【原創】大數據基礎之Spark（7）spark讀取文件split過程（即RDD分區數量）

spark 2.1.1

spark初始化rdd的時候，須要讀取文件，一般是hdfs文件，在讀文件的時候能夠指定最小partition數量，這裏只是建議的數量，實際可能比這個要大（好比文件特別多或者特別大時），也可能比這個要小（好比文件只有一個並且很小時），若是沒有指定最小partition數量，初始化完成的rdd默認有多少個partition是怎樣決定的呢？

以SparkContext.textfile爲例來看下代碼：

org.apache.spark.SparkContext

  /**
   * Read a text file from HDFS, a local file system (available on all nodes), or any
   * Hadoop-supported file system URI, and return it as an RDD of Strings.
   */
  def textFile(
      path: String,
      minPartitions: Int = defaultMinPartitions): RDD[String] = withScope {
    assertNotStopped()
    hadoopFile(path, classOf[TextInputFormat], classOf[LongWritable], classOf[Text],
      minPartitions).map(pair => pair._2.toString).setName(path)
  }

  /**
   * Default min number of partitions for Hadoop RDDs when not given by user
   * Notice that we use math.min so the "defaultMinPartitions" cannot be higher than 2.
   * The reasons for this are discussed in https://github.com/mesos/spark/pull/718
   */
  def defaultMinPartitions: Int = math.min(defaultParallelism, 2)

  /** Get an RDD for a Hadoop file with an arbitrary InputFormat
   *
   * @note Because Hadoop's RecordReader class re-uses the same Writable object for each
   * record, directly caching the returned RDD or directly passing it to an aggregation or shuffle
   * operation will create many references to the same object.
   * If you plan to directly cache, sort, or aggregate Hadoop writable objects, you should first
   * copy them using a `map` function.
   */
  def hadoopFile[K, V](
      path: String,
      inputFormatClass: Class[_ <: InputFormat[K, V]],
      keyClass: Class[K],
      valueClass: Class[V],
      minPartitions: Int = defaultMinPartitions): RDD[(K, V)] = withScope {
    assertNotStopped()

    // This is a hack to enforce loading hdfs-site.xml.
    // See SPARK-11227 for details.
    FileSystem.getLocal(hadoopConfiguration)

    // A Hadoop configuration can be about 10 KB, which is pretty big, so broadcast it.
    val confBroadcast = broadcast(new SerializableConfiguration(hadoopConfiguration))
    val setInputPathsFunc = (jobConf: JobConf) => FileInputFormat.setInputPaths(jobConf, path)
    new HadoopRDD(
      this,
      confBroadcast,
      Some(setInputPathsFunc),
      inputFormatClass,
      keyClass,
      valueClass,
      minPartitions).setName(path)
  }

可見會直接返回一個HadoopRDD，若是不傳最小partition數量，會使用defaultMinPartitions（一般狀況下是2），那麼HadoopRDD是怎樣實現的？

org.apache.spark.rdd.HadoopRDD

class HadoopRDD[K, V](
    sc: SparkContext,
    broadcastedConf: Broadcast[SerializableConfiguration],
    initLocalJobConfFuncOpt: Option[JobConf => Unit],
    inputFormatClass: Class[_ <: InputFormat[K, V]],
    keyClass: Class[K],
    valueClass: Class[V],
    minPartitions: Int)
  extends RDD[(K, V)](sc, Nil) with Logging {
...
  override def getPartitions: Array[Partition] = {
    val jobConf = getJobConf()
    // add the credentials here as this can be called before SparkContext initialized
    SparkHadoopUtil.get.addCredentials(jobConf)
    val inputFormat = getInputFormat(jobConf)
    val inputSplits = inputFormat.getSplits(jobConf, minPartitions)
    val array = new Array[Partition](inputSplits.size)
    for (i <- 0 until inputSplits.size) {
      array(i) = new HadoopPartition(id, i, inputSplits(i))
    }
    array
  }
...
  protected def getInputFormat(conf: JobConf): InputFormat[K, V] = {
    val newInputFormat = ReflectionUtils.newInstance(inputFormatClass.asInstanceOf[Class[_]], conf)
      .asInstanceOf[InputFormat[K, V]]
    newInputFormat match {
      case c: Configurable => c.setConf(conf)
      case _ =>
    }
    newInputFormat
  }

