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From "Jacek Migdal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4218) Delta Encoding of KeyValues (aka prefix compression)
Date Wed, 17 Aug 2011 22:32:27 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13086650#comment-13086650
] 

Jacek Migdal commented on HBASE-4218:
-------------------------------------

So far the implemented interface looks like:
{noformat} 
/**
 * Fast compression of KeyValue. It aims to be fast and efficient
 * using assumptions:
 * - the KeyValue are stored sorted by key
 * - we know the structure of KeyValue
 * - the values are iterated always forward from beginning of block
 * - application specific knowledge 
 * 
 * It is designed to work fast enough to be feasible as in memory compression.
 */
public interface DeltaEncoder {
  /**
   * Compress KeyValues and write them to output buffer.
   * @param writeHere Where to write compressed data.
   * @param rawKeyValues Source of KeyValue for compression.
   * @throws IOException If there is an error in writeHere.
   */
  public void compressKeyValue(OutputStream writeHere, ByteBuffer rawKeyValues)
      throws IOException;
  
  /**
   * Uncompress assuming that original size is known.
   * @param source Compressed stream of KeyValues.
   * @param decompressedSize Size in bytes of uncompressed KeyValues.
   * @return Uncompressed block of KeyValues.
   * @throws IOException If there is an error in source.
   * @throws DeltaEncoderToSmallBufferException If specified uncompressed
   *    size is too small.
   */
  public ByteBuffer uncompressKeyValue(DataInputStream source,
      int decompressedSize)
          throws IOException, DeltaEncoderToSmallBufferException;
}
{noformat}

I also need some kind of interface for iterating and seeking. I haven't got it yet but would
like to have something like:
{noformat}
  public Iterator<KeyValue> getIterator(ByteBuffer encodedKeyValues);
  public Iterator<KeyValue> getIteratorStartingFrom(ByteBuffer encodedKeyValues, byte[]
keyBuffer, int offset, int length);
{noformat}
For me it would work, but for you I might have changing it to something like:
{noformat}
  public EncodingIterator getState(ByteBuffer encodedKeyValues);
class EncodingIterator implements Iterator<KeyValue> {
...
  public void seekToBeginning();
  public void seekTo(byte[] keyBuffer, int offset, int length);
{noformat}

I will figure out how we could share the code.

> Delta Encoding of KeyValues  (aka prefix compression)
> -----------------------------------------------------
>
>                 Key: HBASE-4218
>                 URL: https://issues.apache.org/jira/browse/HBASE-4218
>             Project: HBase
>          Issue Type: Improvement
>          Components: io
>            Reporter: Jacek Migdal
>              Labels: compression
>
> A compression for keys. Keys are sorted in HFile and they are usually very similar. Because
of that, it is possible to design better compression than general purpose algorithms,
> It is an additional step designed to be used in memory. It aims to save memory in cache
as well as speeding seeks within HFileBlocks. It should improve performance a lot, if key
lengths are larger than value lengths. For example, it makes a lot of sense to use it when
value is a counter.
> Initial tests on real data (key length = ~ 90 bytes , value length = 8 bytes) shows that
I could achieve decent level of compression:
>  key compression ratio: 92%
>  total compression ratio: 85%
>  LZO on the same data: 85%
>  LZO after delta encoding: 91%
> While having much better performance (20-80% faster decompression ratio than LZO). Moreover,
it should allow far more efficient seeking which should improve performance a bit.
> It seems that a simple compression algorithms are good enough. Most of the savings are
due to prefix compression, int128 encoding, timestamp diffs and bitfields to avoid duplication.
That way, comparisons of compressed data can be much faster than a byte comparator (thanks
to prefix compression and bitfields).
> In order to implement it in HBase two important changes in design will be needed:
> -solidify interface to HFileBlock / HFileReader Scanner to provide seeking and iterating;
access to uncompressed buffer in HFileBlock will have bad performance
> -extend comparators to support comparison assuming that N first bytes are equal (or some
fields are equal)
> Link to a discussion about something similar:
> http://search-hadoop.com/m/5aqGXJEnaD1/hbase+windows&subj=Re+prefix+compression

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