#1 - o.a.s.shuffle.sort.BypassMergeSortShuffleWriter.write()方法

有意思的是，BypassMergeSortShuffleWriter类是用Java写的，而不是Scala。

  @Override
  public void write(Iterator<Product2<K, V>> records) throws IOException {
    assert (partitionWriters == null);

    //【如果没有数据】
    if (!records.hasNext()) {
      //【写一份空的输出】
      partitionLengths = new long[numPartitions];
      shuffleBlockResolver.writeIndexFileAndCommit(shuffleId, mapId, partitionLengths, null);
      mapStatus = MapStatus$.MODULE$.apply(blockManager.shuffleServerId(), partitionLengths);
      return;
    }

    final SerializerInstance serInstance = serializer.newInstance();
    final long openStartTime = System.nanoTime();

    //【注意会有numPartitions（分区数）个DiskBlockObjectWriter以及FileSegment】
    partitionWriters = new DiskBlockObjectWriter[numPartitions];
    partitionWriterSegments = new FileSegment[numPartitions];

    for (int i = 0; i < numPartitions; i++) {
      //【创建临时块。在上一篇文章的代码#5中做了同样的事】
      final Tuple2<TempShuffleBlockId, File> tempShuffleBlockIdPlusFile =
        blockManager.diskBlockManager().createTempShuffleBlock();
      //【获取块ID和对应文件】
      final File file = tempShuffleBlockIdPlusFile._2();
      final BlockId blockId = tempShuffleBlockIdPlusFile._1();
      //【对每个分区都创建一个DiskBlockObjectWriter。fileBufferSize仍然对应spark.shuffle.file.buffer参数】
      partitionWriters[i] =
        blockManager.getDiskWriter(blockId, file, serInstance, fileBufferSize, writeMetrics);
    }
    // Creating the file to write to and creating a disk writer both involve interacting with
    // the disk, and can take a long time in aggregate when we open many files, so should be
    // included in the shuffle write time.
    writeMetrics.incWriteTime(System.nanoTime() - openStartTime);

    //【如果有数据】
    while (records.hasNext()) {
      final Product2<K, V> record = records.next();
      final K key = record._1();
      //【按key对应的分区，分别写入对应的文件】
      partitionWriters[partitioner.getPartition(key)].write(key, record._2());
    }

    for (int i = 0; i < numPartitions; i++) {
      final DiskBlockObjectWriter writer = partitionWriters[i];
      //【提交写操作，最终获得numPartition个FileSegment】
      partitionWriterSegments[i] = writer.commitAndGet();
      writer.close();
    }

    //【创建输出数据文件与临时数据文件】
    File output = shuffleBlockResolver.getDataFile(shuffleId, mapId);
    File tmp = Utils.tempFileWith(output);
    try {
      //【#2 - 将上面的许多个分区文件合并到临时文件】
      partitionLengths = writePartitionedFile(tmp);
      //【创建索引文件。writeIndexFileAndCommit()方法在上一篇文章中的代码段#9有详细解释】
      shuffleBlockResolver.writeIndexFileAndCommit(shuffleId, mapId, partitionLengths, tmp);
    } finally {
      if (tmp.exists() && !tmp.delete()) {
        logger.error("Error while deleting temp file {}", tmp.getAbsolutePath());
      }
    }
    //【填充MapStatus结果】
    mapStatus = MapStatus$.MODULE$.apply(blockManager.shuffleServerId(), partitionLengths);
  }

之前已经讲过触发bypass机制的条件。从代码可见，在bypass机制下，shuffle write的流程大大简化了。中间没有类似PartitionedAppendOnlyMap那样的缓存（因为没有map端预聚合），也没有数据方面的排序，直接按分区写一批中间数据文件（因为分区数会小于阈值spark.shuffle.sort.bypassMergeThreshold，不会产生过多），然后将它们合并。这种方式实际上颇有一些借鉴hash shuffle的意味。

bypass的含义是“旁路”“支线”，这也符合其绕过了缓存和排序的特征。虽然它中途产生的文件可能会比普通sort shuffle还多，但胜在数据量少，逻辑简单，因此在阈值合适的情况下速度也很快。

下面的方法用于合并文件，同样简单粗暴。

#2 - o.a.s.shuffle.sort.BypassMergeSortShuffleWriter.writePartitionedFile()方法

  /**
   * Concatenate all of the per-partition files into a single combined file.
   *
   * @return array of lengths, in bytes, of each partition of the file (used by map output tracker).
   */
  private long[] writePartitionedFile(File outputFile) throws IOException {
    // Track location of the partition starts in the output file
    final long[] lengths = new long[numPartitions];
    if (partitionWriters == null) {
      // We were passed an empty iterator
      return lengths;
    }

    //【创建合并到临时文件的输出流】
    final FileOutputStream out = new FileOutputStream(outputFile, true);
    final long writeStartTime = System.nanoTime();
    boolean threwException = true;
    try {
      for (int i = 0; i < numPartitions; i++) {
        //【获取FileSegment对应的文件】
        final File file = partitionWriterSegments[i].file();
        if (file.exists()) {
          final FileInputStream in = new FileInputStream(file);
          boolean copyThrewException = true;
          try {
            //【将产生的碎文件复制合并到临时文件中去，并返回长度】
            //【transferToEnabled即spark.file.transferTo参数，默认值true，采用NIO zero-copy方式复制；false就采用传统BIO方式】
            lengths[i] = Utils.copyStream(in, out, false, transferToEnabled);
            copyThrewException = false;
          } finally {
            Closeables.close(in, copyThrewException);
          }
          if (!file.delete()) {
            logger.error("Unable to delete file for partition {}", i);
          }
        }
      }
      threwException = false;
    } finally {
      Closeables.close(out, threwException);
      writeMetrics.incWriteTime(System.nanoTime() - writeStartTime);
    }
    partitionWriters = null;
    //【返回各分区的长度】
    return lengths;
  }

总结

bypass shuffle write流程简图