Question

我希望能够创建一个读取序列文件的自定义InputFormat，但还会在该文件中公开记录所在的文件路径和偏移量。

要退一步，这是用例：我有一个包含可变大小数据的序列文件。密钥几乎不相关，并且值高达几兆字节，包含各种不同的字段。我想将elasticsearch中的一些字段与文件名和偏移量一起索引。这样，我可以从elasticsearch中查询这些字段，然后使用文件名和偏移量返回到序列文件并获取原始记录，而不是将整个内容存储在ES中。

我将整个过程作为单个java程序运行。 SequenceFile.Reader类可以方便地提供getPosition和seek方法来实现此目的。

然而，最终将涉及许多TB的数据，因此我需要将其转换为MapReduce作业（可能仅限Map）。由于序列文件中的实际键是无关紧要的，我希望采用的方法是创建一个自定义的InputFormat，它扩展或以某种方式利用SquenceFileInputFormat，而不是返回实际的键，而是返回一个由文件组成的复合键和抵消。

然而，事实证明这在实践中更加困难。看起来它应该是可能的，但考虑到实际的API以及暴露的内容，它很棘手。有任何想法吗？也许我应该采取另一种方法？

Answer 1

如果有人遇到类似的问题，这就是我提出的解决方案。我最终简单地复制了SequenceFileInputFormat / RecordReader中的一些代码，只是修改它。我原本希望写一个子类或装饰器或者其他东西......这种方式并不漂亮，但它有效：

SequenceFileOffsetInputFormat.java：

import java.io.IOException;
import java.util.List;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hadoop.mapreduce.JobContext;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.TaskAttemptContext;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.FileSplit;
import org.apache.hadoop.mapreduce.lib.input.SequenceFileInputFormat;

public class SequenceFileOffsetInputFormat<V extends Writable> extends FileInputFormat<PathOffsetWritable, V> {

    private static class SequenceFileOffsetRecordReader<V extends Writable> extends RecordReader<PathOffsetWritable, V> {

        private SequenceFile.Reader in;
        private long start;
        private long end;
        private boolean more = true;
        private PathOffsetWritable key = null;
        private Writable k = null;
        private V value = null;
        private Configuration conf;

        @Override
        public void initialize(InputSplit split, TaskAttemptContext context) throws IOException, InterruptedException {
            FileSplit fileSplit = (FileSplit) split;
            conf = context.getConfiguration();
            Path path = fileSplit.getPath();
            FileSystem fs = path.getFileSystem(conf);
            this.in = new SequenceFile.Reader(fs, path, conf);
            try {
                this.k = (Writable) in.getKeyClass().newInstance();
                this.value = (V) in.getValueClass().newInstance();
            } catch (InstantiationException e) {
                throw new IOException(e);
            } catch (IllegalAccessException e) {
                throw new IOException(e);
            }
            this.end = fileSplit.getStart() + fileSplit.getLength();

            if (fileSplit.getStart() > in.getPosition()) {
                in.sync(fileSplit.getStart());
            }

            this.start = in.getPosition();
            more = start < end;

            key = new PathOffsetWritable(path, start);
        }

        @Override
        public boolean nextKeyValue() throws IOException, InterruptedException {
            if (!more) {
                return false;
            }
            long pos = in.getPosition();

            more = in.next(k, value);
            if (!more || (pos >= end && in.syncSeen())) {
                key = null;
                value = null;
                more = false;
            } else {
                key.setOffset(pos);
            }
            return more;
        }

        @Override
        public PathOffsetWritable getCurrentKey() {
            return key;
        }

        @Override
        public V getCurrentValue() {
            return value;
        }

        @Override
        public float getProgress() throws IOException, InterruptedException {
            if (end == start) {
                return 0.0f;
            } else {
                return Math.min(1.0f, (in.getPosition() - start) / (float)(end - start));
            }
        }

        @Override
        public void close() throws IOException {
            in.close();
        }

    }

    @Override
    public RecordReader<PathOffsetWritable, V> createRecordReader(InputSplit split, TaskAttemptContext context) throws IOException, InterruptedException {
        return new SequenceFileOffsetRecordReader<V>();
    }

    @Override
    public List<InputSplit> getSplits(JobContext context) throws IOException {
        return new SequenceFileInputFormat<PathOffsetWritable, V>().getSplits(context);
    }

    @Override
    public long getFormatMinSplitSize() {
        return SequenceFile.SYNC_INTERVAL;
    }


}

PathOffsetWritable.java：

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.WritableComparable;

public class PathOffsetWritable implements WritableComparable<PathOffsetWritable> {

    private Text t = new Text();
    private Path path;
    private long offset;

    public PathOffsetWritable(Path path, long offset) {
        this.path = path;
        this.offset = offset;
    }

    public Path getPath() {
        return path;
    }

    public long getOffset() {
        return offset;
    }

    public void setPath(Path path) {
        this.path = path;
    }

    public void setOffset(long offset) {
        this.offset = offset;
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        t.readFields(in);
        path = new Path(t.toString());
        offset = in.readLong();
    }

    @Override
    public void write(DataOutput out) throws IOException {
        t.set(path.toString());
        t.write(out);
        out.writeLong(offset);
    }

    @Override
    public int compareTo(PathOffsetWritable o) {
        int x = path.compareTo(o.path);
        if (x != 0) {
            return x;
        } else {
            return Long.valueOf(offset).compareTo(Long.valueOf(o.offset));
        }
    }


}

扩展SequenceFileInputFormat以包含文件名+偏移量

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