@Override
  public String toString() {
    String separator = ",";
    return String.format("%s%s%s%s%s%s%s",
        changelogSSP.getSystem(), separator, changelogSSP.getStream(), separator,
        changelogSSP.getPartition().getPartitionId(), separator, changelogOffset);
  }
It is used for serde'ing {@link KafkaStateCheckpointMarker}s, in conjunction with {@link #deserialize(String)}.
@return the String representation of this {@link KafkaStateCheckpointMarker}
samza-core/src/main/java/org/apache/samza/checkpoint/kafka/KafkaStateCheckpointMarker.java