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.client.ClientMmap;
026import org.apache.hadoop.hdfs.client.ClientMmapManager;
027import org.apache.hadoop.hdfs.protocol.LocatedBlock;
028
029/**
030 * A BlockReader is responsible for reading a single block
031 * from a single datanode.
032 */
033public interface BlockReader extends ByteBufferReadable {
034  
035
036  /* same interface as inputStream java.io.InputStream#read()
037   * used by DFSInputStream#read()
038   * This violates one rule when there is a checksum error:
039   * "Read should not modify user buffer before successful read"
040   * because it first reads the data to user buffer and then checks
041   * the checksum.
042   * Note: this must return -1 on EOF, even in the case of a 0-byte read.
043   * See HDFS-5762 for details.
044   */
045  int read(byte[] buf, int off, int len) throws IOException;
046
047  /**
048   * Skip the given number of bytes
049   */
050  long skip(long n) throws IOException;
051
052  /**
053   * Returns an estimate of the number of bytes that can be read
054   * (or skipped over) from this input stream without performing
055   * network I/O.
056   */
057  int available() throws IOException;
058
059  /**
060   * Close the block reader.
061   *
062   * @throws IOException
063   */
064  void close() throws IOException;
065
066  /**
067   * Read exactly the given amount of data, throwing an exception
068   * if EOF is reached before that amount
069   */
070  void readFully(byte[] buf, int readOffset, int amtToRead) throws IOException;
071
072  /**
073   * Similar to {@link #readFully(byte[], int, int)} except that it will
074   * not throw an exception on EOF. However, it differs from the simple
075   * {@link #read(byte[], int, int)} call in that it is guaranteed to
076   * read the data if it is available. In other words, if this call
077   * does not throw an exception, then either the buffer has been
078   * filled or the next call will return EOF.
079   */
080  int readAll(byte[] buf, int offset, int len) throws IOException;
081
082  /**
083   * @return              true only if this is a local read.
084   */
085  boolean isLocal();
086  
087  /**
088   * @return              true only if this is a short-circuit read.
089   *                      All short-circuit reads are also local.
090   */
091  boolean isShortCircuit();
092
093  /**
094   * Get a ClientMmap object for this BlockReader.
095   *
096   * @param opts          The read options to use.
097   * @return              The ClientMmap object, or null if mmap is not
098   *                      supported.
099   */
100  ClientMmap getClientMmap(EnumSet<ReadOption> opts,
101        ClientMmapManager mmapManager);
102}