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    
022    import org.apache.hadoop.fs.ByteBufferReadable;
023    
024    /**
025     * A BlockReader is responsible for reading a single block
026     * from a single datanode.
027     */
028    public interface BlockReader extends ByteBufferReadable {
029    
030      /* same interface as inputStream java.io.InputStream#read()
031       * used by DFSInputStream#read()
032       * This violates one rule when there is a checksum error:
033       * "Read should not modify user buffer before successful read"
034       * because it first reads the data to user buffer and then checks
035       * the checksum.
036       */
037      int read(byte[] buf, int off, int len) throws IOException;
038    
039      /**
040       * Skip the given number of bytes
041       */
042      long skip(long n) throws IOException;
043    
044      /**
045       * Returns an estimate of the number of bytes that can be read
046       * (or skipped over) from this input stream without performing
047       * network I/O.
048       */
049      int available() throws IOException;
050    
051      /**
052       * Close the block reader.
053       *
054       * @throws IOException
055       */
056      void close() throws IOException;
057    
058      /**
059       * Read exactly the given amount of data, throwing an exception
060       * if EOF is reached before that amount
061       */
062      void readFully(byte[] buf, int readOffset, int amtToRead) throws IOException;
063    
064      /**
065       * Similar to {@link #readFully(byte[], int, int)} except that it will
066       * not throw an exception on EOF. However, it differs from the simple
067       * {@link #read(byte[], int, int)} call in that it is guaranteed to
068       * read the data if it is available. In other words, if this call
069       * does not throw an exception, then either the buffer has been
070       * filled or the next call will return EOF.
071       */
072      int readAll(byte[] buf, int offset, int len) throws IOException;
073    
074      /**
075       * @return              true only if this is a local read.
076       */
077      boolean isLocal();
078      
079      /**
080       * @return              true only if this is a short-circuit read.
081       *                      All short-circuit reads are also local.
082       */
083      boolean isShortCircuit();
084    }