org.apache.hadoop.hdfs.BlockReader.java Source code

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Here is the source code for org.apache.hadoop.hdfs.BlockReader.java

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/**
 * 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;

import java.io.Closeable;
import java.io.IOException;
import java.util.EnumSet;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.fs.ByteBufferReadable;
import org.apache.hadoop.fs.ReadOption;
import org.apache.hadoop.hdfs.shortcircuit.ClientMmap;
import org.apache.hadoop.util.DataChecksum;

/**
 * A BlockReader is responsible for reading a single block
 * from a single datanode.
 */
@InterfaceAudience.Private
public interface BlockReader extends ByteBufferReadable, Closeable {

    /* same interface as inputStream java.io.InputStream#read()
     * used by DFSInputStream#read()
     * This violates one rule when there is a checksum error:
     * "Read should not modify user buffer before successful read"
     * because it first reads the data to user buffer and then checks
     * the checksum.
     * Note: this must return -1 on EOF, even in the case of a 0-byte read.
     * See HDFS-5762 for details.
     */
    int read(byte[] buf, int off, int len) throws IOException;

    /**
     * Skip the given number of bytes
     */
    long skip(long n) throws IOException;

    /**
     * Returns an estimate of the number of bytes that can be read
     * (or skipped over) from this input stream without performing
     * network I/O.
     * This may return more than what is actually present in the block.
     */
    int available();

    /**
     * Close the block reader.
     *
     * @throws IOException
     */
    @Override // java.io.Closeable
    void close() throws IOException;

    /**
     * Read exactly the given amount of data, throwing an exception
     * if EOF is reached before that amount
     */
    void readFully(byte[] buf, int readOffset, int amtToRead) throws IOException;

    /**
     * Similar to {@link #readFully(byte[], int, int)} except that it will
     * not throw an exception on EOF. However, it differs from the simple
     * {@link #read(byte[], int, int)} call in that it is guaranteed to
     * read the data if it is available. In other words, if this call
     * does not throw an exception, then either the buffer has been
     * filled or the next call will return EOF.
     */
    int readAll(byte[] buf, int offset, int len) throws IOException;

    /**
     * @return              true only if this is a short-circuit read.
     *                      All short-circuit reads are also local.
     */
    boolean isShortCircuit();

    /**
     * Get a ClientMmap object for this BlockReader.
     *
     * @param opts          The read options to use.
     * @return              The ClientMmap object, or null if mmap is not
     *                      supported.
     */
    ClientMmap getClientMmap(EnumSet<ReadOption> opts);

    /**
     * @return              The DataChecksum used by the read block
     */
    DataChecksum getDataChecksum();

    /**
     * Return the network distance between local machine and the remote machine.
     */
    int getNetworkDistance();
}