決定分區數量的邏輯在getPartitions中，實際上調用的是InputFormat.getSplits，InputFormat是一個接口，

org.apache.hadoop.mapred.InputFormat

public interface InputFormat<K, V> {
    InputSplit[] getSplits(JobConf var1, int var2) throws IOException;

    RecordReader<K, V> getRecordReader(InputSplit var1, JobConf var2, Reporter var3) throws IOException;
}

每種文件格式都有本身的實現類，常見的文件格式avro、orc、parquet、textfile對應的實現類爲AvroInputFormat，OrcInputFormat，MapredParquetInputFormat，CombineTextInputFormat，每一個實現類都有本身的split邏輯，來看下默認實現：

org.apache.hadoop.mapred.FileInputFormat

  /** Splits files returned by {@link #listStatus(JobConf)} when
   * they're too big.*/ 
  public InputSplit[] getSplits(JobConf job, int numSplits)
    throws IOException {
    FileStatus[] files = listStatus(job);
    
    // Save the number of input files for metrics/loadgen
    job.setLong(NUM_INPUT_FILES, files.length);
    long totalSize = 0;                           // compute total size
    for (FileStatus file: files) {                // check we have valid files
      if (file.isDirectory()) {
        throw new IOException("Not a file: "+ file.getPath());
      }
      totalSize += file.getLen();
    }

    long goalSize = totalSize / (numSplits == 0 ? 1 : numSplits);
    long minSize = Math.max(job.getLong(org.apache.hadoop.mapreduce.lib.input.
      FileInputFormat.SPLIT_MINSIZE, 1), minSplitSize);

    // generate splits
    ArrayList<FileSplit> splits = new ArrayList<FileSplit>(numSplits);
    NetworkTopology clusterMap = new NetworkTopology();
    for (FileStatus file: files) {
      Path path = file.getPath();
      long length = file.getLen();
      if (length != 0) {
        FileSystem fs = path.getFileSystem(job);
        BlockLocation[] blkLocations;
        if (file instanceof LocatedFileStatus) {
          blkLocations = ((LocatedFileStatus) file).getBlockLocations();
        } else {
          blkLocations = fs.getFileBlockLocations(file, 0, length);
        }
        if (isSplitable(fs, path)) {
          long blockSize = file.getBlockSize();
          long splitSize = computeSplitSize(goalSize, minSize, blockSize);

          long bytesRemaining = length;
          while (((double) bytesRemaining)/splitSize > SPLIT_SLOP) {
            String[] splitHosts = getSplitHosts(blkLocations,
                length-bytesRemaining, splitSize, clusterMap);
            splits.add(makeSplit(path, length-bytesRemaining, splitSize,
                splitHosts));
            bytesRemaining -= splitSize;
          }

          if (bytesRemaining != 0) {
            String[] splitHosts = getSplitHosts(blkLocations, length
                - bytesRemaining, bytesRemaining, clusterMap);
            splits.add(makeSplit(path, length - bytesRemaining, bytesRemaining,
                splitHosts));
          }
        } else {
          String[] splitHosts = getSplitHosts(blkLocations,0,length,clusterMap);
          splits.add(makeSplit(path, 0, length, splitHosts));
        }
      } else { 
        //Create empty hosts array for zero length files
        splits.add(makeSplit(path, 0, length, new String[0]));
      }
    }
    LOG.debug("Total # of splits: " + splits.size());
    return splits.toArray(new FileSplit[splits.size()]);
  }
  
