@Override
    public String toString() {
      return getClass().getName()
          + "'; sender="
          + getSender()
          + "; processorId="
          + processorId
          + ")";
    }
override in subclass to provide reply data. terminate by returning Token.END_OF_STREAM
geode-core/src/main/java/org/apache/geode/distributed/internal/streaming/StreamingOperation.java