@Override
  public SamzaRelConverter create(SystemStream systemStream, RelSchemaProvider relSchemaProvider, Config config) {
    return new SampleRelConverter(systemStream, (AvroRelSchemaProvider) relSchemaProvider, config);
  }
SampleRelConverter is an {@link AvroRelConverter} which identifies alternate messages as system messages.
This is used purely for testing system messages.
samza-sql/src/test/java/org/apache/samza/sql/util/SampleRelConverterFactory.java