  /** 
   * This function identifies and returns the hosts that contribute 
   * most for a given split. For calculating the contribution, rack
   * locality is treated on par with host locality, so hosts from racks
   * that contribute the most are preferred over hosts on racks that 
   * contribute less
   * @param blkLocations The list of block locations
   * @param offset 
   * @param splitSize 
   * @return array of hosts that contribute most to this split
   * @throws IOException
   */
  protected String[] getSplitHosts(BlockLocation[] blkLocations, 
      long offset, long splitSize, NetworkTopology clusterMap)
  throws IOException {

    int startIndex = getBlockIndex(blkLocations, offset);

    long bytesInThisBlock = blkLocations[startIndex].getOffset() + 
                          blkLocations[startIndex].getLength() - offset;

    //If this is the only block, just return
    if (bytesInThisBlock >= splitSize) {
      return blkLocations[startIndex].getHosts();
    }

    long bytesInFirstBlock = bytesInThisBlock;
    int index = startIndex + 1;
    splitSize -= bytesInThisBlock;

    while (splitSize > 0) {
      bytesInThisBlock =
        Math.min(splitSize, blkLocations[index++].getLength());
      splitSize -= bytesInThisBlock;
    }

    long bytesInLastBlock = bytesInThisBlock;
    int endIndex = index - 1;
    
    Map <Node,NodeInfo> hostsMap = new IdentityHashMap<Node,NodeInfo>();
    Map <Node,NodeInfo> racksMap = new IdentityHashMap<Node,NodeInfo>();
    String [] allTopos = new String[0];

    // Build the hierarchy and aggregate the contribution of 
    // bytes at each level. See TestGetSplitHosts.java 

    for (index = startIndex; index <= endIndex; index++) {

      // Establish the bytes in this block
      if (index == startIndex) {
        bytesInThisBlock = bytesInFirstBlock;
      }
      else if (index == endIndex) {
        bytesInThisBlock = bytesInLastBlock;
      }
      else {
        bytesInThisBlock = blkLocations[index].getLength();
      }
      
      allTopos = blkLocations[index].getTopologyPaths();

      // If no topology information is available, just
      // prefix a fakeRack
      if (allTopos.length == 0) {
        allTopos = fakeRacks(blkLocations, index);
      }

      // NOTE: This code currently works only for one level of
      // hierarchy (rack/host). However, it is relatively easy
      // to extend this to support aggregation at different
      // levels 
      
      for (String topo: allTopos) {

        Node node, parentNode;
        NodeInfo nodeInfo, parentNodeInfo;

        node = clusterMap.getNode(topo);

        if (node == null) {
          node = new NodeBase(topo);
          clusterMap.add(node);
        }
        
        nodeInfo = hostsMap.get(node);
        
        if (nodeInfo == null) {
          nodeInfo = new NodeInfo(node);
          hostsMap.put(node,nodeInfo);
          parentNode = node.getParent();
          parentNodeInfo = racksMap.get(parentNode);
          if (parentNodeInfo == null) {
            parentNodeInfo = new NodeInfo(parentNode);
            racksMap.put(parentNode,parentNodeInfo);
          }
          parentNodeInfo.addLeaf(nodeInfo);
        }
        else {
          nodeInfo = hostsMap.get(node);
          parentNode = node.getParent();
          parentNodeInfo = racksMap.get(parentNode);
        }

        nodeInfo.addValue(index, bytesInThisBlock);
        parentNodeInfo.addValue(index, bytesInThisBlock);

      } // for all topos
    
    } // for all indices

    return identifyHosts(allTopos.length, racksMap);
  }

大體過程以下：

getSplits首先會拿到全部須要讀取的file列表，而後會迭代這個file列表，首先看一個file是否能夠再分即isSplitable（默認是true可能被子類覆蓋），若是不能再split則直接做爲1個split，若是能夠再split，則獲取這個file的block信息，而後綜合根據多個參數來計算出1個split的數據大小即splitSize，而後會將這個file的全部block劃分爲多個split，劃分過程會考慮機架、host等因素，若是是大block，則直接做爲一個split，若是是小block可能多個block合併在一個split裏（這樣可以儘可能減小split數量），最終獲得的split數量即partition數量；

注意：上邊的過程可能被子類覆蓋；