> listReencryptionStatus(long prevId)
      throws IOException;

  /**
   * Set xattr of a file or directory.
   * The name must be prefixed with the namespace followed by ".". For example,
   * "user.attr".
   * <p>
   * Refer to the HDFS extended attributes user documentation for details.
   *
   * @param src file or directory
   * @param xAttr <code>XAttr</code> to set
   * @param flag set flag
   * @throws IOException
   */
  @AtMostOnce
  void setXAttr(String src, XAttr xAttr, EnumSet<XAttrSetFlag> flag)
      throws IOException;

  /**
   * Get xattrs of a file or directory. Values in xAttrs parameter are ignored.
   * If xAttrs is null or empty, this is the same as getting all xattrs of the
   * file or directory.  Only those xattrs for which the logged-in user has
   * permissions to view are returned.
   * <p>
   * Refer to the HDFS extended attributes user documentation for details.
   *
   * @param src file or directory
   * @param xAttrs xAttrs to get
   * @return <code>XAttr</code> list
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  List<XAttr> getXAttrs(String src, List<XAttr> xAttrs)
      throws IOException;

  /**
   * List the xattrs names for a file or directory.
   * Only the xattr names for which the logged in user has the permissions to
   * access will be returned.
   * <p>
   * Refer to the HDFS extended attributes user documentation for details.
   *
   * @param src file or directory
   * @return <code>XAttr</code> list
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  List<XAttr> listXAttrs(String src)
      throws IOException;

  /**
   * Remove xattr of a file or directory.Value in xAttr parameter is ignored.
   * The name must be prefixed with the namespace followed by ".". For example,
   * "user.attr".
   * <p>
   * Refer to the HDFS extended attributes user documentation for details.
   *
   * @param src file or directory
   * @param xAttr <code>XAttr</code> to remove
   * @throws IOException
   */
  @AtMostOnce
  void removeXAttr(String src, XAttr xAttr) throws IOException;

  /**
   * Checks if the user can access a path.  The mode specifies which access
   * checks to perform.  If the requested permissions are granted, then the
   * method returns normally.  If access is denied, then the method throws an
   * {@link org.apache.hadoop.security.AccessControlException}.
   * In general, applications should avoid using this method, due to the risk of
   * time-of-check/time-of-use race conditions.  The permissions on a file may
   * change immediately after the access call returns.
   *
   * @param path Path to check
   * @param mode type of access to check
   * @throws org.apache.hadoop.security.AccessControlException if access is
   *           denied
   * @throws java.io.FileNotFoundException if the path does not exist
   * @throws IOException see specific implementation
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  void checkAccess(String path, FsAction mode) throws IOException;

  /**
   * Get the highest txid the NameNode knows has been written to the edit
   * log, or -1 if the NameNode's edit log is not yet open for write. Used as
   * the starting point for the inotify event stream.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  long getCurrentEditLogTxid() throws IOException;

  /**
   * Get an ordered list of batches of events corresponding to the edit log
   * transactions for txids equal to or greater than txid.
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  EventBatchList getEditsFromTxid(long txid) throws IOException;

  /**
   * Set an erasure coding policy on a specified path.
   * @param src The path to set policy on.
   * @param ecPolicyName The erasure coding policy name.
   */
  @AtMostOnce
  void setErasureCodingPolicy(String src, String ecPolicyName)
      throws IOException;

  /**
   * Add Erasure coding policies to HDFS. For each policy input, schema and
   * cellSize are musts, name and id are ignored. They will be automatically
   * created and assigned by Namenode once the policy is successfully added, and
   * will be returned in the response.
   *
   * @param policies The user defined ec policy list to add.
   * @return Return the response list of adding operations.
   * @throws IOException
   */
  @AtMostOnce
  AddErasureCodingPolicyResponse[] addErasureCodingPolicies(
      ErasureCodingPolicy[] policies) throws IOException;

  /**
   * Remove erasure coding policy.
   * @param ecPolicyName The name of the policy to be removed.
   * @throws IOException
   */
  @AtMostOnce
  void removeErasureCodingPolicy(String ecPolicyName) throws IOException;

  /**
   * Enable erasure coding policy.
   * @param ecPolicyName The name of the policy to be enabled.
   * @throws IOException
   */
  @AtMostOnce
  void enableErasureCodingPolicy(String ecPolicyName) throws IOException;

  /**
   * Disable erasure coding policy.
   * @param ecPolicyName The name of the policy to be disabled.
   * @throws IOException
   */
  @AtMostOnce
  void disableErasureCodingPolicy(String ecPolicyName) throws IOException;

  /**
   * Get the erasure coding policies loaded in Namenode, excluding REPLICATION
   * policy.
   *
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  ErasureCodingPolicyInfo[] getErasureCodingPolicies() throws IOException;

  /**
   * Get the erasure coding codecs loaded in Namenode.
   *
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  Map<String, String> getErasureCodingCodecs() throws IOException;

  /**
   * Get the information about the EC policy for the path. Null will be returned
   * if directory or file has REPLICATION policy.
   *
   * @param src path to get the info for
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  ErasureCodingPolicy getErasureCodingPolicy(String src) throws IOException;

