@Override
    public boolean isSystemMessage(KV<Object, Object> kv) {
      // Return alternate ones as system messages.
      return (i++) % 2 == 0;
    }
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