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From "Keith Turner (JIRA)" <>
Subject [jira] [Created] (ACCUMULO-3913) Add per table sampling
Date Mon, 22 Jun 2015 23:08:00 GMT
Keith Turner created ACCUMULO-3913:

             Summary: Add per table sampling
                 Key: ACCUMULO-3913
             Project: Accumulo
          Issue Type: Bug
            Reporter: Keith Turner
             Fix For: 1.8.0

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

  # Have each RFile store a sample per locality group.  Also store the configuration used
to generate the sample.
  # 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 thats consistent across files.
  # 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.
  # Support generating sample data for RFiles generated for bulk import
  # Support sample data in in memory map
  # 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();

  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.

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