【大数据笔记】--续谈WordCount的Bug

在之前的Blog [http://flyfoxs.iteye.com/blog/2110463]  中讨论了, hadoop在文件切割时,可能会把一个行数据切割成无意义的2块. 如果不做特别处理,这会造成数据的失真及处理错误. 经人指点,发现这个BUG不存在.

 

Hadoop在分割文件后,后期读取中会通过一些规则来保证不会出现把一行数据分割成2行. 下面对这个后期处理机制(LineRecordReader)做一个分析:

 

1)数据分割是由JobClient完成,不是在hadoop集群完成.(并且这个是一个粗分,具体精确的还是依赖Mapper依赖如下规则)

2)数据的分割是由JobClient完成,但是Mapper在处理的时候,不是严格按照这个来处理,

除了第一个Split,其他的Split都是从第一个换行符开始读取

Split的结束是下一个Split的换行符,(太霸道了,除了最后一个,几乎每一都要跨越Split) 

3)针对超长行,有一个理论上的Bug,就是如果有行超过了你限制的长度,那么这一行会有部分数据会被抛弃. 但是这个Bug是理论上的,因为默认值为  Integer.MAX_VALUE .

 

    this.maxLineLength = job.getInt("mapred.linerecordreader.maxlength",  Integer.MAX_VALUE);

 

 

下面的代码可以看出LineRecordReader读取最后一行的时候,并不是严格按照Split的结束而结束. 而是必须要读取到下一个Split的换行符.

代码比较复杂已经添加了注释,如果有不明白的欢迎提问.

 

  public int readLine(Text str, int maxLineLength,
                      int maxBytesToConsume) throws IOException {
    /* We're reading data from in, but the head of the stream may be
     * already buffered in buffer, so we have several cases:
     * 1. No newline characters are in the buffer, so we need to copy
     *    everything and read another buffer from the stream.
     * 2. An unambiguously terminated line is in buffer, so we just
     *    copy to str.
     * 3. Ambiguously terminated line is in buffer, i.e. buffer ends
     *    in CR.  In this case we copy everything up to CR to str, but
     *    we also need to see what follows CR: if it's LF, then we
     *    need consume LF as well, so next call to readLine will read
     *    from after that.
     * We use a flag prevCharCR to signal if previous character was CR
     * and, if it happens to be at the end of the buffer, delay
     * consuming it until we have a chance to look at the char that
     * follows.
     */
    str.clear();
    int txtLength = 0; //tracks str.getLength(), as an optimization
    int newlineLength = 0; //length of terminating newline
    boolean prevCharCR = false; //true of prev char was CR
    long bytesConsumed = 0;
    do {
      //bufferPosn记录了当前Buffer读取到哪个位置,这样当下一次循环时
      int startPosn = bufferPosn; //starting from where we left off the last time
      
      //如果Buffer里面的数据已经处理完毕,则对Buffer清空,重新再从IO流读取数据
      if (bufferPosn >= bufferLength) {
        startPosn = bufferPosn = 0;
        if (prevCharCR)
          ++bytesConsumed; //account for CR from previous read

        //从IO中读取数据处理,只有处理完毕(bufferPosn >= bufferLength)才会再次读取
        //bufferLength记录了从IO中读取了多少个字节的数据
        bufferLength = in.read(buffer);
        if (bufferLength <= 0)
          break; // EOF
      }
      //在For循环总寻找断行符, 兼容MAC, Windows, Linux 多种平台的换行符
      for (; bufferPosn < bufferLength; ++bufferPosn) { //search for newline
    	//判断当前字符是否是'\n'  
        if (buffer[bufferPosn] == LF) {
          //如果是'\r\n'来区分一行, 	newlineLength=2, 如果是'\n'则newlineLength=1
          newlineLength = (prevCharCR) ? 2 : 1;
          ++bufferPosn; // at next invocation proceed from following byte
          break;
        }
        //如果是\r来区分一行,则newlineLength=1
        if (prevCharCR) { //CR + notLF, we are at notLF
          newlineLength = 1;
          break;
        }
        //判断当前字符是否是'\r', 等待下一个循环来组合判断真正的换行符
        prevCharCR = (buffer[bufferPosn] == CR);
      }
      int readLength = bufferPosn - startPosn;
      
      //Buffer最后一个字节就是'\r'
      if (prevCharCR && newlineLength == 0)
        --readLength; //CR at the end of the buffer
      
      
      bytesConsumed += readLength;
      
      //appendLength:在本轮循环中从Buffer中读取的负载长度,去除了换行符
      int appendLength = readLength - newlineLength;
      
      //txtLength:记录了最终返回的str的长度      
      if (appendLength > maxLineLength - txtLength) {
    	//如果添加后,字符串长度超过了一行长度的上限,那么超过的将不会被添加到str
        appendLength = maxLineLength - txtLength;
      }
      
     //将当前Buffer中,指定区间的字符添加到返回值(str)
      if (appendLength > 0) {
        str.append(buffer, startPosn, appendLength);
        txtLength += appendLength;
      }
    //如果在buffer里面没有读取到换行符,并且已经读取的字节数没有超过预定大小,则继续从IO流读取下一批数据  
    } while (newlineLength == 0 && bytesConsumed < maxBytesToConsume);

    if (bytesConsumed > (long)Integer.MAX_VALUE)
      throw new IOException("Too many bytes before newline: " + bytesConsumed);    
    return (int)bytesConsumed;
  }

 

 

 

 

下面的代码可以看出LineRecordReader是如何来判读是否需要忽略第一行

 

public void initialize(InputSplit genericSplit,
                         TaskAttemptContext context) throws IOException {
    FileSplit split = (FileSplit) genericSplit;
    Configuration job = context.getConfiguration();
    this.maxLineLength = job.getInt("mapred.linerecordreader.maxlength",
                                    Integer.MAX_VALUE);
    start = split.getStart();
    end = start + split.getLength();
    final Path file = split.getPath();
    compressionCodecs = new CompressionCodecFactory(job);
    final CompressionCodec codec = compressionCodecs.getCodec(file);

    // open the file and seek to the start of the split
    FileSystem fs = file.getFileSystem(job);
    FSDataInputStream fileIn = fs.open(split.getPath());
    boolean skipFirstLine = false;
    if (codec != null) {
      in = new LineReader(codec.createInputStream(fileIn), job);
      end = Long.MAX_VALUE;
    } else {
      if (start != 0) {
       //只有文件的第一行不能忽略第一行
        skipFirstLine = true;
        --start;
        fileIn.seek(start);
      }
      in = new LineReader(fileIn, job);
    }
    if (skipFirstLine) {  // skip first line and re-establish "start".
      start += in.readLine(new Text(), 0,
                           (int)Math.min((long)Integer.MAX_VALUE, end - start));
    }
    this.pos = start;
  }

 

 

 

 参考文献:

http://blog.csdn.net/bluishglc/article/details/9380087

http://blog.csdn.net/wanghai__/article/details/6583364