What causes "A connection of type PALO is expected" in pentaho/pentaho-kettle?

Select a Palo-type database connection for the step in the dialog.

pentaho/pentaho-kettle · error · KettleException

A connection of type PALO is expected

Error message

A connection of type PALO is expected

What it means

Thrown in the PaloCellOutputData constructor when the supplied DatabaseMeta is not a PALO-type connection. Palo cell output can only run against a Palo OLAP connection, so non-PALO interfaces are rejected before the PaloHelper is created.

Solutions

Select a Palo-type database connection for the step in the dialog.
Programmatically verify databaseMeta.getDatabaseInterface() instanceof PALODatabaseMeta before instantiating.
Ensure the Palo connection definition is exported/imported together with the transformation.

Example fix

// before
new PaloCellOutputData(relationalDbMeta); // throws
// after
if (dbMeta.getDatabaseInterface() instanceof PALODatabaseMeta) {
  data = new PaloCellOutputData(dbMeta);
}

Defensive patterns

Strategy: type-guard

Validate before calling

if (!(dbMeta.getDatabaseInterface() instanceof PALODatabaseMeta)) { throw new IllegalArgumentException("Step requires a PALO connection"); }

Type guard

boolean isPalo(DatabaseMeta dbMeta) { return dbMeta != null && dbMeta.getDatabaseInterface() instanceof PALODatabaseMeta; }

Try / catch

try { data = new PaloCellOutputData(dbMeta); } catch (KettleException e) { /* wrong connection type — reconfigure step */ }

Prevention

Only attach Palo connections to Palo Cell Output steps
Verify connection references after moving transformations between environments
Check connection type in code when building metadata programmatically

When it happens

Trigger: Creating PaloCellOutputData with a DatabaseMeta whose getDatabaseInterface() is not PALODatabaseMeta — the step's connection points to a relational database instead of Palo.

Common situations: Wrong connection selected in the Palo Cell Output dialog; transformation transported between environments losing the Palo connection reference; programmatic construction with an arbitrary DatabaseMeta.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of pentaho/pentaho-kettle@f3058517a1 (2026-09-13). Data as JSON: /api/errors/58828b6d68b2c5c7. Report an issue: GitHub.

Appendix: source

Thrown at plugins/palo/core/src/main/java/org/pentaho/di/trans/steps/palo/celloutput/PaloCellOutputData.java:43

import java.util.List;

import org.pentaho.di.core.database.DatabaseMeta;
import org.pentaho.di.core.database.PALODatabaseMeta;
import org.pentaho.di.core.exception.KettleException;
import org.pentaho.di.core.logging.DefaultLogLevel;
import org.pentaho.di.palo.core.PaloHelper;
import org.pentaho.di.trans.step.BaseStepData;
import org.pentaho.di.trans.step.StepDataInterface;

public class PaloCellOutputData extends BaseStepData implements StepDataInterface {
  public PaloHelper helper;
  public int[] indexes;
  public List<Object[]> batchCache = new ArrayList<Object[]>();

  public PaloCellOutputData( DatabaseMeta databaseMeta ) throws KettleException {
    super();
    if ( !( databaseMeta.getDatabaseInterface() instanceof PALODatabaseMeta ) ) {
      throw new KettleException( "A connection of type PALO is expected" );
    }
    this.helper = new PaloHelper( databaseMeta, DefaultLogLevel.getLogLevel() );
  }
}

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