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     */
018    package org.apache.hadoop.hdfs;
019    
020    import java.io.IOException;
021    import java.net.Socket;
022    
023    /**
024     * A BlockReader is responsible for reading a single block
025     * from a single datanode.
026     */
027    public interface BlockReader {
028    
029      /* same interface as inputStream java.io.InputStream#read()
030       * used by DFSInputStream#read()
031       * This violates one rule when there is a checksum error:
032       * "Read should not modify user buffer before successful read"
033       * because it first reads the data to user buffer and then checks
034       * the checksum.
035       */
036      int read(byte[] buf, int off, int len) throws IOException;
037    
038      /**
039       * Skip the given number of bytes
040       */
041      long skip(long n) throws IOException;
042    
043      void close() throws IOException;
044    
045      /**
046       * Read exactly the given amount of data, throwing an exception
047       * if EOF is reached before that amount
048       */
049      void readFully(byte[] buf, int readOffset, int amtToRead) throws IOException;
050    
051      /**
052       * Similar to {@link #readFully(byte[], int, int)} except that it will
053       * not throw an exception on EOF. However, it differs from the simple
054       * {@link #read(byte[], int, int)} call in that it is guaranteed to
055       * read the data if it is available. In other words, if this call
056       * does not throw an exception, then either the buffer has been
057       * filled or the next call will return EOF.
058       */
059      int readAll(byte[] buf, int offset, int len) throws IOException;
060    
061      /**
062       * Take the socket used to talk to the DN.
063       */
064      Socket takeSocket();
065    
066      /**
067       * Whether the BlockReader has reached the end of its input stream
068       * and successfully sent a status code back to the datanode.
069       */
070      boolean hasSentStatusCode();
071    
072    }