001/**
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *     http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018package org.apache.hadoop.hdfs;
019
020import java.io.IOException;
021import java.util.EnumSet;
022
023import org.apache.hadoop.fs.ByteBufferReadable;
024import org.apache.hadoop.fs.ReadOption;
025import org.apache.hadoop.hdfs.shortcircuit.ClientMmap;
026
027/**
028 * A BlockReader is responsible for reading a single block
029 * from a single datanode.
030 */
031public interface BlockReader extends ByteBufferReadable {
032  
033
034  /* same interface as inputStream java.io.InputStream#read()
035   * used by DFSInputStream#read()
036   * This violates one rule when there is a checksum error:
037   * "Read should not modify user buffer before successful read"
038   * because it first reads the data to user buffer and then checks
039   * the checksum.
040   * Note: this must return -1 on EOF, even in the case of a 0-byte read.
041   * See HDFS-5762 for details.
042   */
043  int read(byte[] buf, int off, int len) throws IOException;
044
045  /**
046   * Skip the given number of bytes
047   */
048  long skip(long n) throws IOException;
049
050  /**
051   * Returns an estimate of the number of bytes that can be read
052   * (or skipped over) from this input stream without performing
053   * network I/O.
054   * This may return more than what is actually present in the block.
055   */
056  int available() throws IOException;
057
058  /**
059   * Close the block reader.
060   *
061   * @throws IOException
062   */
063  void close() throws IOException;
064
065  /**
066   * Read exactly the given amount of data, throwing an exception
067   * if EOF is reached before that amount
068   */
069  void readFully(byte[] buf, int readOffset, int amtToRead) throws IOException;
070
071  /**
072   * Similar to {@link #readFully(byte[], int, int)} except that it will
073   * not throw an exception on EOF. However, it differs from the simple
074   * {@link #read(byte[], int, int)} call in that it is guaranteed to
075   * read the data if it is available. In other words, if this call
076   * does not throw an exception, then either the buffer has been
077   * filled or the next call will return EOF.
078   */
079  int readAll(byte[] buf, int offset, int len) throws IOException;
080
081  /**
082   * @return              true only if this is a local read.
083   */
084  boolean isLocal();
085  
086  /**
087   * @return              true only if this is a short-circuit read.
088   *                      All short-circuit reads are also local.
089   */
090  boolean isShortCircuit();
091
092  /**
093   * Get a ClientMmap object for this BlockReader.
094   *
095   * @param opts          The read options to use.
096   * @return              The ClientMmap object, or null if mmap is not
097   *                      supported.
098   */
099  ClientMmap getClientMmap(EnumSet<ReadOption> opts);
100}