apache/druid · error · IllegalArgumentException

Cannot have empty worker set

Error message

Cannot have empty worker set

What it means

ReadablePartition records which workers hold a given input partition; an empty worker set is meaningless because MSQ needs at least one worker to read each partition. The constructor validates this with an IAE.

Solutions

1. Ensure the worker assignment code only creates ReadablePartitions for partitions actually assigned to >= 1 worker.
2. Verify the MSQ cluster has at least one available worker (middleManager/indexer capacity) before submitting queries.
3. In tests, pass a non-empty worker set, e.g. ImmutableSet.of(0).

Example fix

// before
new ReadablePartition(stageNumber, ImmutableSet.of(), partitionNumber);
// after
new ReadablePartition(stageNumber, ImmutableSet.of(0), partitionNumber);

if (workerNumbers == null || workerNumbers.isEmpty()) {
  throw new IllegalArgumentException("workerNumbers must be non-empty");
}
ReadablePartition p = new ReadablePartition(stageNumber, workerNumbers, partitionNumber);

try {
  return new ReadablePartition(stageNumber, workerNumbers, partitionNumber);
} catch (IllegalArgumentException e) {
  if (e.getMessage() != null && e.getMessage().equals("Cannot have empty worker set")) {
    log.warn("Skipping partition with no assigned workers");
    return null;
  }
  throw e;
}

When it happens

Trigger: Constructing ReadablePartition (directly in tests or via shuffle-spec/partition assignment code) with an empty workerNumbers set - e.g. when partition assignment logic assigns no workers because maxNumSlices is 0 or the worker list was empty.

Common situations: Custom shuffle strategies or unit tests passing an empty ImmutableSet/ImmutableList of workers; cluster had zero workers so the assignment produced empty sets.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• Broadcast input number out of range
• Can not supply empty segments as input, please use either…
• Cannot have fault tolerance without durable storage
• Cannot mix sortable and unsortable key columns
• Cannot shuffle with spec

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/4b94ac9aa9e1b222. Report an issue: GitHub.

Appendix: source

Thrown at multi-stage-query/src/main/java/org/apache/druid/msq/input/stage/ReadablePartition.java:45

/**
 * Represents an output partition associated with a particular stage. Each partition is readable from one or
 * more workers.
 */
public class ReadablePartition
{
  private final int stageNumber;
  private final IntSortedSet workerNumbers;
  private final int partitionNumber;

  private ReadablePartition(final int stageNumber, final IntSortedSet workerNumbers, final int partitionNumber)
  {
    this.stageNumber = stageNumber;
    this.workerNumbers = workerNumbers;
    this.partitionNumber = partitionNumber;

    if (workerNumbers.isEmpty()) {
      throw new IAE("Cannot have empty worker set");
    }
  }

  /**
   * Returns an output partition that is striped across {@code numWorkers} workers.
   */
  public static ReadablePartition striped(final int stageNumber, final int numWorkers, final int partitionNumber)
  {
    final IntAVLTreeSet workerNumbers = new IntAVLTreeSet();
    for (int i = 0; i < numWorkers; i++) {
      workerNumbers.add(i);
    }

    return new ReadablePartition(stageNumber, workerNumbers, partitionNumber);
  }

  /**
   * Returns an output partition that is striped across a set of {@code workerNumbers}.

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