[ACCUMULO-3913] Add per table sampling

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.8.0
• Assignee: Keith Turner
• Reporter: Keith Turner
• Created: 22/Jun/15 23:07
• Updated: 11/Jul/16 02:03
• Resolved: 27/May/16 18:27

Description

I am working on prototyping adding hash based sampling to Accumulo. I am trying to accomplish the following goals in the prototype.

1. Have each RFile store a sample per locality group. Also store the configuration used to generate the sample.
2. Use sampling functions that ensure the same row columns exist across the samples in all RFiles. Hash mod is a good candidate that gives a random sample that's consistent across files.
3. Have scanners support scanning RFile's samples sets. Scan should fail if RFiles have different sample configuration. Different sampling config implies the RFile's sample sets contain a possibly disjoint set of row columns.
4. Support generating sample data for RFiles generated for bulk import
5. Support sample data in the memory map
6. Support enabling and disabling sampling per table AND configuring a sample function.

I am currently using the following function in my prototype to determine what data an RFile stores in its sample set. This code will always select same subset of rows for each RFile's sample set. I have not yet made the function configurable.

public class RowSampler implements Sampler {

  private HashFunction hasher = Hashing.murmur3_32();

  @Override
  public boolean accept(Key k) {
    ByteSequence row = k.getRowData();
    HashCode hc = hasher.hashBytes(row.getBackingArray(), row.offset(), row.length());
    return hc.asInt() % 1009 == 0;
  }
}

Although not yet implemented, the divisor in this RowSample could be configurable. RFiles with sample data would store the fact that a RowSample with a divisor of 1009 was used to generate sample data.

Issue Links

• breaks: ACCUMULO-4363 Invalid class name for RowSampler in ShellServerIT
• is related to: ACCUMULO-4013 Clean up new warnings
• links to: Github Pull Request
• links to: GitHub Pull Request #46
• links to: Link to prototype branch on GH