  /**
   * Unset erasure coding policy from a specified path.
   * @param src The path to unset policy.
   */
  @AtMostOnce
  void unsetErasureCodingPolicy(String src) throws IOException;

  /**
   * Verifies if the given policies are supported in the given cluster setup.
   * If not policy is specified checks for all enabled policies.
   * @param policyNames name of policies.
   * @return the result if the given policies are supported in the cluster setup
   * @throws IOException
   */
  @Idempotent
  @ReadOnly
  ECTopologyVerifierResult getECTopologyResultForPolicies(String... policyNames)
      throws IOException;

  /**
   * Get {@link QuotaUsage} rooted at the specified directory.
   *
   * Note: due to HDFS-6763, standby/observer doesn't keep up-to-date info
   * about quota usage, and thus even though this is ReadOnly, it can only be
   * directed to the active namenode.
   *
   * @param path The string representation of the path
   *
   * @throws AccessControlException permission denied
   * @throws java.io.FileNotFoundException file <code>path</code> is not found
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>path</code>
   *         contains a symlink.
   * @throws IOException If an I/O error occurred
   */
  @Idempotent
  @ReadOnly(activeOnly = true)
  QuotaUsage getQuotaUsage(String path) throws IOException;

  /**
   * List open files in the system in batches. INode id is the cursor and the
   * open files returned in a batch will have their INode ids greater than
   * the cursor INode id. Open files can only be requested by super user and
   * the the list across batches are not atomic.
   *
   * @param prevId the cursor INode id.
   * @throws IOException
   */
  @Idempotent
  @Deprecated
  @ReadOnly(isCoordinated = true)
  BatchedEntries<OpenFileEntry> listOpenFiles(long prevId) throws IOException;

  /**
   * List open files in the system in batches. INode id is the cursor and the
   * open files returned in a batch will have their INode ids greater than
   * the cursor INode id. Open files can only be requested by super user and
   * the the list across batches are not atomic.
   *
   * @param prevId the cursor INode id.
   * @param openFilesTypes types to filter the open files.
   * @param path path to filter the open files.
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(isCoordinated = true)
  BatchedEntries<OpenFileEntry> listOpenFiles(long prevId,
      EnumSet<OpenFilesType> openFilesTypes, String path) throws IOException;

  /**
   * Get HA service state of the server.
   *
   * @return server HA state
   * @throws IOException
   */
  @Idempotent
  @ReadOnly
  HAServiceProtocol.HAServiceState getHAServiceState() throws IOException;

  /**
   * Called by client to wait until the server has reached the state id of the
   * client. The client and server state id are given by client side and server
   * side alignment context respectively. This can be a blocking call.
   *
   * @throws IOException
   */
  @Idempotent
  @ReadOnly(activeOnly = true)
  void msync() throws IOException;

  /**
   * Satisfy the storage policy for a file/directory.
   * @param path Path of an existing file/directory.
   * @throws AccessControlException If access is denied.
   * @throws org.apache.hadoop.fs.UnresolvedLinkException if <code>src</code>
   *           contains a symlink.
   * @throws java.io.FileNotFoundException If file/dir <code>src</code> is not
   *           found.
   * @throws org.apache.hadoop.hdfs.server.namenode.SafeModeException append not
   *           allowed in safemode.
   */
  @AtMostOnce
  void satisfyStoragePolicy(String path) throws IOException;
}

这么多方法我们是无法一一分析的，我们可以分下类：

1. HDFS文件读相关的操作。
2. HDFS文件写以及追加写的相关操作。
3. 管理HDFS命名空间的相关操作。
4. 系统问题与管理相关的操作。
5. 快照相关的操作。
6. 缓存相关的操作。
7. 其他操作。

读数据相关方法

ClientProtocol中与客户端读取文件相关的方法有两个：getBlockLocations()和reportBadBlocks()。

getBlockLocations()方法可以获取HDFS文件执行范围内所有数据块的位置信息，这个方法的参数是HDFS文件的文件名以及读取范围，返回值是文件指定范围内所有数据块的文件名以及它们的位置信息，使用了LocatedBlocks对象来封装。每个数据块的位置信息指的是存储这个数据块副本的所有DataNode的信息，这些DataNode会以与当前客户端的距离远近排序。在读取到位置信息后，在从返回的数据节点读取真正的信息：

@Idempotent
  @ReadOnly(atimeAffected = true, isCoordinated = true)
  LocatedBlocks getBlockLocations(String src, long offset, long length)
      throws IOException;

客户端会调用reportBadBlocks()方法向NameNode汇报错误的数据块，当客户端从数据节点读取数据块且发现数据块的校验和不正确时，就会调用这个方法向NameNode汇报这个错误的数据块信息：

@Idempotent
  void reportBadBlocks(LocatedBlock[] blocks) throws IOException;

写、追加写数据的相关方法

在HDFS客户端操作中最重要的一个部分就是写入一个新的HDFS文件，或者打开一个已有的HDFS文件并执行追加写操作。ClientProtocol里面定义了8个通俗易懂的方法支持HDFS文件的写操作：create()、append()、addBlock、complete()、abandonBlock()、getAddtionnalDatanodes()、updateBlockForPipeline()和updatePipeline()。